513  
FXUS02 KWBC 111545  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1144 AM EDT SAT MAY 11 2019  
 
VALID 12Z TUE MAY 14 2019 - 12Z SAT MAY 18 2019  
 
15 UTC UPDATE...  
 
A BLEND OF THE 00Z ECMWF/CMC AND 06Z GFS WAS USED AS BASIS DURING  
DAYS 3-5 (TUE-THU) FOR THIS FORECAST UPDATE, WITH PARTICULAR  
WEIGHT PLACED TOWARD THE ECMWF/CMC SOLUTIONS. MODELS CONTINUE TO  
SLOWLY CONVERGE ON THE EVENTUAL TRACK OF A SURFACE LOW ALONG/OFF  
THE NORTHEAST U.S. COASTLINE TUE-WED, WHICH SHOWS SOME POTENTIAL  
TO PRODUCE ACCUMULATING SNOWS ACROSS THE HIGHER ELEVATIONS OF  
INTERIOR NORTHERN NEW ENGLAND. THE ECMWF/CMC WERE MORE CLOSELY  
CLUSTERED WITH CONSENSUS ON THE TRACK OF THIS SYSTEM WHILE THE GFS  
WAS A BIT FARTHER SOUTH. SOLUTIONS HANDLE WEAKENING SOUTHERN  
STREAM SHORTWAVE ENERGY CROSSING THE SOUTHERN PLAINS TUE-WED  
SIMILARLY, WITH DIFFERENCES MAINLY CONFINED TO THE AMPLITUDE OF  
THE FEATURE. MODELS SHOW REASONABLY GOOD CONSENSUS ON AN AMPLIFIED  
UPPER TROUGH REACHING THE WEST COAST WED-THU (A LEADING SHORTWAVE  
WED FOLLOWED BY STRONGER HEIGHT FALLS THU). SOLUTIONS DIFFER ON  
THE SPECIFICS OF EMBEDDED SMALLER-SCALE FEATURES WITHIN THE LARGER  
TROUGH, FAVORING GRADUALLY INCREASING ENSEMBLE WEIGHTING. STRONGER  
HEIGHT FALLS CROSSING THE ROCKIES BY LATE NEXT WEEK SHOULD INDUCE  
SURFACE CYCLOGENESIS ACROSS THE HIGH PLAINS BY FRI NIGHT-SAT.  
LARGER DIFFERENCES BEGIN TO EMERGE FROM THU ONWARD WITH THE  
EVOLUTION/TIMING OF A FEW NORTHERN STREAM SHORTWAVES. THE CHAOTIC  
AND SOMEWHAT BLOCKED UPPER-LEVEL FLOW ACROSS CANADA REDUCES  
PREDICTABILITY FOR SMALLER SCALE FEATURES. THUS, TRENDED THE  
FORECAST TOWARD ECENS/NAEFS ENSEMBLE MEANS DURING DAYS 6-7  
(FRI-SAT).  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 0525 UTC)...  
 
TONIGHT'S FORECASTS WERE DERIVED FROM A BLEND OF THE LATEST  
DETERMINISTIC 12Z GFS /12Z ECMWF BLENDED WITH THE 12Z ECMWF/18Z  
GEFS ENSEMBLE MEANS DAYS 3-7.  
MODELS ARE SLOWLY CONVERGING ON THE LATITUDE OF THE LOW DEVELOPING  
OFF THE NEW ENGLAND COAST MON NIGHT-TUE. THE 12Z ECMWF/ECMWF  
ENSEMBLE MEAN/12Z UKMET REMAIN FARTHER NORTH THAN THE GFS/18Z GEFS  
MEAN/12Z CANADIAN. THE FORECASTS USED AN INTERMEDIATE SOLUTION TO  
PROVIDE CONTINUITY AND A CONSENSUS BASED SOLUTION.  
 
THE MODELS SHOW AN UPPER LEVEL LOW NEAR THE AZ/NM BORDER WITH  
MEXICO 12Z MON THAT SLOWLY MOVES EAST AND SHEARS INTO AN UPPER  
TROUGH AS IT PROGRESSES ACROSS THE SOUTHERN PLAINS/LOWER  
MISSISSIPPI VALLEY THROUGH EARLY NEXT WEEK. WELL CLUSTERED  
FORECASTS LEND THE SITUATION TO A CONSENSUS-BASED FORECAST.  
 
