Severe Weather Discussion for May 16 – 22, 2011

Severe Weather Discussion for May 16 – 22, 2011.
Valid:  05/15/2011.

Highlights:

–  Thunderstorms are possible late Wednesday across western Oklahoma.

–  Severe thunderstorms are possible Thursday and Friday across much of Oklahoma.

–  Thunderstorms are possible Saturday across eastern Oklahoma.

–  Severe thunderstorms are possible Sunday across western/central Oklahoma.

–  Temperatures will be in the low 70s Monday-Tuesday, upper 70s Wednesday – Friday, mid 80s Saturday – Sunday.

Probabilities:

–  Severe Thunderstorms in Oklahoma:  75%
–  Storm Chase:  75%

Discussion:

Model guidance indicates thunderstorm chances returning to the state by Wednesday.  Until then, very pleasant conditions will exist across the state Monday and Tuesday as unseasonably cool May conditions continue.  By early Wednesday, a large trof will develop across the western U.S. with subsequent surface reflection east of the mountains.  Airmass modification is expected to commence on Wednesday but will likely be slower than the GFS progs.  NAM dewpoint progs for late Wednesday are more reasonable given current frontal intrusion into the Gulf.  Due to increasing deep layer moisture, surface dewpoints upper 50s and 850mb @12C, a few thunderstorms are possible given CAPE values around 750 J/Kg.  Primary area will be western Oklahoma.  While a severe thunderstorm or two seems possible, overall chances at this juncture negate introducing the potential.

Beyond Wednesday models forecast the upper trof over the western U.S. to edge eastward towards the region and eventually eject northeast over the region on Friday.  Upper flow does not become meridional as was the case last week, with the subtropical jet suppressed farther south.  Deep layer moisture continues to increase across the region with dewpoints in the upper 60s and low 70s by Friday.  850mb dewpoints near 15C will also be in-place.  GFS CAPE values correspondingly increase in this time period as stronger flow aloft overspreads this potentially unstable airmass.  GFS develops a squall line Thursday afternoon in western Oklahoma and progresses it eastward across the state.  This seems suspicious given what should be the presence of a strong EML (cap), given time of year.  Indeed the 12z NAM 84 hr panel has higher 700mb temperatures than the GFS and this may translate to lower cap temperatures.  Tough call at this point, but will need to monitor.  AFDOUN hits things better than I could regarding the severe potential Thursday-Friday as there are several potential outcomes these two days.  Current thinking yields the potential for severe weather both days, along with the potential for a medium end event.  I have set the probabilities for this “potential” and will start special discussions if necessary.  SPC has not outlined an area in the Day 4-8 product, which is good given blocking pattern and uncertainties regarding the break-down of said pattern.  Rainfall potential looks good with this system and it could be supercells or end up like last week with widespread thunderstorms.  Either way, we need the rain, especially western OK.

Storm chances on Saturday look low as the primary feature will be northeast of the region and flow weaker across the state.  However, a dryline should remain present and there could be enough forcing to generate a thunderstorm or two.  If timing of the current system changes, severe chances could enter the forecast.  Sunday is also contingent on timing but currently appears that severe chances exists east of a dryline located in far western Oklahoma.  Models indicate a shortwave trof ejecting northeast into Kansas during the late afternoon with stronger flow located across the region.  Model CAPE values are AOA 3500 J/Kg, but 7 days out this will likely change.  Time of year points to severe chances and will go with this for now.  Active pattern continues into next week with additional severe chances.  Chase potential looks good, as with last week.  The finer details will likely remain elusive until the morning-of.

Temperatures are expected to be below normal Monday-Tuesday and then above normal the remainder of the week.  GFS seems to cool early on and went above guidance.  Latter half of the week looks good.

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