Severe Weather Discussion for April 7 and 9-11, 2008.
– Active severe weather day is possible for much of Oklahoma on
Monday. Primary threats are tornadoes, destructive hail, and damaging
– Significant excessive rain event is possible late Wednesday into
Friday for the eastern half Oklahoma into MO/KS/AR.
– Chase Status is increased to Level 3
Significant changes to yesterday's forecast for Monday and hence the
special issuance. SPC Day 2 has a slight risk posted for Oklahoma
with a 30%/10% hatched area. This really caught my attention and it
would seem that the 12z model runs are why we have this change. 12z
NAM has trended towards the GFS on the setup for Monday. Models are
very similar in their depictions regarding the atmosphere above 850mb.
As is usually the case, it is the near surface to 850mb area that
are different. Despite these differences, there is enough confidence
to raise the chase level and wording.
Models now indicate 300mb-200mb winds of 100kts with 400mb winds
around 65kts. 500mb flow is a little weaker than I'd expect around
50kts, but this isn't a big deal. 700mb flow of 40kts with 850mb of
25kts will provide favorable speed shear for 0-3km rotation.
Directional shear looks to be impressive with NAM helicity values of
300 to 450 m^2/s^2. There are some questions regarding this setup and
moisture is a big one. NAM is much more robust with near surface
through 850mb moisture while the GFS is a little less at 850mb
indicating 12C dewpoints. I think this would go more to storm type
than anything, with the GFS favoring LP supercells and the NAM more
around the classic. NAM CAPE values are around 2000 J/Kg in Oklahoma
with LIs to -7 and EHI of 5.93, all of these are good for early April.
Another question is surface feature placement. Models disagree on the
surface low location and resultant warm front. There is confidence
that the warm front will be in in Oklahoma and probably near or just
north of I-40. Favorable west to east orientation of this boundary
will provide a favorable anchor for storms in this area. These storms
will have enhanced tornado potential given stronger helicity due to
backed winds near the boundary. The dryline looks to be in western
Oklahoma, probably near Hollis. This is why I picked Hobart, but I
reserve the right to change that location. NAM develops convection
during the late afternoon hours and spreads it east. There is some
signs that convection may develop a little farther east, so my Hobart
location may be modified.
NAM and GFS indicate that Oklahoma will be unseasonable cool Wednesday
morning and that the warm sector will likely stay south of the state.
While this will keep us out of the significant severe weather threat,
it will provide a favorable setup for an excessive rainfall event.
12z/18z GFS continues prior trends of 3-5 inches widespread across
Oklahoma/AR/north Texas, and some parts of MO/KS. Forecast reasoning
remains the same and the 84hr panels of the NAM are showing a similar
event. Central and south Texas face a significant severe weather
threat and this will be reviewed in more detail once I can get past
Monday. SPC has the entire region in the Day 4 outlook, but I really
think they'll end up with Texas in the high threat area. Oklahoma may
see severe thunderstorms late Wednesday into Thursday as elevated
instability will be favorable for large hail.
Risk area for any part of OK/TX (valid: 04/07 Day 1, Day 3):
Slight: 100%, 100%
Moderate: 100%, 50%
High: 15%, N/A
Level 3 – Looking to chase on Monday, initial target area is Hobart.
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