Severe Weather Discussion for December 12 – 18, 2011

Severe Weather Discussion for December 12 – 18, 2011.
Issued:  12/11/2011.
Highlights:
–  Severe thunderstorms are possible on Wednesday, showers are possible on Tuesday.
–  Slight chance of rain/freezing rain on Saturday/Sunday.  Best potential for rain is southeast Oklahoma.
–  Temperatures will be in the low 50s Monday, upper 50s Tuesday, low 60s Wednesday, mid 50s Thursday, mid 40s Friday, low 40s Saturday, and upper 30s Sunday.
Probabilities:
–  Severe Thunderstorms in Oklahoma:  20%
–  Storm Chase:  < 5%
–  Winter Precip:  15%
Discussion:
Complex forecast this week with two at least low-end events and potential for a near medium-end event both times.  Model issues make forecasting quite difficult, even for the Wednesday event and certainly for next weekend.
Split flow will continue this week with southern stream systems diving south over California and then ejecting eastward across the region.  Two such systems are expected with the first Wednesday and the next during the weekend.  The first system has been consistently progged by all models to increase moisture across the state with low 60s likely by Wednesday afternoon.  The WAA pattern should result in widespread clouds across the state with heating seemingly minimal.  However, given the trend for 2011, severe weather prevails.  I’m not going to ignore the setup, although the primary threat will be large hail.  Medium-end events can occur in December, I believe it was December 12, 1991, which was a moderate risk.  Although nothing came of it due to high shear and very low thermo but SPC still did a tornado watch.  Upper system passes with little change in airmass.  Temperatures will be a little cooler absence WAA pattern.
Next upper system approaches the region next weekend and the models are in somewhat better agreement.  ECMWF and GFS keep swapping viewpoints with each other.  At this time the GFS’s progression is more typical of what we’ve seen this fall and what is expected next weekend.  The issue becomes the surface pattern, which none of the models seem to handle well.  The 500mb northern jet isn’t well defined and a significant cold air dump doesn’t seem likely.  However, temperatures will be below freezing during precipitation times next Friday-Sunday, such that freezing rain is possible.  Model soundings show the moisture in the lower few thousand feet of the atmosphere with little chance for snow.  At this time do not see a significant winter weather event, but a little of the freezing rain goes a long ways!  This is true for travelers, with minimal sun angle and surface heating.
Both setup will be monitored but at this juncture, special discussions are not anticipated.
Upper system clears the region on Sunday with quiet conditions for a few days.  GFS shows another southern stream system approaching the region around Christmas eve and no I’m not going there.
Followed the NAM somewhat on temperatures.  Wednesday will either be in the low 60s or near record breaking low 70s.  I’m sticking with the low 60s for now.  Beyond Wednesday went with general pattern, which agrees best with OUN and ECMWF.  GFS seems off and a little high in general.
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