**A RARE F-2 TORNADO EVENT IN BEAR, IDAHO**

**Melissa M. Hurlbut**

*WFO Jacksonville, Florida (former affiliation WFO Boise, Idaho)*

November 2006

ABSTRACT

A variety of real time and model data were studied to properly assess
the evolution of a significant microburst and tornado event in the Boise
County Warning Forecast Area (CWFA). A squall line that formed along a
cold front produced damaging winds and severe hail across Baker County,
Oregon. As the southern end of this squall line continued to evolve, a
strong F2 tornado was spawned over Adams County, Idaho, which damaged
structures, destroyed 5,000 acres of a national forest, and caused one
injury. Data limitations in the West, including terrain induced beam
blockage, significant radar spacing and limited real time surface data,
make severe weather decision making particularly challenging.
Climatological records show that tornadic events in the Intermountain
West are rare. Therefore, with limited data available for storm
interrogation and the rarity of tornadic events in the Intermountain
West, proper pre-storm environmental analysis and utilization of
available data sets throughout the event are especially crucial. This
paper documents the pre-storm environment and the evolution of a
tornadic cell through Bear, Idaho.

1.  **Introduction**

On 4 June 2006, a quickly developing squall line swept across central
Oregon and Idaho, which caused one injury and extensive damage to
structures and a national forest. The southern portion of the line
produced significant microburst damage across Baker and Adams counties,
quarter sized hail in Cuprum, Idaho, and a strong F2 tornado in Bear,
Idaho.

Aerial surveys of the tornado damage conducted by the Payette National
Forest Service revealed that up to 5,000 acres of timber were blown
down, snapped in half, or uprooted. The path was 21 kilometers (13
miles) long with a 0.8 kilometer (half mile) width in some locations
(figure 1). Houses in Bear, Idaho, where the tornado first touched down,
suffered substantial roof damage. The extensive damage, and the length
and width of the path, indicated that this tornado was at least F2
intensity (Fujita, 1981) and could have reached weak F3 strength in some
locations. Climate records show this tornado was the strongest known
tornado in Idaho since 1940 (NCDC 2006).

Forecasting tornadic events in Western locations can be particularly
challenging. Data available to a warning forecaster is lacking due to
several factors. Lower elevation angles from the WSR-88D radar are often
blocked by terrain. This is especially problematic at larger distances
where storm structure cannot be viewed below three kilometers
(approximately 10,000 ft) above ground level (AGL). Radar data from
neighboring offices are often limited due to the distance between
radars. Over complex terrain, surface observation sensors are placed far
apart as well. Furthermore, in sparsely populated areas, fewer "ground
truth" reports from storm spotters are available (Fujita 1989 et al).

Climate records indicate an average of three tornadoes are reported per
year in Idaho, most of which are below F2 strength. Many of these
reports come from the populated Snake River Valley. While tornadoes
likely occur over the higher terrain, many of these events are not seen
or reported due to sparse populations.

Records from 1880-2000
([www.tornadoproject.com](http://www.tornadoproject.com/)) indicate that
no tornadoes in Idaho history have reached F4 or greater strength. In
addition, only two F3 tornadoes and nine F2 tornadoes have been
recorded. National Climactic Data Center (NCDC) records from 1950
through the end of April 2006 indicate nine tornadoes in Idaho have
reached F2 strength. NCDC records show an increase in reports over the
past 50 years. This is likely due to an increase in population rather
than strictly an increase in the occurrence of these events. In
addition, developments in technology, such as the introduction of the
WSR-88D, have also led to an increase in detection of severe events
(Fujita 1989 et al).

Most Idaho tornado events occur in the late spring and summer during the
late afternoon when surface instability is usually at its highest. Since
damaging winds are the most common severe storm threat in the West, it
is also possible that many of the tornadoes are "gustnadoes";
short-lived, shallow vortices that develop along a gust front as it
coincides with a strong updraft (Wakimoto, R., and J.W. Wilson, 1989).

Given that tornadic events in the West are rare, a warning meteorologist
must carefully examine and understand the environment prior to event
onset and monitor environmental changes over time. Potential impacts of
terrain (figure 2) on the near storm environment are also vital to know.
These impacts may enhance or decrease tornado potential.

