**A Collaborative Approach to Better Understanding Northwest Flow Snow**

**in the Southern Appalachians**

**Steve Keighton, Laurence Lee, Blair Holloway, David Hotz, Steven
Zubrick, Jeffrey Hovis, Gary Votaw, Baker Perry, Gary Lackmann, Sandra
Yuter, Charles Konrad, Douglas Miller, and** **Brian Etherton**

 

*Abstract*

 

*Upslope-enhanced snowfall events during periods of northwesterly flow
in the southern Appalachians have been recognized as a significant
winter* *forecasting problem* *for some time. However,* *only in recent
years has this problem received noteworthy attention by both the
academic and operational communities.  The fascinating meteorology of
these events includes the detailed topographic influences, as well as*
*a linkage between the upstream Great Lakes and resultant* *southern
Appalachian snowfall.  A unique collaborative team has recently formed,
working towards the goals of improving* *the physical understanding of
the mechanisms at work in these events, as well as developing more
accurate forecasts and more detailed climatologies.  The literature
shows only limited attention to this problem through the 1990s. However,
with modernization of the National Weather Service (NWS) in the
mid-1990s came opportunities to bring more attention to new or poorly
understood forecast problems. These opportunities included
meteorologists at additional forecast offices often co-located with
universities, the deployment of the WSR-88D* *radar network, expansion
of the surface observational network in both* *space and time, improved
access to sophisticated numerical models, and expansion of the spotter
and cooperative observer networks. *

 

*A collaborative team, consisting of faculty from five universities and
meteorologists from six NWS forecast offices, has established an
ongoing, structured dialogue* *to further understanding and improve
forecasting of these events. The team utilizes* *a variety of
communication strategies to discuss* *emerging* *research* *findings,
review recent events, and share data and* *ideas. The ultimate goal is
to continue fostering working relationships between research and
operational meteorologists and climatologists as well as students, all
with a common motivation of continually improving forecasts and
understanding of this important phenomenon. This group may serve as a
model for other collaborative efforts between the research and
operational communities interested in a common forecast problem.*

 

**AFFILIATIONS: ** KEIGHTON - National Weather Service, Blacksburg, VA;
LEE and HOLLOWAY - National Weather Service, Greenville-Spartanburg, SC;
HOTZ - National Weather Service, Morristown, TN; ZUBRICK - National
Weather Service, Sterling, VA; HOVIS - National Weather Service,
Charleston, WV; VOTAW - National Weather Service, Jackson, KY; PERRY -
Appalachian State University, Boone, NC; LACKMANN and YUTER - North
Carolina State University, Raleigh, NC; KONRAD - University of North
Carolina at Chapel Hill, NC; MILLER - University of North Carolina at
Asheville, NC; ETHERTON - University of North Carolina at Charlotte, NC.

**CORRESPONDING AUTHOR:** Steve Keighton, National Weather Service, 1750
Forecast Dr., Blacksburg, VA 24060

Email: stephen.keighton@noaa.gov

**A Collaborative Approach to Better Understanding Northwest Flow Snow**

**in the Southern Appalachians**

**Steve Keighton, Laurence Lee, Blair Holloway, David Hotz, Steven
Zubrick, Jeffrey Hovis, Gary Votaw, Baker Perry, Gary Lackmann, Sandra
Yuter, Charles Konrad, Douglas Miller,** **and Brian Etherton**

**I. Introduction**

Snowfall associated with periods of upslope northwest flow in the
southern Appalachians has been a forecast problem for many years. The
highly variable nature of the snowfall in terms of duration, spatial
distribution, and amount is such that the observational and forecast
techniques until fairly recently were not conducive to providing useful
advance notice of these events. Frequently, forecasts of post-cold
frontal weather mentioned only "snow flurries" which implied no
accumulation. Indeed, flurries often occurred, but many events also
included several hours of snow and heavy snow showers resulting in
sufficient accumulation to cause travel problems and to close schools
and businesses. The purpose of this article is to describe the unique
collaborative arrangement that has been organized to address the
northwest flow snow (NWFS) problem in the southern Appalachian region,
and to document the advantages to research and operations that have
subsequently come about.

Keeter et al. (1995) provide an overview of the effects of topography on
winter storms in the southeast United States. However, NWFS in the
southern Appalachians is not specifically addressed in their work.  St.
Jean et al. (2004) examine northwest flow regimes that produce
orographically enhanced snowfall in the northeastern United States, but
until recently very little attention was devoted to NWFS in the southern
Appalachians.  Initial climatological NWFS studies (e.g., Perry 2006,
Perry et al. 2007b) noted the potential contribution of the upstream
Great Lakes, consistent with the findings of Sousounis and Fritsch
(1994), who first emphasized the regional character of the lake
aggregate influence.

