Quantitative Precipitation Forecast
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000 FOUS30 KWBC 250021 QPFERD EXCESSIVE RAINFALL DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 819 PM EDT SUN SEP 24 2017 ...VALID 01Z MON SEP 25 2017 - 12Z MON SEP 25 2017... ...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL... MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM 40 E MMCU 90 WSW MRF 30 E GDP 50 NNW HOB 40 NNE TCC 35 N LAA 40 SSW ITR 25 S PIR 15 ESE KGWR 10 NNE BRD 10 NE CBG 10 WSW FRM 15 SSW OFF 25 SW SLN 10 NW CSM 30 SW SJT 100 SW DRT 90 NNW MMTC. SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM 90 ENE MMCU 15 NW MRF 30 NNE INK 35 SW MCK 20 N BBW 15 WSW ATY 10 SSE OVL 15 W FET 20 SSW RSL 50 SSW CDS 10 NNE 6R6 150 WNW MMMV. 0100 UTC UPDATE FOR THE REMAINDER OF THE OVERNIGHT PD...HAVE FOCUSED SLIGHT RISK AREA FROM PORTIONS OF THE TX/OK PANHANDLES NEWD ALONG NE/SW ORIENTED COLD FRONT THAT IS EXPECTED TO MOVE SLOWLY EWD SUN NIGHT. WHILE LITTLE IN THE WAY OF SIGNFICANT HEIGHT FALLS ARE EXPECTED AHEAD OF ASSOCD HI AMPLITUED UPPER TROF OVER THE GT BASIN...20 TO 30 KT SSWLY LOW LEVEL JET DIRECTED INTO THE FRONT..IN COMBINATION WITH FAVORABLE DIFFLUENT RT ENTRANCE REGION JET SUPPORT..1.5"+ PWS..AND WEAK INSTABILITY..WOULD TEND TO FAVOR AREAS FROM THE TX/OK PANHANDLE REGION NEWD THROUGH KS AND CONTINUING NEWD THROUGH IA/SRN MN FOR POTNL REPEATING CONVECTIVE CELLS OVERNIGHT. RECENT HRRR RUNS PARTICULARLY HIT PORTIONS OF SW INTO CNTL TX PARTICULARLY HARD WITH OVER 5" RAINS WHILE EARLIER 12Z RUNS HAD THE AXIS OF HEAVIER RAINS A BIT TO THE EAST AND NOT QUITE AS EXTREME WITH AMOUNTS..WHICH GENLY SUPPORTS MAINTAINING A SLIGHT RISK OF EXCESSIVE RAINS HERE GIVEN THE SPREAD. FARTHER SOUTH...IT IS DIFFICULT TO FIND A STG FOCUS FOR ORGANIZED CONVECTION THOUGH PARAMETERS ARE IN PLACE THAT WOULD LEND ITSELF TOWARD LOCALIZED HEAVIER RAIN AMOUNTS..IE SELY LOW LEVEL UPSLOPE FLOW AND 1000 J/KG ML CAPES WITHIN BROADLY DIFFLUENT UPPER FLOW REGIME. GIVEN BROAD MODEL SPREAD IN PLACEMENT AND AMOUNTS HERE...TRIMMED SLIGHT RISK AREA TOWARD WHERE THERE WAS A BIT MORE CONSENSUS FOR HEAVIER RAIN AMOUNTS OVERNIGHT. SULLIVAN INITIAL DISCUSSION ...GREAT PLAINS / CENTRAL U.S... MODERATE AND EMBEDDED HEAVIER RAINFALL IS FORECAST SUNDAY FROM TEXAS TO MINNESOTA. THIS WAS DRIVEN BY A VERY DEEP TROUGH IN THE WEST AND EQUALLY STOUT DOWNSTREAM RIDGE OVER THE GREAT LAKES. RAINFALL INTENSITY WAS TEMPERED...HOWEVER...BY MARGINAL LAPSE RATES IN THE WARM SECTOR AND THE BULK OF DEEP LAYER ASCENT REMAINING BACK IN THE COLD SECTOR. THUS...IT MAY BE THAT RAIN RATES REMAIN SOMEWHAT MARGINAL AS HAD BEEN OBSERVED ON SATURDAY. TWO AREAS STAND OUT AS SEEING GREATER ODDS OF ORGANIZED HEAVY RAINFALL. SUFFICIENT FRONTOGENESIS AND CONVERGENCE OF THE LOW LEVEL FLOW...ESPECIALLY JUST TO THE COOL SIDE OF THE SURFACE FRONT...SHOULD ALLOW FOR NEAR CONTINUAL RAIN/THUNDERSTORM DEVELOPMENT FROM WESTERN KANSAS THROUGH CENTRAL NEBRASKA INTO EASTERN SOUTH DAKOTA. ADDITIONAL CLUSTERS OF THUNDERSTORMS / SMALL MCS ACTIVITY WILL LIKELY OCCUR DOWN THROUGH WEST TEXAS AND EASTERN NEW MEXICO...BLEEDING OVER INTO WESTERN OKLAHOMA WITH TIME. AT THESE MORE SOUTHERLY LATITUDES THE STEERING FLOW INCLUDES MORE WESTERLY COMPONENT...ALLOWING CONVECTION TO PUSH OUT INTO THE WARM SECTOR. AT THE SAME TIME HOWEVER...INSTABILITY ACTUALLY DECREASES WITH DISTANCE INTO THE WARM SECTOR...SO THERE ARE SOME COMPETING FACTORS AT PLAY. ONE SIGNAL OF NOTE IS THE INCREASE OF QPF OVER THE BIG BEND AREA IN TEXAS. THE WRF-ARW WAS PARTICULARLY AGGRESSIVE THERE...AND OTHER GUIDANCE TRENDED UPWARDS AS WELL. THE WPC TREND WAS MORE CONSERVATIVE...GIVEN THE POTENTIAL FOR OUTFLOW AND PROPAGATION TO SUPPRESS ANY EXTREME VALUES...BUT THERE IS POTENTIAL FOR LOCAL INTENSE RAIN RATES...AS THE UPPER PATTERN OPENS UP AND IS MORE DIFLUENT ACROSS THIS AREA...WHILE ALSO TAPPING INTO MOISTURE ASSOCIATED WITH TROPICAL STORM PILAR. OVERALL THE SITUATION MIGHT BE BEST DESCRIBED BY A MARGINAL RISK OF EXCESSIVE RAINFALL AT THIS TIME...BUT THERE WILL BE SOME OVERLAP OF NEW RAINFALL ATOP AREAS THAT RECEIVED A WETTING RAIN ON SATURDAY. THEREFORE WE MAINTAINED THE INHERITED NARROW SLIGHT RISK AREA FROM SOUTHWEST TEXAS TO SOUTHWEST MINNESOTA. BURKE $$

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