Quantitative Precipitation Forecast
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FOUS30 KWBC 210052
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
851 PM EDT FRI MAY 20 2016

...VALID 01Z SAT MAY 21 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
20 N 6V3 35 WSW I16 30 SE JKL 15 S JKL 10 NW JKL 35 WSW HTS
20 NW HTS 40 WSW UNI 10 SE CMH 15 NNW ZZV 20 SW PHD 15 SE PHD
HLG 20 WNW MGW 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 20 N 6V3.


21/0100 UTC UPDATE

ACROSS MT...HI PRES RIDGING SWD THROUGH WRN MT HAS BROUGHT COOLER
AND MORE STABLE AIR INTO THAT REGION SUGGESTING ANY CONVECTIVE
HEAVY RAIN THREAT WILL OCCUR FARTHER EAST ACRS MT ALONG COLD FRONT
WHERE ML CAPES HAVE INCREASED TO BETWEEN 1000 TO 2000 J/KG AS OF
LATE AFTN. 12Z HI RES GUIDANCE AND RECENT HRRR PCPN SUGGEST ANY FF
THREAT WOULD BE MARGINAL ACRS THE AREA WITH ISOLD SHORT TERM
RAINFALL AMOUNTS PERHAPS APCHG 1.5 INCH.

ACRS THE MID ATLC REGION...TRIMMED WRN PORTION OF PREVIOUS SLIGHT
RISK AREA GIVEN CURRENT PROGRESSION OF MAIN HEAVY RAIN CORE AHEAD
OF PRIMARY LOW MOVING NEWD THRU TN/KY.  MODEL CONSENSUS FAVORS
HEAVIEST RAIN OF 1 TO 1.5 INCHES OCCURRING IN THE UPSLOPE AREAS OF
WRN VA AND NEIGHBORING AREAS OF WV THROUGH THE OVERNIGHT..BUT
MAINTAINED SLIGHT RISK FARTHER WEST GIVEN THE VERY LOW FF GUIDANCE
VALUES THRUGH MUCH OF WV.

ACRS NRN CA...SNOW LEVELS HAVE LOWERED SUFFICIENTLY UNDER THE CORE
OF THE UPR LOW MOVING THROUGH NRN CA TO SUPPORT REMOVING ANY FF
THREAT FOR THE REMAINDER OF THE NIGHT THERE THOUGH SOME SPOTTY
HEAVIER CONVECTIVE CELLS WILL BE PSBL IN THE VALLEY FEEDING OFF OF
RESIDUAL WEAK CAPE.

SULLIVAN



20/1900 UTC UPDATE

ONLY SOME SLIGHT CHANGES MADE TO THE PREVIOUS SLIGHT RISK AREA
FROM THE TN VALLEY NORTHEASTWARD INTO THE UPPER OH VALLEY AND
CENTRAL APPALACHIANS.  THE SOUTHERN EDGE OF THE PREVIOUS SLIGHT
RISK AREA WAS TRIMMED NORTHWARD FOR CURRENT RADAR TRENDS.

TO THE SOUTH AND SOUTHEAST OF THIS---THE SEE TEXT REGION WAS
REMOVED.  THE SECOND IN THE SERIES OF ORGANIZED AREAS OF
CONVECTION AFFECTING THE EASTERN GULF COAST EXPECTED TO PRESS
EASTWARD FROM THE FL PANHANDLE INTO NORTH FLORIDA PRIOR TO THE
BEGINNING OF THE FORECAST PERIOD.  IN THE WAKE OF THIS---MORE
UNCERTAINTIES AS TO WHETHER THERE WILL BE ADDITIONAL CONVECTION
AHEAD OF THE SOUTHEASTWARD MOVING COLD FRONT MOVING THROUGH THE
SOUTHEAST TONIGHT INTO SATURDAY.  THE LATEST WPC QPF TRENDED
LIGHTER FROM THE PREVIOUS FORECASTS--CLOSER TO THE HI RES ARW
WHICH OFTEN SHOWS THE BEST VERIFICATION SKILL.  THE LIGHTER TREND
IS SUPPORTED BY THE 1200 UTC ECMWF.  THE NAM CONEST REMAINS THE
HEAVIEST WITH TOTALS FROM SOUTH GA INTO NORTH FL...WHILE THE NAM
IS MUCH LIGHTER.

NO CHANGES MADE TO THE SEE TEXT REGIONS OVER THE NORTHERN SIERRA
AND THE NORTHERN ROCKIES OF NW MT.


ORAVEC

20/1500 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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