735  
FXUS02 KWBC 080700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
300 AM EDT WED MAY 08 2019  
 
VALID 12Z SAT MAY 11 2019 - 12Z WED MAY 15 2019  
   
..OVERVIEW
 
 
EXPECT MEAN TROUGHING ALOFT TO SETTLE OVER EASTERN NORTH AMERICA,  
CONSISTING OF A LEAD AMPLIFYING TROUGH REACHING THE  
PLAINS/MISSISSIPPI VALLEY DURING THE WEEKEND AND CONTINUING  
EAST/NORTHEAST THEREAFTER FOLLOWED BY A MORE SHALLOW TROUGH WHOSE  
CORE SHOULD TRACK NORTH OF THE GREAT LAKES BY THE MIDDLE OF NEXT  
WEEK. THE FIRST TROUGH WILL SPREAD MOISTURE NORTHEASTWARD OVER  
THE EASTERN U.S. WHILE GIVING EASTERN TEXAS AND THE LOWER  
MISSISSIPPI VALLEY AT LEAST A BRIEF REPRIEVE FROM A HEAVY RAINFALL  
THREAT THAT SHOULD EXTEND THROUGH SAT. MEANWHILE THE PATTERN OVER  
THE WEST TAKES ON A REX BLOCK APPEARANCE DURING THE WEEKEND/EARLY  
NEXT WEEK AS AN UPPER RIDGE DRIFTS OVER THE NORTHERN HALF OF THE  
REGION AND AN UPPER LOW SLOWLY TRACKS OVER/NEAR THE DESERT  
SOUTHWEST AND SOUTHERN ROCKIES. THE UPPER LOW WILL PROMOTE  
MULTIPLE DAYS OF PRECIPITATION OVER SOUTHERN PARTS OF THE WEST  
WITH ACTIVITY EVENTUALLY EXTENDING INTO THE SOUTHERN PLAINS AND  
VICINITY AS THE FEATURE APPROACHES AND REACHES THE REGION.  
FARTHER WESTWARD THE MODELS AND ENSEMBLE MEANS CONTINUE TO SHOW A  
TROUGH AMPLIFYING OFF THE WEST COAST DURING MON-WED.  
 
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
FOR THE FULL FORECAST DOMAIN A BLEND OF MOSTLY OPERATIONAL  
GUIDANCE EARLY FOLLOWED BY A TREND TO 60 PERCENT 18Z GEFS/12Z  
ECMWF MEANS BY DAY 7 WED PROVIDED THE BEST STARTING POINT TO  
REFLECT THE MOST AGREEABLE ASPECTS OF THE FORECAST WHILE  
ACCOUNTING FOR OTHER CASES WHERE SPREAD STILL EXITS. THIS  
APPROACH YIELDED A SOLUTION THAT MAINTAINED OR ONLY MODESTLY  
REFINED CONTINUITY.  
 
FOR THE EVOLUTION OVER THE EASTERN U.S. FROM SAT ONWARD, GUIDANCE  
SEEMS TO BE MAKING ONLY A GRADUAL ATTEMPT TO CONVERGE TOWARD AN  
INTERMEDIATE SOLUTION FOR THE SYSTEM APPROACHING/REACHING THE  
GREAT LAKES DURING THE WEEKEND. THERE ARE STILL MEANINGFUL  
MEDIUM-SMALLER SCALE DETAIL DIFFERENCES ALOFT THAT TEND TO HAVE  
FAIRLY LOW PREDICTABILITY SEVERAL DAYS OUT IN TIME. THE GFS IS  
STILL ON THE STRONGER SIDE OF THE SPREAD BUT ASIDE FROM THE 00Z  
RUN (WHICH HAS AN ODD EVOLUTION ALOFT SUN-MON) IT HAS SHOWN SOME  
TRENDS TOWARD WEAKENING OVER THE LAKES WITH GREATER EMPHASIS ON  
LOW PRESSURE TO THE EAST.  
 
REGARDING THE OVERALL EASTERN TROUGH ALOFT AND THE LEADING LOW  
PRESSURE THAT EVENTUALLY TRACKS OFF THE NORTHEASTERN COAST, THE  
ECMWF/CMC HAVE BEEN THE MOST AMPLIFIED ALOFT WITH SURFACE LOW  
PRESSURE GENERALLY ON THE NORTHWESTERN SIDE OF THE FULL SPREAD  
(SIMILARLY FOR THE EXTENT OF ASSOCIATED MOISTURE). THUS FAR AN  
INTERMEDIATE SOLUTION APPEARS BEST AS TRENDS HAVE NOT YET  
DISPLAYED A PRONOUNCED SHIFT TO EITHER SIDE OF THE ENVELOPE.  
 
FARTHER UPSTREAM GUIDANCE HAS BECOME BETTER DEFINED OVER THE PAST  
COUPLE DAYS FOR A FRONT THAT SHOULD REACH THE NORTHERN PLAINS BY  
DAY 5 MON AND CONTINUE SOUTH/EAST THEREAFTER. FOR THE TIME FRAME  
INVOLVED THERE ARE TYPICAL DIFFERENCES IN SOUTHWARD EXTENT.  
 
