Quantitative Precipitation Forecast
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420 FOUS30 KWBC 051413 QPFERD EXCESSIVE RAINFALL DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 913 AM EST MON DEC 05 2016 ...VALID 15Z MON DEC 05 2016 - 12Z TUE DEC 06 2016... ...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL... MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM 10 SE KBQX 10 NW DWH GLH DCU 25 W CEU 25 W CAE 55 E HXD 10 NNE NRB 70 WNW PIE. SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM 20 S KGVX 25 ESE LBX 10 NE EFD 45 E CXO 40 NNE JAS 20 NE TVR 30 ENE JAN 20 NNW GPT 20 NW NSE 30 SW LSF 10 NNE MCN 15 NW JYL SVN 35 ESE VAD 25 SW 40J 75 SSE AAF. MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM 25 SSE KXIH 25 NNE GLS 30 SE JAS 20 SE ACP 25 ENE LFT 25 N 7R3 25 E 7R3 30 NNE GSM 15 S KSPR. 1500 UTC UPDATE ONLY SOME SLIGHT CHANGES MADE TO THE PREVIOUS EXCESSIVE RAINFALL POTENTIAL ACROSS THE CENTRAL GULF COAST. THE MODERATE RISK AREA WAS TRIMMED SLIGHTLY ON THE WESTERN SIDE---CONFINED TO COASTAL WESTERN LA. MODEL QPF TRENDS IN THE HI RES RUNS IS FOR THE HEAVIEST PRECIP POTENTIAL TO BEGIN TO SHIFT FARTHER EASTWARD LATE MORNING INTO AFTERNOON ALONG THE GULF COAST AS SHORTWAVE ENERGY MOVING NORTHEASTWARD AHEAD OF THE UPSTREAM CLOSED LOW ALSO SHIFTS FARTHER DOWNSTREAM. THE BEST INSTABILITY REMAINS OFFSHORE---WITH THE HEAVIEST RAINFALL RATES LIKELY TO ALSO REMAIN OFFSHORE. HOWEVER---SOME OF THE HEAVIER RATES MAY BRUSH COASTAL SECTIONS OF WESTERN TO CENTRAL LA---WITH THE PREVIOUS MODERATE RISK AREA MAINTAINED. ELSEWHERE---NO CHANGES MADE DOWNSTREAM ACROSS THE EASTERN GULF AND INTO THE SOUTHEAST. ORAVEC PREVIOUS DISCUSSION ...SOUTHERN U.S AND GULF COAST... OVERALL EXPECT TO SEE A COMPLEX CONVECTIVE EVOLUTION TODAY...WITH A FEW AREAS OF POSSIBLE EXCESSIVE RAINFALL DEVELOPING. AS OF EARLY MONDAY MORNING CURRENTLY HAVE A BROAD SWATH OF STEADY RAINFALL OVER PORTIONS OF CENTRAL TEXAS DEVELOPING IN THE STRONG WARM AIR ADVECTION AHEAD OF THE UPPER LOW. THIS INLAND PRECIPITATION IS DISPLACED WELL NORTH OF THE SURFACE FRONT AND BETTER INSTABILITY...ALTHOUGH DO NOTE SOME WEAK ELEVATED INSTABILITY. EXPECT THIS AXIS OF SHOWERS AND EMBEDDED CONVECTION TO GENERALLY MOVE EASTWARD INTO CENTRAL LA THIS MORNING AND TOWARDS CENTRAL MS/AL BY THIS AFTERNOON. WOULD NOT EXPECT THIS RAINFALL BE TO BE ALL THAT INTENSE GIVEN THE LACK OF INSTABILITY AS MENTIONED ABOVE. HOWEVER WILL CONTINUE A SLIGHT RISK ACROSS PORTIONS OF CENTRAL LA AND SOUTHWEST MS TO ACCOUNT FOR THIS MORNING CONVECTION...AS RECENT RAP RUNS CONTINUE TO SHOW SOME ELEVATED INSTABILITY WORKING NORTH THROUGH THE MORNING HOURS AND RECENT RADAR DOES SEEM TO SHOW AN UPTICK IN CONVECTIVE INTENSITY WITH THE ACTIVITY STREAMING FROM EAST TEXAS TOWARDS CENTRAL LA. ALSO AT LEAST A PORTION OF THIS SLIGHT RISK AREA HAS SEEN SEVERAL INCHES OF RAIN OVER THE PAST FEW DAYS...WHICH MAY RESULT IN SOME RUNOFF ISSUES SHOULD RATES GET HIGH ENOUGH. FURTHER EAST INTO AL WILL JUST CARRY A MARGINAL RISK AS THIS MORNING/AFTERNOON ACTIVITY SHOULD WEAKEN AS IT HEADS INTO THIS REGION GIVEN THE LACK OF INSTABILITY. EXPECT MORE INTENSE CONVECTION TO FOCUS CLOSER TO THE GULF COAST CLOSER TO THE WARM FRONT WHERE THE BETTER INSTABILITY IS LOCATED...WHICH WILL SLOWLY LIFT NORTHWARD THROUGH THE DAY. CAN ALREADY SEE THIS CONVECTION FORMING OFF AND NEAR THE TEXAS COAST THIS MORNING. THE LATEST HRRR AND HIGH RES ARW APPEAR TO BE