550  
FOUS30 KWBC 200845  
QPFERD  
 
EXCESSIVE RAINFALL DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
344 AM EST TUE FEB 20 2018  
 
VALID 12Z TUE FEB 20 2018 - 12Z WED FEB 21 2018  
 
 
MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM  
HNB 10 ENE DYR 15 SSW PBF 15 W LFK 10 SSE 3T5 15 SE RND  
10 SSE ERV 35 W 6R9 15 NW 7F9 15 NNW 7F9 20 WNW 1F0 15 NE GOK  
30 E WLD 25 ESE UKL 20 NNE IRK JVL 15 ESE HTL 30 SSW CWWX  
30 NNE CXPC 20 WSW CYWA 40 SE CWRK 10 S CXDI 10 SW FDY HNB.  
 
SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM  
IND 15 ENE M30 15 ESE JBR 35 N ELD 35 S PSN 25 E GTU 20 N BMQ  
15 E 7F9 10 NW GLE 10 WSW ADH 30 ENE SWO 25 ENE CNU 35 S IRK  
15 E SQI 20 SSW RQB 30 N CWLS 10 SSW CYKF 10 SSW TTF IND.  
 
MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM  
10 E PSN 25 SE ACT 25 NNE 05F 15 W FWS 15 N AQR 25 ENE GMJ  
35 SW AIZ 25 ESE VIH 10 NNE FAM 20 NW CGI 30 NW BYH 15 E LRF  
35 NW DTN 10 E PSN.  
 
MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM  
10 ESE PNT 10 SW BIV 25 ENE GRR 10 SW RNP 10 SE JXN 20 N GUS  
MTO 20 S DEC 15 N DEC 10 ESE PNT.  
 
...EASTERN PLAINS / MID-UPPER MS VALLEY / GREAT LAKES / UPPER OHIO  
VALLEY...  
 
A SYNOPTIC PATTERN THAT SHOULD SPARK GREAT CONCERN OVER HEAVY  
RAINFALL CONTINUES TO COME INTO PLACE, WITH LARGE SCALE  
AMPLIFICATION OF THE WESTERN U.S. TROUGH, BLOCKED BY A 594 DM  
RIDGE IN THE ATLANTIC, ALLOWING FOR PROLONGED, ELONGATED MOISTURE  
TRAJECTORIES EMANATING FROM THE CARIBBEAN BEFORE ENCOUNTERING A  
SLOW MOVING FRONTAL ZONE IN THE MIDDLE OF THE NATION. MOISTURE  
QUALITY LOOKS MORE LIKE MARCH OR APRIL. PRECIPITABLE WATER  
INCREASING TO AT LEAST 1.50 INCHES SOUTHERN GREAT PLAINS AND  
TAPERING ONLY TO AROUND 1.30 INCHES TO THE SOUTHERN GREAT LAKES  
WOULD REPRESENT MAX VALUES FOR LATE FEBRUARY PER GEFS M-CLIMATE  
DATA. EXPECT REPEATED WAVES OF MODERATE RAINFALL, AND EMBEDDED  
INCREASINGLY HEAVY RAINFALL THROUGH WED/THU IN SOME AREAS,  
RESULTING IN A WIDE SWATH THAT WILL INDUCE LOCAL FLASH FLOODING  
AND MAY OVERWHELM SOME MAIN STEM RIVERS. ESPECIALLY CONCERNED IN  
THE SOUTHERN PLAINS / LOWER MISSISSIPPI VALLEY WITH THE GREATEST  
AVAILABLE INSTABILITY, AND THE SOUTHERN GREAT LAKES WHERE DEEP  
FROST DEPTH/FROZEN GROUND AND SOME SNOW MELT WATER WILL EXACERBATE  
THE RUNOFF/SHORT-TERM FLOODING POTENTIAL.  
 
ALTHOUGH THE QPF WAS NOT OVERLY COMPLICATED ON THE LARGE SCALE  
WITH THE SYNOPTIC PATTERN DRIVING THE SITUATION, OF GROWING  
CONCERN PER THE 00Z GUIDANCE IS THE INCREASED W-E SPREAD WITH THE  
AXIS OF HEAVIEST RAINFALL, PARTICULARLY OVER THE SOUTHERN  
PERIPHERY WHERE THE CAMS (INCLUDING THE HIGH-RES MEANS) ARE  
TRENDING A BIT FASTER WITH THE EASTWARD PROGRESSION IN LIGHT OF  
(MOST LIKELY) SOME CONVECTIVE FEEDBACK. FOR DAY 1, WPC UTILIZED A  
BLEND OF THE ECMWF, IN-HOUSE ENSEMBLE QPF, AND THE PREVIOUS  
FORECAST, WHICH CONTINUES TO KEEP THE MAX QPF AXIS A LITTLE  
FARTHER WEST THAN THE ENSEMBLE OF HIGH-RES GUIDANCE. THIS IDEA  
REPRESENTS A RELATIVELY SMOOTH TRANSITION FROM CONTINUITY  
(PREVIOUS FORECAST), AS GIVEN THE HIGHLY-AMPLIFIED PATTERN  
(DEEP-LAYER S-SW FLOW) ALONG WITH LIMITED INSTABILITY, FEEL MORE  
CONFIDENT IN A SLOWER EASTWARD PROGRESSION. THE RESULT IS A  
CONTINUAL BROAD SWATH OF 1.0 TO 2+ INCH AREAL AVERAGE 24-HOUR  
RAINFALL TOTALS FROM TEXAS TO MICHIGAN, WITH THE HEAVIEST AMOUNTS  
(AREAL-AVERAGE 3+ INCHES) FOCUSED FROM NORTHEAST TX-EASTERN OK  
INTO NORTHWEST AR AND SOUTHERN MO, WHERE MODEST DEEP-LAYER  
INSTABILITY WILL BE PRESENT TO ENHANCE THE RAINFALL RATES. OVER  
THESE AREAS, PER THE MAJORITY OF HIGH-RES CAMS, LOCALIZED 24 HOUR  
TOTALS EXCEEDING 5 INCHES ARE LIKELY -- AND AS SUCH WPC MAINTAINED  
A BROAD MODERATE RISK AREA OF EXCESSIVE RAINFALL IN THE DAY 1  
EXCESSIVE RAINFALL OUTLOOK, LARGELY SIMILAR TO THE AREA ADVERTISED  
IN YESTERDAY'S DAY 2 ERO.  
 
THE OTHER MODERATE RISK AREA WAS NOTED OVER PORTIONS OF THE UPPER  
MIDWEST (EASTERN IL, NORTHWEST IN, AND SOUTHWEST LOWER MI) BASED  
ON THE ANTECEDENT HEAVIER RAINFALL/LOWERING FFG VALUES THROUGH  
EARLY TUE AND THUS HIGHER PROBABILITIES OF 3-6 HOURLY QPF>FFG  
EXCEEDANCE.  
 
HURLEY  

 
 
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