Weekly Severe Weather Discussion

Severe Weather Discussion for August 17 – 23, 2009.
Valid: 08/16/2009.

Highlights:

– Showers and thunderstorms are possible Monday – Thursday.
Temperatures will be in the upper 90s Monday, mid 90s Tuesday, low 90s
Wednesday, mid 90s Thursday, and low 90s Friday – Sunday.

– The tropics have gotten real active with three system in the
Atlantic right now. Claudette should make landfall tonight and I
think Ana was just downgraded again. Bill is the only thing of
interest and it appears Bill will miss the U.S. but we'll see on that
one.

Probabilities:

– Severe Thunderstorms in Oklahoma: 10%
– Storm Chase: < 5%
– Tropical Development: > 75%

Discussion:

August is supposed to be easy forecasting, but not this go around!
Northern stream jet will continue to be active with two systems
expected to traverse the northern U.S. this week. The first system is
well handled by the NAM/GFS/ECMWF to move from North Dakota into
eastern Canada by Wednesday. This will put some push on a cold front
in Kansas. This boundary will likely maintain a chance of showers and
thunderstorms during the first half of the week. While an outflow
boundary may become dominate, a southward push much beyond northern
Oklahoma isn't likely. I agree with OUN that the GFS is probably too
dry and I have gone with the NAM, which also happens to be persistence
for this summer. Pops are kept Monday – Thursday, and I've left pops
out this next weekend. Can't rule out an MCS, but flow will be
meridional and will be tough to get high plains convection into
Oklahoma.

Models do start to diverge beyond Day 4 with respects to the 500mb
pattern. The end result is gen4erally the same. The NAM holds back
more 500mb trofing on Wednesday than the GFS but as noted this has
little impact on Oklahoma. The big story seems to be the rare
development (at least in a normal year) of an eastern U.S. longwave
trof by Friday. As this system develops in the central U.S., a cold
front will move through the state on Thursday, stalling in north Texas.

Said eastern U.S. longwave trof will also weaken the subtropical ridge
over the western Atlantic. Current indications are this will allow
Bill to turn north and recurve without much impact on the U.S. The
GFS does take Bill very close to the northeast U.S., but that is still
a long ways out. Ana is back to a depression and not much of a threat
right now. Indications are it will move very close to Hispaniola,
which should destroy it.

Temperatures are very tricky this week and I'm just looking to get
beat down. Monday is likely to be warm like today, but Tuesday could
be complicated by convection. I have further decreased temperatures
on Wednesday with anticipation of more convection. Thursday is also
tricky since the cold front will be close and more convection may be
possible. I haven't followed models too much on temperatures through
Thursday, but did so Friday – 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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