The previous lack of attention to NWFS in the weather analysis and
forecasting literature may be due in part to the absence of an
observational and climatological basis for identifying the events. In
addition, the application of numerical weather prediction models to
study the mesoscale features and processes associated with NWFS has not
occurred until recently. Furthermore, a need for studying these unique
events has become increasingly apparent because of the significant
public and economic impact caused by the highly variable spatial and
temporal distribution of snowfall in this region (much of which is
experiencing rapid growth in population and winter tourism).

To more specifically define NWFS in the southern Appalachian region,
these events occur during periods of moist, upslope flow in association
with northwest low-level wind, often resulting in significant snowfall
in the absence of any non-orographic synoptic scale upward vertical
motion. Figure 1 shows the synoptic surface map during the heart of a
significant NWFS event in December 2003, with the cold front well
offshore and only a weak low pressure center near the Mid-Atlantic
coast. Snowfall totals can range from a trace to over a meter from event
to event or even within a single event. Very sharp snow accumulation
gradients along the spine of the southern Appalachians are created by
the superposition of the synoptic scale flow across the long, but rather
narrow, mountain range and the smaller scale upward and downward motions
induced by the numerous ridges and valleys. Figure 2 shows the high
variability in the final snowfall totals for the same December 2003
event depicted synoptically in Figure 1, while Figure 3 includes the
details of the southern Appalachian terrain for comparison. Most of the
higher ridges extend above 1000m MSL, with a few peaks extending above
1500m, the highest of which reaches a little over 2000m.

Several occurrences of NWFS during the early and mid 1990s led to the
eventual collaboration discussed here. The National Weather Service
(NWS) field structure was reorganized so that individual Warning and
Forecast Offices (WFO) had smaller areas of responsibility than the
previous Weather Service Forecast Offices (WSFO) that served entire
states or portions of large states. This allowed forecasters to focus on
smaller areas and to become more familiar with the local climatology,
terrain features, and the interaction of weather systems with terrain.
Also, the new WFOs were equipped with improved data processing and
computing capabilities which included the use of locally run mesoscale
forecast models. The implementation of the WSR-88D radar network also
provided improved surveillance of precipitation systems. At the same
time, a mechanism was put in place to bring together forecasters in both
the public and private sectors with scientists at colleges,
universities, and atmospheric science research laboratories. One such
initiative was the 1994 collocation of the WFO at Raleigh, NC with the
Department of Marine, Earth, and Atmospheric Sciences at North Carolina
State University (NCSU). This union was the culmination of a gradual
evolution of collaboration that started in the 1970s and steadily
matured through the 1980s into the 1990s. By the late 1990s, the
Collaborative Science, Technology, and Applied Research (CSTAR) program
provided funding for researchers at NCSU to collaborate with
meteorologists at nearby WFOs with the goal of studying
topographically-forced weather systems in the Carolinas and Virginia.

The initial CSTAR efforts did not examine northwest flow snow, but
operational forecasters and university researchers worked together to
identify and study difficult regional forecast problems. The
relationships that were forged and the positive results that were
obtained from these collaborative efforts led to additional CSTAR
funding in 2003 that supported an effort to improve quantitative
precipitation forecasting in the southeast U.S. Even though NWFS was not
a primary CSTAR topic, the program provided a means for interested
forecasters and researchers to discuss this challenging wintertime
precipitation forecast problem. CSTAR workshops created an opportunity
for the sharing of NWFS studies which introduced participants to the
interesting multi-perspective (viz., observational, climatological,
meteorological) examination of the phenomenon. The meetings strengthened
existing NCSU-NWS ties, and improved collaboration and communication
among the parties who study the various aspects of northwest flow snow
events (e.g., Perry 2006; Perry and Konrad 2006b). Recent collaborations
among Appalachian State University (ASU), North Carolina State
University (NCSU), and the University of North Carolina at Asheville
(UNCA) have brought more advanced observational tools such as vertically
pointing radar and mobile sounding equipment into the southern
Appalachians.

As WFOs improved their own understanding of NWFS and became acquainted
with what neighboring offices and researchers were discovering, the
opportunity arose to create a small group outside the formal CSTAR
umbrella.   This article will describe the major goals of this
collaboration group, how it functions and communicates, and the benefits
it has provided to both the research and operational communities in the
southern Appalachian region.   This NWFS collaborative group promotes
the documentation and study of individual events, shares observational
and research information, identifies topics requiring additional
research, and develops forecast strategies and methodologies.

