092  
FXUS02 KWBC 120509  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
109 AM EDT SUN MAY 12 2019  
 
VALID 12Z WED MAY 15 2019 - 12Z SUN MAY 19 2019  
 
 
TONIGHT'S FORECASTS WERE DERIVED FROM A BLEND OF THE LATEST  
DETERMINISTIC 12Z ECMWF BLENDED WITH THE 12Z ECMWF/18Z GEFS  
ENSEMBLE MEANS DAYS 3-7, WITH LESS WEIGHTING ON THE 12Z  
DETERMINISTIC CANADIAN GLOBAL MODEL.  
 
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/12Z CANADIAN GLOBAL REMAIN FARTHER NORTH  
THAN THE GFS/18Z GEFS MEAN WED. THE FORECASTS GAVE MORE WEIGHTING  
TO THE MAJORITY CLUSTER, AND LESS TO THE OPERATIONAL 18Z GFS, NOW  
THE SOUTHERN MOST SOLUTION. THIS SYSTEM MOVES ACROSS THE CANADIAN  
MARITIMES AND ADJACENT COASTAL WATERS THU.  
 
ELSEWHERE, MODELS HAVE TYPICAL TIMING/AMPLITUDE DIFFERENCES WITH  
NORTHERN STREAM ENERGY CROSSING THE MIDWEST/GREAT LAKES 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 CANADIAN/ECMWF  
MODELS AND THE 18Z GEFS/12Z ECMWF ENSEMBLE 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. THE 18Z GFS WAS  
EXCLUDED FROM THE BLEND AS THE UPSTREAM EAST PACIFIC OCEAN LOW WAS  
HALF A DAY BEHIND THE PACK OF THE OTHER MODELS/MEANS THU 16 MAY TO  
12Z FRI 17 MAY.  
 
HEIGHT FALLS CROSSING THE ROCKIES ON TO THE PLAINS BY LATE NEXT  
WEEK SHOULD INDUCE SURFACE CYCLOGENESIS ACROSS THE NORTHERN PLAINS  
FRI NIGHT-SAT, DRIFTING SLOW EAST NORTHEAST TOWARDS THE UPPER MS  
VALLEY SUN 19 MAY. GOOD CLUSTERING AMONG THE 12Z ECMWF/18Z GEFS  
MEAN/12Z ECMWF ENSEMBLE MEAN LED TO A CONSENSUS OF THESE  
FORECASTS. THE 00Z GFS CLUSTERS WELL WITH THIS GROUP OF FORECASTS  
LOW POSITIONS AS WELL.  
   
..WEATHER HIGHLIGHTS/THREATS
 
 
EXPECT SHOWERS OVER THE NEW ENGLAND MID WEEK AS LOW PRESSURE IS  
SLOW TO MOVE OFF THE NORTHEAST COAST. 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.  
 
SHOWERS/STORMS ALSO DEVELOP IN THE VICINITY OF A STATIONARY FRONT  
LINGERING OVER THE SOUTHERN 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. HIGHER ELEVATION SNOWS DEVELOP AS COLDER AIR ALOFT MOVES  
INTO THE NORTHWEST US, STARTING IN NORTHERN CA/WESTERN OR THU AND  
THEN SPREADING INTO THE HIGHER ELEVATIONS OF THE NORTHERN ROCKIES  
AND NORTHERN GREAT BASIN NEXT WEEKEND.  
 
WITH LOW PRESSURE MOVING SLOWLY EAST ACROSS THE PLAINS NEXT  
WEEKEND, RETURN MOIST SOUTHERLY FLOW PROVIDES FUEL TO ENHANCE  
SHOWER/THUNDERSTORM ACTIVITY, WITH POTENTIAL FOR LOCALLY HEAVY  
RAIN TO DEVELOP OVER PORTIONS OF CENTRAL TEXAS.  
 
TEMPERATURE-WISE, HIGHS OF 10-15 F ABOVE NORMAL ARE FORECAST OVER  
MT/WY/WESTERN DAKOTAS 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 NORTHEAST WED WILL KEEP HIGHS 10-15F OR  
SO BELOW NORMAL OVER NORTHERN NEW ENGLAND. GRADUAL MODERATION IS  
EXPECTED AS THE TROUGH MOVES OFFSHORE THU 16 MAY, WITH SLIGHTLY  
ABOVE TEMPERATURE FRI INTO THE WEEKEND. TEMPERATURES SEVERAL  
DEGREES ABOVE NORMAL DEVELOP OVER THE SOUTHEAST LATE IN THE WEEK  
AND CONTINUE THROUGH THE WEEKEND.  
 
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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