Quantitative Precipitation Forecast
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FOUS30 KWBC 090029
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
728 PM EST FRI DEC 08 2017

...VALID 01Z SAT DEC 09 2017 - 12Z SAT DEC 09 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
55 SSW CTY 15 NE GNV 20 S SGJ 10 NNW TTS 20 SW MLB 35 ENE RSW
20 SE RSW APF 30 SW APF.


...FL...

THERE ARE TWO OPPORTUNITIES FOR HEAVY TO LOCALLY EXCESSIVE
RAINFALL ACROSS FL DURING THE OVERNIGHT HOURS.

THE FIRST IS NEAR CEDAR KEY OF THE WEST COAST OF FL....WHERE A
BATCH OF CONVECTION EARLY THIS EVENING PRODUCE LOCAL 1.50 TO 2.00
INCHES OF RAINFALL (BASED ON THE KTBW STORM TOTAL RAINFALL
PRODUCT).  THE MESOSCALE FEATURE ASSOCIATED WITH THIS CONVECTIVE
CLUSTER HAS MOVED INLAND...AND THE CONVECTION WEAKENED AS IT LOST
ITS INSTABILITY SOURCE.

ANOTHER BATCH OF CONVECTION DEVELOPING OVER THE EASTERN GULF OF
MEXICO WEST OF A CEDAR KEY TO TAMPA LINE IS TAPPING 1500-2500 J/KG
OF MLCAPE...AS 30 TO 40 KNOT LOW LEVEL WINDS TRANSPORTS 1.75/2.00
INCH PRECIPITABLE WATER AIR TO THE COAST SOUTH OF A QUASI
STATIONARY FRONT ACROSS NORTH FL. THE CONVECTION IS EXPECTED TO
FOLLOW THE AXIS OF BEST INSTABILITY...COMING ASHORE BETWEEN THE
ABOVEMENTIONED POINTS. GIVEN THE DEPTH OF THE MOISTURE
(PRECIPITABLE WATER VALUES ARE NEARLY TWO STANDARD DEVIATIONS
ABOVE THE MEAN)...HOURLY RAINFALL RATES WITH THE GULF CONVECTION
COULD APPROACH 2.00 INCHES AS THE CONVECTION NEARS THE SHORELINE.

THERE IS A MULTI MODEL HIGH RESOLUTION SIGNAL (SEEN IN THE MOST
RECENT HRRR...12Z REGIONAL GEM AND 12Z NSSL WRF FOR 2.00 TO 4.00
INCHES OF QPF WITH THE CONVECTION THROUGH ABOUT 06Z. ONE AND THREE
HOUR FLASH FLOOD GUIDANCE VALUES ARE FAIRLY HIGH (GENERALLY ABOVE
3.50 INCHES)...SO TRAINING OF THE CONVECTION WOULD BE NECESSARY TO
INITIATE FLASH FLOODING. HOWEVER...SHOULD THE NEXT BATCH OF
CONVECTION TRAIN OVER AREAS THAT RECEIVED HEAVY RAINFALL
EARLIER...THE FLASH FLOOD THREAT COULD BECOME ENHANCED.

THE SECOND AREA OF CONCERN IS NEAR THE TAMPA METRO
AREA...GENERALLY BETWEEN 09/08Z AND 09/12Z. THE QUASI STATIONARY
FRONT OVER NORTH FL WILL BECOME A COLD FRONT AS LOW PRESSURE OF
THE NORTHEAST FL/SC COAST DEEPENS. THE FRONT BECOMES ORIENTED
SOUTHWEST TO NORTHEAST AFTER 06Z. AHEAD OF THE FRONT...A 30 TO 40
KNOT LOW LEVEL SOUTHWEST FLOW IS EXPECTED TO FOCUS THE DEEP
MOISTURE AND MARGINAL TO LOCALLY MODERATE INSTABILITY OVER THE
TAMPA BAY REGION. AGAIN...THERE IS A MULTI HIGH RESOLUTION MODEL
SIGNAL FOR .LOCAL 2.00 TO 4.00 INCH RAINFALL AMOUNTS...WITH SOME
NORTH/SOUTH SPREAD N THE PLACE,...BASED ON FRONTAL TIMING.
AS IS THE CASE FURTHER NORTH...FLASH FLOOD GUIDANCE VALUES ARE
RELATIVELY HIGH...SO TRAINING COULD BE REQUIRED TO INITIATE FLASH
FLOODING. THE MOST RECENT RAP SOLUTIONS DO SHOW PROPAGATION
VECTORS SLOWING AFTER 06Z...WHICH COULD FOSTER TRAINING JUST AHEAD
OF THE COLD FRONT.

HAYES
$$





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