Severe Weather Discussion for April 18 – 24, 2011

Severe Weather Discussion for April 18 – 24, 2011.
Issued:  04/17/2011.

Highlights:

–  Severe thunderstorms are possible Tuesday afternoon and evening across eastern Oklahoma, generally east of HWY 75.

–  Severe thunderstorms are also possible Thursday-Friday and Sunday across parts of the state.

–  Extreme fire danger is likely on Tuesday.

–  Temperatures will be in the upper 80s on Monday, low 90s Tuesday, low 70s
Wednesday, mid 70s Thursday, low 80s Friday, mid 70s Saturday and upper 70s Sunday.

Probabilities:

–  Severe Thunderstorms in Oklahoma:  50%
–  Storm Chase:  20%

Discussion:

Complex forecast this week as I wish to avoid a repeat of last week’s discussion.  Models seem to agree that the 500mb flow will be somewhat less amplified than last week, other than Tuesday.  The continuation of drought conditions across Oklahoma will yield hot temperatures part of the week and extreme fire danger.  If current progs verify, some relief is possible across the eastern half of Oklahoma by Saturday.  Unfortunately, western Oklahoma looks to remain drought stricken for a while longer.

Very dry air is located across Oklahoma with a somewhat deep penetration into the Gulf from the front on Friday.  The result of this front will be a slow return of moisture into the work week.  18z NAM/GFS agree that moisture return will occur on Tuesday ahead of a shortwave trof.  This system will move across Oklahoma late Tuesday and may induce another severe weather episode.  Moisture looks to be in-place and with daytime heating, afternoon CAPE values will be in the 2500 J/Kg range.  Wind fields will again be favorable for rotating storms, although not the significance of last Thursday.  SPC Day 3 looks good and a moderate risk also seems a good possibility given the threat.  I may end up doing special discussions, but for now will let things ride.  The threat is very real and appears that destructive hail, damaging winds, and a few tornadoes are all possible.  The dryline is expected to surge eastward to near HWY 75 by late Tuesday with thunderstorms developing during the late afternoon hours.  Models are a little different on the cold front, but for now that is not a significant issue to the severe weather threat.  Once again, Oklahoma faces a significant fire threat to the west as the dryline screams across the state.

After Tuesday the forecast continues to be increasingly complex partly due to model variability but also seasonable heat/moisture near Oklahoma through at least Friday.  A low amplitude shortwave trof will approach the region on Thursday and move through on Friday.  Models disagree on the timing of this system along with placement of surface features.  GFS is very aggressive with the cold front, while the ECMWF is the least aggressive.  Another issue complicating the forecast is the potential for WAA late Wednesday into Thursday and if the front hangs up.  Given the ongoing drought, I seriously doubt the warm front will hang up south of Oklahoma.  I went with severe chances Thursday due to WAA as the approaching shortwave trof influences the region.  The ECMWF and to some extent GFS show severe chances for Friday.  The big difference between the two models is the stronger system shown on the GFS and resultant cold front.  GFS sends a cold front through the state on Friday with cool conditions Saturday and Sunday while the ECMWF keeps Oklahoma in the warm sector and develops southwest flow aloft.  GFS brings another weak shortwave trof through the region on Sunday while the ECMWF has a much stronger representation in the southwest U.S.  Both models would seemingly point towards at least low-end severe chances.

Tuesday appears the big severe weather threat for the state this week, but additional threats may develop towards the end of the week.  I’m not sure about the chase potential, but can’t be any worse than last week.  Temperature wise, I went with the NAM through Wednesday, GFS Thursday, and ECMWF Friday-Sunday.  We’ll see how badly I bite it next Sunday.

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