Quantitative Precipitation Forecast
Issued by NWS

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662
FOUS30 KWBC 270102
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
901 PM EDT TUE JUL 26 2016

...VALID 01Z WED JUL 27 2016 - 12Z WED JUL 27 2016...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...




CENTRAL TO LOWER MID ATLANTIC REGION
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

0100 UTC UPDATE...

RECENT RADAR AND SATELLITE TRENDS SHOW A SLIGHT UPTICK IN
CONVECTION ACROSS CENTRAL AND SOUTHEAST VA INTO NORTHEAST
NC...EAST OF THE DECAYING MCS OVER WV. WHILE HEAVILY WEIGHTED
TOWARD THE SURFACE/BOUNDARY LAYER (GIVEN THE WEAK MID LEVEL LAPSE
RATES)...MIXED LAYER CAPES REMAIN QUITE STRONG FOLLOWING SUNSET
OVER THIS AREA (BETWEEN 2000-3000 J/KG)...HOWEVER...AGAIN EXPECT A
DECREASING TREND AS THE EVENING PROGRESSES GIVEN THE 5.5-6.0 C/KM
700-500 MB LAPSE RATES. THROUGH 06Z...DEEP LAYER SHEAR AROUND
20-25 KTS ALONG WITH THE AFOREMENTIONED INSTABILITY AND HIGH PW
CONTENT (1.75-2.00") WILL BE SUPPORTIVE OF LOOSELY ORGANIZED MULTI
CELL STRUCTURES...THE STRONGEST OF WHICH WILL BE HIGHLY EFFICIENT
WITH HOURLY RAINFALL RATES NEAR 2".


SOUTH DAKOTA/NEBRASKA
~~~~~~~~~~~~~~~~~~~~~

2100 UTC UPDATE...

CONVECTIVE DEVELOPMENT (EVOLVING MCS) WITH THE HEART OF THE
HEAVIEST RAINFALL CORES NOW PUSHING THROUGH EASTERN SD HAS
NECESSITATED AN UPDATE TO THE PREVIOUS EXCESSIVE RAINFALL OUTLOOK.
SPECIFICALLY...THE AIRMASS BEHIND THE MCS OVER WESTERN AND CENTRAL
SC HAS BEEN MODIFIED SOMEWHAT (INCREASED CIN) AND WOULD TAKE A
WHILE TO RECOVER. THIS HAS BEEN REFLECTED IN THE LATEST RAP/HRRR
GUIDANCE AS WELL AS IN WPC`S QPF...I.E. THE IDEA THAT ADDITIONAL
HEAVIER TOTALS AFTER 00Z WILL BE FAVORED OVER NORTHERN NE INTO
SOUTHERN SD...WHERE THE BEST POOLING OF DEEP-LAYER INSTABILITY
RESIDES...AHEAD OF NEXT MCV WHICH IS CURRENTLY ALONG THE TRI-STATE
BORDER OF MT/ND/SD. THE STRONG FORWARD (DOWNWIND) PROPAGATION
EARLY WILL LIMIT THE FLASH FLOOD THREAT...ESPECIALLY OVER THE SAND
HILLS IN NORTHERN NE AND ELSEWHERE UNAFFECTED BY THE INITIAL MCS
(I.E. WHERE THE 1/3 HR FFG WILL BE HIGHER). HOWEVER...THE
STRENGTHENING SOUTHERLY LLJ OVERNIGHT (40+ KTS AT 850 MB PER THE
MODELS) WOULD ENHANCE THE PROSPECTS OF MORE UPWIND PROPAGATION
(WEAK EASTERLY CORFIDI VECTORS) AND THUS CELL TRAINING AFTER
SUNSET. THEREFORE...A MARGINAL OR MORE ISOLATED RISK OF EXCESSIVE
RAINFALL (RUNOFF ISSUES) WILL REMAIN DURING THE OVERNIGHT HOURS.


SOUTHERN LOUISIANA
~~~~~~~~~~~~~~~~~~

NUMEROUS AFTERNOON AND EVENING THUNDERSTORMS WILL ASCEND ACROSS
MUCH OF THE CENTRAL GULF COAST WITHIN AN AREA OF HIGH PRECIPITABLE
WATER VALUES...AT OR ABOVE 2"...AROUND THE PERIPHERY OF A WEAK BUT
EXPANSIVE UPPER LOW IN THE GULF OF MEXICO.  SEA BREEZE CONVERGENCE
AND DIFFERENTIAL HEATING WILL BE MAIN TRIGGERS/FOCI FOR
CONVECTION, THOUGH THE RAMPING UP OF BROAD LOW-LEVEL INFLOW OFF
THE GULF OF MEXICO TO THE TUNE OF 20-30 KTS IS A CONCERN TUESDAY
NIGHT INTO WEDNESDAY MORNING AS THE WARM SEA SURFACE TEMPERATURES
KEEP THE SURFACE-BASED CAPES IN THE 2000-3000 J/KG RANGE.  THE
MESOSCALE GUIDANCE SHOWS A STRONG SIGNAL FOR HIGH AMOUNTS IN THIS
REGION, SHOWING LOCAL MAXIMA IN THE 4-7" RANGE.  ELSEWHERE IN THE
COUNTRY, THIS WOULD BE SUFFICIENT FOR A SLIGHT TO MODERATE RISK
AREA.  IN LOUISIANA, THE SWAMPY COASTAL PLAIN MUDDIES THE PICTURE.
 FIGURE A SEE TEXT/MARGINAL RISK WAS SUFFICIENT FOR THE TIME
BEING, WHICH CAN BE REVISITED/UPGRADED LATER ON, IF NEEDED.


HURLEY/MUSHER/ROTH
$$





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