Wednesday unseasonal SE USA severe?

[Ed Mahmoud]

Not a lot of directional difference between 850 and 500 mb flow, but note 50 knot low level flow forecast by Euro Wednesday morning.

12Z GFS not radically different.

18Z GFS also shows strong 850 mb flow South of frontal low over SE USA








I'm not sure there will be severe weather, instability doesn't seem outstanding, but unseasonably strong low level winds and thunderstorms leads me to believe there is potential for fairly widespread winds approaching severe limits.


This is, of course, amateur and unofficial.

[Ed Mahmoud]

SPC has a SLIGHT RISK for the area:

...GA INTO THE CAROLINAS...

THE LATEST ECMWF INDICATES THAT AFOREMENTIONED SHORT WAVE TROUGH AND
ATTENDANT SURFACE LOW WILL BE MUCH MORE INTENSE/DYNAMIC THAN THAT OF
THE GFS/NAM. WHILE THERE IS SOME QUESTION AS TO HOW UNSTABLE WARM
SECTOR ENVIRONMENT WILL BECOME GIVEN WEAK MIDLEVEL LAPSE RATES...THE
BOUNDARY LAYER WILL BE VERY MOIST. OF MORE IMPORTANCE...WIND FIELDS
WILL BE SEASONABLY STRONG GIVEN THE DYNAMIC NATURE OF SYNOPTIC
SYSTEM. THUS...IT APPEARS SETUP MAY BE SUPPORTIVE OF EMBEDDED
SUPERCELL AND/OR BOWING STRUCTURES CAPABLE OF A FEW TORNADOES AND
DAMAGING WINDS.

[Weatherfreak14]

I think this maybe a pretty decent severe weather episode.... Heres a tidbit from the NWS in Charleston...

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
-- Changed Discussion --MEDIUM RANGE GUIDANCE IS IN GOOD AGREEMENT THROUGHOUT THE EXTENDED
PERIOD THIS AFTERNOON. A WARM FRONT IS EXPECTED TO LIFT SLOWLY
NORTHWARD TUESDAY NIGHT AND THROUGH THE FORECAST AREA WEDNESDAY...AS
AN AREA OF LOW PRESSURE APPROACHES FROM THE WEST. THIS AREA OF LOW
PRESSURE WILL TRACK THROUGH NEAR OR JUST NORTH OF THE FORECAST
AREA WEDNESDAY EVENING...THEN OFFSHORE AND INTO THE ATLANTIC
OVERNIGHT. WILL CARRY LIKELY POPS ACROSS THE AREA TUESDAY NIGHT
AND WEDNESDAY GIVEN DEEP MOISTURE AND IMPRESSIVE DYNAMIC FORCING
IN PLACE ACROSS THE AREA AND RUN WITH CHANCE POPS WEDNESDAY NIGHT.

IN FACT...WEDNESDAY AFTERNOON/EVENING COULD END UP TURNING INTO
QUITE AN IMPRESSIVE SEVERE WEATHER EVENT IF ENOUGH INSTABILITY IS
REALIZED. THE FORECAST AREA WILL BE UNDER THE INFLUENCE OF ENTRANCE
REGION FORCING FROM A 100+ KT H25 JET...A STRONG SHORTWAVE TROUGH
WILL BE APPROACHING THE AREA...AND WITH WITH A SURFACE LOW TRACKING
NEAR OR JUST NORTH OF THE FORECAST AREA...EVEN MEAGER INSTABILITY
MAY BE ENOUGH TO ALLOW FOR SUPERCELL DEVELOPMENT. ADDITIONALLY...0-1
KM HELICITY VALUES ARE FORECAST TO BE AROUND 200 M2/S2 WITH ENERGY
HELICITY INDEX VALUES BETWEEN 1.5 AND 2. MAIN HAZARDS FROM ANY
THUNDERSTORMS THAT DEVELOP WILL BE STRONG DAMAGING WINDS AND
ISOLATED TORNADOES.

[Ed Mahmoud]

SPC has a SLIGHT RISK for the area:

...GA INTO THE CAROLINAS...

