**January 19^th^, 2002**

# INTRODUCTION / LARGE SCALE FORCING

Our system of interest was a relatively weak surface low pressure system
moving northeast off of the mid-Atlantic coast. The system appeared to
be located in the right entrance region of a 300-mb jet maximum moving
east across New England. There did not appear to be a second, southern
branch upper-level jet. The flow was of relatively low-amplitude,
without any major upper level troughs. The surface pmsl, 300 mb heights
and isotachs, and 700 mb heights shown below are all 9 hour forecasts
from the Eta model verifying at 21z on the 19^th^.

![](media/image1.png){width="5.991666666666666in"
height="2.6597222222222223in"}

Surface pmsl and 850 mb temperatures

![](media/image2.png){width="5.993055555555555in"
height="3.029861111111111in"}

300 mb heights and wind speed.

![](media/image3.png){width="5.992361111111111in"
height="3.446527777777778in"}

Eta 09 hr forecast 700 mb heights verifying at 1/19 21z.

The following figure shows a 12 hr eta forecast of potential vorticity
in the layer from 500 to 300 mb, verifying at 1/19 21z. The main lobe of
PV is shown back across the western Ohio Valley, with only a weak
eastward extension shown farther east across the mid-Atlantic region.

![](media/image4.png){width="5.995138888888889in"
height="4.678472222222222in"}

09 hr eta forecast 500-300 mb PV verifying 1/19 21z.

The large-scale geostrophic forcing for upward motion in this case can
by implied from the following figure. Divergence of Q vectors is shown
in the layer from 500 to 400 mb. Recall that upward motion is favored in
areas where the divergence is negative. Recall also that the vertical
motion implied by the divergence of Q does not include the effects of
stability, which in the real atmosphere are significant (instability
increases vertical motion given equal forcing). In addition, the
divergence of Q will not do a good job capturing the details of the
vertical motion field in locations where there are strong ageostrophic
winds, ie near fronts and jet streaks. In a case like this, where fronts
and jet streaks are important, the divergence of Q cannot accurately
predict the total vertical motion field, instead it should be thought of
as depicting the "large-scale" forcing for vertical motion. In this
case, a very weak, banded pattern of forcing is indicated across
Pennsylvania and New York State. This is an indication that the forcing
for the snow band in question was probably not "large-scale", but was
associated with smaller scale features (such as a front or jet streak).

![](media/image5.png){width="5.995138888888889in"
height="4.678472222222222in"}

09 hr eta forecast 500-400 mb divergence of Q verifying 1/19 21z.

The patterns shown above resulted in relatively narrow, weak bands of
upward vertical motion and saturation. The figure below shows the mean
layer vertical velocity and relative humidity from 600 to 500 mb,
verifying at 1/19 21z. Note that the vertical velocity shown in this
figure is the total model forecast vertical velocity, and includes the
effects of stability and ageostrophic flow. The vertical motion fields
are still fairly weak, but at least they indicate a maxima in the area
where the heavy snow band developed.

![](media/image6.png){width="5.995138888888889in"
height="4.678472222222222in"}

09 hr eta forecast 600-500 mb layer average omega and rh verifying 1/19
21z.

**MODEL QPF VS OBSERVATIONS**

Radar loops indicated that a band of moderate to heavy snowfall
developed on the northern edge of the system across northern
Pennsylvania and southern New York during the afternoon on the 19^th^.
The band eventually weakened and pivoted to the southeast during the
evening.

![](media/image7.png){width="5.9944444444444445in"
height="3.6979166666666665in"}

![](media/image8.png){width="5.9944444444444445in"
height="3.6979166666666665in"}

![](media/image9.png){width="5.9944444444444445in"
height="3.6979166666666665in"}

Since the band eventually pivoted southeast across Pennsylvania, QPF
totals across Pennsylvania and southern New York were fairly uniform.
However, very high snow to liquid ratios over southern New York resulted
in an unexpected 6 to 12 inch snowfall across southern New York, despite
an average of only around 0.25 inches of liquid in that area. The
following figure shows the total observed precipitation for this event
(mm) through 1/20 00z. Note that the movement of the band resulted in an
even distribution of precipitation from southern New York through
Pennsylvania.

![](media/image10.png){width="3.252083333333333in"
height="2.7708333333333335in"}

Observed precipitation (mm) through 1/20 00z.

![](media/image11.png){width="4.098611111111111in" height="4.125in"}

Observed snowfall through 1/20 06z. Note the band of 8 to 12 inches of
snow across the southern tier of New York.

