869  
FXUS02 KWBC 061600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1159 AM EDT MON MAY 06 2019  
 
VALID 12Z THU MAY 09 2019 - 12Z MON MAY 13 2019  
 
...MULTI-DAY HEAVY RAINFALL THREAT FOR EASTERN/SOUTHEASTERN TEXAS  
AND THE LOWER MISSISSIPPI VALLEY LATER THIS WEEK...  
 
   
..OVERVIEW
 
 
SIGNIFICANT PRECIPITATION WILL BE IN THE FORECAST ACROSS PORTIONS  
OF THE SOUTHERN AND EASTERN U.S. DURING THE PERIOD. A STORM  
SYSTEM REACHING THE MIDWEST BY THE START OF THE PERIOD EARLY THU  
AND TRACKING NORTHEAST WITH TIME WILL BRING ONE AREA OF ENHANCED  
RAINFALL ACROSS THE GREAT LAKES/NORTHEAST LATE THIS WEEK, ALONG  
WITH A THREAT OF STRONG TO SEVERE CONVECTION IN THE WARM SECTOR  
(SOUTHEAST TEXAS NORTHEASTWARD TO THE MIDWEST). MEANWHILE THERE  
IS STILL A PRONOUNCED SIGNAL FOR HEAVY RAINFALL POTENTIAL FROM  
EASTERN/SOUTHEASTERN TEXAS THROUGH THE LOWER MISSISSIPPI VALLEY  
AND TO SOME DEGREE EXTENDING FARTHER NORTHEASTWARD THU THROUGH THE  
WEEKEND. THIS THREAT CORRESPONDS TO GUIDANCE CONTINUING TO SHOW  
THE MIDWEST STORM'S TRAILING FRONT STALLING NEAR AND NORTHEAST  
FROM THE WESTERN GULF COAST, WITH SOME DEGREE OF LOW LEVEL GULF  
INFLOW PROVIDING MOISTURE WHILE IMPULSES PROGRESS ALONG IN  
SOUTHWESTERLY FLOW ALOFT. AT THE SAME TIME UPPER TROUGHING OVER  
THE WEST WILL BRING MULTIPLE DAYS OF PRECIPITATION TO LOCATIONS  
FROM CENTRAL-SOUTHERN CALIFORNIA INTO THE SOUTHERN HALF OF THE  
ROCKIES WITH HIGHEST TOTALS OVER AREAS WITH FAVORABLE TERRAIN  
ENHANCEMENT. MODEST SNOW AT THE END OF THE SHORT RANGE OVER  
COLORADO WILL CONTINUE INTO THE START OF THE MEDIUM RANGE PERIOD  
BEFORE DIMINISHING.  
 
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
THE MAJORITY OF GUIDANCE SHOWS A SIMILAR THEME FOR THE EVOLUTION  
OF MEAN FLOW ALOFT, AN EASTERN PACIFIC RIDGE BUILDING INTO THE  
NORTHWEST STATES WHILE SPLIT FLOW PREVAILS TO THE SOUTHEAST--A  
MEAN TROUGH SETTLING OVER EAST-CENTRAL CANADA INTO THE UPPER  
MIDWEST/GREAT LAKES AND A DEVELOPING UPPER LOW TRACKING NEAR THE  
WESTERN U.S.-MEXICO BORDER (WITH ANOTHER ATTENDANT WEAK UPPER LOW  
ROUNDING ITS BASE). THE RELATIVE AGREEMENT IN MEAN FLOW CONCEALS  
SIGNIFICANT DIFFERENCES/UNCERTAINTY IN SOME DAY-TO-DAY DETAILS.  
 
GREATEST DIFFERENCES OVER THE PAST COUPLE DAYS HAVE INVOLVED  
NORTHERN STEAM FLOW, BOTH WITHIN THE MEAN TROUGH OVER SOUTHERN  
CANADA AND NORTHERN U.S. AS WELL AS NORTH PACIFIC FLOW HEADING  
TOWARD/INTO NORTH AMERICA. THE MOST NOTICEABLE MULTI-DAY TREND  
HAS BEEN FOR STRONGER RIDGING TO BUILD OVER THE NORTHWEST AND TO  
PERSIST FOR A LONGER TIME. UNCERTAIN DETAILS OF NORTH PACIFIC  
FLOW ROUNDING THE RIDGE CONTINUE TO CAUSE DIFFICULTY IN RESOLVING  
THE TIMING AND AMPLITUDE OF A POTENTIAL SURFACE LOW/FRONTAL SYSTEM  
DROPPING FROM CANADA INTO THE NORTHERN U.S. LATE WEEK ONWARD.  
BASED ON THE FULL ARRAY OF GUIDANCE GFS RUNS MAY BE SOMEWHAT  
STRONG/FAST WITH THE SYSTEM BUT THE 00Z ECMWF/ECMWF MEAN CONTINUED  
TO BE SLOWEST/WEAKEST. ACROSS THIS EXPANSE OF NORTHERN LATITUDES  
A BLEND/COMPROMISE APPROACH PROVIDES REASONABLE CONTINUITY WHILE  
REFLECTING LATEST TRENDS FOR THE NORTHWEST RIDGE ALOFT, WITH A  
RELIANCE ON THE PREVIOUS SHIFT THAT FELL NICELY BETWEEN THE SPREAD  
OF QUICKER/SLOWER SOLUTIONS (GFS/ECMWF) SOMEWHAT NEAR THE 00Z  
CANADIAN AT TIMES BEFORE IT BECAME SLOWER THAN THE ENSEMBLE  
CONSENSUS. LOW CONFIDENCE FOR THAT SYSTEM GIVEN ITS HISTORY.  
 
