Flash Flood Guidance
Issued by NWS

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000
AWUS01 KWNH 231104
FFGMPD
FLZ000-231700-

MESOSCALE PRECIPITATION DISCUSSION 0137
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
704 AM EDT SUN APR 23 2017

AREAS AFFECTED...SOUTH FLORIDA

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 231100Z - 231700Z

SUMMARY...CONCERN FOR STALLED CONVERGENCE LOCATION FOR PERSISTENT
HIGHLY EFFICIENT THUNDERSTORM REDEVELOPMENT.

DISCUSSION...GOES-E AND 16 WV LOOPS DENOTE AMPLIFYING WAVE
CROSSING THROUGH THE SOUTHERN TIP OF FL ATTM WITH NICELY
ANTICYCLONICALLY CURVED OUTFLOW JET ACROSS THE CENTRAL PENINSULA
SUPPORTING RIGHT ENTRANCE ASCENT AS WELL AS MODEST UPPER LEVEL
DIFFLUENCE FOR CONTINUED CONVECTIVE UPSCALE GROWTH.  RADAR MOSAIC
AND SURFACE OBSERVATIONS ALSO DEPICT STRONG SELY FLOW IN THE WARM
CONVEYOR BELT OFF THE SE FL COAST AND GIVEN SFC FLOW CONFLUENCE
OFF THE NW TIP OF GRAND BAHAMAS PROVIDING A WELL DEMARCATED SFC
MST CONVERGENCE BAND WITHIN THE CONVEYOR FOCUSING THE BEST CELLS
TOWARD THE NW TOWARD MARTIN/SE ST. LUCIE COUNTIES. THIS IS WHERE
THE CELLS STALL AND MASS CONVERGE INTO A LARGER COMPLEX AT THE
INTERSECTION OF THE COOL CONVEYOR/DEFORMATION ZONE THAT EXTENDS
BACK ACROSS LAKE OKEECHOBEE TO SANIBEL ISLAND.  WITH THE TRACK OF
THE UPPER LEVEL FEATURE EXPECTED THROUGH THE S TIP OF FL STRAITS
TOWARD GRAND BAHAMA WITH SOME CONTINUED STRENGTHENING...THERE
SHOULD BE LIMITED LOCATION CHANGE OF THE BEST MASS CONVERGENCE
BETWEEN THE TWO CONVEYOR BELTS EVEN AS THE WESTERN PORTION OF THE
DEFORMATION ZONE SHIFTS SOUTH AND EAST ACROSS THE EVERGLADES.

WHILE INSTABILITY IS CURRENTLY AT DIURNAL MINIMA...POCKETS ALONG
THE GULF STREAM ARE AROUND 1000-1250 J/KG AND IS SUFFICIENT TO AID
SOME SHALLOW CONVECTIVE GROWTH.  GIVEN THE NEAR RECORD TPWS IN THE
VICINITY AOA 2.0" AND PROFILES INDICATING A NEARLY SATURATED
ENVIRONMENT THROUGH THE BULK OF THE CONVECTIVE LAYER RAINFALL
EFFICIENCY SHOULD BE QUITE HIGH WITH RATES UP TO 2"/HR WITH HIGHER
AMOUNTS IF/WHEN SOME ADDITIONAL SFC/LOW LEVEL HEATING CAN INCREASE
INSTABILITY.  WITH EXPECTATIONS OF 3-5 HOURS OF THIS STATIONARY
NATURE...TOTALS UP TO 5" ARE POSSIBLE WITH LOCALLY HIGHER AMOUNTS
PARTICULARLY IF BEST CONVERGENCE AREA SHIFTS FURTHER ASHORE.
HI-RES ENSEMBLE (HREF V.2) NEIGHBORHOOD PROBABILITIES OF EXCEEDING
FFG AOA 20% WHICH GIVEN THE HIGH FFG IS A LARGER THAN EXPECTED
PERCENTAGE PROVIDING SOME CONCERN FOR POSSIBLE LOCALIZED FF
CONDITIONS.

GALLINA

...PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT...

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

ATTN...RFC...SERFC...

LAT...LON   27838035 27107992 26088002 25718003 25678024
            25888048 26078097 25978211 26198237 26718240
            26898207 27098185 27568082



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