Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 251437
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
936 AM EST SAT FEB 25 2017

...VALID 15Z SAT FEB 25 2017 - 12Z SUN FEB 26 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 NNW CYSC 35 NNE BML 10 SSW HIE VSF 15 W PSF 15 SE MPO
15 SE SEG 25 WNW IPT 15 E DSV 20 S CWQP 35 NW CWGH CYND
15 NE CMAN.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 NNW AVP 15 SSW ITH 25 SE SYR 30 W SCH 20 NNE MSV 30 NNW AVP.


1500 UTC UPDATE

NO CHANGES MADE TO THE PREVIOUS EXCESSIVE RAINFALL POTENTIAL
OUTLOOK.

ORAVEC


..NORTHERN MID ATLANTIC / NEW YORK / NEW ENGLAND...

A MID LATITUDE CYCLONE WAS TRACKING THROUGH THE GREAT LAKES ON ITS
WAY INTO EASTERN CANADA. EXPECT DEEPER...NEAR SURFACE BASED
CONVECTION TO BREAK OUT OVER THE MID ATLANTIC AND NORTHEAST DURING
THE DAY AS HEATING OCCURS AND HEIGHT FALLS CONTINUE TO SPREAD
EAST. ON FRIDAY THE NAM CONUS NEST HAD DONE A VERY GOOD JOB OF
PREDICTING THE MESOSCALE DETAILS OF THE OBSERVED CONVECTION...AND
AGAIN TODAY PLACEMENT OF 0.50-INCH-PLUS HOURLY RAIN RATES IN THE
NAM CONUS NEST IS FAIRLY WELL REPRESENTATIVE OF OUR
EXPECTATIONS...AND ALIGNS WITH THE STORM PREDICTION CENTER
CONVECTIVE OUTLOOK. GIVEN THAT THE EVENT WILL BE COMPRISED OF A
PROGRESSIVE UPPER TROUGH AND HIGH SHEAR ENVIRONMENT WITH A LATE
WINTER DIURNAL CYCLE...THERE ARE SEVERAL FACTORS ARGUING AGAINST
THERE BEING MUCH TRAINING OR REPEATING OF CONVECTION OVER A GIVEN
POINT...AND RAPID INDIVIDUAL CELL MOTION WILL LIMIT THE TOTAL
RAINFALL POTENTIAL.

THE RISK OF EXCESSIVE RAINFALL...THEREFORE...IS BUILT AROUND
HYDROLOGIC CONDITIONS. WE CONFINED MARGINAL RISK TO THE AREA FROM
NORTHERN PENNSYLVANIA NORTHWARD...IN AN AREA REMOVED FROM D1
DROUGHT CONDITIONS...WHERE SNOW MELT WILL ALSO CONTRIBUTE TO
GROUND SATURATION AND A HIGHER PERCENTAGE OF SURFACE RUNOFF. IN
AREAS FARTHER SOUTH...THE INSTANTANEOUS RAIN RATES MAY BE OF SOME
CONCERN DOWN TOWARD MD/DC/VA...BUT WITH STORM TOTAL AMOUNTS MAINLY
UNDER AN INCH...AND RECENT DROUGHT CONDITIONS...THE ODDS OF FLASH
FLOODING APPEAR LOW. OUTPUT FROM THE NATIONAL WATER MODEL HELPS TO
CONFIRM SOME OF THESE THOUGHTS...WITH QPF RESULTING IN ABOVE
NORMAL STREAM FLOWS IN THE MODEL OVER NORTHERN PA NORTHWARD...AND
MUCH LESS RESPONSE SEEN OVER THE REST OF THE MID ATLANTIC.

WE ALSO EMBEDDED A SMALL SLIGHT RISK AREA OVER SOUTHERN NEW
YORK...WHERE FLASH FLOOD GUIDANCE VALUES WERE ESPECIALLY LOW...AND
THERE ARE GOOD ODDS OF CONVECTIVELY ENHANCED RAIN RATES THIS
AFTERNOON...INCLUDING EVIDENCE FROM THE LATTER HOURS OF RECENT
HRRR RUNS.

BURKE
$$





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