**2. Synoptic and Mesoscale Environment**

On the morning of 4 June 2006, a negatively-tilted short wave trough
moved across central Washington and Oregon (figure 3). A vorticity
maximum advected over central Oregon, with convection already developing
ahead of this maximum as early as 1200 UTC (all times in UTC). A jet
streak quickly moved northeast across Oregon, which placed Baker County
and the West Central mountains in the favorable left exit region (figure
4). A northwest to southeast oriented 850 mb thermal ridge stretched
across the Snake River valley ahead of the cold front.

AMSU Satellite data showed that precipitable water (PW) values were over
an inch off the coast, while the 1200 sounding at Boise showed 0.88
inches (22.352 mm), which was 160% of normal. Strong southwesterly flow
ahead of the short wave trough enabled the transfer of mid-level
moisture over the higher terrain.

Surface dew points indicated low-level moisture was ample for
convection, with most locations reporting mid 40s to lower 50s
throughout the morning (figure 5). Dew points in the area of interest
were mainly in the lower 50s. The unmodified 1200 sounding at Boise
suggested 711 J/kg of CAPE and a lifted index of -3 could be achieved at
the time of maximum surface heating. However, 1800 model data suggested
that over 1,500 J/kg of CAPE could be realized in association with the
upper trough.

The modified sounding at KBOI (figure 6) showed that the environment was
already favorable for hail formation with mid-level shear and sufficient
CAPE in the hail growth zone. Ample CAPE throughout the entire layer
existed. Additionally, daytime heating in combination with the rapid
cooling aloft associated with the passing upper trough would create very
steep lapse rates later in the day, which would further increase the
threat of large hail and strong downburst winds.

A strong veering wind profile with 0-6 km bulk shear values of 60-70 kt
was also suggested in model data. The hodograph from 0-6 km showed a
turning of the shear vector that would favor right moving supercells.
Further analysis of the hodograph showed that any increase in
southeasterly to southwesterly wind speeds, or backing of the low-level
winds, would more than double the helicities (figure 7).

Previous studies in the northwestern United States (Evenson and Johns,
1995) have shown that a 500 mb negatively tilted trough and a northwest
to southeast oriented 850 mb thermal ridge are favorable conditions for
a significant severe weather event. On this day, the best moisture
associated with the incoming short wave would be placed across the
northern halves of Oregon and Idaho.

**3. Storm Evolution**

As the cold front progressed across central Washington and Oregon, a
squall line formed along the boundary. By approximately 1605 this line
began to move into Baker County (figure 8). Radar limitations at this
distance and over complex terrain affected storm interrogation. The
southern end of the squall line was located 115 nm from the radar and
the lowest slice, 0.5º, was sampling at four kilometers (approximately
13,000 ft) above ground level (AGL). Therefore, the evolution of the
lowest portions of the storm below this elevation could not be detected
by radar.

Velocity data at the 0.5 degree elevation angle showed indications of
weak mid-level rotation, while storm structure shown on reflectivity
data may have been unrepresentative of the severity (figure 9).
Reflectivity cores were below 50 dBZ at the freezing level and above.
Dew points at this time were still in the lower 50s which would favor
severe weather due to instability. The strongest reflectivities of the
storm were located in the valley near Sumpter, Oregon. At 1630, two
separate calls from trained spotters in this location revealed that the
line was producing estimated winds of 60 mph and 0.75 inch hail.

By 1650 a rear-inflow notch (RIN) and bow echo first became apparent,
supporting spotter reports of severe winds (figure 10). Mesonet and
METAR data showed local temperatures in the mid to upper 60s with dew
points in the lower 50s. Surface winds were still from a southeasterly
to southerly direction, which supported increased helicities. Velocity
data revealed a tightening couplet in the mid levels of the storm.

By 1717, reflectivity data began to show the first indication of large
hail with 60 to 65 dBZ cores at 4.7 km (15,300 ft) above sea level
(ASL), and 55 to 60 dBZ cores 6.9 km (22,500 ft) ASL (figure 11).
Outbound velocities at the 0.5 degree level had increased to nearly 30
kt while storm top divergence had increased to 70 kt.

By 1734, this storm core continued to grow and suggested increased
severity. 60 to 65 dBZ cores (figure 12) were observed at 7.3 km (24,000
ft). At this point the storm began to deviate to the right. The 50 dBZ
core extended to nearly 9.1 km (30,000 ft).

