Severe Weather Discussion for March 21 – 27, 2011

Severe Weather Discussion for March 21 – 27, 2011.
Issued:  03/20/2011.

Highlights:

–  A few severe thunderstorms are possible Tuesday late afternoon/evening across central/eastern Oklahoma.  Primary threats are large hail and damaging winds.

–  Thunderstorms are possible late Friday into Saturday.

–  High temperatures will be in the upper 70s on Monday, near 80F Tuesday, mid 70s Wednesday, 70F Thursday, low 70s Friday, mid 70s Saturday and 60F Sunday.

Probabilities:

–  Severe Thunderstorms in Oklahoma:  30%
–  Storm Chase:  20%

Discussion:

Deterministic guidance continues to struggle this week as blocking develops across Canada.  Models are finally in decent agreement regarding Tuesday but rapidly diverge thereafter with the GFS and ECMWF much different by Saturday.  This creates a major headache temperature wise as the GFS argues for low 80s and the ECMWF for low 60s.

Upper system now moving onto the California coast will translate northeast into the central U.S. by Tuesday.  Mid/upper 50s dewpoints are spread across the state today and little additional moisture will move northward due to ongoing surface ridging @ Florida.  As the upper system lifts out on Tuesday, winds will increase at all levels.  An EML will suppress development much of the day, but the 18z NAM finally develops some convection near Oklahoma.  CAPE values are near 1500 J/Kg and directional/speed shear is sufficient for organized convection.  As such, expect a few severe thunderstorms along the dryline late Tuesday.  The GFS and NAM differ on the dryline Oklahoma and I favor the NAM at this point.  SPC Day 3 looks good.

Wednesday is seemingly a quiet day with cooler temperatures as a cold front slips into the region.  Model divergence makes the forecast beyond Wednesday quite difficult.  ECMWF is much weaker with several shortwave trofs traversing the central U.S. than the GFS.  This leads to a southward progression of a cold front into the area late Friday and cool weekend.  The GFS is some 15 degrees warmer than the ECMWF Friday and Saturday as it brings a stronger trof into the central U.S. on Saturday.  This would seemingly increase severe weather chances but given the big difference between the models, I’ll go a little lower for now.  One of the continuing hazards of issuing this product once a week.

NAM is good on temperatures through Wednesday, followed the GFS on Thursday, then blended the medium range models Friday/Saturday, and then GFS on Sunday.  Probabilities are set to climo as much as anything.  Confidence on storms Tuesday is decent but very low regarding Saturday.

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