**II. Cross-discipline collaboration group**

Since forming in the fall of 2005, the Northwest Flow Snow Group has
continued collaborating on research and operational aspects of the
northwest flow snow problem via a number of means, and has also grown in
size. Members of the academic community participating in the group in
various capacities come from several universities in the North Carolina
state system, including: NCSU, ASU, UNCA, UNC at Charlotte, and UNC at
Chapel Hill. They include meteorologists specializing in synoptic scale
meteorology, numerical modeling, and instrumentation, and also include
climatologists from two geography departments. National Weather Service
participation in the group primarily consists of the Science and
Operations Officers, as well as one or two forecasters from a cluster of
forecast offices which have responsibility for the southern Appalachian
region. These offices include Baltimore/Washington (LWX),
Blacksburg/Roanoke, VA (RNK), Greenville/Spartanburg, SC (GSP),
Morristown/Tri-Cities, TN (MRX), Jackson, KY (JKL), and Charleston, WV
(RLX) (Fig. 3). Collectively, discussion centers on how to learn from
and improve on various mesoscale modeling configurations, promote
further understanding of the role of the Great Lakes in providing
supportive moisture and thermal profiles, and explore how unique local
instrumentation can provide new insights into the NWFS forecast problem.
Improved knowledge of the climatology of NWFS events includes
computation of snow-to-liquid ratios, expansion of snow spotter
networks, and better utilization of high resolution satellite imagery.
Better definition of the relative roles of orographic forcing versus the
vertical motion from instability and synoptic and mesoscale forcing of
the free atmosphere is also sought. The diversity of the expertise
within the group allows for effective sharing of knowledge and
identification of collaborative opportunities for potential new research
projects for students and faculty, and ultimately helps the NWS offices
improve forecasting methodologies for NWFS events.

The collaboration among group members has been more or less continuous
since the initial formation of group, regardless of whether a specific
formal research effort is in progress or not. The methods of
communication are diverse, and have effectively maintained an
enthusiastic environment for sharing and learning. These methods
include: 1) routine (typically monthly) conference calls; 2) an email
listserv discussion forum with a broader audience than the typical
conference call group, designed primarily for operational discussion of
upcoming or recent events (this broader audience includes additional
forecasters from the cluster of NWS forecast offices as well as
meteorologists from the Hydrometeorological Prediction Center and
Eastern Region Scientific Services Division); 3) simple web-based event
reviews to facilitate discussion and develop an historical archive, and;
4) a group web page used for central storage of these event reviews,
conference call minutes, presentations on preliminary research results,
published references, and operational links, including local mesoscale
model data and a variety of observations \[Available from:

[**http://www.erh.noaa.gov/gsp/localdat/NWFS_discussion_group/nwfs_discussion_group.html**](http://www.erh.noaa.gov/gsp/localdat/NWFS_discussion_group/nwfs_discussion_group.html)\].

As long as there is continued interest and new research opportunities
present themselves, this group is expecting to maintain frequent
communication. We anticipate over the next couple of years that the
group will help develop additional publications on various aspects of
the NWFS forecast problem, facilitate enhancements to local model
configurations (potentially including a "collaborative ensemble"
approach), continued expansion of ground truth reporting networks,
development of new operational forecasting methods, and ongoing specific
research, data collection, and field projects as funding allows. The
coordination and collaboration afforded by the group enables us to
improve physical understanding of NWFS processes while engaged in
operational forecasting of the events. This arrangement provides results
that could not be anticipated without the unique collaboration and
relationships that have developed with the formation of this group.

**III. Scientific and Operational Results**

*A. Climatology*

Previous climatological research has demonstrated that NWFS events are
common along the windward slopes and at higher elevations of the
southern Appalachian Mountains, averaging nearly 15 events per season
and accounting for over 50 percent of mean annual snowfall totals (Perry
2006; Perry et al. 2007a). At the higher elevation windward slopes, such
as Mt. LeConte, TN (1951 m, 250 cm mean annual snowfall), NWFS
contributes 57% of mean annual snowfall, totaling 143 cm (Perry et al.
2007a). Interestingly, 60% of all NWFS events analyzed during a 50-year
period were accompanied by synoptic-scale subsidence at the 700-hPa
level, highlighting the importance of orographic processes during these
snowfall events (Perry 2006). As a result of the pronounced topographic
forcing, the spatial patterns of snowfall for a typical NWFS event
display complex patterns that are strongly related to the topography
(Fig. 4). In particular, NWFS totals are most positively correlated with
the topographic variables of elevation and exposure to the northwest
(Perry and Konrad 2006b).

 

Schmidlin (1992) initially suggested that the Great Lakes play an
important role in enhancing NWFS events in association with favorable
air trajectories, particularly in the mountains of West Virginia. In a
longer-term climatological analysis, Perry et al. (2007b) found that
upstream air trajectories with a Great Lakes connection were tied to
higher composite mean and maximum point snowfall totals along higher
elevation windward slopes when compared with other northwest
trajectories. They noted little effect on snowfall totals at lower
elevations or along leeward slopes. Upstream air trajectories with a
Great Lakes connection were also tied to higher values of moisture and
humidity in the lower troposphere, as well as a thicker moist layer (202
hPa depth from the surface to the top of the moist layer with the Great
Lakes connection versus 140 hPa depth for the non-Great Lakes
connection). In some instances, the spatial patterns of snowfall were
strongly influenced by the upstream air trajectories, such that maximum
snowfall totals occurred in association with favorable air trajectories
extending downwind from Lake Michigan. Observational evidence from
satellite and radar imagery (Fig. 5) of banding at various scales during
many NWFS events strongly supports an important connection to the Great
Lakes as well. A classic example of the snowfall variability is depicted
in a false color polar orbiter satellite image during the 6 March 2001
event, which resulted in 10-30 cm of snowfall in higher elevations and
along northwestern slopes of the southern Appalachians (Fig. 5).

