Quantitative Precipitation Forecast
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000
FOUS30 KWBC 291354
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
953 AM EDT WED APR 29 2015

...VALID 15Z WED APR 29 2015 - 12Z THU APR 30 2015...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...




1400 UTC UPDATE

LOWER FLORIDA KEYS

CHANGES TO THE PREVIOUS EXCESSIVE RAINFALL POTENTIAL OUTLOOK WERE
TO PLACE THE EMPHASIS FOR HEAVIEST RAINS ACROSS THE LOWER KEYS AS
OUTFLOW FROM THE CURRENT ACTIVITY HAS STABILIZED AREAS FARTHER TO
THE NORTH AND HAS PUSHED THE BEST CONVERGENCE TO THE SOUTH AND
SOUTHWEST.  IN THE SHORT TERM---HEAVY RAINFALL AMOUNTS LIKELY OVER
THE LOWER KEYS---WITH POTENTIAL FOR INTENSE RAINFALL RATES OVER AN
HOUR OR SO SUPPORTING RUNOFF ISSUES IN URBANIZED
AREAS---ESPECIALLY AROUND KEY WEST.  HI RES GUIDANCE IS SHOWING
MORE STABLE CONDITIONS ALSO PUSHING INTO THE LOWER KEYS BY EARLY
AFTERNOON AT THE LATEST.  AFTER THIS---UNCERTAINTIES AS TO WHETHER
THERE WILL BE ANOTHER ROUND OF CONVECTION TO AFFECT THE KEYS AS
LATEST HRRR AND HI RES ARW ARE NOT HANDLING THE CURRENT OUTFLOW
WELL.  THE HI RES ARW/NMM DO SHOW POTENTIAL FOR ANOTHER ROUND OF
CONVECTION PRIOR TO 0000 UTC WITH CAPES RETURNING TO 1500 TO 2000
J/KG.  HOWEVER---THE CURRENT OUTFLOW MAY PUSH THE NEXT AXIS
FARTHER TO THE SOUTH---LIMITING THE VERY HEAVY RAINFALL POTENTIAL
FOR THE LOWER KEYS.  AT THE MOMENT---THE GREATEST THREAT FOR
RUNOFF ISSUES FOR THE LOWER KEYS WILL BE OVER THE NEXT HOUR OR
TWO---WITH LESS CONFIDENCE ON ADDITIONAL CONVECTION LATE THIS
AFTERNOON.

ORAVEC




PREVIOUS DISCUSSION


...SOUTHERN FLORIDA/KEYS...

MULTIPLE ROUNDS OF CONVECTION ARE EXPECTED TODAY ACROSS SOUTH
FLORIDA AND THE KEYS WITHIN A BROAD DIFFLUENT FLOW PATTERN ALOFT
AHEAD OF A SLOW MOVING UPPER TROUGH AND SURFACE COLD FRONT. AN MCV
AND ASSOCIATED WIDESPREAD AREA OF CONVECTION WILL PUSH THROUGH
SOUTH FL EARLY THIS AM...CAPITALIZING ON PWS OF 2-2.25 INCHES AND
MUCAPES BETWEEN 1000-2000 J/KG IN PRODUCING AN EXPANDING AREA OF
HEAVY RAINFALL. ORIENTATION AND STRENGTH OF LOW LEVEL INFLOW
(20-30 KTS) WITH THE STRONG WSW 0-6 KM BULK SHEAR (40-50 KTS)
WOULD SUPPORT MORE DOWNWIND PROPAGATION OF CELLS THAN
OTHERWISE...HOWEVER THE EXPANDING PCPN SHIELD WILL NEVERTHELESS
RESULT IN ISOLATED RAINFALL RATES OF 2-4 INCHES WITHIN A FEW HOURS
PER SOME OF THE LATEST HIGH RESOLUTION GUIDANCE (INCLUDING THE
HRRR). WHILE THESE AMOUNTS ARE GENERALLY AT OR BELOW THE CURRENT 3
HOURLY FLASH FLOOD GUIDANCE (3-4 INCHES)... THE HEAVY RAINFALL
EARLY THIS MORNING WITH THE MCS WOULD SET THE STAGE FOR AN
ENHANCED LOCALIZED EXCESSIVE RAINFALL THREAT WITH ANY ADDITIONAL
AREAS OF ORGANIZED CONVECTION LATER TODAY.

THE 30-35 KT LOW LEVEL ONSHORE SOUTH-SOUTHWESTERLY FLOW OFF THE
GULF OF MEXICO LATER THIS MORNING AND AFTERNOON WILL KEEP PW
VALUES MUCH ABOVE AVERAGE...2 TO 3 STANDARD DEVIATIONS ABOVE THE
MEAN...ACROSS SOUTH FLORIDA WHILE MU-CAPE VALUES ARE FORECAST TO
REMAIN WITHIN THE 1000-2000 J/KG RANGE.  WITH THE CONTINUED
FAVORABLE DYNAMICAL SETUP AHEAD OF THE UPPER TROUGH/SURFACE
FRONT...ADDITIONAL AREAS OF HEAVY RAINFALL...PARTICULARLY OVER THE
FAR SOUTHERN PENINSULA AND ACROSS THE KEYS...COULD PRODUCE
ISOLATED RUNOFF ISSUES IN ANY URBANIZED AREAS.

HURLEY


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