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Untitled Document
An Elevated
Supercell in the Columbia Basin on 19 May 2002
Ron Miller, WFO Spokane WA
Introduction
On the evening of May 19th, 2002,
a long-lived supercell thunderstorm tracked across eastern Washington. The storm
moved across a sparsely populated area after sunset, so no damage was reported.
But the supercell was impressive from a structure and evolution standpoint.
Discussion
On the afternoon of May 19th, deep
south-southeasterly flow existed over most of the Pacific Northwest with a large
negative tilt trough offshore. This synoptic pattern is well known for being
a favorable pattern for severe weather in the eastern Washington and northern
Idaho area. This was well documented by Evenson and Johns in 1995.
Examination of the model forecasts
for the afternoon (00Z 20 May) indicated good instability (Fig
1) over the Idaho panhandle with strong 0-6km shear (40-50kts). This pointed
the forecasters to the possibility of organized convection over the panhandle.
Low-level (0-3km) shear was also strong, further suggesting the possibility
of organized lines. Over eastern Washington, stronger shear existed. But the
drier low-levels minimized the available CAPE, with a large area of CIN present
that would be difficult to overcome with daytime heating. An Eta forecast sounding
from Moses Lake in the Columbia Basin (Fig 2) showed
this less than impressive sounding..
Convection developed over the central
Idaho mountains and moved north into the panhandle. An organized west-east line
developed and produced severe wind damage in several locations. By the end of
the event, the convective line had moved into Canada (Fig
3). Meanwhile, a lone thunderstorm developed over the mountains of northeast
Oregon (see Fig 3) and moved towards the north-northwest
into the low elevations of central Washington. While this area was unfavorable
for initiating convection due to the strong cap, the thunderstorm that moved
into this region was elevated and did not initially rely on the boundary layer.
As the storm continued to develop,
it appears that the downdrafts generated by the storm were able to mix the low
level air and thus remove the cap. The forecast straight-line hodograph (Fig
2) indicated that significant rotation was not to be expected unless the
storm could move considerably left or right of the mean wind. At 0437Z some
rotation was noted (Fig 4) but it was only
in the weak mesocyclone category. Also note that the rotation was aloft, 16-21kft
AGL. Several times during it's life the storm showed the classic structure of
new updraft development. At 0449Z a new updraft is clearly evident on the southeast
flank of the storm (Fig 5). Just 17 minutes
later, a strong reflectivity gradient formed on the northeast side of the storm
(Fig 6). This is an atypical region of the storm
for this feature, but the low-level inflow was from the east-northeast as shown
in the VWP (Fig 7). The storm reached full maturity
at 0518Z (Fig 8) with 65 dbZ echoes present
above 12kft AGL.
Evenson, E. C. and R. H. Johns: 1995:
Some Climatological and Synoptic Aspects of Severe Weather Development in the
Northwestern United States. Natl. Wea. Dig., 20, 34-50.
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