Quantitative Precipitation Forecast
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FOUS30 KWBC 250308
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1108 PM EDT MON APR 24 2017

...VALID 0308Z TUE APR 25 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
170 SE HXD 25 SSE HXD 10 N NBC 15 NNW OGB 10 NNE MRN 15 WNW ROA
20 S W99 20 ESE W99 20 S OKV 10 NW OMH 15 SSW CHO 15 NE LYH DAN
AVC 10 WNW FFA 45 ENE HSE 35 E HSE.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
60 S MRH 40 ESE ILM 10 ENE EYF 20 SSE HBI 10 SE TDF 25 E OCW
30 SSE HSE.


03Z UPDATE:

RECENT TRENDS SUGGESTS THE FLASH FLOODING RISK IS ON THE
DECREASE...THUS OPTED TO REMOVE THE MODERATE RISK AREA. WILL KEEP
A SLIGHT RISK ACROSS PORTIONS OF CENTRAL NC. CONTINUED STEADY
RAINFALL OVER INLAND PORTIONS OF NC IS FALLING ON SATURATED
SOILS...RESULTING IN CONTINUED FLOODING ISSUES OVER THE NEXT
SEVERAL HOURS. CURRENT INDICATIONS ARE THAT BY AROUND 08Z OR SO
THE STEADIER RAINFALL WILL BEGIN TO PUSH NORTH OF THIS AREA.
CLOSER TO THE NC COAST WE ARE SEEING SOME HEAVIER RAINFALL
RATES...ALTHOUGH EVEN HERE RATES ARE CURRENTLY STRUGGLING TO GET
MUCH ABOVE 0.5" IN AN HOUR GIVEN THE LACK OF INSTABILITY. STILL A
CHANCE SOME 1"-1.5" IN AN HOUR TOTALS MOVE INLAND RIGHT ALONG THE
NC COASTLINE OVER THE NEXT SEVERAL HOURS. THUS IN GENERAL...WHILE
SOME FLASH FLOOD THREAT STILL EXISTS ALONG THE NC COAST...THE
MAGNITUDE OF THE THREAT APPEARS LOWER THAN IT WAS THIS AFTERNOON.
-CHENARD

1500 UTC UPDATE

CHANGES TO THE PREVIOUS EXCESSIVE RAINFALL POTENTIAL OUTLOOK WERE
HEAVILY BASED ON LATEST RADAR TRENDS.  THE MARGINAL RISK AREA WAS
EXPANDED WEST SOUTHWESTWARD INTO FAR NORTHEAST GEORGIA AND
SOUTHWEST SC FOR THE DEVELOPMENT OF A SLOW MOVING RAIN BAND CLOSER
TO THE MID TO UPPER LEVEL LOW CENTER IN THE VICINITY OF THE
CENTRAL AL/GA BORDER AREA.  THE HRRR SIMULATED RADAR SUGGESTS THIS
AREA MAY WEAKEN BY EARLY AFTERNOON---WITH POTENTIAL FOR ADDITIONAL
BANDS TO THE WEST OF THE PRIMARY EASTERN BAND FORMING OVER EASTERN
SC.    ELSEWHERE---THE GRADIENT OF THE OUTLOOK RISK AREAS WAS
TIGHTENED UP NORTHEAST SC FOR THE ONGOING NORTHEAST PUSH OF THE
FARTHER EASTERN CONCENTRATED INFLOW BAND.  PLEASE SEE WPC`S
MESOSCALE PRECIPITATION DISCUSSION #0141 VALID UNTIL 1745 UTC FOR
ADDITIONAL INFORMATION ACROSS THE SOUTHEAST.

ORAVEC

PREVIOUS DISCUSSION


...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 BACK BUILDING 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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