Weekly Severe Weather Discussion

Severe Weather Discussion for November 30 – December 6, 2009.
Valid: 11/29/2009.

Highlights:

– Slight chance of showers Monday morning in southern Oklahoma.
Another chance of rain for the Oklahoma late Tuesday into Wednesday.
Light snow flakes are possible in far southern Oklahoma early Wednesday.

– Showers are possible on Sunday in eastern Oklahoma.

– Temperatures will be in the upper 50s on Monday, mid 50s Tuesday,
mid 40s Wednesday, upper 40s Thursday, low 50s Friday, mid 50s
Saturday-Sunday.

– Tropical Storm season ends on Monday, 2009 saw 9 tropical storms,
out of those 3 hurricanes, and finally one major hurricane.

Probabilities:

– Severe Thunderstorms in Oklahoma: < 5%
– Storm Chase: < 5%
– Winter Precip: 5%

Discussion:

12z ECMWF continues to be the best performer in the medium range, with
the NAM also a good reference in the short-term. GFS is noted, but
for the most part is not used in this forecast. Temperatures will be
a little below normal, but feel cool due to the relatively warm/dry
November.

The general 500mb pattern this week will be characterized by an
amplified northern stream ridge just off the western U.S. coast. This
feature will result in the northern jet stream moving southeast across
the central U.S. as a longwave trof develops Wed-Fri. This will setup
episodes of Canadian air movements into the central U.S. Add to this
mix a cutoff/closed southern stream low over AZ/NM this afternoon.

Ensemble plots show that the models are in good agreement through
Thursday and rapidly diverge after that point. The latter part of the
forecast is somewhat low confidence due to model disagreement, but
weather impacts are not expected either way.

Northern stream medium wavelength trof is progged to move into the
northeast U.S. by Monday morning. Cold front from this morning was in
response to said wave and expect warmer conditions on Monday as skies
clear due to dry air advection. Tuesday will also be quiet, but by
evening the southwest U.S. closed/cutoff low will move close enough to
influence conditions across OK/TX. Initial impacts will be cloud
cover and have adjusted temperatures down on Tuesday. This system
moves through the region on Wednesday with rain expected primarily
across central/southern Oklahoma late Tuesday through Wednesday. I
have followed NAM/ECMWF timing, along with NWS forecasters. There
appears to be a window for snow in southern Oklahoma Wednesday morning
as the upper system approaches and a Canadian cold front moves through
Oklahoma. Impacts are not expected, but light snow seems possible
right now.

Previously noted Canadian cold front will be pushed southward by
another northern stream shortwave trof moving east on Thursday. Air
in Canada is not particularly cold ATTM and this high originates in
western Canada. FROPA is expected Wednesday morning, so temperatures
remaining steady under cloudy skies seems likely. Thursday will
provide more clearing as the southwest U.S. system rapidly departs and
skies clear. CAA will still be ongoing with isobars connecting to
central Canada. Daytime temperatures will likely remain in the 40s,
even with full sun.

Friday, the surface high is centered over the region with light winds
expected and a little warmer. Sensible weather is likely to be low
Friday-Sunday as the 500mb pattern transitions. I did put a slight
chance of showers in for Sunday as a small nod to the GFS. Said model
is more progressive with the 500mb pattern and developing a longwave
trof in the west. The ECMWF also appears to support this solution but
on a slightly slower time schedule.

Interesting to note that during the week, cold air starts to load in
Canada and the Artic Oscillation goes negative per the CDC ensemble
plots. This is similar to what we saw in the rather cool/wet October.
GFS hints at another Canadian air plunge into the central U.S. on
Sunday 12/06. The slower ECMWF would also seem to support such a
plunge on Monday/Tuesday as it shows a high developing in northwest
Canada late in the week. This air would seem to have more of a bite
than the one expected on Wednesday, should it even materialize. As a
final note, models tend to show a southwest U.S. system moving east
post cold front passage. If true, could make for a spicy time period
around 12/07-12/10. However, it is all very low confidence and
knowing my luck we'll be in the 60s with clear skies.

I followed the NAM MOS through Wednesday and ECMWF through the end of
the week. I stayed away from the GFS mainly late in the week. The
early part of the GFS looked good. Several of the ensembles strayed
from the operational solution Thursday-Sunday.

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About Putnam Reiter

Putnam has been storm chasing since 1990 and is a co-founder of Hook-Echo.com. For his day job, Putnam works in emergency management for information technology.
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