*B. Observations*

Several recent collaborative initiatives have greatly improved our
observational networks, leading to real-time forecasting benefits as
well as improved understanding of the physical processes and spatial
patterns of snow accumulation associated with NWFS.  Expansion of the
Community Collaborative Rain, Hail, and Snow (CoCoRaHS) Network (Reges
et al. 2008) to the southern Appalachians has significantly improved the
density and quality of snowfall, snow water equivalent (SWE), and snow
density measurements in some areas. CoCoRaHS data, when combined with
observations from NWS cooperative observers (Fig. 6), and other reports
from local communication centers, law enforcement, and the general
public, have been very helpful in capturing the mesoscale variability of
NWFS as well as providing data for use in the analysis of model
predictions.

 

A vertically-pointing METEK Micro Rain Radar (MRR) (Peters et al. 2002),
PARSIVEL disdrometer (Löffler-Mang and Joss 2000; Löffler-Mang and
Blahak 2001; Yuter et al. 2006), Pluvio weighing precipitation gauge,
and a research-quality meteorological station have been operational
since October 2006 at two sites on Poga Mountain (1018 and 1137 m) along
the NC-TN border (Fig. 6)**.** The 2006-2007 snow season was quite
active, with 20 snow events identified. Most of the snow events were
shallow, with 14 out of 20 storms having 3 dBZ radar echo heights less
than 2.5 km above ground level. Ninety-one percent of the total SWE
occurred in association with NWFS. When inversions were present, snow Z
layer depth was strongly correlated to inversion height (Yuter and Perry
2007). The cellular nature of both the precipitation and velocity fields
and the shallowness of the precipitating layer are evident in NWFS
cases, highlighted here in the 7 April 2007 event (Fig. 7). Since fall
speeds of snow are usually \~2 m s^-1^, Doppler velocities of less than
to 2 m s^-1^ indicate likely updrafts (Yuter and Houze 2003).

 

A mobile sounding unit was deployed to the Poga Mountain site for the
2007-2008 season to release rawinsondes during snow events. The sounding
data will be used to determine moist layer temperature and thickness, as
well as to document the presence and height of a capping inversion. An
additional PARSIVEL disdrometer, Pluvio weighing precipitation gauge,
and other meteorological sensors have also been installed on the summit
of nearby Beech Mountain (1678 m) to analyze the effects of elevation on
particle size and fall speed, snowfall, snow water equivalent, and new
snowfall density. In support of the 2007-2008 snow season activities,
the State Climate Office of NC has also developed a near real-time web
interface to bring together the aforementioned observational datasets
for the forecasting and academic communities, as well as the general
public
[http://www.nc-climate.ncsu.edu/cronos/poga/map.php](http://www.ncclimate.ncsu.edu/cronos/poga/map.php)).  \
\
*C. Mesoscale Modeling*

As earlier climatological studies (Perry 2006; Perry et al. 2007b)
demonstrated that the Great Lakes can contribute to NWFS, some
scientific questions arose that were best addressed using numerical
model simulations of individual cases. For instance, what fraction of
the NWFS precipitation is attributable to the Great Lakes? What physical
processes control the varying contribution of the lakes to NWFS, and how
does the synoptic scale pattern relate to this variability? Also, how,
specifically, do the lakes alter precipitation processes in the upslope
flow region? To what extent does moistening versus destabilization of
the upstream air mass by the lakes alter the intensity and distribution
of NWFS? How is the pattern and strength of orographic lift affected by
the upstream presence of the Great Lakes?

In order to isolate the lake contributions to NWFS, model experiments
that effectively removed the Great Lakes were performed (hereafter
referred to as "No-Lake"

simulations[^1]), and compared to full-physics control simulations
(denoted "Control" simulations) that included the lakes (Holloway 2007).
These simulations utilized the WRF-EMS, at a 24km horizontal resolution,
and with the Betts-Miller-Janic (BMJ) convective parameterization
scheme. Three events were selected for modeling studies and were chosen
to represent a variety of synoptic flow patterns and NWFS intensity. The
quantitative contribution of the Great Lakes influence was found to be
highly variable between events.  An NWFS event with an intermediate lake
influence took place on 10-11 February 2005. Liquid equivalent
precipitation totals for this event in the control simulation ranged
from 0.15 to 0.30 in. (4 to 8 mm) along the spine of the Appalachian
mountains (not shown). As expected, when the lakes were removed from the
model atmosphere, large negative precipitation differences are found
over and immediately downwind of the lakes (Fig. 8).  Area-averaged
precipitation differences between the No-Lake and control simulations
demonstrate that the lakes provided a significant contribution to
precipitation in the southern Appalachian region during this event, with
up to 0.16\" (4 mm) less water equivalent precipitation produced in the
areas of heaviest NWFS in the No-Lake simulation relative to the
control. In some locations, the No-Lake simulation actually produced
more precipitation than the control (Fig. 8); these localized areas were
presumably due to shifts in patterns of ascent and compensating
subsidence.  For this event, the No-Lake simulation exhibited an
area-averaged precipitation reduction of 40% at locations in the
southern Appalachians.  In another event, comparison of No-Lake and
Control simulations revealed little difference in precipitation totals,
due to more westerly upstream flow trajectories (not shown). 