THE LATEST ECMWF INDICATES THAT AFOREMENTIONED SHORT WAVE TROUGH AND
ATTENDANT SURFACE LOW WILL BE MUCH MORE INTENSE/DYNAMIC THAN THAT OF
THE GFS/NAM. WHILE THERE IS SOME QUESTION AS TO HOW UNSTABLE WARM
SECTOR ENVIRONMENT WILL BECOME GIVEN WEAK MIDLEVEL LAPSE RATES...THE
BOUNDARY LAYER WILL BE VERY MOIST. OF MORE IMPORTANCE...WIND FIELDS
WILL BE SEASONABLY STRONG GIVEN THE DYNAMIC NATURE OF SYNOPTIC
SYSTEM. THUS...IT APPEARS SETUP MAY BE SUPPORTIVE OF EMBEDDED
SUPERCELL AND/OR BOWING STRUCTURES CAPABLE OF A FEW TORNADOES AND
DAMAGING WINDS.

Not much change

...GA INTO THE CAROLINAS...

MODEL DIFFERENCES STILL EXIST WITH REGARD TO THE INTENSITY AND
TIMING OF AFOREMENTIONED SHORT WAVE TROUGH AND SURFACE LOW. THE GFS
REMAINS MOST PROGRESSIVE WITH THE SURFACE AND UPPER SYSTEMS WHEN
COMPARED TO THE NAM...SHORT RANGE ENSEMBLE MEAN AND 12Z ECMWF
SOLUTIONS. ANOTHER UNCERTAINTY IS THE DEGREE OF INSTABILITY THAT
WILL DEVELOP ACROSS WARM SECTOR OWING TO WEAK LAPSE RATES AND THE
POTENTIAL FOR WIDESPREAD CLOUDS AND PRECIPITATION. THUS...SEVERE
THREAT IS CONDITIONAL ON A SLOWER...MORE AMPLIFIED SYNOPTIC SYSTEM
AND SUFFICIENT INSTABILITY TO INITIATE AND SUSTAIN SURFACE-BASED
STORMS.

LATEST MODEL GUIDANCE DOES SUGGEST THAT MOIST BOUNDARY LAYER
CONDITIONS /DEWPOINTS IN THE 70S/ IN COMBINATION WITH DIABATIC
HEATING SHOULD RESULT IN POCKETS OF STRONGER INSTABILITY WITH
MLCAPES APPROACHING 1000 J/KG. TSTMS ARE EXPECTED TO INCREASE IN
COVERAGE AND INTENSITY BY AFTERNOON FROM ALONG AND AHEAD OF SURFACE
LOW SWWD ALONG TRAILING FRONT AS FORCING FOR ASCENT ATTENDANT TO
MIGRATORY SHORT WAVE TROUGH ACTS ON DESTABILIZING AIR MASS. THE
GREATEST SEVERE WEATHER THREAT /INCLUDING THE POTENTIAL FOR
SUPERCELLS AND/OR BOWING STRUCTURES/ WILL EXIST ALONG THE SURFACE
LOW TRACK WHERE SEASONABLY STRONG WIND FIELDS WILL RESULT IN A
CORRIDOR OF STRONG LOW AND DEEP-LAYER SHEAR. THE PRIMARY SEVERE
THREATS WILL BE DAMAGING WINDS AND A FEW TORNADOES.

[jdray]

AFD JAX has some interest in it, but not worried:

000
FXUS62 KJAX 120800
AFDJAX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JACKSONVILLE FL
400 AM EDT TUE AUG 12 2008

...SEVERE STORMS ARE POSSIBLE LATE TODAY THROUGH WEDNESDAY...
...PERIODS OF HEAVY RAINFALL TODAY AND WEDNESDAY...

.SYNOPSIS...WATER VAPOR IMAGERY AND RUC MODEL SHOW 500 MB CLOSED LOW
JUST OFF THE NJ AND NY COASTS WITH TROUGHING EXTENDING SW INTO FL.
TO THE W...SHORTWAVE TROUGH AND OTHER UPPER DISTURBANCES ARE ACROSS
TX...LA...MS AND AL PRODUCING SCT CONVECTION MAINLY IN TX. 00Z JAX
SOUNDING SHOWS PWAT HAS INCREASED ABOUT 60% FROM 24 HRS AGO...NOW AT
1.99 INCHES AND MEAN 0-6KM FLOW IS 290 DEG AT 17 KT. RADAR SHOWS A
FEW LIGHT SHOWERS OVER THE CWA AS HIGH MOISTURE AND WEAK LIFT STREAM
EWD IN THE NEAR ZONAL FLOW AT 500 MB. AT SFC...MSAS SHOWS A NEARLY
STATIONARY FRONTAL BOUNDARY WITH 1 OR 2 FRONTAL WAVES OVER SC TO
ACROSS GA/AL/MS WITH WEAK HIGH PRESSURE RIDGE OVER TN AND VA AT 1015
MB.

