Weekly Severe Weather Discussion

Severe Weather Discussion for April 19 – 25, 2010.
Valid: 04/18/2010.

Highlights:

– Severe thunderstorms are possible in western and central Oklahoma on
Thursday.

– Severe thunderstorms are possible in eastern Oklahoma on Friday.

– Isolated thunderstorms are expected late Tuesday into early
Wednesday across central and northern Oklahoma.

– Temperatures will be in the upper 60s Monday, mid 70s
Tuesday-Wednesday, upper 70s Thursday, low 70s Friday, mid 70s
Saturday, near 70F Sunday.

Probabilities:

– Severe Thunderstorms in Oklahoma: 50%
– Storm Chase: 40%
– Winter Precip: < 5%

Discussion:

Severe weather probabilities are finally at climatologly given the
rather limited activity so far this year. Given SPC Day 4-8 prog and
18z GFS/NAM 12z ECMWF, special discussions will likely be needed
starting tomorrow.

As noted above, models are in decent agreement regarding the evolution
of a western U.S. closed low. As always, there are differences in the
details, but the overall pattern is similar amounts the models through
Friday. Larger differences do come into play on Saturday and Sunday
as the ECMWF is slower than the GFS with the closed low and associated
trof. This forecast will lean towards the faster GFS these days and
this really has limited impact overall on Oklahoma.

Current small shortwave trof will leave the region later today as
shortwave ridging develops across the region. Clouds will be slow to
burn-off on Monday, so expect temperatures to be held down a little.
Ridging increases a little on Wednesday but a stationary front is
expected to be in the state. This will likely focus storm development
late Tuesday into early Wednesday. Models generally agree on this and
so POPs are added for this time period.

Interesting and potential medium-end event starts to take shape
Thursday and continues into Friday. Models develop a large closed low
in the southwest U.S. by late Tuesday and slowly move it east.
Surface ridging has decreased in the Gulf with 60-65F dewpoints, which
are expected to increase by Wednesday with 65-70F. NAM shows moisture
return into the region with low 60s dewpoints and 850mb dewpoints of
10C across the region. GFS seems at odds on moisture return, which is
a typical bias of this model. I expect better moisture return by late
Thursday than currently advertised by this model. This is noted by
the limited movement of the 65-70F dewpoints between Thursday morning
and late afternoon. Either way, good moisture return is expected
leading up to late Thursday across the state. GFS develops QFP along
the dryline early Thursday and spreads it east during the day with
additional development later in the day, but lowered instability
numbers. This appears to be another bias given the unexpectedly
cooler 850-700mb temperatures. There is disagreement between the NAM
and GFS at 84 hours with regards to this parameter. The NAM appears
to have a much better handle on temperatures and subsequent EML (cap).
I'd think the cap will be stronger than the GFS indicates and the
18z GFS seems to be catching onto this as it has much less QPF than
the 12z run. Although, this really doesn't carry through to Thursday
afternoon. Winds look good at all levels and sfc-500mb crossover is
greater than 45 degrees. Given these factors, there appears decent
confidence of a medium-end event on Thursday and probabilities are set
for such. As always, the finer details will be important for chasing
and storm mode/development.

GFS favors eastern Oklahoma for severe thunderstorm development on
Friday and this appears reasonable. The upper system starts to move
eastward with a closed low over northwest Oklahoma. Even though the
upper system is vertically stacked, sfc-500mb crossover in far eastern
Oklahoma remains at least 45 degs at early afternoon. The ECMWF would
seemingly bring the threat a little westward, but I'm going with the
GFS for now. Given the expected development on Thursday and possible
cap erosion, widespread thunderstorms may completely end the severe
weather threat on Friday. This will be handled in special discussions
if necessary.

Beyond Friday, the weather quiets down a lot with a pleasant weekend.
I hope I do better with temperatures next weekend.

Regarding temperatures, I didn't deviate too much from the models
other than Thursday/Friday. The weekend is really a guess, but
lowering temperatures behind the front along with possibly some
mid-level clouds makes sense right now. In the longer term, the 18z
GFS shows a similar system developing for the 29-30th, so we may
finally get into an active pattern.

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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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