Quantitative Precipitation Forecast
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596
FOUS30 KWBC 201427
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1027 AM EDT FRI MAY 20 2016

...VALID 15Z FRI MAY 20 2016 - 12Z SAT MAY 21 2016...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 WSW TYS 15 WNW DNN 20 NNW 8A0 15 NW 1M4 25 E TUP 40 W TUP
30 SE UTA 20 S MEM 30 SSW MKL 40 SW HOP HOP 20 NE LOU 10 S VTA
10 E MGW 20 SSE 2G4 15 WSW OKV 20 NW CJR 15 NNW OMH CHO
30 SSW CHO 25 NNE LYH 15 NW LYH ROA 10 SSE BCB 10 SE HLX
10 WSW HLX 15 W HLX 10 ESE MKJ BLF 40 NNE LNP 20 SSW 1A6
10 WSW TYS.


1500 UTC UPDATE

CHANGES TO THE PREVIOUS SLIGHT RISK AREA OVER TN WERE TO EXTEND
THE WESTERN PORTION 50-75 NM TO THE SOUTHWEST INTO FAR NORTHEAST
MS---WESTERN TN AND FAR NW AL TO ACCOUNT FOR CONVECTION THAT IS
ENHANCING THIS MORNING ACROSS THESE AREAS.  PLEASE SEE WPC`S
MESOSCALE PRECIPITATION DISCUSSION #0223 VALID UNTIL 1800 UTC FOR
FURTHER INFORMATION ACROSS THIS AREA.

THE SLIGHT RISK AREA WAS TRIMMED BACK TO THE NORTHWEST OVER
PORTIONS OF THE MID ATLANTIC FROM NORTH CENTRAL NC INTO CENTRAL VA
AFTER COORDINATION WITH WFO LWX.  FFG VALUES ARE RELATIVELY HIGH
ACROSS THE AREA WHERE THE SLIGHT RISK WAS ADJUSTED AND THOUGHTS
ARE THAT PRECIP WILL BE MOSTLY STRATIFORM ACROSS THESE AREAS.

ORAVEC


PREVIOUS DISCUSSION


...TENNESSEE AND KENTUCKY INTO PARTS OF OH/WV/VA/NC...

A RELATIVE MAXIMUM SWATH OF RAINFALL IS FORECAST INTO NORTHERN
TN/CENTRAL KY...GENERALLY FROM NASHVILLE TO
LEXINGTON. THE MODEL GUIDANCE WAS ABSOLUTELY UNANIMOUS IN THIS
ASPECT. GLOBAL AND MESOSCALE MODELS ALIKE APPEAR TO BE LATCHING
ONTO MODERATE SYNOPTIC FORCING WHERE SOME SENSE OF UPPER JET
COUPLING COINCIDES WITH A FEED OF SEASONABLY MOIST AND
SUFFICIENTLY UNSTABLE AIR. THE CONVERGENT NORTHERN EXTENT OF A LOW
LEVEL JET IS FORECAST TO TRACK STEADILY ALONG A PATH FROM
WESTERN/NORTHERN TN INTO CENTRAL KY AND SOUTHERN OH. MODELS
GENERALLY AGREE TO 1 TO 2 INCH AREAL AVERAGE RAINFALL ALONG THIS
SWATH...WHILE THEIR FORECASTS HAVE DRIED
CONSIDERABLY OVER EASTERN TN...PERHAPS OWING BOTH TO CHANGES IN
ASCENT AND SOME MOISTURE ROBBING ASPECT OF THE GULF COAST
CONVECTION. FARTHER EAST THE FORECAST IS MORE STABLE...WITH
PRECIPITATION SURROUNDING A COASTAL LOW POISED TO AFFECT COASTAL
PARTS OF THE CAROLINAS...AND ADDITIONAL RAINFALL...SOME
CONVECTIVE...BREAKING OUT FROM THE MOUNTAINS TOWARD THE
PIEDMONT/COASTAL PLAIN IN NC/VA AS WELL DEFINED SYNOPTIC FORCING
OVERSPREADS THE REGION THIS EVENING/TONIGHT. GIVEN A SEASONABLY
MOIST AIRMASS...WITH INSTABILITY AT THE HIGH END OF LOW OR LOW END
OF MODERATE...FLASH FLOODING APPEARS POSSIBLE...AT LEAST LOCALLY.


...GEORGIA AND SOUTH CAROLINA...

A WELL FORMED BOW ECHO WITH IMPRESSIVE RADAR PRESENTATION WAS
TRACKING ALONG THE GULF COAST AT 08Z. THE BROAD MESOSCALE
CONVECTIVE VORTEX ASSOCIATED WITH THIS WAS LOCATED INLAND...AND
MEAN STEERING FLOW IN THE 600-400 MB LAYER SHOULD GUIDE THE MCV
TOWARD SOUTHERN/CENTRAL GEORGIA AND WESTERN SOUTH CAROLINA BY
AFTERNOON. NONE OF THE HI-RES MODELS IS HANDLING THIS
IDEALLY...ALTHOUGH ALL HAVE A HEAVY RAINFALL SIGNAL SCATTERED
AROUND THE SOUTHEAST U.S...AND ALL SHOW VARYING EVOLUTION AND
TIMING. WE HAVE TAKEN THE APPROACH OF EXTRAPOLATING FROM
OBSERVATIONAL TRENDS...SUGGESTING THAT THE MOST LIKELY SCENARIO TO
PRODUCE A FLASH FLOOD CONCERN WOULD BE RENEWED AFTERNOON/EVENING
CONVECTION FORCED AHEAD OF THE MCV AND NORTH OF A QUASI-STATIONARY
FRONT WHERE EASTERLY LOW LEVEL WINDS WILL PROVIDE INFLOW OF VERY
MOIST AIR...WITH PW ABOVE 1.50 INCHES. THOUGH FLASH FLOOD GUIDANCE
NUMBERS ARE HIGH...RAIN RATES COULD EXCEED 2 INCHES PER HOUR...AND
CELL MERGERS/ORGANIZATION COULD BOOST 3-HOUR TOTALS OVER FFG.
CONFIDENCE IN THE SCENARIO IS ONLY MODEST...HOWEVER...SO WILL NOT
INTRODUCE A RISK AREA AT THIS TIME.


...NORTHERN CALIFORNIA...

SLOW CELL MOTIONS AND SUFFICIENT MOISTURE BENEATH A COLD UPPER LOW
MAY LEAD TO AN ISOLATED FLASH FLOOD CONCERN IN THE WESTERN
FOOTHILLS OF THE HIGH SIERRAS.


...NORTHERN MONTANA...

SHOULD AN MCS BECOME ESTABLISHED NEAR THE INTERNATIONAL
BORDER...EAST OF THE MOUNTAINS...INFLOW WILL BE STRONG THIS
EVENING/OVERNIGHT...SUPPORTING A MENTIONABLE RISK OF EXCESSIVE
RAINFALL. CONFIDENCE IN THE DETAILS IS LOW...HOWEVER.


BURKE
$$





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