December Storms
(B.McNoldy)
As many of you know, there was an impressive
display of December Fury over the Central Plains on the evening and
night of December 2, 1999. The squall line and resulting severe
thunderstorms were spawned by a front and dryline moving through the
area.
High temperatures that day reached into the
mid-60's in much of OK, with dewpoints in the mid to upper 50's. Surface
winds were out of the south at about 30kts, and 300mb winds were out of
the southwest at 100kts. Based on the 12/3 00Z Norman sounding, the CAPE
was about 1500 J/kg, the Lifted Index was around -7, SWEAT was 485, K
was 38, and the Total Totals Index was 60. The classic indices pointed
to a likelihood of numerous thunderstorms, some severe with isolated
tornadoes. That night, the indices were perfectly on target.
The cold front (the section of interest anyway)
was along a line from Alva,
OK to Altus AFB, OK at 0130Z. The dryline was along a line from Vernon,
TX to Fort
Stockton, TX at that time. The synoptic setup was very similar to that
of the May 3rd
tornado outbreak in central OK.
At about 1500 CST (21Z), the first storms
started to erupt in western OK. Within a few hours, a major squall line
stretching from Wichita to the OK/TX state line was racing across the
Great Plains. Due to a field experiment I'm involved with at CSU, I've
been forecasting for the ARM site in north central OK for the past few
weeks. I was on duty for 10 hours that day, so I was watching hints of
the squall line since the first visible images arrived in the morning
(not convective at that point). I called RapidScan on GOES-10 for the
20Z-23Z period for the experiment (NOT involving severe storms), but as
it turns out, that was also the pre-storm period. Anyone watching
GOES-10 imagery just before things started to pop noticed the 7-minute
imagery interval. Unfortunately, the sun was setting just as things
really started to get active.
During the course of the evening, much of
Oklahoma was covered by severe thunderstorm watches, a large number of
counties under severe thunderstorm warnings, and there were a handful of
tornado warnings. There were 5 tornadoes reported on the ground between
Ponca City and Oklahoma City in the 00Z-02Z time frame, according to SPC
records.
It is remarkable how similar this event was to
the OKC outbreak 7 months ago. The temps and dewpoints were much lower
than the classic expected numbers for a powerful squall line (65/58 in
December, 72/68 in May). The cloud deck was either solid or slightly
broken most of the day in both cases (i.e., it wasn't clear before the
storms came --- lapse rates not too steep).
The tornadoes --though less numerous-- formed in the same places as
they did in May too. Just something to keep in mind next time you're
chasing, traveling, etc.
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