The model experiments reveal that the influence of the Great Lakes is
not exclusively limited to that of a moisture source. In No-Lake
simulations of NWFS events, the Froude number of the lower-tropospheric
flow impinging on the southern Appalachians was decreased relative to
that in the control simulation. This indicates that the lakes may
indirectly contribute to the ability of the air stream impinging on the
Southern Appalachians to ascend the barrier, resulting in enhanced
orographic lift.

Higher resolution experiments, capable of explicitly resolving
convective precipitation (e.g., 2 km grid spacing), are desirable to
more closely examine changes to the character of the snowfall due to the
upstream lake influence.  Nevertheless, the coarser simulations
described above, provide a quantitative estimate of the enhancement that
can result with a favorable Great Lakes trajectory, and confirm the
importance of this aspect during NWFS prediction. It should also be
noted that the studies mentioned here are only a starting point to
further understanding NWFS events in the southern Appalachians. It is
anticipated that future modeling research will address additional
questions related to the upstream lake influence on the character of the
downstream response. More cases with varying backward trajectories and
lake influences need to be examined to better understand this regional
forecasting problem.

Another avenue through which mesoscale models have been used by the
group is through the distribution of the workstation Weather Research
and Forecasting Environmental Modeling System (WRF-EMS) (Rozumalski
2006) into WFOs. Local WFOs are now using the Workstation WRF--EMS to
run domain--specific forecasts at a finer grid spacing than that of
current operational models. The model can be tuned and configured as an
office desires and the output data are often made available for public
view through the internet. The purpose of these local models is to allow
for better prediction of regional scale meteorological phenomena such as
NWFS events. Output from these high resolution models over the last
couple of seasons has confirmed the ability to capture the detailed
influence from the terrain quite accurately, but has also shown some
potential skill in identifying mesoscale banded structures associated
with trajectories off the Great Lakes. An 18‑hour forecast of 1‑hour
snowfall accumulation from a 5 km resolution WRF-EMS model running at
the Charleston, WV (RLX) WFO, for 27 February 2008 (Fig. 9), depicts the
resulting NWFS bands along the mountains. The northeast-to-southwest
oriented line of higher snow accumulation, with a sharp gradient on the
southeast edge, aligns precisely with the NW upslope locations (Fig. 3),
and an elongated northwest-to-southeast oriented band appears to be
extending from the vicinity of Lake Michigan.

The locally run mesoscale models are discussed through the group\'s
conference calls and listserv in order to coordinate changes in model
configuration and to gather any feedback on the effectiveness of the
output. Partially due to this coordination, a diversity of local WRF
configurations exists across the participating WFOs.  This includes
various domains sizes, horizontal grid spacing ranging from 4 to 12 km,
two different dynamical core packages, explicit vs. parameterized
convection schemes, different precipitation physics packages and
boundary layer schemes, and various methods for model initialization and
provision of boundary conditions. A few WFOs are running local versions
of the Workstation Eta model as well.

There is momentum within the NWFS group and the NWS in general to more
carefully coordinate local model configurations with the goal of
creating a high resolution pseudo-ensemble cluster of models within a
small region sharing similar forecast challenges. The path this group
has already taken toward local model coordination could be important
groundwork for a more formal, organized regional ensemble across the
southern Appalachians in the near future.

 

*D. Developing operational forecasting methods*

  

Armed with a greater knowledge of the various aspects of NWFS,
forecasters have naturally incorporated new techniques into different
stages of the forecast process.  For example, first-hand experience with
actual events and familiarity with numerous case studies have resulted
in a synoptic scale pattern recognition that allows forecasters to
usually anticipate NWFS snow events several days in advance.  The
recognition of the potential for an event is followed by analysis of
various numerical models available to the WFOs (including the local
models described earlier). Specifically, fields such as moisture depth
in relation to the vertical temperature structure, stability, wind
direction, wind speed, duration of the moist upslope flow, and the
presence or absence of short wave troughs embedded in the northwest
flow, are most carefully considered.  Recent work by Perry et al.
(2007a) and Holloway (2007) has demonstrated the value of assessing
trajectory analyses using HYSPLIT[^2] model (Draxler and Hess 1998)
runs to anticipate the possible Great Lakes connection (Fig. 10)
\[available online at
[http:www.arl.noaa.gov/ready/hysp_info.html](http://www.arl.noaa.gov/ready/hysp_info.html)\]. 
The climatology of snowfall distribution (Fig. 4), and increasing
knowledge of local terrain by forecasters in the modernized WFOs (as
higher resolution maps and map backgrounds become available in
operational software), has provided forecasters with a basis for
adjusting model QPF output.  As increasing knowledge about local
terrain, NWFS climatology, and model performance are continually shared
through this collaboration, local WFOs have felt more confident in
created pre-defined "edit areas" used in the preparation of gridded
weather forecasts to help more precisely depict where significant
snowfall is likely in these scenarios.

