WESTERN REGION TECHNICAL ATTACHMENT
NO. 97-11
MARCH 25, 1997
A DECEMBER SEVERE THUNDERSTORM
IN SOUTHWEST IDAHO
Chris Gibson - NWSFO Boise, Idaho
Introduction
During the afternoon of December 10, 1996, a number of strong to severe thunderstorms
developed over southwest Idaho. One of the cells produced severe hail and exhibited
supercell characteristics including a long-lived rotating updraft and storm splitting. This
convective outbreak provided forecasters at NWSFO Boise an early opportunity to
examine and utilize new features of the WSR-88D Build 9.0 software load. The intent of
this Technical Attachment (TA) is to both document unusually strong wintertime convection
in southwest Idaho and to provide a glimpse of the potential benefits of new displays
produced by the Build 9.0 software.
Synoptic Setting
The synoptic pattern of the afternoon considered here, can best be described using the
"kata" type cold front conceptual model of Browning and Monk (1982). The kata, or "split"
front configuration has been shown to produce significant, and sometimes severe weather
east of the Rocky Mountains (Hobbs et al., 1990). In southwest Idaho, this pattern has
been associated with a number of severe convective weather episodes, although these
outbreaks usually occur during the spring and summer.
On December 10, 1996, a kata-type cold front moved across Idaho. The upper-level front
traversed the region during the morning hours, providing partial clearing over southwest
Idaho as cooler and drier air moved in aloft. The surface cold front lagged behind the
upper-level feature and was still west of the region by mid-afternoon. In advance of the
surface front, unusually mild air was advected into the area by low-level southerly flow.
The configuration of (relatively) warm and moist air at low levels with dry and cool air aloft
can result in an environment with significant convective instability. Modification of the
0000 UTC 11 December Boise sounding yielded a lifted index of -5 and a CAPE of 500-600 J/KG. Thus, the environment was characterized by conditionally unstable air and any
lifting by the terrain, or the approaching surface front would likely result in convection.
With the jetstream positioned over the region, vertical wind shear was significant. The
Boise hodograph (Fig. 1) was relatively unidirectional. The magnitude of the shear, as
measured by the length of the hodograph from the surface to 6 km was greater than 60
knots. A value around 50 knots is generally used at Boise as a threshold for development
of organized convection (supercellular traits).
The Storm
Isolated, intense thunderstorms developed across the region after 2000 UTC. One in
particular was long-lived, producing severe hail and strong straight line winds. The cell
developed in extreme eastern Oregon and moved northeast across southwest Idaho at
about 45 mph. As the storm moved across the populated region near Boise, storm relative
velocity data indicated a weak, broad but persistent cyclonic circulation at low to mid levels
within the storm. At 2215 UTC, the AWOS at the Caldwell, ID (25 miles west of Boise)
reported a wind gust to 54 mph. During the next 10 minutes hail up to .5 inches in
diameter was reported by weather spotters. By 2224 UTC, the cell had moved into
northern Ada County and was over the communities of Star and Eagle (Fig. 2). Maximum
hail size within the storm was estimated at 1.25 inches by the hail algorithms.
The Cell Trends display (new for software Build 9.0), provided interesting details about the
storm severity and evolution (Fig. 3). Note that the radar derived storm top is only about
20,000 feet and the height of the maximum reflectivity (DBZM HT) was increasing and
decreasing in a "pulsing" manner. The hail detection algorithms indicated a rapid rise in
the probability of hail and severe hail contained within the storm after 2149 UTC. Values
of VIL and maximum reflectivity increased through the period, with the VIL rising
dramatically after the 2149 UTC volume scan. A severe thunderstorm warning was issued
at 2227 UTC for northern Ada County. At 2245 UTC a weather spotter reported .75 inch
hail in extreme northern Ada County. A severe thunderstorm warning was issued for Boise
County at 2250 UTC.
As the cell reached its maximum intensity over northern Ada County, it appears to have
split. Storm relative velocity data at 2214 UTC (Fig. 4, quadrant 1) shows the weak low-
level cyclonic meso-cyclone just southwest of Star. By 2229 UTC (quadrant 2), the
cyclonic rotation center had moved into a region of ground clutter suppression and cannot
be seen. However, a center of anti-cyclonic rotation is evident on the west flank of the
storm. This is believed to be the center of a newly formed left moving cell which weakened
rapidly (quadrant 3). One kilometer resolution satellite data (not shown) also indicates the
formation of a new cell on the north flank of the original storm. The observed splitting
behavior of this storm agrees well with the results of numerical simulations of storm
development and evolution in moderate to strong unidirectional shear (Weisman and
Klemp 1986).
As the storm moved northeast into Boise County the circulation re-emerged from the area
of clutter suppression (quadrant 4). At 2310 UTC, a spotter reported 1 inch diameter hail
at the location indicated on the figure. The cell dissipated over northern Boise County
over two hours after it had formed in extreme eastern Oregon.
Summary
Although December thunderstorms are rare in Boise, averaging one December
thunderstorm day every 10 years, unusually vigorous convection developed near the city
on December 10, 1996. One cell produced severe weather and contained features
normally associated with supercells. The new software load for the WSR-88D, including
the Cell Trends display and Hail Detection Algorithms, proved very useful for monitoring
these storms. It is apparent that this technology will play a significant role in our abilities
to detect and provide warnings for severe thunderstorms during the upcoming convective
season.
Acknowledgment
I'd like to thank Dr. David Billingsley and the other reviewers for their excellent suggestions
regarding this TA.
References
Browning, K. A., and G. A. Monk, 1982: A Simple Model for the Synoptic Analysis of
Cold Fronts. Quart. J. R. Met. Soc., 108, 435-452.
Hobbs, P. V., J. D. Locatelli and J. E. Martin, 1990: Cold Fronts Aloft and the
Forecasting of Precipitation and Severe Weather East of the Rocky Mountains.
Wea. and Forecasting, 5, 613-626.
Weisman, M. L. and J. B. Klemp, 1986: Characteristics of Isolated Convective Storms.
Mesoscale Meteorology and Forecasting (P. S. Ray, ed.), Amer. Meteor. Soc.,
Boston, 341-358.