Quantitative Precipitation Forecast
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000 FOUS30 KWBC 021447 QPFERD EXCESSIVE RAINFALL DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1046 AM EDT SUN AUG 02 2015 ...VALID 15Z SUN AUG 02 2015 - 12Z MON AUG 03 2015... ...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL... SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM 20 NW TAD 25 WSW TAD 15 W RTN 15 W LVS 15 SE SAF 15 SSW SAF 20 NE AEG AEG 25 W AEG 10 ESE GNT GNT 35 NNW GNT 30 NE GUP 30 NNE RQE 50 N RQE 60 SW CEZ 55 S 4BL 40 SSW 4BL 40 SW 4BL 55 NE PGA 50 SSW 4HV 40 SSW 4HV 30 SW 4HV 15 SW 4HV 4HV 15 E 4HV 20 SW CNY 10 SSE CNY 15 E CNY 25 ENE CNY 20 WSW GJT 10 ENE GJT 10 SW RIL 10 W EGE 10 WNW CCU 30 SE LXV 40 SW FCS 30 NNE VTP 20 NW TAD. SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM VVG 10 NW LAL 10 W SRQ 90 SSE AAF 10 SW 40J 45 SE VLD 20 SW VQQ 25 SE GNV VVG. ...FLORIDA... DEEP TROPICAL MOISTURE RESIDING BENEATH A BROAD UPPER LEVEL TROUGH WILL FEED SURGES OF CONVECTION THAT TRACK WEST TO EAST ACROSS THE CENTRAL AND NORTHERN FLORIDA PENINSULA. THE GREATEST CONCERN IS JUST TO THE SOUTH OF A SURFACE LOW PRESSURE CENTER...WHERE LOW LEVEL CONVERGENCE AND LOW LEVEL INFLOW OFF THE GULF WILL BE MAXIMIZED FOR QUITE SOME TIME...AS THE LOW MOVES VERY SLOWLY EASTWARD. THIS INCLUDES THE STRETCH OF COAST FROM NEAR AND NORTH OF TAMPA BAY UP THROUGH CEDAR KEY AND STEINHATCHEE. CONVECTION HERE MAY BE SHALLOW AT TIMES...BUT STILL HIGHLY EFFICIENT AT RAINFALL PRODUCTION GIVEN WARM RAIN PROCESSES AND TRAINING. EFFICIENCY MAY BE ENHANCED DURING THE OVERNIGHT HOURS WHEN THE BOUNDARY LAYER DECOUPLES AND STRONGER LOW LEVEL SOUTHWESTERLY WINDS WILL CONTINUE TO PROGRESS ONSHORE. A BASIN AVERAGE OF 3 TO 4 INCHES OF QPF SHOULD BE EXPECTED BETWEEN KCTY AND KBKV. IT SHOULD BE NOTED THAT SOME OF THE 00Z HIGH RESOLUTION GUIDANCE...SUCH AS THE 00Z WRF ARW AND 00Z REGIONAL GEM...SHOW THE POTENTIAL FOR AN ADDITIONAL 6 TO 9 INCHES OF RAINFALL WHERE TRAINING OCCURS. THERE IS SOME MODEL SPREAD ON WHERE THE QPF MAXIMUM OCCURS. ELSEWHERE...THE VERY HIGH MOISTURE CONTENT ON THE LARGER SCALE SHOULD SUPPORT LOCALLY INTENSE RAIN RATES THROUGHOUT FLORIDA...BUT THERE ARE FACTORS COMBATING A MORE ORGANIZED LARGE SCALE EXCESSIVE RAIN THREAT. LAPSE RATES HAD GROWN LESS STEEP WITH TIME AFTER A FEW DAYS OF ACTIVE CONVECTION...AND LAPSE RATES WERE NOT BEING REPLENISHED GIVEN RELATIVELY LITTLE LIFT ASSOCIATED WITH THE WEAK TROUGH / SHEAR ZONE. DAYTIME CONVECTION WILL TEND TO PROPAGATE WITH DECENT FORWARD MOTION ALONG OUTFLOWS...CUTTING DOWN ON RESIDENCE TIME OVER A GIVEN POINT. ...COASTAL SOUTH CAROLINA AND GEORGIA... A NARROW ZONE NEAR THE COAST MAY RECEIVE TWO ROUNDS OF HEAVY RAIN THROUGH MONDAY MORNING. THUNDERSTORMS DEVELOPING DURING AFTERNOON HEATING WILL MOVE VERY SLOWLY. LATER IN THE NIGHT SOUTHERLY LOW LEVEL FLOW WILL INCREASE UP TO AND POSSIBLY ONTO THE COAST OF SOUTH CAROLINA...AHEAD OF AN APPROACHING WEAK BUT WELL DEFINED LOW LEVEL CYCLONE MOVING THROUGH NORTHERN FLORIDA. MODELS SUCH AS THE 