 

During the near-term, or nowcasting phase of an event, multiple
observational data sets are used to support short term forecast
updates.  WSR-88D reflectivity data provide real-time displays of the
areal distribution and intensity of the snowfall across the mountains
and upstream of western slopes.  The patterns of radar return in
combination with an assessment of the stability characteristics provide
forecasters with a basis for anticipating convective enhancement of
precipitation.  Recently, forecasters have had access to the Poga
Mountain vertically pointing radar data to assist in evaluating the type
and intensity of precipitation associated with precipitation elements
viewed on radar. In addition, a satellite enhancement curve that detects
stratiform cloud top temperatures in the range favorable for dendritic
ice crystal growth is used alongside the radar data to ascertain where
snow is occurring.  

Based on the gradual accumulation of experience and new information
revealed in formal studies, forecasters in the office cluster are
preparing "playbooks' specifically for NWFS events.  The playbooks are
designed to provide assistance in determining which data sets to analyze
at various phases of the forecast process and to identify implications
for modifying the forecast based on the outcome of the analyses.  We
anticipate that prior to the winter season of 2008-09, we will be able
to develop one or more standard playbooks covering NWFS forecast
methodologies. The incorporation of this collective knowledge and
available data sets will be shared with all WFOs in this cluster, and
perhaps with interested media partners in the region as well. The
purpose of developing such methodologies is to ensure that all
forecasters within a particular office and across all impacted offices
are considering the appropriate data and reaching similar conclusions.  
A consistent analysis and decision-making strategy among members of the
cluster enhances the likelihood of a collaborated forecast across County
Warning and Forecast Area boundaries.   Standard methodologies at
neighboring offices also provide a common starting point for training
new staff members not yet exposed to this forecast challenge.

The findings from the collaborated research have been incorporated into
the WFO training programs through office presentations, the Weather
Event Simulator (WES), office weather briefings, etc.  The WES is a
great training tool to pass the increased understanding of NWFS to the
operational forecasters.  Science and Operations Officers have the
forecasters work through a NWFS event and utilize the new forecasting
techniques to determine potential snow accumulations across the forecast
area. 

**IV. Conclusions**

 

Several benefits to the operational forecasting community in the
southern Appalachians stemmed from this Northwest Flow Snow
collaborative group. These include:

> direct involvement in applied research with the academic community,
>
> the opportunity for near-real-time feedback and discussion of
> impending, ongoing, and recent events,
>
> access to experimental data sets in near real-time,
>
> sharing ideas for improved forecast methods,
>
> the shared use of local mesoscale models among offices, and
>
> ultimately developing improved common forecast methodologies that are
> accepted and used throughout the region.

Furthermore, the benefits of such a collaboration go well beyond the
potential improvements to specific forecasts in northwest flow snow
regimes (which have already begun to take place, as described in this
paper). Relationships between operational and academic meteorologists
and climatologists have been established that will likely pave the way
for future collaborations. The personal connections and communication
infrastructure developed here can also be applied to a variety of other
potential forecast problems of mutual interest.

From the university standpoint, the collaborative arrangement that has
developed for the NWFS problem provides several benefits: (i)
operational forecasters are able to help steer research directions in a
way that maximizes societal benefit, (ii) the close scrutiny of weather
events by forecasters in real time provides an operational framework for
theoretical aspects of the research, and (iii) the collaboration
provides graduate and undergraduate students with an opportunity to gain
experience and make contacts in the professional forecasting ranks. This
in turn makes it easier to attract top students to the participating
university programs.

 

The routine frequency of the communication and methods that the group
has used have helped to maintain interest and energy on this particular
forecast problem, even in between formal funding windows. In fact, these
efforts have helped to provide opportunities for new formal research,
and may provide the framework for establishing new field research
efforts in this region. It is hoped that sharing the successes of this
particular group will inspire similar collaborative groups to form and
work together on other unique forecast and research problems in other
regions.

**Disclaimer**\
Mention of a commercial company or product does not constitute an
endorsement by the National Weather Service.  Use of information from
this publication concerning proprietary products or tests of such
products for publicity or advertising purposes is not authorized.

**References**

 

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modeling\
system for trajectories, dispersion, and deposition. Aust. Meteor.\
Mag., 47, 295--308.

