Quantitative Precipitation Forecast
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FOUS30 KWBC 261436
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1035 AM EDT TUE JUL 26 2016

...VALID 15Z TUE JUL 26 2016 - 12Z WED JUL 27 2016...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 N BBW 20 WNW GLD 15 SSW SNY 45 SE IEN 40 SW PHP 40 NNE D07
20 NE BIS 20 WSW K46D 15 NNW JMS 15 SW K2D5 25 N HON 30 SE MHE
10 N BBW.



15Z UPDATE...

WPC KEPT TWO SEE TEXT AREAS... ADDED ONE SEE TEXT AND ENDED UP
CUTTING DOWN TO ONE SLIGHT THREAT IN THE MIDWEST. WPC KEPT TWO SEE
TEXT AREAS IN SOUTHEAST VA/NERN NC AND ALONG MUCH OF THE CENTRAL
GULF TO WESTERN GULF COAST. HI-RES MODELS INCLUDING SOME GLOBAL
MODELS SUGGEST DIURNAL CONVECTION DEVELOPING INVOF PRE-FRONTAL/LEE
SIDE TROUGH WITH LIGHT STEERING CURRENT COULD PRODUCE LOCALLY
HEAVY RAINS. MEANWHILE A WEAK MID TO UPPER LOW AND INCREASING
ONSHORE FLOW WILL RESULT IN MORE THAN NORMAL WIDESPREAD SEA BREEZE
ACTIVITY. IT APPEARS THE MOST OPPORTUNE AREAS FOR ENHANCED
SHOWERS/STORMS AND LOCALLY HEAVY RAINS FROM SERN LA AND SRN MS/AL
AND WRN FL PANHANDLE... ACROSS SOUTHEAST TX BETWEEN HOU/IAH TO CRP
AND ANOTHER LOCAL MAX BETWEEN AUS AND BRO EAST OF LRD. WPC ADDED A
SEE TEXT FOR SOUTH CENTRAL IN INTO WEST CENTRAL OH FOR PERSISTING
CONVECTIVE HEAVY RAIN... WHICH IS ALREADY ONGOING AND NEAR A
QUASI-STATIONARY FRONT. SOME OF THE MODELS... GFS AND ARW CAPTURE
THIS SMALL POCKET OF CONVECTIVE RAINS ADVANCING DOWNSTREAM OVER
TIME WITH LOCALLY HEAVY AMOUNTS. FINALLY WPC WENT AHEAD AND
SIMPLIFIED THE AREA ACROSS THE MIDWEST WITH JUST A SLIGHT THREAT
FROM SOUTH CENTRAL ND THROUGH CENTRAL SD INTO WEST CENTRAL NE AND
EXTREME NERN CO. SHORT WAVE TROUGH OVER MT/WY WILL SLIDE
DOWNSTREAM AND REACH THE ERN DAKOTAS BY WED MORNING. ONGOING
CONVECTION OUT AHEAD OF THE SMALL SCALE FEATURE SHOULD GRADUALLY
GIVE WAY TO A POSSIBLE MORE ORGANIZED CONVECTIVE CLUSTER. THIS
GENERAL CORRIDOR APPEARS SUPPORTED BY THE OVERNIGHT GFS/ECMWF AND
ARW/SSEO MEAN WITH SOME LOCAL 2-4 INCH AMOUNTS.

PREVIOUS DISCUSSION BELOW...

SOUTH DAKOTA/NEBRASKA
~~~~~~~~~~~~~~~~~~~~~
A NEGATIVELY-TILTED TROUGH MOVING OUT OF THE NORTHERN ROCKIES
SHOULD HELP SPUR THE DEVELOPMENT OF A LOW-LEVEL CYCLONE ACROSS
WESTERN SD WHICH IS EXPECTED TO LEAD TO THE INITIATION OF AN
ORGANIZED CONVECTIVE COMPLEX THIS AFTERNOON/EVENING, GIVEN
FAVORABLE PARAMETERS SUCH AS 2000+ J/KG CAPES, BROAD LOW-LEVEL
INFLOW OF 20-40 KTS, AND PRECIPITABLE WATER VALUES OF 1.25-1.75".
LITTLE TO NO CAPPING INVERSION APPEARS TO BE PRESENT, WHICH
IMPLIES THAT A MORE SOUTHWESTERLY SOLUTION IS POSSIBLE.  USED A
COMPROMISE OF THE AVAILABLE GUIDANCE, MESOSCALE AND OTHERWISE, TO
COME UP WITH THE INITIATION POINT.  IT IS POSSIBLE THE CONVECTION
MOVING THROUGH EASTERN MT EARLY THIS MORNING COULD START THE BALL
ROLLING.  GFS-BASED FORWARD PROPAGATION VECTORS FROM THE 18-00Z
CYCLES INDICATED MOTION TO THE SOUTHEAST TO SOUTH-SOUTHEAST OF 20
KTS, SO ATTEMPTED TO MOVE THE SYSTEM ALONG AT THAT RATE OF SPEED,
WHICH IS QUICKER THAN SEEN IN THE MODEL GUIDANCE AVAILABLE.
HOURLY RAIN RATES UP TO 2", WITH LOCAL AMOUNTS IN THE 3-5" RANGE,
ARE CONSIDERED POSSIBLE WHERE CELLS MERGE OR PERIODS OF CELL
TRAINING SET UP.  SHOULD ANY PROGRESSIVE LINEAR/CURVED CONVECTIVE
BANDS ORGANIZE WITHIN THIS AREA, FORWARD PROPAGATION COULD BE
QUICKER THAN EXPECTED DUE TO ASSOCIATED COLD POOL DEVELOPMENT.  A
SLIGHT RISK OF EXCESSIVE RAINFALL WAS SPLIT AROUND THE SAND HILLS
OF NE, WHICH SHOULD BE ABLE TO ABSORB THE EXPECTED MAGNITUDE OF
RAINFALL.  A SEE TEXT/MARGINAL RISK AREA WAS PLACED OVER THE SAND
HILLS THEMSELVES, SINCE ANY FLASH FLOOD ISSUES THERE ARE EXPECTED
TO BE MORE ISOLATED.


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.


NORTHEAST 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.

MUSHER/ROTH
$$




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