Quantitative Precipitation Forecast
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FOUS30 KWBC 240825
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
424 AM EDT MON APR 24 2017

...VALID 12Z MON APR 24 2017 - 12Z TUE APR 25 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
105 SE HXD 70 SE HXD 45 SE HXD 15 NNW NBC 25 SW CAE 20 W CAE
15 E AVL 15 WNW ROA 20 S W99 20 ESE W99 20 S OKV 10 NW OMH
15 SSW CHO 15 NE LYH DAN AVC NGU 15 NE NTU 35 ENE FFA 40 E HSE.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
95 SE CHS 40 S CHS 15 SSE MMT 35 NNE CUB 10 S EXX 10 NE GSO
15 NNE RWI 30 NE NKT 50 SSE MRH.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
45 SSE MYR 40 SW MYR 45 WSW MYR UDG 10 NNW 45J 15 ESE HRJ OAJ
10 S NCA 20 E SUT 35 S SUT 45 SSE MYR.


...SOUTHERN-CENTRAL APPALACHIANS / SOUTHEASTERN U.S. / MID
ATLANTIC...

A HEAVY RAINFALL EVENT WILL CONTINUE TODAY...FOCUSED MAINLY ACROSS
PORTIONS OF NORTH AND SOUTH CAROLINA. OVERALL AN IMPRESSIVE
SETUP...WITH BOTH CONDITIONS ALOFT AND IN THE LOWER LEVELS
CONDUCIVE TO HEAVY CONVECTIVE RAINFALL. IN THE UPPER LEVELS THE
CLOSED LOW MOVING SOUTHEASTWARD WILL CONTINUE TO TAKE ON MORE OF A
NEGATIVE TILT RESULTING IN INCREASED ASCENT OVER NC/SC. ALSO NOTE
IMPRESSIVE DIFFLUENCE AT 250 MB...WHICH SHOULD AID IN THE
MAINTENANCE OF CONVECTION THROUGH THE DAY. IN THE LOWER LEVELS A
STRONG COASTAL FRONT EXTENDS ALONG THE CAROLINA COAST...WITH
INSTABILITY ALONG/SOUTH AND JUST NORTH OF THIS BOUNDARY. STRONG
EASTERLY FLOW INTO AND OVER THIS BOUNDARY WILL HELP MAINTAIN A
STRONG CONVERGENCE AXIS...AND ALSO HELP MAINTAIN INSTABILITY INTO
THE BOUNDARY. PRECIPITABLE WATER VALUES WILL CONTINUE TO INCREASE
TO AROUND CLIMATOLOGICAL RECORD VALUES ACROSS PORTIONS OF SC/NC AS
MOISTURE FROM THE TROPICAL WAVE OFF OF FLORIDA IS WRAPPED WEST
INTO THE CLOSED LOW.

THE EXPECTATION IS THAT THE HEAVY CONVECTIVE LINE MOVING ACROSS SC
AS OF 06Z WILL CONTINUE SLOWLY MOVING EAST INTO THE MORNING.
HOWEVER AS THE CLOSED LOW DIVES SOUTHEAST AND BECOMES INCREASINGLY
NEGATIVELY TILTED...EXPECT THE NORTH/SOUTH CONVERGENCE AXIS
RESPONSIBLE FOR THIS CONVECTIVE LINE TO SLOW ITS EASTWARD
PROGRESSION. THIS SHOULD ALLOW FOR CONTINUED CONVECTIVE
REGENERATION OVER EASTERN SC THIS MORNING...WITH THE ACTIVITY
SLOWLY MOVING EASTWARD INTO SOUTHERN NC WITH TIME. THE COMBINATION
OF PROLONGED SYNOPTIC SUPPORT AND ONLY SLOW EASTWARD PROGRESSION
OF THE CONVERGENCE AXIS...AND WIND FIELDS CONDUCIVE TO SOME
SOUTHWARD BACKBUILDING OF CONVECTION INTO THE INSTABILITY...BOTH
POINT TOWARDS THE LIKELIHOOD OF TRAINING CONVECTION. WHICH GIVEN
THE VERY HIGH PWAT VALUES...WILL LEAD TO SOME IMPRESSIVE RAINFALL
TOTALS INTO THIS EVENING.

