Quantitative Precipitation Forecast
Issued by NWS

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000
FOUS30 KWBC 280304
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1103 PM EDT SAT JUN 27 2015

...VALID 0303Z SUN JUN 28 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
25 SSE ELM 25 N RME 20 SW BTV 15 WNW LEB 10 SE ORE TAN 10 W MVY
15 SSE BID 20 ESE MJX 10 W CGE 10 WNW NYG 15 W FDK 25 SSE ELM.


DAY 1...

03Z UPDATE...

TRIMMED BACK THE SOUTHERN PORTION OF THE PREVIOUS OUTLOOK AREA
BASED ON THE LATEST TRENDS WITH STLT/RADAR/OBS AND SHORT-RANGE
GUIDANCE TRENDS. WITH ONLY A FEW ADDITIONAL BANDS SHOWERS AHEAD OF
THE FRONT (TOWARD BACK EDGE OF THE WARM CONVEYOR BELT)...WHERE THE
RAINFALL RATES ARE CONSIDERABLY LESS INTENSE AS COMPARED TO
EARLIER IN THE EVENING...HAVE ALSO DROPPED THE MODERATE RISK AREA
WHICH HAD BEEN IN PLACE ACROSS AREAS ALONG AND EAST OF I-95 IN
NORTHERN VA AND MUCH OF SOUTHERN/EASTERN MD. HURLEY



PREVIOUS DISCUSSION FOLLOWS...

...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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