Quantitative Precipitation Forecast
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FOUS30 KWBC 131326
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
926 AM EDT FRI OCT 13 2017

...VALID 15Z FRI OCT 13 2017 - 12Z SAT OCT 14 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...



...CHANGES TO THE EXCESSIVE RAINFALL OUTLOOK DEFINITION AND
THRESHOLD PROBABILITIES...

NOTE TO USERS...BEGINNING WITH THE 15Z DAY 1 EXCESSIVE RAINFALL
OUTLOOK TODAY...THE DEFINITION OF THIS PRODUCT CHANGES FROM A
POINT PROBABILITY TO A NEIGHBORHOOD PROBABILITY. BY CAPTURING MORE
AREA AROUND A POINT...THE PROBABILITIES ASSOCIATED WITH EACH
RESPECTIVE CATEGORY WILL NECESSARILY INCREASE. SEE THE GRAPHIC
LEGEND FOR THE NEW PROBABILITIES. THE PRODUCT WILL LOOK AND BEHAVE
THE SAME AS IT ALWAYS HAS...BUT STUDIES SHOW THAT THE RISK
CONTOURS WPC DRAWS ARE BETTER CALIBRATED TO THE NEW DEFINITION AND
HIGHER ASSOCIATED PROBABILITIES. THESE CHANGES BETTER ALIGN THE
EXCESSIVE RAINFALL OUTLOOK WITH OTHER OUTLOOKS FROM NATIONAL
CENTERS...AND THE PRODUCT IS NOW DEFINED AS THE PROBABILITY OF
RAINFALL EXCEEDING FLASH FLOOD GUIDANCE WITHIN 40 KM...25
MILES...OF A POINT.


...SOUTH FLORIDA...

IN THE 15Z UPDATE WE INTRODUCED A SMALL MARGINAL RISK AREA ALONG
THE SOUTHERN PORTION OF THE EAST COAST OF FLORIDA. FOR DAYS
PERSISTENT EASTERLY FLOW HAS RESULTED IN A SHOWERY REGIME. THE
ASSOCIATED MID LEVEL TROUGH IS NOT PARTICULARLY STRONG...SO
SYNOPTIC SUPPORT FOR HEAVY RAIN IS RELATIVELY MINIMAL...BUT IF
THERE IS TO BE A CONCENTRATED PERIOD OF HEAVIER RAIN IT IS LIKELY
TO OCCUR TODAY. THE DEEP LAYER TROUGH WILL DRIFT WESTWARD. THE
SURFACE OBSERVATIONS AND VAD WIND PROFILES SHOW THE TROUGH HAD
MOVED INLAND OVER THE PENINSULA...WITH ENHANCED CONVERGENCE NEAR
THE TROUGH AND ENHANCED EASTERLY WIND SPEEDS AT 850 MB COMING
ONSHORE. WITHIN THIS ENHANCED FLOW A NARROW PLUME OF PRECIPITABLE
WATER VALUES APPROACHING 2.00 INCHES WAS LOCATED IN THE AREA
BETWEEN MELBOURNE AND MIAMI...EAST OF AND OVER LAKE OKEECHOBEE.
THE RAP PRODUCES VERY STEADY STATE LOW LEVEL CONVERGENCE ACROSS
THIS ZONE THROUGHOUT THE DAY...AND A FEW HI-RES MODELS DO PRODUCE
2 TO 4 INCHES OF RAIN. WITHOUT ANY WESTERLY FLOW IN THE COLUMN THE
EVENT MAY NOT TEND TO STALL OR BECOME VERY WELL FOCUSED...BUT THE
PERSISTENCE OF OCCASIONAL HEAVY DOWNPOURS WITHIN THE NARROW PW
PLUME COULD RESULT IN A FEW WATER RELATED IMPACTS.


...LOWER LAKE MICHIGAN...

ON DAY 2...SATURDAY...WPC IS CARRYING A SLIGHT RISK OF EXCESSIVE
RAINFALL. LEADING INTO THE EVENT...IT APPEARS THAT THERE IS SOME
RISK THAT HEAVY RAIN AND WEST TO EAST TRAINING OF ECHOES COULD
BEGIN LATE IN THE DAY 1 PERIOD OVER NORTHERN ILLINOIS...AND
POSSIBLY STRETCHING ACROSS LAKE MICHIGAN INTO LOWER MICHIGAN. THE
TAIL END OF A SHORTWAVE AND UPPER JET STRUCTURE SWEEPING NOW
ACROSS THE NORTHERN PLAINS WILL LAY DOWN ACROSS LOWER LAKE
MICHIGAN BY FRIDAY NIGHT...AND BEGIN TO TAP INTO SOUTHWESTERLY
RETURN FLOW...UPGLIDE ON THE 300 K ISENTROPIC SURFACE...AND MUCAPE
OF SEVERAL HUNDRED J/KG. THE HI-RES MODEL QPF SIGNAL RAMPS UP
CONSIDERABLE JUST BEYOND THE END OF THE DAY 1 PERIOD AT 12Z
SATURDAY...BUT GIVEN THE INGREDIENTS MOVING INTO PLACE LATE
TONIGHT...WE CHOSE TO LEAN INTO THE EVENT WITH A SMALL MARGINAL
RISK AREA.

BURKE

$$





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