.SHORT TERM...TODAY-THURSDAY.
MODELS SHOW DEEP LAYER MOISTURE CONTINUING OVER THE AREA AND IN FACT
INCREASING WITH PWATS GOING ABOVE 2 INCHES. SHORTWAVE TROUGH OVER
THE FL PANHANDLE AT 12Z WILL SHIFT EWD SUPPORTING LIFT IN THE
AREA...WITH ANOTHER STRONGER SHORTWAVE TROUGH INITIALLY IN TX TO
SHIFT EWD INTO AL 00Z TONIGHT THEN INTO THE CWA BY 12Z WED. AT
SFC...MODELS CONTINUE TO SHOW THE CWA IN THE WARM SECTOR AND
INDICATE THE FRONT WILL OSCILLATE OVER S CENTRAL GA TODAY (PERHAPS
SAG TO NEAR THE ALTAMAHA RIVER) THEN LIFT NWD TONIGHT AS A SFC LOW
CURRENTLY OVER TX MOVES INTO MS/AL TONIGHT CENTRAL GA WED MORNING.
THE NAM IS NOTICEABLY SLOWER WITH THE MAIN SFC LOW AND WILL PREFER
THE GFS AS ITS CLOSER TO THE ECMWF MODEL. FAIRLY TIGHT PRESSURE
GRADIENT SETS UP LATER TONIGHT AND INTO WED WITH 850 MB WINDS
MAXIMIZED AT 50-60 KT OVER SE GA WED MORNING THEN SHIFTS SWD INTO NE
FL WED AFTERNOON. A 1004 MB SFC LOW EXPECTED TO TRACK FROM CENTRAL
GA WED MORNING AND OFF THE SC/NC CAROLINA COASTS WED NIGHT.
ATTENDANT COLD FRONT EXPECTED TO PUSH THROUGH THE CWA EARLY WED
EVENING THEN SWD INTO THE SRN CWA INTO EARLY THU MORNING. BOTH NAM
AND GFS IN AGREEMENT SHOWING FRONT BECOMING QUASI-STATIONARY THU
WITH A SHARP N TO S MOISTURE GRADIENT. BEST RAIN CHANCES SHOULD BE
ACROSS OUR SRN ZONES BY THEN.

[Ed Mahmoud]

NAM forecast sounding for Charleston, SC early tomorrow afternoon. Not super unstable, but lots of wind energy...

[JonathanBelles]

an early fall severe weather season or just a piece of energy with the right conditions?

[Ed Mahmoud]

Tornado Watch already...

[Weatherfreak14]

Tornado Watch already...

Ill be out chasing in SE south carolinia today.... Hopefully to spot some tornados.

[tropicana]

good luck weatherfreak! stay safe down there!

-justin-

[Ed Mahmoud]

A Tornado Watch, in August, in Florida, without a landfalling tropical system involved. Has to be pretty rare.

[Ed Mahmoud]

Updated SPC SWODY1 snip...

...SOUTHEAST STATES...
A RATHER STRONG AND PROGRESSIVE SHORTWAVE TROUGH IS MOVING ACROSS THE SOUTHEAST STATES THIS MORNING...PROVIDING SHOWERS AND THUNDERSTORMS TO PARTS OF FL/GA/SC. LOCAL VAD DATA AND MORNING RAOBS SHOW 50-60 KNOT WEST-SOUTHWESTERLY LOW LEVEL WINDS WHICH ARE RESULTING IN FAVORABLE LOW LEVEL VERTICAL SHEAR FOR OCCASIONAL UPDRAFT ROTATION.
WEAK LAPSE RATES AND SUBSTANTIAL CLOUD COVER ARE GREATLY LIMITING CAPE VALUES TO BELOW 500 J/KG. NEVERTHELESS...THREAT FOR ISOLATED DAMAGING WIND GUSTS OR TORNADOES WILL CONTINUE THROUGH MUCH OF THE AFTERNOON AND EARLY EVENING UNTIL STORMS MOVE OFFSHORE. REFERENCE WW 825 FOR FURTHER DETAILS.

[CrazyC83]

Definitely a strange event. If this keeps up, we may see a major outbreak in September...

[Ed Mahmoud]

Definitely a strange event. If this keeps up, we may see a major outbreak in September...

And the Autumn like cold fronts/digging troughs aren't done.


I think this could have an affect on tropical season, September 21st type (1938) events in the Northeast may come before late September, before the SSTs have started to cool, with digging, maybe negative tilt troughs...