Five minutes later, outbound velocities at the 0.5 degree level
increased to nearly 40 kt and expanded in area (figure 13). Storm top
divergence increased to 80 kt. The storm maintained this magnitude of
storm top divergence over the next hour. A damaging downburst was soon
reported by a spotter located 6.2 km (10 miles) north of Halfway,
Oregon.

By 1805, outbound velocities at the 0.5 degree level again increased,
with speeds near 50 kt. The storm began to show an increase in
rotational velocities while strong outbound velocities had been
maintained (figure 14). The Velocity Azimuth Display (VAD) wind profile
at KBOI showed that winds had increased from a south to southwest
direction (figure 15). Mesonet data from a valley to the east indicated
surface winds from the southeast, which would create a veering profile
ahead of the storm. Adjusting the hodograph with these winds more than
doubled the SRH to 250 m²/s², which led to increased tornado potential.
The storm approached Hells Canyon, where it could encounter channeling
moisture (Monteverdi, Doswell, and Lipari, 2003). Temperatures in this
area were in the upper 50s with dew points in the lower 50s. At this
time, a Remote Automated Wind Sensor (RAWS) station at Flagstaff Hill
reported a wind gust to 80 mph.

As the storm moved over the canyon around 1809 to 1814, the detected
rotation appeared to weaken. However, outbound velocities remained
strong (figure 16), and for the first time in the storm\'s existence, 50
to 55 dBZ cores extended up to 10.4 km (34,000 ft) (figure 17). From
here the storm progressed quickly. At 1827, the storm displayed a clear
RIN and rotational velocities jumped up to 40 kt (figure 18). Within the
next few minutes, rotational velocities increased to 45 kt. Eyewitness
reports indicated the tornado touchdown time was near 1830. Quarter
sized hail was also reported from Cuprum, Idaho. Ten minutes after
touchdown, the storm finally began to weaken.

**4. Summary and Conclusions**

In the Bear tornado case, environmental analysis showed that conditions
were favorable for damaging winds, large hail, and tornado formation.
Utilizing all datasets, a forecaster could continue to evaluate how this
near-storm environment progressed.

In this case, Mesonet data showed that dew points remained high while
temperatures were increasing. VAD wind profile data showed an increase
in south to southwest winds and, an increase in southeasterly winds at
the surface ahead of the storm would increase the SRH.

Widespread cloud cover inhibited satellite utility. Although radar data
were unavailable beneath three kilometers (approx. 10,000 ft), there
were indications of the storm's increasing severity. Persistent rotation
in SRM and base velocity data and continual ground truth reports of
severe weather indicated that the storm was maintaining strength. The
storm was relatively unimpeded by rough terrain as it traversed across a
valley. An increase in storm inflow and a rapid increase in reflectivity
values as the storm passed over Hell's Canyon were noted shortly before
the cell became tornadic.

Examining these important near-storm environmental clues where data
sources are limited is vital in severe weather episodes. By utilizing
all datasets and keeping a constant state of awareness of how the
environment is evolving, a forecaster can use the information available
to make better warning decisions.

**5. Acknowledgements**

The author would like to thank Peter Wolf for his constructive criticism
and valuable insights into this case. The author would also like to
thank Angela Enyedi, Jay Breidenbach, George Skari, and Charles Redman
for their helpful insights as well. Additional thanks goes to Timothy
Barker for archiving and providing data for this case.

**6. References**

Evenson, E.C., and Johns, R.H., 1995: Some Climatological and Synoptic
Aspects of Severe Weather Development in the Northwestern United States.
*National Weather Digest*, 20, 1, 34-50.

Fujita, T.T., 1981: Tornadoes and downbursts in the context of
generalized planetary scales. *J. Atmos. Sci.*, **38**, 1511-1634.

Fujita, T.T., 1989: The Teton-Yellowstone Tornado of 21 July 1987.
*Monthly Weather Review,* **117**, 1913-1940.

Monteverdi, J.P., Doswell, C.A., Lipari, G.S., 2003: Shear Parameter
Thresholds for Forecasting Tornadic Thunderstorms in Northern and
Central California. *Monthly Weather Review*, 18, 357-370.

NCDC, 2006: Storm data.
*<http://www4.ncdc.noaa.gov/cgi-win/wwcgi.dll?wwevent~storms>.*

Tornado Project:
[*http://www.tornadoproject.com/*](http://www.tornadoproject.com/),
tornadoes by state: Idaho.

