Severe Weather Discussion 2009-3-3

Severe Weather Discussion for March 23, 2009.
Valid: 03/22/2009.

Highlights:

– Active severe weather day is possible across much of Oklahoma on
Monday. The most enhanced area is from Oklahoma City to Ponca City,
per SPC's moderate risk. Obviously eastern Kansas/Nebraska/South
Dakota will all join in on the severe weather. Primary threats in
Oklahoma are large hail and damaging winds. However, if discrete
storms can form and organize, a tornado threat will develop. SPC
hints at a strong tornado threat for central and north-central Oklahoma.

– Chase Status is increased to Level 3.

Discussion:

No changes to prior forecast package or reasoning. The main thing to
note is the models finally got a clue about moisture return. So, it
is the NAM = 1 and Putnam = 1, I'll take the hit for the speed of the
upper system. Gulf has not been cleaned out for two weeks and while a
ridge is present, all it has to do is move. It isn't like we're
trying to get 65F dewpoints back into the Gulf, they're already
in-place. As mentioned the past few discussions, rapid moisture
return is expected on Monday with upper 50s dewpoints common across
Oklahoma by late afternoon. Moisture will be deep with greater than
10C up to 800mb. Models have also gotten a clue that we'll likely
warm up with temperatures in the mid/upper 70s. These values yield
CAPEs of 700-1200 J/Kg across the region Monday afternoon. This is
sufficient for severe thunderstorms and continues the prior thinking
of a high shear, low CAPE day. I wouldn't be surprised if afternoon
CAPE values end up a little higher than currently progged.

The NAM's dryline position still looks best, along the far western
part of Oklahoma. 12z plot has it from Fairview to Altus. This looks
good and think the ultimate location will be quite close. BTW – this
run also show 60F dewpoints moving into Oklahoma City around 7pm.
Another interesting situation is that after sunset, CAPE values will
hold steady or increase. This will occur as cooler temperatures
arrive aloft. So, ongoing storms should continue into the evening
hours.

NAM UVV plot show an area of deep layer UVV's by 7pm across southwest
Oklahoma and northwest sections. This is very interesting and I think
what will get the event started by 4-5pm. I know that some prefer
north-central Oklahoma and I can't rule it out, but that is also the
area where a squall line is likely to form first. Farther south,
storms should remain isolated longer, assuming they start that way.
Tornado potential will exists given deep layer shear. This potential
may be enhanced by early evening in central Oklahoma northward (per
SPC) as the low-level jet increases, along with LCL's decreasing. NAM
shows 60-65kt low-level jet Monday night.

Either way, it appears that Monday could be a very long day and the
group is up for night chasing. We'll be testing some new software
this go around, hopefully we'll stay out of the hail cores. Did I
mention that large hail is a big threat? SPC risked part of southern
Oklahoma in a 10% hatched area just for the large hail threat. The
10% hatched area and 45% area co-located in the moderate risk area.
This looks to be their representation of the strong tornado threat.
Indeed this appears to be a good product, especially considering the
rapid moisture return. NAM 850mb map shows 13C at OKC by 7pm.

PRIND: A dryline will be located over western Oklahoma during the
mid-afternoon hours. Isolated storm development will occur by 4pm
along this feature. Storms will move northeast with elevated storms
screaming to the north at 45-50mph. BUFKIT data for OKC shows storm
motion from SW at 35mph. By early evening a long squall line is
expected to develop with damaging winds the primary threats. A
residual severe threat exists for southeast Oklahoma on Tuesday, but
certainly not the significance of tomorrow.

Probabilities:

– Risk area for any part of OK/TX (valid: 03/23, Day 1):

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

Chase Status:

Level 3 – looking along I-40 to Weatherford and will drift around as
needed. Time to leave around 15:00.

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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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