ELSEWHERE, MODELS HAVE TYPICAL TIMING/AMPLITUDE DIFFERENCES WITH  
NORTHERN STREAM ENERGY DIVING FROM THE UPPER MS VALLEY INTO THE  
MIDWEST/GREAT LAKES TUE NIGHT TO WED AND CROSSING THE CENTRAL  
APPALACHIANS/MID ATLANTIC/COASTAL WATERS THU, WITH POTENTIAL LOW  
PRESSURE DEVELOPMENT OFF THE COAST FRI 17 MAY. THERE REMAINS A  
LARGE LATITUDINAL SPREAD AS TO WHERE LOW PRESSURE DEVELOPS ON THE  
SURFACE FRONT. A BLEND OF THE 12Z GFS/ECMWF MODELS AND THEIR MEANS  
WAS USED UNTIL BETTER AGREEMENT DEVELOPS.  
 
AGREEMENT AMONGST THE MODELS/ENSEMBLES SUPPORTS AN AMPLIFIED UPPER  
TROUGH REACHING THE WEST COAST BY WED 15 MAY, WITH A SECONDARY  
TROUGH REACHING THE WEST COAST THU 16 MAY, WHICH THEN MOVES INLAND  
INTO THE GREAT BASIN AND TOWARDS THE PACIFIC NORTHWEST COAST FRI  
17 MAY. MODELS DIFFER ON THE EXACT TIMING AND PHASING OF THE  
TROUGH, WITH THE ECMWF/ECMWF ENSEMBLE MEAN FASTER TAKING THE UPPER  
TROUGH ACROSS THE ROCKIES ON TO THE HIGH PLAINS. BLENDING WITH THE  
SLOWER TIMING OF THE GEFS WAS USED FOLLOWING HISTORICALLY SLOWER  
MOTION OF HIGHER AMPLITUDE UPPER TROUGHS.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
EXPECT SHOWERS OVER THE NEW ENGLAND EARLY IN THE WEEK AS  
CONSOLIDATION OF LOW PRESSURE OFF THE NORTHEASTERN COAST OCCURS,  
WITH THE SYSTEM MOVING SLOWLY. SOME OF THE PRECIPITATION COULD  
FALL IN THE FORM OF SNOW OVER HIGHEST ELEVATIONS OF NORTHERN NEW  
HAMPSHIRE AND WESTERN MAINE AS THE CORE OF THE UPPER TROUGH PASSES  
THROUGH.  
 
A COLD FRONT AND FRONTAL WAVE MOVING EAST ACROSS THE UPPER MS  
VALLEY, UPPER MIDWEST/GREAT LAKES/OHIO VALLEY SHOULD PRODUCE  
SHOWERS DURING THE EARLY TO MIDDLE PORTIONS OF NEXT WEEK BEFORE  
THE SYSTEM MOVES INTO THE NORTHEAST LATER IN THE WEEK.  
 
THE UPPER LOW TRACKING FROM THE WESTERN U.S.-MEXICO BORDER INTO  
THE SOUTHERN PLAINS WILL BE RESPONSIBLE FOR LOCALLY ENHANCED AREAS  
OF SHOWERS/STORMS OVER THE SOUTHERN PLAINS AND WESTERN GULF COAST  
IN THE EARLY TO MIDDLE PORTIONS OF NEXT WEEK. SHOWERS/STORM ALSO  
DEVELOP IN THE VICINITY OF A STATIONARY FRONT LINGERING OVER THE  
FL PENINSULA IN THE MIDDLE TO LATTER PORTIONS OF NEXT WEEK.  
 
THE UPPER TROUGH NEARING THE WEST COAST SHOULD BRING INCREASED  
CLOUDS/PRECIPITATION TO THE PACIFIC NORTHWEST AND NORTHERN CA WED  
15 MAY, CONTINUING INLAND INTO THE GREAT BASIN AND NORTHERN  
ROCKIES THU 16 MAY, AND THEN THE PLAINS ON FRI 17 MAY AND SAT 18  
MAY.  
 
SHORT-RANGE WARMTH OVER THE NORTHWEST WILL CONTINUE INTO THE FIRST  
HALF OF NEXT WEEK,  
WITH HIGHS OF 10-15F ABOVE NORMAL OVER THE INTERIOR NORTHWEST AND  
NORTHERN ROCKIES TUE, CONTINUING IN MT/WY WED. THE WARM CORE  
MOVES EAST INTO THE NORTHERN TO CENTRAL PLAINS THU WITH COOLING  
OVER THE NORTHWEST. WITH THE APPROACHING TROUGH, CA/OR/WA WILL  
SEE A REVERSAL TO COOLER THAN NORMAL TEMPERATURES IN THE MIDDLE TO  
LATTER PORTIONS OF NEXT WEEK.  
 
THE SYSTEM AFFECTING THE MID ATLANTIC, GREAT LAKES, AND NORTHEAST  
TUE WILL KEEP HIGHS 10-15F OR SO BELOW NORMAL, CONTINUING ON WED  
IN NORTHERN MAINE. GRADUAL MODERATION IS EXPECTED AS THE TROUGH  
MOVES OFFSHORE THU 16 MAY.  
 
PETERSEN  
 
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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