Severe Weather thread 9/17 - 9/20

[dkmac]

Day 1:


DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1227 AM CDT THU SEP 17 2009

VALID 171200Z - 181200Z

...NO SVR TSTM AREAS FORECAST...

...GULF STATES...

LATEST MODEL GUIDANCE CONTINUES TO SUGGEST UPPER LOW WILL GRADUALLY
FILL AND BECOME MORE DISJOINTED WITH TIME AS IT RETROGRADES SLOWLY
WWD ACROSS THE ARKLATEX INTO THE SRN PLAINS. WHILE SWLY MID LEVEL
FLOW IS EXPECTED TO CONTINUE ACROSS THE CNTRL GULF STATES...H5
SPEEDS WILL GRADUALLY WEAKEN TO AROUND 15KT AND LARGE SCALE ASCENT
WILL BECOME LESS FOCUSED. EVEN SO...NUMEROUS SHOWERS AND
THUNDERSTORMS WILL ONCE AGAIN DEVELOP FROM THE LA/MS/AL/FL
COAST...NWD INTO AR AND SRN TN. FORECAST SOUNDINGS DEPICT WEAK
VEERING PROFILES THAT SHOULD FAVOR SLOW MOVING CONVECTION THAT AT
TIMES MAY EXHIBIT WEAK ROTATION. HOWEVER...IN THE ABSENCE OF STEEP
LAPSE RATES OR STRONGER FLOW...IT APPEARS THE PROBABILITY FOR
ORGANIZED SEVERE THUNDERSTORMS WILL BE LOWER THAN DURING THE DAY
WEDNESDAY.

...SRN ROCKIES...

BOTH THE GFS/NAM GUIDANCE SUGGEST UPPER LOW NEAR THE 4-CORNERS WILL
DIG SLOWLY SSEWD TO A POSITION NEAR THE NM BORDER OVER SERN AZ.
WITH H5 TEMPERATURES ON THE ORDER OF MINUS 10-12 C...ANY HEATING
ACROSS THIS REGION WILL RESULT IN BUOYANCY MORE THAN ADEQUATE FOR
DEEP CONVECTION. NUMEROUS SHOWERS AND THUNDERSTORMS WILL DEVELOP BY
EARLY-MID AFTERNOON OVER THE HIGHER TERRAIN...THEN SPREAD SEWD ALONG
AN AXIS OF MODEST INSTABILITY TOWARD NCNTRL MEXICO. IF EARLY
MORNING CLOUD CANOPY IS NOT TOO RESTRICTIVE...STRONG BOUNDARY LAYER
HEATING MAY RESULT IN MORE INSTABILITY THAN CURRENTLY FORECAST BY
00Z GUIDANCE. ADDITIONALLY...WITH ONGOING CONVECTION ACROSS NM
OUTFLOW FROM THIS ACTIVITY MAY FORCE LOW LEVEL MOISTURE WWD BENEATH
APPROACHING UPPER LOW. IF SO...A FEW SEVERE THUNDERSTORMS COULD
DEVELOP ACROSS SRN NM AND PERHAPS EXTREME SERN AZ. LOW
PROBABILITIES MAY NEED TO BE ADDED TO THIS REGION IN LATER OUTLOOKS.

..DARROW/GRAMS.. 09/17/2009


http://www.spc.noaa.gov/products/outlook/day1otlk_1200.gif

[nightrider]

http://www.spc.noaa.gov/products/exper/day4-8/day48prob.gif

ZCZC SPCSWOD48 ALL
ACUS48 KWNS 170845
SPC AC 170845

DAY 4-8 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0345 AM CDT THU SEP 17 2009

VALID 201200Z - 251200Z

...DISCUSSION...
MEDIUM-RANGE MODELS CONTINUE TO REVEAL THE EXPANSION OF A LARGE
MID-LEVEL TROUGH INTO THE CENTRAL U.S. DAYS 4-5 /SUN. AND MON. SEPT.
21-22/...AND ARE RELATIVELY CONSISTENT WITH LOCATION AND TIMING OF
THE ASSOCIATED COLD FRONT THROUGH DAY 5. HOWEVER...FROM DAY 5
ONWARD...MODEL AGREEMENT DIVERGES SIGNIFICANTLY. THE GFS EVOLVES
THE TROUGH INTO A VERY SLOWLY-MOVING CUT-OFF LOW OVER THE SRN HIGH
PLAINS REGION...WHERE IT LINGERS THROUGH THE END OF THE PERIOD.
MEANWHILE...THE ECMWF RETROGRADES THE LOW WWD ACROSS THE FOUR
CORNERS REGION...AND EVENTUALLY INTO THE ERN GREAT BASIN.
THEREFORE...THIS INCREASING UNCERTAINTY BEYOND DAY 5 PRECLUDES ANY
CONFIDENT ASSESSMENT OF SEVERE POTENTIAL BEYOND EARLY NEXT WEEK.

PRIOR TO THIS...ANY SEVERE THREAT ACROSS THE CENTRAL U.S. SHOULD
REMAIN LIMITED DAY 4 /SUN. SEPT. 20/. HOWEVER...MODELS CONTINUE TO
HINT AT A POTENTIAL SEVERE EVENT DAY 5 /MON. SEPT. 21/...AS THE
UPPER TROUGH DIGS SEWD ACROSS THE PLAINS IN CONJUNCTION WITH A
SURFACE COLD FRONT. THE FRONT IS PROGGED TO LIE FROM THE UPPER MS
VALLEY SSWWD INTO OK...AND THEN WSWWD INTO FAR W TX MON.
AFTERNOON...WITH A MARGINALLY TO MODERATELY UNSTABLE PRE-FRONTAL
AIRMASS EXPECTED. WHILE THE STRONGEST FLOW ALOFT SHOULD REMAIN TO
THE COOL SIDE OF THE SURFACE FRONT...SHEAR SHOULD BE SUFFICIENT FOR
AN ORGANIZED SEVERE EVENT FROM ROUGHLY MO SWWD ACROSS PARTS OF OK
AND NWRN AR INTO CENTRAL TX. ATTM...IT APPEARS THAT EVOLUTION INTO
A SQUALL LINE WOULD BE POSSIBLE...WHICH WOULD THEN MOVE SEWD ACROSS
THE OZARKS/ARKLATEX REGION OVERNIGHT.

..GOSS.. 09/17/2009

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[dkmac]

Day 4 thru 8:


ZCZC SPCSWOD48 ALL
ACUS48 KWNS 190821
SPC AC 190821

DAY 4-8 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0321 AM CDT SAT SEP 19 2009

VALID 221200Z - 271200Z

...DISCUSSION...

LATEST ECMWF/GFS GUIDANCE AND ENSEMBLE MEMBER DATA NOW DIVERGE
SIGNIFICANTLY BEYOND D4 /TUE SEP 22ND/ WITH THE EVOLUTION OF CUT-OFF
LOW. THE GFS RETREGRADES SYSTEM WWD INTO THE SWRN DESERTS BEFORE
WEAKENING IT. IN CONTRAST...THE ECMWF MAINTAINS THE CIRCULATION
OVER THE CNTRL CONUS INTO D7 /FRI SEP 25TH/ BEFORE PHASING IT WITH A
HIGHER LATITUDE IMPULSE. REGARDLESS OF SOLUTION...THE PROSPECT FOR
AN ORGANIZED SEVERE WEATHER EVENT APPEAR RATHER LOW.

..MEAD.. 09/19/2009