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Untitled Document
A Severe Thunderstorm event
on June 10, 2003 in the Inland Northwest
Ron Miller, WFO Spokane WA
Introduction
A common weather pattern in the Pacific
Northwest during the spring is cold upper troughs. These troughs and their associated
steep lapse rates result in considerable diurnal convection. Typically at least
one factor limits the convective development so that the thunderstorms don't
become severe. These factors are often limited low-level moisture or surface
temperatures too cool. However there are situations where all the right ingredients
are in place to generate severe thunderstorms.
Discussion
On June 10th, 2003, a cold upper
trough moved into the Inland Northwest from Canada. The water vapor imagery
at 1730Z on this day (Fig 1) shows good darkening
associated with this trough implying a vigorous system with a good tropopause
fold. Storms which move into this direction (i.e. northwesterly flow) are typically
produce more wind than precipitation owing to the fact that they come from moisture
deficient region. On this day, however, low-level moisture was already in place.
Note the METAR and MesoWest observations at 18Z (Fig
2) with dew points in the lower and middle 50s over northeast Washington
and the northern Idaho Panhandle.
The Eta model showed that resulting
CAPE values (Fig 3) would be between 500-1000
J/kg in this area (CAPE values are yellow contours). However, this was offset
somewhat by the lack of lower-tropospheric shear. The Eta model showed that
the 0-6 km shear was moderate (around 40 kts) over the areas where the CAPE
was near-zero. But over the unstable area associated with the upper trough,
the shear was markedly less, below the 35 kts generally needed for mesocyclone
development.
The convection that developed during
the afternoon hours was fairly intense, in agreement with the forecast CAPE
values. But due to the lack of shear, most of the storms failed to become organized.
A few storms were able to generate some super-cell characteristics. One of these
storms was located near Newport, Washington.
The storm developed west of Newport
and was the result of an outflow boundary from a nearby storm. The thunderstorm
rapidly developed. By 2217Z a core of 60 dbZ extended more than 10,000 AGL (Fig
4), but with little or no tilt discernable. Even so, the storm did exhibit
some rotation in the mid-levels (Fig 5).
This rotation persisted in the mid-portion of the storm and did not show any
tendency to descend to the lower levels. At this same time, the storm had excellent
storm-top divergence (STD). Figure 6 shows
the SRM data on the 7.5 degree tilt, which was about 34,000 AGL. The outbound
velocity was greater than 50 kts with inbound velocities of 22 to 30 kts. This
results in a STD of around 80 kts. In a study of hail events in the Boise, ID
area, Jewell (2000) found STD of 75 kts or greater to be a good indicator of
large hail. The WSR-88D Hail algorithm at this 2217Z scan estimated a hail size
of 1.25". Two inch diameter hail was reported by a spotter near Newport.
Summary
The combination of a cold upper trough
and surface dewpoints in the mid 50s resulted in a severe weather event on June
10, 2003. While most of the severe thunderstorms produced 0.75"-1.00"
hail, one storm developed multi-cellular characteristics with some mid-level
rotation and very strong updrafts. Maximum hail size from this storm was 2"
which corresponded with a storm top divergence of around 80 kts.
References
Jewell, Ryan E. D., 2000: Using Storm
Top Divergence Signatures as Large Hail Indicators in the Boise CWA. Western
Region Technical Attachment, No. 00-14.
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