The BUFKIT Eta forecast sounding shown below indicates that southern New
York had a somewhat unusual temperature profile for a snowstorm during
this event. Note that the lack of strong QG forcing associated with this
event meant that there were no layers of strong warm advection, and
therefore no pronounced warm layers in the forecast sounding. As a
result the sounding was roughly moist adiabatic through a deep layer.
The result was a favorable temperature (around --15 degrees C) for snow
growth by deposition in the mid-levels of the sounding, combined with a
favorable temperature (just below 0 degrees C) for crystal growth by
aggregation in the lower levels of the sounding.

![](media/image12.png){width="6.872222222222222in" height="5.15625in"}

The following figure is a time cross section of Eta forecast temperature
and relative humidity at BGM. Note the long period where high relative
humidity values combined with a deep layer of near --15 degrees C across
the area.

![](media/image13.png){width="5.990972222222222in"
height="4.520138888888889in"}

BUFKIT depiction of Eta forecast relative humidity and temperature at
BGM.

# MODEL DIAGNOSTICS

1\) FORCING FOR UPWARD MOTION

A 9 hour eta forecast of 850 mb moisture convergence verifying at 1/19
21z indicates a strong maximum over the Delmarva peninsula area, where
the southern area of heavy precipitation occurred. Enhanced southerly
flow occurred in this region where there was a strong low-level gradient
of temperature and moisture, resulting in moisture convergence, upward
motion and precipitation.

![](media/image14.png){width="5.991666666666666in"
height="2.6597222222222223in"}

850 mb moisture convergence and 850 mb wind barbs at 21z on the 19^th^.

However, looking at only level when trying to diagnose the potential for
upward motion can be deceiving. Note that little if any forcing is
indicated across northern Pennsylvania and southern New York by this
figure.

The following figure shows a cross section of frontogenesis from
Philadelphia to Rochester for 21z on the 19th. The frontogenesis slopes
upward from south to north across the region. Note a strong maximum
located along the southern part of the front, and a secondary maximum
located to the north, near the heavy snow band. Reasons for this double
maximum will be explored later.

![](media/image15.png){width="5.99375in" height="2.884027777777778in"}

Cross section of frontogenesis (frnt(tmpc,obs)) at 21z on the 19^th^.
The cross section is from PHL to ROC.

A plan view of frontogenesis at 625 mb at 21z on the 19^th^ shows that
the frontogenesis at that level lines up very well with the location of
the snow band.

![](media/image16.png){width="5.9847222222222225in"
height="3.923611111111111in"}

Frontogenesis at 625 mb at 21z 1/19.

The components of frontogenesis can be clearly seen on the next two
figures. The first figure shows 625 mb stretching deformation and wind
barbs, as forecast by the Eta at 21z 1/19. The second figure shows the
625 mb forecast temperature in degrees C. Note that the area of maximum
deformation corresponds to an area of enhanced temperature gradient over
southern New York. The axis of dilatation implied by the wind barbs
would be roughly parallel to the isotherms, indicating strong
frontogenesis. Finally, note how well then --15 degree C isotherm lines
up with the band of heavy snow shown in earlier figures.

![](media/image17.png){width="5.995138888888889in"
height="4.678472222222222in"}

Stretching deformation and wind barbs at 625 mb, 1/19 21z.

![](media/image18.png){width="5.995833333333334in"
height="3.1569444444444446in"}

Temperature (degrees C) at 625 mb at 21z 1/19.

The stretching deformation maximum across northern Pennsylvania and
southern New York was caused by confluence between a southwesterly flow
across Pennsylvania and a more westerly flow farther to the north. The
next figure shows ageostrophic wind barbs and heights at 625 mb
verifying at 1/19 21z

![](media/image19.png){width="5.995138888888889in"
height="4.678472222222222in"}

Ageostrophic wind barbs and heights verifying 1/19 21z.

The strong southeasterly ageostrophic flow over Pennsylvania apparently
resulted in a confluent (and convergent) flow across northern
Pennsylvania and southern New York. The combination of confluence and
the ambient temperature gradient resulted in the frontogenesis maximum.
For insight into what caused the strong ageostrophic southeast flow,
consider the next two figures. In the first figure, 500 mb isotachs are
shown, along with 500 mb divergence. Note the strong speed maximum
approaching the area from the southwest. Pennsylvania and southern New
York are in the left exit region of this speed maximum. The thermally
indirect circulation in the exit region is indicated by the divergence
in the left exit region over Pennsylvania and southern New York, and by
the ageostrophic southeast flow below the exit region shown in the
proceeding figure. In the second figure, ageostrophic wind barbs and
heights are shown at 500 mb. Note that, in contrast to the first figure,
the ageostrophic flow is from the north across southern New York and
Pennsylvania. The figure taken at 625 mb appears to be showing a view of
the lower branch of the circulation, while the figure taken at 500 mb
appears to be showing a view of the upper branch of the circulation.

