Weekly Severe Weather Discussion

Severe Weather Discussion for April 20 – 26, 2009.
Valid: 04/19/2009.

Highlights:

– Severe thunderstorms are possible on Sunday. Degree of moisture
return and possible mid-level flow would indicate the potential for a
medium-end event. Potential continues beyond this forecast period.

– Temperatures will be very pleasant all week. Monday will see a
high near 80F, Tuesday upper 70s, Wednesday-Thursday mid 80s,
Friday-Saturday upper 70s, Sunday low 80s.

Probabilities:

Severe Thunderstorms in Oklahoma: 35%
Storm Chase: 25%
Winter Precip: < 5%

Discussion:

Models are in good agreement through much of the upcoming seven days.
I have tried to make sense of the 12z ECMWF and 18z GFS at D7 since
sensible weather impacts appear to increase. Given the consistent
ECMWF, time of year, and possible moisture return, I've gone a little
higher on the probabilities.

Closed low which brought an EF1 tornado to Logan County and much
needed rain for the area has opened up to a shortwave trof. Said
shortwave trof will continue east and phase with the northern jet
stream. The result will be an eastern U.S. longwave trof by late
Monday. This development will whack the Gulf, sending tropical
moisture past the Yucatan. Climatology does not support such an
event, but the models are quite clear on it and the 500mb pattern
supports it. Despite the upcoming violation of the Gulf, moisture
return does commence by late in the week. However, all POPs will be
zero through Saturday.

By Friday the eastern U.S. longwave trof clears the east coast and the
central U.S. ridge deamplifies. The flow starts to buckle on Saturday
as a western U.S. trof approaches. This is where the GFS and ECMWF
start to diverge. The GFS has more upstream ridging over the Pacific
than does the ECMWF. The GFS develops a western U.S. closed low and
leaves it out there until it essentially dissipates. The GFS also has
less northern stream energy over western Canada at this time period
than the ECMWF. The ECMWF ends at 168 hours, but at that time appears
to be more progressive. These factors along with the recent
consistency and performance of the ECMWF lead me to stay with it for
D6-7.

I don't like talking about medium-end events at D7, but I can't ignore
the GFS's moisture fields and climatology. Ensembles are also in
serious disagreement by 120hrs and this appears to be driven by
downstream ridging across the southeast U.S. The GFS appears too
aggressive with the ridging, developing a 588dm ridge over south
Texas. The ECMWF does have a 588dm ridge over Mexico.

No significant changes in temperature are expected this week. A weak
frontal passage is expected late Monday with little cool air behind
it. NAM temperatures are used through Wed, persistence Thu-Sun.
500mb ridging will make the middle of the week fell like late May.
Although, after an unseasonably cool April, I think this will be
enjoyed.

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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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