GUIDANCE STILL SHOWS A COMPLEX EVOLUTION TAKING PLACE ALONG AND TO  
THE SOUTH OF THE CALIFORNIA COAST, AS INTERIOR WEST ENERGY PUSHES  
SOUTHWEST TO INTERACT WITH AN OFFSHORE UPPER LOW MOVING EASTWARD  
ALONG 30N LATITUDE. THERE WILL BE VARIOUS WAYS THAT THIS  
EVOLUTION COULD ULTIMATELY STRAY FROM CONSENSUS BUT SO FAR THE  
MAJORITY/INTERMEDIATE SOLUTION HAS BEEN CONSISTENT IN SHOWING THE  
CENTER OF THE UPPER LOW REACHING NEAR THE SOUTHERN ARIZONA BORDER  
BY SUN. DETERMINISTIC MODELS GENERALLY AGREE ON SLIDING THE  
FARTHER WEST UPPER LOW EASTWARD UNDERNEATH THE FORMING UPPER LOW  
OVER COASTAL CA FRIDAY THEN INTO NW MEXICO AND SE AZ EARLY  
SATURDAY, PROVIDING ANOTHER MECHANISM FOR PRECIPITATION. THE 00Z  
ECMWF/ECMWF MEAN MAY BE A BIT TOO QUICK TO OPEN UP/EJECT THE  
FEATURE AFTER SUN GIVEN THE RIDGE BUILDING OVER THE NORTHWEST  
(GEFS MEAN WAS STRONGER WITH THE UPPER RIDGE INTO THE OR COAST SAT  
AND SLOWER WITH THE SPEED OF THE UPPER LOW INTO NW MEXICO).  
 
FOR THE STORM TRACKING NORTHEAST FROM THE MIDWEST, UKMET RUNS WERE  
STILL NOTABLE FOR BEING ON THE STRONG/NORTHWEST FRINGE OF THE  
MODEL SPREAD. OTHERWISE CONSENSUS HAS TRENDED A BIT WEAKER FROM  
PRIOR CYCLE BUT TO A DEGREE THAT IS GENERALLY WITHIN THE NOISE  
RANGE. MOST SOLUTIONS CONTINUE TO SHOW A TRIPLE POINT WAVE OFF  
NEW ENGLAND BY EARLY SAT AS THE SYSTEM PROGRESSES OUT TO SEA.  
ALONG THE TRAILING FRONT THAT WILL BECOME THE FOCUS FOR HEAVY  
RAINFALL OVER THE SOUTH, SOME UNCERTAINTY IN WAVINESS/FRONTAL  
POSITION ARISES TOWARD SUN-MON IN RESPONSE TO DETAILS OF THE UPPER  
TROUGH TO THE NORTH. THESE WILL LIKELY HAVE TO WAIT UNTIL THE  
SHORT RANGE TO BE RESOLVED BUT THE OVERALL PICTURE SHOULD STAY THE  
SAME.  
 
FOR THE FULL FORECAST DOMAIN A GENERAL MODEL BLEND EARLY IN THE  
PERIOD TRENDING TOWARD A BLEND AMONG THE 06Z GFS/GEFS MEAN AND 00Z  
ECMWF/ECMWF MEAN ALONG WITH CONTINUITY PROVIDED A REASONABLE  
STARTING POINT FOR REPRESENTING THE DESIRED CONSENSUS OR  
INTERMEDIATE SOLUTION FOR FEATURES OF INTEREST. SOME ADJUSTMENTS  
WERE MADE TO FINE TUNE SOME DETAILS WHERE THE CONSENSUS BLEND  
APPEARED TOO WASHED OUT. THIS APPROACH PROVIDED FAIRLY GOOD  
CONTINUITY WHILE AWAITING ANY POSSIBLE ADJUSTMENTS OVER AREAS OF  
GREATEST UNCERTAINTY.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE STORM TRACKING NORTHEASTWARD FROM THE MIDWEST THU-FRI WILL  
PRODUCE AN AREA OF RAINFALL THAT MAY BE HEAVY AT TIMES OVER THE  
GREAT LAKES/NORTHEAST AS WELL AS THUNDERSTORMS (SOME STRONG TO  
SEVERE) IN THE WARM SECTOR FROM THE SOUTHEASTERN PLAINS/LOWER  
MISSISSIPPI VALLEY INTO THE OHIO VALLEY/LOWER GREAT LAKES, AND  
POSSIBLY EXTENDING FARTHER EASTWARD WITH TIME. CONSULT STORM  
PREDICTION CENTER PRODUCTS FOR LATEST INFO ON THE SEVERE WEATHER  
FORECAST. THEY CURRENTLY HIGHLIGHT AN AREA OF POTENTIAL SEVERE  
WEATHER ON THURSDAY FROM EASTERN/SOUTHEASTERN TEXAS NORTHEASTWARD  
TO THE MIDWEST (IL/IN/OH). IN THE NORTHERN PART OF THE PRECIP  
SHIELD THERE MAY BE LOCALIZED AREAS OF SNOW AT TIMES, WITH BEST  
POTENTIAL (THOUGH WITH FAIRLY LOW PROBABILITY IN ABSOLUTE TERMS)  
OVER THE WESTERN UP OF MICHIGAN AND EXTREME NORTHERN WISCONSIN.  
 