HANDLING THIS ACTIVITY BEST AND SUGGEST HEAVY CONVECTION WILL STREAM NORTHWARD TOWARDS THE FAR SOUTHEAST TX COAST AND INTO SOUTHERN LA THROUGH THE MORNING HOURS. IMPRESSIVE PRECIPITABLE WATER VALUES AND LOW LEVEL MOISTURE TRANSPORT SHOULD SUPPORT HEAVY RAINFALL RATES WITH THIS CONVECTION. THINK THERE IS THE POTENTIAL FOR LOCALIZED AMOUNTS OF 2-5" ACROSS THIS AREA THROUGH THE MORNING HOURS. THE POTENTIAL FOR HIGH RATES AND REPEAT CONVECTION THROUGH THE MORNING HOURS RESULTING IN SOME TRAINING WARRANTS A SLIGHT RISK ACROSS THE AREA. HOWEVER GIVEN ANTECEDENT RAINFALL OPTED TO CONTINUE WITH A MODERATE RISK FROM FAR SOUTHEAST TX INTO SOUTHERN LA. THERE IS CHANCE THE HEAVIEST RAINFALL THIS MORNING WILL REMAIN EAST OF THE AREAS THAT HAVE RECEIVED THE HEAVIEST ANTECEDENT RAINFALL...WHICH WOULD LOWER THE FLASH FLOOD RISK. HOWEVER FEEL THE RISK IS STILL HIGH ENOUGH TO WARRANT THE CONTINUATION OF THE MODERATE RISK FOR NOW. WILL REASSESS BY MID MORNING. AS WE HEAD THROUGH THE AFTERNOON HOURS WOULD ALSO EXPECT TO SEE CONVECTIVE COVERAGE INCREASE FURTHER EAST ALONG THE GULF COAST INTO THE FLORIDA PANHANDLE. RECENT MODEL GUIDANCE CONTINUES TO SUGGEST THE PLUME OF MAX PRECIPITABLE WATER AND LOW LEVEL MOISTURE TRANSPORT WILL FUNNEL INTO THE FL PANHANDLE AND SOUTHERN GA THIS AFTERNOON. THIS MOISTURE PLUME IS THE RESULT OF ENHANCED LOW LEVEL FLOW IN BETWEEN THE UPPER LOW TO THE WEST AND THE UPPER HIGH OVER THE SOUTHERN ATLANTIC. IN FACT ANTICIPATE PWAT VALUES TO SURPASS 2" ACROSS THE FL PANHANDLE INTO SOUTHERN GA...WHICH WILL APPROACH CLIMATOLOGICAL MAX VALUES. MEANWHILE...ALOFT THE FLOW WILL BECOME INCREASINGLY DIVERGENT IN THE RIGHT ENTRANCE REGION OF THE UPPER JET. THUS WOULD APPEAR TO BE A GOOD SETUP FOR WIDESPREAD CONVECTION CAPABLE OF PRODUCING HEAVY RATES. WILL NOT NECESSARILY SEE A LOT OF BACKBUILDING CONVECTION...BUT EXPECTED WIDESPREAD COVERAGE SHOULD RESULT IN SOME TRAINING POTENTIAL. THUS WHILE OVERALL WPC QPF WAS A MULTI MODEL GLOBAL AND HIGH RES BLEND...PREFERENCE FOR QPF WAS GIVEN TO THE WETTER MODELS ACROSS THIS REGION...GENERALLY THE HRRR AND OTHER AVAILABLE HIGH RES MODELS. A MULTI MODEL HIGH RES SIGNAL EXISTS FOR POCKETS OF 2-4" FROM THE FL PANHANDLE INTO SOUTHERN GA. WILL THUS CONTINUE WITH A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE FLORIDA PANHANDLE INTO SOUTHEAST GA. AS WE HEAD INTO TONIGHT AND THE UPPER LOW BEGINS TO EJECT INTO THE LOWER MS VALLEY EXPECT TO SEE ADDITIONAL CONVECTION FORM ALONG THE COLD FRONT FROM EASTERN LA INTO MS...MOVING INTO AL AND GA OVERNIGHT. COULD SEE SOME LOCALLY HEAVIER RATES WITH THIS ACTIVITY. MEANWHILE WILL CONTINUE TO SEE A BROAD AREA OF SHOWERS AHEAD OF THE UPPER LOW PROGRESS INTO THE MID ATLANTIC AND SOME WRAP AROUND RAIN ON THE BACKSIDE OF THE LOW FROM AR INTO THE WESTERN OH/TN VALLEY. OVERALL THE TREND AMONGST ALL THE GUIDANCE WAS FOR A SLIGHTLY FURTHER EAST LOW TRACK...THUS DID CUT BACK QPF AMOUNTS SOME ACROSS THE NORTHWEST EDGE OF THE PRECIPITATION SHIELD TO COME MORE IN LINE WITH THE CURRENT MODEL CONSENSUS. OVERALL RAINFALL ASSOCIATED WITH THIS ACTIVITY OUTSIDE OF THE SLIGHT RISK AREAS WILL EITHER LACK SUFFICIENT INSTABILITY OR GENERALLY BE TOO PROGRESSIVE TO WARRANT ANYTHING MORE THAN A MARGINAL RISK AT THIS TIME. CAPPED THE NORTHERN EDGE OF THE MARGINAL RISK AT THE EXPECTED INSTABILITY GRADIENT SUGGESTED BY THE CONSENSUS OF HIGH RES MODELS. CHENARD $$

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