SOME MODEL DIFFERENCES EXIST WITH REGARDS TO THE EXACT AXIS OF
HIGHEST QPF AMOUNTS. AT LEAST SOME OF THE DIFFERENCES ARE RELATED
TO EXACTLY HOW MUCH CONVECTION IS ABLE TO PROPAGATE EASTWARD WITH
TIME. THE HIGH RES ARW PROPAGATES THE MOST...PUSHING THE HEAVIEST
TOTALS MORE INTO SOUTHERN NC. THE HIGH RES NMMB ON THE OTHER HAND
IS THE LEAST PROGRESSIVE SOLUTION...KEEPING THE HEAVIEST AMOUNTS
OVER CENTRAL SC. BASED ON RECENT RADAR TRENDS THINK THE SLIGHTLY
MORE PROGRESSIVE NSSL WRF AND HIGH RES ARW ARE PROBABLY CLOSER TO
TO BEING CORRECT. WPC QPF GENERALLY FOLLOWED A MULTI MODEL
BLEND...ALTHOUGH WAS WEIGHTED TOWARDS THE HIGH RES CONSENSUS.
THOUGHT MUCH OF THE NON CAM GUIDANCE (GFS/NAM/ECMWF) MAY HAVE BEEN
A BIT TOO FAR INLAND WITH THEIR HIGHEST AMOUNTS...AND PROBABLY A
BIT TOO FAR SOUTHWEST...AS THEY MAY BE FAILING TO CORRECTLY
PROPAGATE CONVECTION SLOWLY EASTWARD. LOOKING AT AREAL AVERAGED
AMOUNTS MAXING IN THE 4-6" RANGE ALONG THE NC/SC BORDER NEAR THE
COAST. WOULD ANTICIPATE SOME ISOLATED 6-10" TOTALS IN THIS
VICINITY AS WELL WHERE TRAINING AND CELL MERGERS END UP MAXIMIZED.
WILL CONTINUE WITH A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS
THIS AREA WITH RAINFALL RATES AND DURATION LIKELY ENOUGH TO RESULT
IN SOME FLASH FLOOD ISSUES TODAY...WITH SOME SIGNIFICANT ISSUES
POSSIBLE. THE FACT THAT THE CLOSED LOW WILL REMAIN MOVING WILL
PREVENT RAINFALL TOTALS FROM BEING EVEN MORE EXTREME AND CAPS THIS
EVENT IN THE MODERATE RISK CATEGORY.

THIS MODERATE RISK AREA WILL BE SURROUNDED BY A SLIGHT RISK. THE
SLIGHT RISK ACCOUNTS FOR CONVECTION FURTHER SOUTHWEST ACROSS SC
CLOSER TO THE CLOSED LOW...AS WELL AS PORTIONS OF EAST CENTRAL NC
WHERE CONVECTION SHOULD EVENTUALLY INCREASE IN COVERAGE...BOTH
ALONG THE COASTAL FRONT AND AHEAD OF THE MAIN CONVECTIVE AXIS AS
IT MOVES EAST. THE SLIGHT RISK AREA GENERALLY FOLLOWS THE EDGE OF
EXPECTED ISOLATED TO SCATTERED 2-3" AMOUNTS IN THE 12Z-12Z DAY 1
PERIOD. EXTENDED A MARGINAL RISK FURTHER NORTH INTO PORTIONS OF
THE SOUTHERN APPALACHIANS. IN GENERAL RAINFALL INTENSITY SHOULD BE
ON A DECREASE ACROSS THIS AREA AFTER 12Z AS THE SYSTEM BEGINS TO
CUTOFF A BIT FROM THE FLOW AND WE START TO LOSE THE BETTER
MOISTURE AND DYNAMICS. HOWEVER THE COMBINATION OF SATURATED SOILS
AND CONTINUED LIGHT TO MODERATE RAINFALL WILL LIKELY LEAD TO SOME
CONTINUING WATER ISSUES...ESPECIALLY INTO FAVORED UPSLOPE AREAS.

CHENARD


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