MESO: Mesoscale Discussion 1916

ACUS11 KWNS 261644
SWOMCD
SPC MCD 261644
FLZ000-261915-

MESOSCALE DISCUSSION 1916
NWS STORM PREDICTION CENTER NORMAN OK
1144 AM CDT THU SEP 26 2013

AREAS AFFECTED...SOUTH FLORIDA

CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY

VALID 261644Z - 261915Z

PROBABILITY OF WATCH ISSUANCE...20 PERCENT

SUMMARY...ISOLATED SEVERE THUNDERSTORMS WITH DAMAGING WIND GUSTS
WILL BE POSSIBLE THIS AFTERNOON. WHILE CONVECTIVE TRENDS WILL
CONTINUE TO BE MONITORED...THE ISSUANCE OF A WW IS CURRENTLY
UNLIKELY.

DISCUSSION...LATEST SURFACE OBSERVATIONS AND VISIBLE SATELLITE
IMAGERY SUGGEST THAT A COLD FRONT CONTINUES ADVANCING SWD OVER THE
FL PENINSULA...EXTENDING ROUGHLY FROM THE COAST OF BREVARD COUNTY TO
THE COAST OF HILLSBOROUGH COUNTY. FARTHER S ACROSS SOUTH
FLORIDA...THE AIR MASS CONTINUES TO DESTABILIZE AS SFC TEMPERATURES
HAVE CLIMBED INTO THE MIDDLE/UPPER 80S TO AROUND 90 DEGREES AMIDST
DEWPOINTS IN THE MIDDLE 70S. MODIFICATIONS TO THE 12Z MIAMI RAOB TO
ACCOUNT FOR THESE CONDITIONS YIELDS MLCAPE ON THE ORDER OF 2000-3000
J/KG WITH NEAR ZERO CINH. COMMENSURATE WITH DIURNALLY STRENGTHENING
SENSIBLE HEAT FLUXES FROM THE DIABATICALLY-HEATED SFC
LAYER...DEEPENING PBL CIRCULATIONS ARE ALLOWING TOWERING CU TO
BECOME INCREASINGLY AGITATED ALONG NUMEROUS CONFLUENCE AXES
PRECEDING THE FRONT. LIGHTNING DATA SUGGEST THAT EMBEDDED UPDRAFTS
ARE NOW EXTENDING SUFFICIENTLY DEEP INTO ICING LAYERS ALOFT TO
SUPPORT CG STRIKES. A CONTINUED UPTICK IN BOTH THE INTENSITY AND
COVERAGE OF THUNDERSTORMS WILL OCCUR DURING THE AFTERNOON AS
INSOLATION SUPPORTS ADDITIONAL DESTABILIZATION.

FORCING FOR ASCENT WILL BE NEUTRAL ABOVE LOW-LEVEL ASCENT ATTENDANT
TO THE FRONT...CONFLUENCE AXES...AND PBL CIRCULATIONS...AND
MID-LEVEL LAPSE RATES ARE ONLY MODEST PER OBSERVED 12Z RAOBS -- E.G.
AROUND 6 C/KM IN THE 700-500-MB LAYER. HOWEVER...ISOLATED DAMAGING
WIND GUSTS WILL BE OF CONCERN. THIS IS ILLUSTRATED BY THE 12Z MIAMI
RAOB...WHICH INDICATES ABUNDANT DRY AIR SURMOUNTING A DEEP MOIST
LAYER EXTENDING TO AROUND 650 MB. WITH THIS DEEP MOIST LAYER
EXTENDING WELL ABOVE THE TOPS OF PBL CIRCULATIONS INTO THE FREE
ATMOSPHERE...THE DRY AIR ALOFT SHOULD NOT BE DELETERIOUS IN MIXING
OUT LOW-LEVEL MOISTURE THUS ALLOWING HIGH BUOYANCY TO BE MAINTAINED.
AT THE SAME TIME...LOW THETA-E CORRESPONDING TO DOWNDRAFTS
INITIATING WITHIN THE MID-LEVEL DRY AIR WILL BE SUBJECTED TO LARGER
NEGATIVE BUOYANCY WHILE DESCENDING TO THE SFC...BOLSTERING DOWNDRAFT
STRENGTH...WITH DCAPE VALUES OVER 1000 J/KG IN MANY AREAS.
RELATIVELY HIGH PW OF 1.8-2.0 INCHES PER GPS DATA WILL ALSO AID
ABUNDANT WATER LOADING IN SUPPORT OF STRONG MICROBURSTS.

ALSO...THE PRESENCE OF 30-35 KT WNWLY/NWLY FLOW IN THE 2.5-5-KM AGL
LAYER OVERLYING 10-20 KT WLY/S PER MIAMI VWP DATA WILL YIELD
SUFFICIENT DEEP SHEAR FOR ENHANCED CONVECTIVE ORGANIZATION AND
PERHAPS THE EVOLUTION OF MULTICELL CLUSTERS. ISOLATED SEVERE WIND
GUSTS WILL BE POSSIBLE WITH THE MOST INTENSE STORMS...WITH THIS
ACTIVITY LIKELY SPREADING ESEWD ACROSS THE SRN FL PENINSULA THROUGH
THE AFTERNOON. AREAS ALONG THE EAST COAST FROM PALM BEACH COUNTY TO
MIAMI-DADE COUNTY APPEAR TO HAVE THE GREATEST POTENTIAL OF
EXPERIENCING THIS ACTIVITY WHERE ANTECEDENT SFC TEMPERATURES WILL BE
WARMEST AND EAST-COAST SEA BREEZE CIRCULATIONS WILL BE MODESTLY
ENHANCED. LACKING DEEP FORCING FOR ASCENT AND THE SMALL SPATIAL AREA
OF SVR THREAT SHOULD BE THE PRIMARY LIMITING FACTORS IN A
HIGHER-COVERAGE SVR THREAT FROM ENSUING...THOUGH CONVECTIVE TRENDS
WILL BE MONITORED.

..COHEN/CORFIDI.. 09/26/2013

...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...MFL...MLB...TBW...

LAT...LON 26188173 27338125 27588034 26458007 25438026 25268095
26188173

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