![](media/image20.png){width="5.991666666666666in"
height="3.222916666666667in"}

500 mb isotachs (shaded) and divergence verifying at 1/19 21z.

![](media/image21.png){width="5.995138888888889in"
height="4.678472222222222in"}

500 mb heights and ageostrophic wind barbs, verifying 1/19 21z.

A cross section of forecast omega and frontogenesis indicates a maximum
of upward vertical motion located just above the axis of maximum of
frontogensis. The upward vertical motion maximum is fairly shallow,
extending along and just above the frontal zone.

![](media/image22.png){width="5.995138888888889in"
height="4.678472222222222in"}

Frontogenesis (black lines) and omega (purple lines).

One interesting aspect of this event appears to be that the upward
motion associated with the event was quite shallow. Note that a sloping
band of subsidence appears just above the upward motion maximum
associated with the frontal boundary. Another band of upward motion then
appears above this subsidence, in the upper levels of the troposphere.

The figure below shows the relationship between the location of the
maximum of upward vertical motion and wind speed. Note again that the
band of upward vertical motion maximum is fairly shallow.

![](media/image23.png){width="5.995138888888889in"
height="4.678472222222222in"}

Cross section of omega and isotachs verifying at 1/19 21z.

A cross section of divergence and wind speed taken along the same line
from PHL to ROC shows the relationship between divergence, the location
of the frontal zones, and the upper level jet. As would be expected,
upward motion is forecast in areas where convergence changes to
divergence with height. Also, note that areas of low-level convergence
indicate the location of the low-level frontal zones. The data in the
following figure indicate that there are really two frontal zones
associated with this event, a mid-level front across the north located
underneath the upper level jet, and a low-level front farther to the
south. Convergence and frontogenesis maxima are associated with both
fronts.

![](media/image24.png){width="5.995138888888889in"
height="4.678472222222222in"}

Eta 12 hour forecast of divergence (black lines) and wind speed (purple
lines) from PHL to ROC verifying at 1/20 00z.

A cross section taken along a longer line, from ORF to ROC, better shows
the structure of the upper jet. Note that the speed max at 500 mb is
really just part of a single jet that slopes southwestward from above
300 mb down through 500 mb. In this figure, the jet not only slopes
downward from left to right, but also slopes downward out of the page.
As a result, this cross section is taken in the exit region of the jet
around 500 mb, and in the entrance region of the jet at 300 mb. The
sloped band of divergence between 400 and 700 mb is associated with the
left exit region of this sloped jet, while the divergence farther up in
the troposphere is associated with the right entrance region of the same
jet at upper levels.

![](media/image25.png){width="5.995138888888889in"
height="4.678472222222222in"}

Isotachs (black lines) and divergence (purple) verifying at 1/19 21z.

2\) ASSESSING INSTABILITY

An Eta forecast cross section of frontogenesis and theta, taken from PHL
to ROC and verifying at 1/19 21z is shown below. Note the frontal zone
sloping from south to north. The double maximum of frontogenesis is
indicated, with a weaker maximum located to the north near the heavier
snow band. An area of weak stability is located above the band of
frontogenesis (indicated by the theta lines being spread relatively far
apart).

![](media/image26.png){width="5.9944444444444445in"
height="4.743055555555555in"}

The following figure shows a cross section of theta-e and momentum,
taken across the frontal zone along a line from PHL to ROC at 1/19 21z.
Note the narrow band of potential symmetric instability (PSI), in the
zone just above the frontal zone, where the theta-e lines slope more
steeply than the momentum lines. In fact, the theta-e lines appear to
"fold over" on themselves in this case, indicating the presence of
conditional instability, especially over the central and southern part
of the cross section. In the area above northern Pennsylvania and
southern New York, the atmosphere appears to be just about neutral.

![](media/image27.png){width="5.995138888888889in"
height="4.678472222222222in"}

A cross section of theta-e and momentum, taken from PHL to ROC and
verifying 1/19 21z.

A cross section of frontogenesis and EPV indicates that a narrow band of
negative EPV was associated with this band of PSI (and conditional
instability). Again, note that this band of weak stability is located
just above the frontal surface.