Holloway, B. S., 2007: The role of the Great Lakes in northwest flow
snowfall events in the southern Appalachian Mountains. M.S. thesis,
Dept. of Marine, Earth, and Atmospheric Sciences, North Carolina State
University, 204 pp. \[Available online at
<http://www.lib.ncsu.edu/theses/available/etd-11022007-181739/>.\]

 

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Winter weather forecasting throughout the eastern United States. Part
III: The effects of topography and the variability of winter weather in
the Carolinas and Virginia. *Wea. Forecasting*, **10**, 42--60.

Löffler-Mang, M., and J. Joss, 2000: An optical disdrometer for
measuring size and velocity of hydrometeors. *J. Atmos. Oceanic*
*Technol.,* **17,** 130--139.

------, and U. Blahak, 2001: Estimation of the equivalent radar
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*Meteor.,* **40,** 843--849.

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pp.

National Climatic Data Center, 1992:  *Storm Data.* Vol. 34, No. 5, 112
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NOAA's HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT)
model. \[http:www.arl.noaa.gov/ready/hysp_info.html\]

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the southern Appalachians. Ph.D. Dissertation, Dept. of Geography,
University of North Carolina at Chapel Hill, 176 pp.

Perry, L. B., and C. E. Konrad II, 2006a: Synoptic patterns associated
with the record snowfall of 1960 in the southern Appalachians, *Proc.
63^rd^ Eastern Snow Conference,* Newark, DE, 55--64.

\_\_\_\_\_\_\_\_\_, and \_\_\_\_\_\_\_\_\_\_\_\_, 2006b: Relationships
between NW flow snowfall and topography in the southern Appalachians,
USA. *Climate Research,* **32**, 35--47.

 

Perry, L. B, C. E. Konrad, D. Hotz, and L. G. Lee, 2007a: Synoptic
classification of snowfall events in the Great Smoky Mountains, USA.
*Proc. 64^th^ Eastern Snow Conference*, 207-215.\
\
Perry, L. B., C. E. Konrad, and T. W. Schmidlin, 2007b: Antecedent
upstream air trajectories associated with northwest flow snowfall in the
southern Appalachians. *Wea. Forecasting*, **22**, 334--352.

Peters, G., B. Fischer, and T. Andersson, 2002: Rain observations with a
vertically looking Micro Rain Radar (MRR). *Boreal Environment
Research*, 7, 353--362.

Reges, H. W., R. C. Cifelli, N. J. Doesken, and J. Turner, 2008:
CoCoRaHS (The Community Collaborative Rain, Hail and Snow Network) --
The accidental network: evolving collaborations. Preprints, *17^th^*
*Symposium on Education,* New Orleans, LA, Amer. Meteor. Soc., 1.3.  

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http://strc.comet.ucar.edu/wrf/wrf_userguide.htm\]

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**Figure Captions**

Fig. 1. Surface analysis, including sea-level pressure (4 hPa
intervals), subjective frontal analysis, and areas of precipitation
(green shading) for 1200 UTC, 19 December 2003.  Dashed blue lines
correspond to 32ºF and 0ºF isotherms; orange dashed lines are trough
axes. \[Image generated from the National Center for Environmental
Prediction's Daily Weather Map web site:
www.hpc.ncep.noaa.gov/dailywxmap/frame.html\]

Fig. 2. Storm total snowfall (cm.), 18-20 December 2003. The outlined
regions represent the boundaries of WFO county forecast/warning areas
involved in this collaboration.

Fig. 3. Southern Appalachian region and terrain, with participating NWS
office areas of responsibility, as well as universities involved in the
NWFS collaboration group.

Fig. 4. Snowfall distribution during a typical NWFS event. Subjective
descriptions on the legend refer to relative storm total accumulations.

Fig. 5.  AVHRR false color image from NOAA-16 polar orbiter on 6 March
2001, at 1826 UTC. Large white streak extending from Lake Michigan
(upper-left portion of image) to the TN/NC border is largely stratus,
but likely some snow cover as well. Narrower streaks oriented
northwest-to-southeast with an apparent trajectory from the Great Lakes
region can also be seen just to the northeast of this wide band.

Fig. 6. Regional topographic map with CoCoRaHS and NWS cooperative
observer stations. The location of the Poga Mt. field site is also
identified.\
\
Fig. 7. Time-height plots of MRR data from 20 UTC on 6 April 2007 to 12
UTC on 7 April 2007 from Poga Mountain. a) reflectivity, and b) Doppler
velocity. Positive velocities are downward. This storm yielded 147 mm of
snowfall with 8.1 mm snow water equivalent.\
\
\
Fig 8.  Precipitation difference between experimental No-Lake simulation
and a control simulation using the WRF-ARW model for the 10-11 February
2005 NWFS event.  Negative differences (warm colors) correspond to areas
with reduced precipitation in the No-Lake simulation relative to the
control. Units are in inches.

Fig. 9.  One-hour snowfall accumulation forecast (inches), valid on 27
February 2008 at 1200 UTC, from the 5 km operational WRF-EMS at WFO
Charleston, WV.