Wakimoto, R., and J.W. Wilson, 1989: Nonsupercell tornadoes. *Mon. Wea.
Rev*., **117**, 113-1140.

![](media/image1.png){width="5.993055555555555in"
height="4.429861111111111in"}

Figure 1: The blue outlines the tornado path through Adams County.

![](media/image2.png){width="5.991666666666666in"
height="3.736111111111111in"}

Figure 2: High resolution topography image. The area of interest is
outlined by a white polygon, which includes Baker and Adams Counties
(labeled in red).

![](media/image3.png){width="5.9944444444444445in" height="5.6875in"}

Figure 3: Map of the northwest displaying 500 mb heights, shown in pink,
and 250 mb winds at 1200 UTC, shown in blue.

![](media/image4.png){width="5.990277777777778in" height="5.71875in"}

Figure 4: 1200 UTC water vapor image of jet streak moving across
southern Oregon, shown by the white arrow. The area outlined in yellow
is the Boise County Warning Area.

![](media/image5.png){width="5.997916666666667in"
height="5.699305555555555in"}

Figure 5: 1200 UTC ASOS plot, shown in pink, and Mesonet dew point
temperatures, shown in blue. The area of interest is outlined in red.

![](media/image6.jpeg){width="5.9944444444444445in"
height="4.6409722222222225in"}

Figure 6: Modified 1200 UTC sounding at KBOI.

![](media/image7.png){width="5.995138888888889in"
height="5.7340277777777775in"}

Figure 7: 1200 UTC Hodograph at KBOI modified with VAD winds at 1815
UTC.

![](media/image8.png){width="5.990972222222222in" height="5.71875in"}

Figure 8: kcbx Composite Reflectivity of the squall line moving into
Baker County at 1605 UTC, which is outlined by the white polygon.

![](media/image9.png){width="5.9944444444444445in"
height="5.961111111111111in"}

Figure 9: 1605 UTC showing kcbx 0.5 degree SRM, 0.5 degree reflectivity,
Mesonet dew point data (shown in orange), and composite reflectivity.
The area of interest is outlined in white.

![](media/image10.png){width="5.991666666666666in"
height="5.794444444444444in"}

Figure 10: kcbx 0.5 degree SRM image at 1650 UTC showing the RIN and a
tightening rotation.

![](media/image11.png){width="6.166666666666667in"
height="5.956944444444445in"}

Figure 11: 1717 UTC displaying the RIN and increase in reflectivities.
Shown are the kcbx 0.5, 1.3, and 2.4 degree reflectivities, and the 0.5
degree SRM.

![](media/image12.png){width="6.583333333333333in"
height="6.239583333333333in"}

Figure 12: 1734 UTC kcbx 0.5, 1.3, 2.4, and 3.1 degree slices,
displaying an increase in reflectivities.

![](media/image13.png){width="5.995833333333334in" height="5.725in"}

Figure 13: 1739 UTC kcbx 0.5, 1.3, 2.4, and 3.1 degree SRM showing a
large area of strong outbound velocities and increased storm top
divergence, which are outlined in a white circle.

![](media/image14.png){width="5.995833333333334in"
height="2.8243055555555556in"}

Figure 14: 1805 UTC kcbx 0.5 and 1.5 degree SRM showing increase in
rotational velocities, which is outlined in white.

![](media/image15.png){width="5.9944444444444445in"
height="5.947916666666667in"}

Figure 15: kcbx VAD Wind profile from 1743 UTC through 1827 UTC showing
an increase in south to southwest winds.

![](media/image16.png){width="5.993055555555555in"
height="5.7027777777777775in"}

Figure 16: 1814 UTC kcbx 0.5, 1.3, 2.4, and 3.1 degree SRM showing
increased outbound velocities and storm top divergence, which are
outlined in white.

![](media/image17.png){width="5.997222222222222in"
height="5.684027777777778in"}

Figure 17: 1827 UTC kcbx display of 50 to 55 dBZ cores extending up to
10.4 km.

![](media/image18.png){width="5.998611111111111in"
height="5.718055555555556in"}

Figure 18: 1827 UTC kcbx 0.5, 1.3, 2.4, and 3.1 degree SRM showing a
rapid increase in rotational velocity and storm top divergence