CONSENSUS HAS HELD UP FAIRLY WELL FOR THE UPPER LOW TRACKING ALONG  
THE WESTERN U.S.-MEXICO BORDER AND THEN IN WEAKER FORM OVER THE  
SOUTHERN ROCKIES/PLAINS. MULTI-DAY HISTORY HAS GENERALLY FAVORED  
SOLUTIONS IN THE MIDDLE TO SLOWER HALF OF THE SPECTRUM.  
 
FOR THE TROUGH AMPLIFYING OFF THE WEST COAST THE PRIMARY  
CONSIDERATION WAS EXCLUSION OF THE CMC WHOSE LATEST RUNS ARE  
QUESTIONABLY FAST TO BRING HEIGHT FALLS INTO THE NORTHWEST. BY  
DAY 7 WED SOME GFS RUNS BECOME SOMEWHAT FASTER THAN CONSENSUS WITH  
AT LEAST A PORTION OF THE TROUGH AS WELL. BEYOND THAT, INDIVIDUAL  
MODELS/ENSEMBLE MEMBERS STILL SUGGEST CONSIDERABLE UNCERTAINTY  
OVER HOW ENERGY WILL BE DISTRIBUTED WITHIN THE TROUGH--FAVORING AN  
ENSEMBLE MEAN/BLEND APPROACH.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE COMBINATION OF WAVINESS ALONG A STALLED FRONT OVER THE WESTERN  
GULF COAST/LOWER MISSISSIPPI VALLEY, LOW LEVEL GULF INFLOW, AND  
HEIGHT FALLS ALOFT WILL EXTEND THE HEAVY RAINFALL THREAT OVER THE  
REGION INTO SAT. THEN NORTHEASTWARD PROGRESSION OF LOW PRESSURE  
SHOULD BRING DRIER WEATHER FOR A TIME. CONFIDENCE REMAINS LOWER  
THAN DESIRED FOR THE AXIS OF HEAVIEST RAINFALL OVER THE EAST FROM  
SAT NIGHT ONWARD--AS FAR NORTHWEST AS THE OHIO VALLEY/NORTHEAST OR  
AS FAR SOUTH AS THE TENNESSEE VALLEY/MID-ATLANTIC. DEPENDING ON  
EXACT EVOLUTION AT THE SURFACE AND ALOFT OVER/OFF THE NORTHEAST,  
HIGHEST TERRAIN OF NEW ENGLAND COULD SEE A LITTLE SNOW FROM THIS  
SYSTEM. FARTHER WEST, EXPECT A WEAKENING SYSTEM TO BRING MOSTLY  
LIGHT AMOUNTS FROM THE NORTHERN PLAINS INTO GREAT LAKES DURING THE  
WEEKEND. THE NEXT FRONT DROPPING SOUTHEAST FROM THE NORTHERN  
PLAINS MAY BE ACCOMPANIED BY SOME RAINFALL BY NEXT TUE-WED.  
 
THE UPPER LOW AFFECTING THE SOUTHERN PARTS OF THE WEST WILL BRING  
AREAS OF TERRAIN-ENHANCED PRECIPITATION (MOSTLY RAIN) FROM THE  
WEEKEND INTO EARLY NEXT WEEK. EXPECT RAINFALL TO INCREASE OVER  
THE SOUTHERN PLAINS MON-WED AS THE FEATURE APPROACHES AND REACHES  
THE AREA. HOWEVER THERE IS CURRENTLY POOR AGREEMENT REGARDING THE  
COVERAGE/INTENSITY OF POTENTIAL SHOWERS AND THUNDERSTORMS.  
 
OTHER PORTIONS OF THE WEST MAY SEE SCATTERED RAINFALL AT TIMES,  
AND THEN CLOUDINESS/MOISTURE SHOULD INCREASE ALONG THE WEST COAST  
STATES TOWARD TUE-WED AS THE UPPER TROUGH APPROACHES. LOW  
CONFIDENCE DETAILS OF THE TROUGH WILL DETERMINE THE  
COVERAGE/INTENSITY OF ANY ASSOCIATED PRECIPITATION.  
 
VERY WARM TEMPERATURES WILL CONTINUE OVER THE NORTHWEST THROUGH  
THE WEEKEND WITH BROAD COVERAGE OF PLUS 10-20F ANOMALIES (POSSIBLY  
LOCALLY HIGHER ON SAT). THERE MAY BE BETTER POTENTIAL FOR RECORD  
HIGHS/WARM LOWS IN THE SHORT RANGE PERIOD THROUGH FRI BUT SOME  
POTENTIAL MAY EXTEND INTO THE WEEKEND. THE APPROACHING UPPER  
TROUGH WILL LEAD TO A GRADUAL MODERATING TREND NEXT WEEK. ON THE  
OTHER HAND THE SYSTEM AFFECTING THE SOUTHERN PARTS OF THE WEST  
WILL KEEP THE SOUTHWEST INTO SOUTHERN PLAINS BELOW NORMAL FOR  
HIGHS WITH SOME MINUS 10-20F ANOMALIES. EASTERN U.S. TEMPERATURES  
WILL VARY WITH SYSTEM PROGRESSION.  
 
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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