Severe Weather Discussion for November 7 – 13, 2011.

Severe Weather Discussion for November 7 – 13, 2011.
Issued:  11/06/2011.
Highlights:
–  Severe thunderstorms are likely Monday afternoon and evening mainly in western and parts of central Oklahoma.  Primary threats are destructive wind, large hail, and a few tornadoes.  SPC indicates the potential for a strong tornado.  Given the current Day 2 outlook, a moderate risk may be in the offering tomorrow.
–  Severe thunderstorms are possible Tuesday morning and afternoon across central and eastern Oklahoma.  Destructive wind and large hail are the primary threats, with continued potential for a few tornadoes.
–  Slight chance of showers on Saturday, better chance of widespread showers and some thunderstorms on Sunday.
–  Temperatures will be near 70F Monday – Tuesday, upper 50s Wednesday, low 60s Thursday, mid 60s Friday, upper 60s Saturday, and mid 60s Sunday.
Probabilities:
–  Severe Thunderstorms in Oklahoma:  50%
–  Storm Chase:  < 5%
–  Winter Precip:  < 5%
Discussion:
Interesting setup for Monday as a late season severe weather episode appears to be on-tap.  Models are in good agreement on the overall setup of a shortwave trof ejecting eastward across the state late Monday into Tuesday.  Decent moisture, characterized by surface dewpoints in the low 60s will move northward through Oklahoma during the overnight hours into Monday.  The big question for severe thunderstorms on Monday continues to be the potential of widespread rainfall.  18z NAM has continued the trend of reducing morning rainfall and increasing instability parameters Monday afternoon.  As is always the case, it is a gamble as to which scenario will verify.  For now, the potential is in-place for severe thunderstorms on Monday with concerns of an active severe weather day, especially given time of year.  SPC has issued a slight risk with 30% and 10% hatched areas for much of western and parts of central Oklahoma.  This is reasonable and see no reason to deviate.  Setup has some similarities to February 10, 2009, so will be interesting to see how this plays out.  One big difference is the dryline will be near I-27 Monday afternoon.  Either way, thunderstorm development is likely Monday afternoon and storms will progress rapidly eastward during the late afternoon and evening hours.  Very strong jet streaks aloft will create potential for destructive winds while speed/directional shear will create an environment favorable for large hail.  SPC is also targeting a strong tornado threat, mainly in the area of SW OK/NW TX given expected better moisture and heating.  18z NAM does develop some QPF near OKC Monday afternoon, but not sure the reason for it.  If storms develop overnight, then outflow boundaries will be a player on Monday.
Beyond Monday, the upper system will be moving across Oklahoma on Tuesday with a dryline moving east of I-35 by early afternoon.  NAM/GFS are at odds but the NAM has shifted eastward, closer to the GFS position of a dryline near OK/AR border at 00z (6pm).  As such, parts of eastern Oklahoma will be in a severe threat on Tuesday, but the significance is muddied due to expected convection on Monday.
Upper trough then exits the area with a cold front moving across Oklahoma late Tuesday.  Wednesday-Friday will be mild and dry, with a warming trend Thursday and Friday.  GFS and ECMWF both forecast another shortwave trof to dig into the mean longwave trof position over the western U.S. by Saturday.  A weak shortwave trof will eject eastward on Saturday and may influence Oklahoma late in the day.  Models have no QPF for this time period, but I added POPs anyway given warm advection regime that is likely to setup as mid 50s dewpoints move through north Texas.
A stronger shortwave trof will eject eastward on Sunday, which should lead to widespread showers and thunderstorms across the state.  ECMWF is a little slower but the overall trend is the same.  There is the potential for most of the rainfall to occur just beyond this forecast period.  A severe weather threat may develop given temperatures in the upper 60s and dewpoints in the low 60s.  Indeed the GFS has 1000 J/Kg CAPE values across western Oklahoma.  For now, I’m going to leave the severe chances low.  GFS winds are very strong with 200MB 120-140kts and 500mb 70-80kts.  We’ll see what comes of it, but next Sunday is likely to be windy at the very least.
Temperatures, oh boy these can be fun.  I went with near 70F Monday and Tuesday given expected location of surface features and that the cold front on Tuesday may be late in the afternoon.  As such, a warmup behind the dryline appears likely.  Wednesday will be cool with a warming trend into Saturday.  Sunday is rough since I’m thinking showers will be widespread and keep temperatures down, I went with mid 60s. Severe probability is set to moderate risk for Monday and Tuesday.  If this was October or April/May, we’d be chasing tomorrow but time of year doesn’t support it for us.
I must say, it is quite nice to have a western U.S. longwave trof, been too long!
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