THE FAVORABLE SETUP OF A STALLING FRONT, LOW LEVEL GULF INFLOW,  
AND IMPULSES ALOFT CARRIED BY SOUTHWESTERLY MEAN FLOW, ARE  
MAINTAINING THE THREAT FOR HEAVY RAIN ACROSS PARTS OF THE SOUTH.  
PRECIPITABLE WATER VALUES NEAR 2 INCHES (+2 TO +3 SIGMA) AND  
STRENGTHENING/INCOMING SUBTROPICAL JET (LESS ENSEMBLE AGREEMENT)  
WILL PROVIDE AN ENVIRONMENT CONDUCIVE FOR HEAVY RAIN. THE BEST  
POTENTIAL FOR HIGHEST 5-DAY TOTALS STILL EXISTS OVER EASTERN TEXAS  
THROUGH LOUISIANA WITH SIGNIFICANT TOTALS ALSO EXTENDING FARTHER  
NORTHEASTWARD. GUIDANCE CONTINUES TO SHOW THE POSSIBILITY FOR AT  
LEAST 5-10" ACCUMULATIONS AT SOME LOCATIONS. THE FRONT MAY DRIFT  
A LITTLE SOUTHEASTWARD LATE IN THE PERIOD TO TONE DOWN THE HEAVY  
RAINFALL THREAT.  
 
FROM FRI ONWARD A SYSTEM DROPPING OUT OF CANADA MAY BRING MOSTLY  
LIGHT TO LOCALLY MODERATE RAINFALL TO SOME LOCATIONS FROM THE  
NORTHERN PLAINS INTO THE GREAT LAKES. THUS FAR GUIDANCE HAS NOT  
BEEN SUFFICIENTLY CONSISTENT OR SIMILAR TO PROVIDE MEANINGFUL  
CONFIDENCE IN TIMING/COVERAGE OF PRECIPITATION.  
 
WITHIN THE AREA OF TERRAIN-ENHANCED PRECIPITATION FROM  
CENTRAL-SOUTHERN CALIFORNIA EASTWARD INTO THE ROCKIES, THE BEST  
POTENTIAL FOR HIGHER ELEVATION SNOW WILL BE CENTERED OVER THE  
ROCKIES OF COLORADO THU INTO FRI BEFORE HEIGHTS ALOFT RISE OVER  
THAT AREA. THERE IS STILL UNCERTAINTY OVER DETAILS OF THE UPPER  
LOW(S) THAT MAY TRACK ALONG THE SOUTHWESTERN U.S./NORTHERN MEXICO  
BORDER BUT THE FAVORED AVERAGE OF GUIDANCE DURING THE WEEKEND INTO  
EARLY NEXT WEEK YIELDS A FAIRLY CONSISTENT FORECAST FOR THE TIME  
BEING. RAINFALL MAY INCREASE IN COVERAGE/INTENSITY OVER THE  
SOUTHERN HIGH PLAINS EARLY NEXT WEEK AS THE FEATURE APPROACHES.  
TIMING REMAIN UNCERTAIN THAT FAR OUT IN THE PERIOD.  
 
THE UPPER RIDGE BUILDING INTO THE NORTHWEST WILL BRING MULTIPLE  
DAYS OF WELL ABOVE NORMAL TEMPERATURES--GENERALLY PLUS 10-20F  
ANOMALIES AND MAYBE LOCALLY HIGHER. SOME DAILY RECORDS FOR  
HIGHS/WARM LOWS WILL BE POSSIBLE WITH SOME AREAS APPROACHING 90F.  
ON THE OTHER HAND MUCH BELOW NORMAL HIGHS (MINUS 15-25F ANOMALIES)  
WILL PUSH DOWN THROUGH THE CENTRAL/SOUTHERN HIGH PLAINS LATE THIS  
WEEK AND SLOWLY MODERATE THEREAFTER. TO A LESS EXTREME EXTENT THE  
SOUTHWEST WILL SEE COOL DAYTIME HIGHS AS WELL GIVEN THE EXTENSIVE  
CLOUD COVER AND SCATTERED RAIN. THE EAST WILL BE WARM LATE THIS  
WEEK AHEAD OF THE MIDWEST SYSTEM, FOLLOWED BY A TREND TOWARD MORE  
NORMAL READINGS.  
 
FRACASSO/RAUSCH  
 
ADDITIONAL 3-7 DAY HAZARDS INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDICES ARE AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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