Severe Weather Discussion – 2008-10-3

Severe Weather Discussion for May 1, 2008.
Valid: 04/30/2008.

Highlights:

– Active severe weather event is possible late Thursday afternoon into
the evening. Primary threats are large hail, isolated tornadoes, and
damaging winds.

Discussion:

I find myself in agreement with SPC again as their Day 2 outlooks
appears right on the money. Overall setup is not favorable for
chasing and as such the chase level is left at normal.

12z NAM/GFS are in good agreement for this event and have resolved
many of the issues noted yesterday, especially the double barrel
850/925mb lows. Model progs continue to develop a large closed low
across the central U.S. by Thursday afternoon. This system will
mature during the day as the surface low matures and the system starts
to become vertically stacked by 00z Friday (7pm Thursday). The net
result is the 850mb low will lift northeast during the day with a
strong push on the dryline. Said dryline will mix to I-35 and
probably a little east of that location. The eastward extent is
outside of climatology for early May, but is in keeping with the
structure of this system. Another issue, also outside of climatology,
is the marginal moisture return expected for this event. 22z surface
analysis indicates mid 50s dewpoints just now on-shore at Houston.
NAM and GFS both show PW values at or below 2.0 across much of
Oklahoma, except for the eastern 1/3rd to 1/4 where 2.5 can be found.

The proverbial nail in the coffin for this event is the significant
westward displacement of the 300mb-200mb jet streaks. While the
NAM/GFS disagree a little on the speeds, they are both in the ballpark
of < 50kts during much of the day at these levels. It is only near
and after sunset that the jet streams meet the dryline over eastern
Oklahoma. In fact the 12z GFS shows a 100kt 250mb jet punching over
the dryline at 1am Friday. Of course, by this time the cold front
will likely have overtaken much of the dryline as the surface low has
occluded. SPC covered many of the above items and noted that none of
the operational models develop QPF along the dryline. While this is
not always the best gauge of an event, it is worth noting, especially
when compared to all the other negatives.

I am still inclined to believe that isolated severe thunderstorms will
develop by late afternoon around 6-7pm along the dryline, which should
be from Osage County to Shawnee to Ardmore. Any storms that can
survive and move east, will have a high likelihood of severe weather
with medium-end impacts. Low level flow will be quite favorable for
rotation, especially with any storms that develop after sunset.

A squall line is likely to develop during the evening hours and move
east across eastern Oklahoma. This line will have damaging wind
potential and maybe a gustnado or two.

SPC continued the slight risk and I do agree with this approach.
There is not enough confidence that the cap can be breach and even if
it does, storms should be isolated. An upgrade to a moderate is also
a consideration depending on afternoon cap strength and deep layer
moisture return.

Probabilities:

Risk area for any part of OK/TX (valid: 05/01 Day 1):

Slight: 100%
Moderate: 75%
High: 0%

Chase Status:

Level 1 – Normal

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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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