Quantitative Precipitation Forecast
Issued by NWS

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000
FOUS30 KWBC 271458
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1057 AM EDT SAT JUN 27 2015

...VALID 15Z SAT JUN 27 2015 - 12Z SUN JUN 28 2015...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 NNE ROA 15 NNW W99 15 WSW PIT CAK 25 ENE FDY 20 S JYM
15 S PTK 15 N CXHM GTB 20 S MPV 10 ESE BVY 30 ESE BID 10 ESE OXB
15 SE RWI 20 S VUJ 15 S UKF 20 NNE ROA.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
OMH 20 WNW CJR 10 SSE MRB 10 N FDK 20 SE THV 10 WNW APG
10 SE MTN 15 N 2W6 30 SW 2W6 15 NNW OFP OMH.


DAY 1...

...MID ATLANTIC FROM MARYLAND SOUTHWARD...

IN THE 15Z UPDATE A SMALL AREA WAS UPGRADED TO MODERATE RISK OF
EXCESSIVE RAINFALL SURROUNDING THE D.C. METRO AREA AND UP THROUGH
NORTH-CENTRAL AND NORTHEAST MARYLAND...INCLUDING BALTIMORE. THIS
IS AN AREA WERE FLASH FLOOD GUIDANCE VALUES ARE PARTICULARLY
LOW...AS LOW AS 1 INCH IN 6 HOURS IN THE URBAN AREAS...AND MODEL
AGREEMENT IS EXCEPTIONALLY GOOD. THE 00Z HIGH RESOLUTION MODELS
PLACED HEAVIER RAIN IN THIS REGION THAN ANYWHERE ELSE...WITH SOME
INDICATING LOCAL TOTALS GREATER THAN 5 INCHES. THE 10 TO 12Z RUNS
OF THE HRRR SHOW IDENTICAL PLACEMENT TO THIS LOCAL PRECIPITATION
MAXIMUM...WHICH GIVES US CONFIDENCE IN MAKING THE UPGRADE. THERE
ARE A FEW OFFSETTING FACTORS...SUCH AS A NARROW INSTABILITY AXIS
AND THE DEEPER LOW LEVEL MOISTURE HAVING BEEN SUPPRESSED BY A
RECENT FRONT WHICH SETTLED INTO NORTH CAROLINA. ATLANTIC MOISTURE
WILL BE CHANNELED INLAND...HOWEVER...ALONG A NEWLY DEVELOPING WARM
FRONT ACROSS NORTHERN VIRGINIA AND SOUTHERN MARYLAND...AND A
LENGTHY PERIOD OF SUSTAINED SYNOPTIC LIFT SHOULD OFFSET THE
TENDENCY FOR QUICK INDIVIDUAL CELL MOTIONS...RESULTING IN SOME
TRAINING. WITH PRECIPITABLE WATER VALUES BETWEEN 1.75 AND 2.00
INCHES...AND HIGH FREEZING LEVELS AROUND 13 THOUSAND FEET...WARM
RAIN PROCESSES WITHIN DEEP CONVECTION SHOULD YIELD VERY HIGH RAIN
RATES.

IN NORTH CAROLINA THE RICHEST LOW LEVEL MOISTURE WAS IN PLACE NEAR
A REMNANT FRONTAL BOUNDARY FROM SOUTH THROUGH SOUTHWEST OF RALEIGH
TO NORTH OF MOREHEAD CITY. THIS BOUNDARY IS EXPECTED TO LIFT
SLIGHTLY AND MAY LOSE SOME OF ITS DEFINITION AS THE GRADIENTS
WITHIN THE NORTHERN JET STREAM BECOME BETTER FOCUSED OVER
VA/MD...BUT THE GRADIENT IN LOW LEVEL MOISTURE IS LIKELY TO RESULT
IN AN AXIS OF GREATER COVERAGE OF DEEP CONVECTION THIS
AFTERNOON/EVENING. THE HIGH RESOLUTION MODELS ARE SPLIT...WITH
SOME LIFTING INTO NORTHERN NC/SOUTHERN VA....AND OTHERS LIKE THE
NSSL WRF LOCATING THE AXIS IN ITS EARLY MORNING POSITION FARTHER
SOUTH. THE SSEO FORMS A NICE COMPROMISE WHICH MAY BE REALISTIC
GIVEN THE FRONT HAS ONLY A SHORT TIME TO LIFT BEFORE THE NEXT
CONVECTIVE CYCLE. IN ANY CASE...A SLIGHT RISK WAS MAINTAINED OVER
CENTRAL NC WHERE SOILS HAD BEEN MOISTENED BY RECENT WET WEATHER.


...UPPER OHIO VALLEY / MID ATLANTIC NORTH OF MARYLAND /
NORTHEAST...

A WELL-ADVERTISED AMPLIFYING/ANOMALOUS UPPER LEVEL TROUGH ACROSS
THE EASTERN CONUS WILL MAINTAIN A BROAD MOD-HEAVY RAINFALL THREAT
ACROSS THE MID ATLANTIC...NORTHEAST...AND THE NORTHEAST TODAY AND
TONIGHT. IMPRESSIVE COOL-SEASON DYNAMIC FORCING WILL COUPLE WITH
ANOMALOUS PW TO PRODUCE A RISK OF RAINFALL EXCEEDING FLASH FLOOD
GUIDANCE...THOUGH MAINLY ON THE 6-HOUR TIME SCALE OWING TO WEAK
INSTABILITY. STRENGTHENING/DEEPENING SYNOPTIC SCALE ASCENT AND
LOW-MID LAYER WAA/THETA-E ADVECTION WILL ACCOMPANY THE ELONGATED
WARM CONVEYOR BELT INTO THE MID ATLANTIC-NORTHEASTERN SEABOARD AND
DEEPENING TROWAL ZONE ACROSS THE NORTHERN OH VALLEY AND NEW YORK
INTO SOUTHERN NEW ENGLAND. STRENGTHENING S-SE 850-700 MB FLOW
WITHIN THE MATURING CYCLONE WILL LEAD TO MOISTURE FLUX ANOMALIES
ON THE ORDER OF 4 STANDARD DEVIATIONS ABOVE NORMAL AND PW
ANOMALIES OF AROUND 2 STANDARD DEVIATIONS ABOVE NORMAL ACROSS THE
MID ATLANTIC AND NORTHEAST STATES. AREAL-AVERAGE TOTALS THROUGH
SUNDAY MORNING ARE FORECAST AT 1-3 INCHES. ISOLATED HIGHER TOTALS
ARE POSSIBLE...ESPECIALLY EASTWARD FACING ELEVATED TERRAIN FROM
CENTRAL/EASTERN PA TOWARD NEW ENGLAND...THAT WILL CATCH THE
ANOMALOUS EASTERLY WINDS OFF THE ATLANTIC.

BURKE/HURLEY
$$





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