![](media/image28.png){width="5.995138888888889in"
height="4.678472222222222in"}

A cross section of frontogenesis (black lines) and EPV (purple lines,
only negative values are plotted). Note the band of negative EPV located
just above the sloping band of frontogenesis.

The following figure shows a cross section of negative EPV and relative
humidity, verifying at 1/19 21z. Note that the negative EPV occurs along
the vertical saturation gradient, with high values at the bottom of the
negative EPV zone, and low values at the top.

![](media/image29.png){width="5.995138888888889in"
height="4.678472222222222in"}

A 12 hr eta forecast of EPV (only negative values plotted) and relative
humidity, verifying at 1/19 21z.

The following figure shows the relationship between vertical velocity
and negative EPV for this case. The negative EPV layer is located within
a zone of upward vertical velocity.

![](media/image30.png){width="5.995138888888889in"
height="4.678472222222222in"}

12 hr eta forecast of vertical velocity and epv (only negative values
plotted) verifying at 1/19 21z.

The proceeding figures indicate that a layer of conditional instability
developed above a sloping frontal zone in this case. The layer was
shallow, but located in a position where the upward motion associated
with the large scale dynamics of the system, as well as the circulation
induced by frontogenesis became co-located with the instability. The
result was that the circulation associated with the mid-level
frontogenesis became strengthened and localized, leading to the
intensification of the snow band.

A cross section of theta-e advection taken from PHL to ROC shows
positive advection along the frontal boundary with weak negative
advection above in the southwest flow. The result is decreasing
stability above the frontal boundary. This differential advection of
theta-e is apparently responsible for the development and maintenance of
the instability shown in the previous few figures.

![](media/image31.png){width="5.995138888888889in"
height="4.678472222222222in"}

Cross section of theta-e advection. Note the positive values below and
especially along the frontal boundary, with negative values above the
front. The result is decreased stability above the front.

Summary:

This is an interesting an event from several perspectives. Despite the
fact that the QPF with this event was fairly even distributed across Pa
and southern New York, radar loops indicate that the event was
characterized by a fairly narrow band of moderate to heavy snow. The
band eventually moved across a large area, resulting in the even
distribution of QPF. Despite the even distribution of QPF, the snowfall
amounts varied widely. It is hypothesized that the main reason for the
heavy snow falling over northern Pennsylvania and southern New York was
a favorable temperature profile across that area. Farther south, the
temperature profile was warmer, and not as favorable for snow growth.

The mechanisms that produced the narrow, heavy band of snow with this
case are another interesting aspect of the case. The moderately heavy
precipitation that fell over the Delmarva peninsula to the south of the
band apparently were associated with strong low-level moisture
convergence and lift along a strong, low-level baroclinic zone. The
moderate to heavy precipitation in a narrow band farther to the north
was apparently generated by frontogenesis and convergence that developed
along a second, northern frontal zone located directly underneath the
upper-tropospheric jet. Vertical motion was maximized above this second
northern frontal boundary as 1) an area of divergence associated with
the left-exit of a 500 mb speed maximum moved over the top of the jet
(the speed maximum was associated with a single, sloping jet stream
which sloped downward from the northeast toward the southwest), and 2)
the area above the front became destabilized due to strong differential
theta-e advection associated with a dry slot advancing eastward across
the area in the moderately strong southwesterly flow. (This instability
would have acted to intensify and focus the lift along the axis of
frontogenesis).

As far as Phil's ideas regarding narrow, heavy snow bands on the
northern edge of systems: this system appeared to be similar to the case
in South Dakota in that there was no coupled jet circulation in
evidence. In Phil's case, two jets were present; they were just too far
apart to be coupled. In this case, there was apparently only one jet, so
no coupling could occur. Like in Phil's case, this was a case where the
flow was relatively straight (ie no major upper level trof). As a
result, upward motion generated by changes in wind speed along the flow
(jet streaks) would be allowed to dominate the upper level divergence
fields, without interference from curvature effects.

An important aspect that determined the configuration of the
precipitation echoes with this event was the lack of strong, widespread
forcing that would typically be associated with a deep mid to upper
level trough. When a deep mid to upper level trough is present,
curvature effects associated with the upper level flow result in large
scale upward motion forcing and large areas of light to moderate
precipitation. In this case, the upward motion was confined to the areas
where vertical motion associated with mid-level frontogenesis, in
combination with instability, acted to produced a more localized area of
moderate precipitation.