Severe Weather Discussion for March 14 – 21, 2011

Severe Weather Discussion for March 14 – 21, 2011.
Issued:  03/13/2011.

Highlights:

–  Severe thunderstorms are possible on Sunday across western/central Oklahoma.  Thunderstorms are possible across parts of the state on Saturday.

–  Temperatures will be in the low 50s on Monday, mid 60s Tuesday, mid 70s Wednesday-Sunday.

–  Spring starts Saturday evening.

Probabilities:

–  Severe Thunderstorms in Oklahoma:  25%
–  Storm Chase:  20%
–  Winter Precip:  < 5%

Discussion:

This week presents a forecast that seems easy but I’m thinking it will be much tougher by D7.  Models are inconsistent even at D3 on temperatures.  I have sided with the NWS/GFS but the NAM certainly is colder.  The flow will flatten out early this week with zonal flow the primary pattern through Thursday.  There will be periodic perturbation in the zonal flow and given decent moisture, isolated showers cannot be ruled out the latter half of the week.  I left pops out Thursday and Friday because I didn’t want to shotgun them, especially when higher and more organized storm chances seem to exist next weekend.

Deterministic guidance has been consistent in developing stronger southwest flow across the region late week into the weekend as a Pacific shortwave trof moves onshore.  18z GFS is consistent with prior runs in ejecting this system into the southern plains late Sunday while the 12z ECMWF is slower, about 24 hours such.  Problem for me is this forecast is issued only once a week and I have to cover all my bases.  So, I have gone a little higher on severe and chase probabilities than might otherwise be expected.  Lack of recent significant frontal intrusions into the Gulf should allow at least decent (upper 50s/low 60s) dewpoints to return later this week across Oklahoma.  A surface ridge will sit over Florida resulting in modified Gulf moisture return part of the week.  This seems the main limiting factor to higher octane moisture moving northward on Sunday.  Despite this, there should be enough moisture for thunderstorms on Sunday with the potential for severe weather.  SPC Day 4-8 hits this potential and for now will let the probabilities stand.  Special discussions may be needed if a medium-end event indeed develops.

For temperatures I didn’t get fancy this week and have to wonder if they’ll actually verify.  The GFS would suggest the mid 70s while the NAM would argue for lower temperatures Tue/Wed.

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