Quantitative Precipitation Forecast
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FOUS30 KWBC 262123
QPFERD

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

...VALID 21Z TUE JUL 26 2016 - 12Z WED JUL 27 2016...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...




NORTHEAST KY/SOUTHERN OH/WESTERN WV
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

2100 UTC UPDATE...

RECENT TRENDS SHOW AN UPTICK IN CONVECTION ACROSS PORTIONS OF FAR
NORTHERN KENTUCKY THIS AFTERNOON...SOUTHERN OHIO AND PORTIONS OF
WEST VIRGINA. WHILE THERE IS A LOW-LEVEL BOUNDARY IN THE
VICINITY...INFLOW AND CONVERGENCE REMAINS WEAK. HOWEVER DIURNAL
HEATING AND SYNOPTIC ASCENT AHEAD OF SHORTWAVE/MCV EVIDENT ON
VISIBLE SATELLITE OVER IL...HAVE BEEN ENOUGH TO SUPPORT INCREASED
CONVECTIVE COVERAGE. DEEP LAYER SHEAR AROUND 20 KTS IS ALSO
SUPPORTIVE OF LOOSELY ORGANIZED MULTI CELL STRUCTURES.

STORM MOTIONS OFF TO THE EAST AT 15-20 KTS SHOULD GENERALLY LIMIT
THE OVERALL FLASH FLOOD THREAT INTO THIS EVENING.HOWEVER CONTINUED
INCREASED STORM COVERAGE WILL LIKELY LEAD TO AT LEAST A FEW CELL
MERGERS AND TRAINING AS THE CONVECTION MOVES OFF TO THE EAST.
STORMS SHOULD BE EFFICIENT RAINFALL PRODUCERS GIVEN DEEP WARM
CLOUD DEPTHS AND HIGH PRECIPITABLE WATER VALUES. IN FACT RECENT
GPS OBS SHOW
PRECIPITABLE WATER AROUND 2.2" ACROSS SOUTHERN OHIO AND
VICINITY...WHICH IS AROUND THE RECORD HIGH VALUE. NORMALLY WOULD
NOT EXPECT A FLASH FLOOD THREAT FROM A SETUP LIKE THIS...HOWEVER
THE NEAR RECORD PWAT VALUES SUGGEST THAT A MARGINAL FLASH FLOOD
THREAT MAY EXIST. AS THE COMBINATION OF INCREASED STORM COVERAGE
AND HIGH PWAT VALUES SHOULD LEAD TO ISOLATED RAINFALL AMOUNTS OF
3-4" IN A FEW HOURS...WHICH MAY BEGIN TO LOCALLY EXCEED FLASH
FLOOD GUIDANCE. THE HRRR AND PARALLEL HRRR APPEAR TO NOW HAVE A
DECENT HANDLE ON THIS CONVECTION.

EXPECT THE CONVECTIVE COVERAGE AND INTENSITY TO DROP OFF
APPRECIABLY AFTER SUNSET (FOLLOWING PEAK DIURNAL HEATING) GIVEN
THE THAT THE STRONG DEEP-LAYER INSTABILITY (MIXED LAYER CAPES
2000-3000 J/KG AS OF 2100 UTC) IS HEAVILY WEIGHTED TOWARD
SURFACE/BOUNDARY LAYER HEATING (AS 700-500 MB LAPSE RATES REMAIN
WEAK -- 5.5-6.0 C/KM AFTER SUNSET).


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.


EASTERN NC/SOUTHEAST VA
~~~~~~~~~~~~~~~~~~~~~~~

SCATTERED TO NUMEROUS THUNDERSTORMS ARE EXPECTED NEAR AND AHEAD OF
A COLD FRONT...WHICH STALLS ALONG THE OHIO VALLEY AND JUST SOUTH
OF THE MASON/DIXON LINE TODAY.  WEAK UPPER LEVEL DIFFLUENCE,
MLCAPES POTENTIALLY RISING TO 3000 J/KG, AND HIGH PW VALUES
(2-2.25") ALONG/AHEAD OF THE FRONT WILL MAKE FOR A FAVORABLE
ENVIRONMENT FOR THUNDERSTORM DEVELOPMENT ACROSS PORTIONS OF VA AND
NC.  THE FLOW IN THE REGION IS NEARLY UNIDIRECTIONAL OUT OF THE
WEST, AND WITH AN APPROACHING FRONTAL ZONE, IT APPEARS TO BE A
GOOD SETUP FOR HEAVY RAIN IN SOUTHEAST VA/NORTHEAST NC.  THE
HIGH-RES GUIDANCE SUPPORTS LOCALLY HIGHER TOTALS (2-4")...WHICH
GIVEN THE THERMODYNAMIC ENVIRONMENT...WOULD FALL VERY QUICKLY
(I.E. WITHIN 2-3 HOURS). SINCE IT HAS BEEN A WHILE SINCE IT RAINED
HEAVILY IN THIS AREA, WHICH HAS SOME EMBEDDED SWAMPS, FIGURED A
SEE TEXT/MARGINAL AREA APPEARED BEST HERE.

HURLEY/MUSHER/ROTH
$$





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