00Z WRF-ARW AND WRF-NMM PLACE HEAVY RAINFALL ONSHORE IN SC LATE TONIGHT...WHILE OTHERS REMAIN JUST OFFSHORE. QUESTIONS ABOUT PLACEMENT AND RECENT TRENDS TOWARD POOR LAPSE RATES IN THE LOWER TO MIDDLE LEVELS CAUSED WPC TO STOP SHORT OF ADDING A SLIGHT RISK FOR THIS AREA...BUT RICH MOISTURE WILL SUPPORT ISOLATED VERY HEAVY RAIN RATES WHEREVER CONVECTION DOES FORM. ...WESTERN UNITED STATES... MONSOONAL MOISTURE IN THE SOUTHWEST STATES REMAINS CONNECTED TO A PLUME EMANATING FROM THE EASTERN PACIFIC UP THROUGH SONORA. PORTIONS OF THE MOISTURE ARE THEN DIVERTED INTO THE GREAT BASIN AND SOUTHERN PLAINS...FOLLOWING A MID LEVEL SHEAR AXIS LAID OUT ROUGHTLY WNW TO ESE. THE HIGH MOISTURE CONTENT AIR...COMBINED WITH MARGINAL INSTABILITY AND LIFT FROM A SHORT WAVE CROSSING THE SOUTHERN ROCKIES AND GREAT BASIN...IS EXPECTED TO PRODUCE AN AREA OF SCATTERED TO BROKEN CONVECTION...WHICH COULD PRODUCE HEAVY TO LOCALLY EXCESSIVE RAINFALL. THE BEST THRUST OF MONSOONAL MOISTURE SHOULD OCCUR ACROSS NM INTO THE SOUTHERN ROCKIES AND GREAT BASIN (AS SEEN ON THE LATEST HIGH AND MID LEVEL LAYERED PRECIPITABLE WATER IMAGES). THERE IS GOOD MODEL AGREEMENT THAT A SHORT WAVE TRACKS NORTH OUT OF AZ THROUGH NV AND UT DURING THE AFTERNOON AND EVENING HOURS. THE SYNOPTIC SCALE ASCENT ASSOCIATED WITH THE SHORT WAVE SHOULD ALLOW CLUSTERS OF CONVECTION TO BLOSSOM ACROSS WESTERN CO (INCLUDING THE SAN JUAN MOUNTAINS)...NORTHERN NM AND SOUTHEAST UT. PRECIPITABLE WATER VALUES HERE SHOULD RANGE BETWEEN TWO AND THREE STANDARD DEVIATIONS ABOVE THE MEAN...INCREASING THE THREAT FOR FLASH FLOODING. SOME OF THE HIGH RESOLUTION GUIDANCE SUGGESTS LOCAL 2.00 INCH QPF AMOUNTS ARE POSSIBLE WITH STORMS THAT TAP THE HIGH MOISTURE AIR...AND BASED ON WHAT OCCURRED ACROSS WESTERN CO SAT...THIS SEEMS POSSIBLE. THERE IS SOME UNCERTAINTY CONCERNING HOW WIDESPREAD THE HEAVY RAIN THREAT WILL BE...BUT WESTERN CO AND NORTHERN NM SEEM TO BE IN THE AREAS WITH THE BEST COMBINATION OF LIFT/MOISTURE. WHILE BASIN AVERAGE QPF AMOUNTS BETWEEN 0.25 AND 0.50 INCHES MAY BE COMMON IN THE HIGHER TERRAIN...HIGHER AMOUNTS IN AREAS WHERE ONE AND THREE HOUR FLASH FLOOD GUIDANCE VALUES ARE AN INCH OR LESS WILL CAUSE PROBLEMS. A SLIGHT RISK AREA WAS ISSUED FOR THIS AREA. FARTHER WEST...INSTABILITY AND MOISTURE COULD SUPPORT SCATTERED CONVECTION WITH LOCALLY HEAVY RAINFALL ACROSS PORTIONS OF NV INTO NORTHERN CA AND WESTERN OR. PRECIPITABLE WATER VALUES FROM NORTHERN CA/SOUTHWEST OR THROUGH MUCH OF NV SHOULD TOP OUT BETWEEN TWO AND THREE STANDARD DEVIATIONS ABOVE THE MEAN....AND MARGINAL INSTABILITY SHOULD INITIATE CONVECTION MAINLY OVER THE TERRAIN. SOME OF THE MORE AGGRESSIVE HIGH RESOLUTION GUIDANCE (LIKE THE 00Z NAM CONEST) SUGGESTS THAT LOCAL 1.00+ INCH AMOUNTS ARE POSSIBLE ACROSS THE TERRAIN OF CENTRAL NV. WITH A SHORT WAVE CROSSING THE REGION FOR SYNOPTIC SCALE ASCENT...LOCAL QPF AMOUNTS OF THIS MAGNITUDE ARE PLAUSIBLE. GIVEN THE HIGH MOISTURE CONTENT OF THE AIR...A SEE TEXT WAS PLACED OVER NV. HAYES/BURKE $$

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