Fig. 10. Air parcel backward trajectory analysis for 10 February 2005,
from NOAA\'s Air Resources Laboratory HYSPLIT model, using the 40 km Eta
model data analysis system (EDAS) in this example. Two trajectories are
shown, one ending over western NC at the 1500m AGL level (green), and
the other ending at the same location at the 500m AGL level (blue).
Vertical profiles of the two trajectories are shown in the bottom inset
portion, with the trajectory ending time/altitude on the left side, and
the origination time/altitude of the trajectories 60 hrs prior on the
right side.

![](media/image1.png){width="5.948611111111111in"
height="4.223611111111111in"}

Fig. 1. Surface analysis, including sea-level pressure (4 hPa
intervals), subjective frontal analysis, and areas of precipitation
(green shading) for 1200 UTC, 19 December 2003.  Dashed blue lines
correspond to 32ºF and 0ºF isotherms; orange dashed lines are trough
axes. \[Image generated from the National Center for Environmental
Prediction's Daily Weather Map web site:
www.hpc.ncep.noaa.gov/dailywxmap/frame.html\]

![](media/image2.jpeg){width="5.0in" height="4.0in"}

Fig. 2. Storm total snowfall (cm,), 18-20 December 2003.  The outlined
regions represent the boundaries of WFO county forecast/warning areas
involved in this collaboration.

![](media/image4.jpeg){width="4.004861111111111in"
height="4.004861111111111in"}

Fig. 3. Southern Appalachian region and terrain, with participating NWS
office areas of responsibility, as well as universities involved in the
NWFS collaboration group.

 

![](media/image5.jpeg){width="5.915972222222222in"
height="4.721527777777778in"}

Fig. 4. Snowfall distribution during a typical NWFS event. Subjective
descriptions on the legend refer to relative storm total accumulations.

 

![](media/image6.png){width="6.0in" height="4.5in"}

Fig. 5.  AVHRR false color image from NOAA-16 polar orbiter on 6 March
2001, at 1826 UTC. Large white streak extending from Lake Michigan
(upper-left portion of image) to the TN/NC border is largely stratus,
but likely some snow cover as well. Narrower streaks oriented
northwest-to-southeast with an apparent trajectory from the Great Lakes
region can also be seen just to the northeast of this wide band.

 

![](media/image8.jpeg){width="5.997222222222222in"
height="4.8590277777777775in"}

Fig. 6. Regional topographic map with CoCoRaHS and NWS cooperative
observer stations. The location of the Poga Mt. field site is also
identified.

Fig. 7. Time-height plots of MRR data from 20 UTC on 6 April 2007 to 12
UTC on 7 April 2007 from Poga Mountain. a) reflectivity, and b) Doppler
velocity. Positive velocities are downward. This storm yielded 147 mm of
snowfall with 8.1 mm snow water equivalent.

 

![](media/image11.png){width="4.322916666666667in"
height="5.300694444444445in"}

Fig 8.  Precipitation difference between experimental No-Lake simulation
and a control simulation using the WRF-ARW model for the 10-11 February
2005 NWFS event.  Negative differences (warm colors) correspond to areas
with reduced precipitation in the No-Lake simulation relative to the
control. Units are in inches.

![](media/image12.png){width="5.957638888888889in"
height="4.469444444444444in"}

Fig. 9.  One-hour snowfall accumulation forecast (inches), valid on 27
February 2008 at 1200 UTC, from the 5 km operational WRF-EMS at WFO
Charleston, WV.

![](media/image13.jpeg){width="5.951388888888889in"
height="7.373611111111111in"}

Fig. 10. Air parcel backward trajectory analysis for 10 February 2005,
from NOAA\'s Air Resources Laboratory HYSPLIT model, using the 40 km Eta
model data analysis system (EDAS) in this example. Two trajectories are
shown, one ending over western NC at the 1500m AGL level (green), and
the other ending at the same location at the 500m AGL level (blue).
Vertical profiles of the two trajectories are shown in the bottom inset
portion, with the trajectory ending time/altitude on the left side, and
the origination time/altitude of the trajectories 60 hrs prior on the
right side.

[^1]: ^1^ Removal of the influence of the Great Lakes in the No-Lake
    simulations was accomplished by altering the WRF model surface layer
    parameterization scheme to set the fluxes of heat and moisture to
    zero over the Great Lakes.

[^2]: Results from the research on NWFS climatology, findings from
    observational studies, insights from model sensitivity tests, and
    analysis of real-time local mesoscale models are routinely shared
    among forecasters and researchers.   Information is exchanged in
    local seminars, operational discussions stemming from listserv
    postings, one-on-one mentoring by team members with their staffs,
    individual review of event summaries that are posted on the NWFS
    group website or local intranet sites, and incorporation of peer
    reviewed research into forecast strategies and methodologies. ^2^
    NOAA\'s [HY]{.underline}brid
    [S]{.underline}ingle-[P]{.underline}article [L]{.underline}agrangian
    [I]{.underline}ntegrated [T]{.underline}rajectory model