586  
FXUS02 KWBC 130556  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
156 AM EDT MON MAY 13 2019  
 
VALID 12Z THU MAY 16 2019 - 12Z MON MAY 20 2019  
 
 
TONIGHT'S FORECASTS WERE DERIVED FROM A BLEND OF THE LATEST 12Z  
ECMWF/18Z GEFS ENSEMBLE MEANS WITH LESS WEIGHT ON THE OPERATIONAL  
ECMWF AND 12Z CANADIAN GLOBAL DAYS 3-7. THE GFS HAS MADE  
SIGNIFICANT RUN TO RUN CHANGES 12-18Z WITH THE MIDWEST CYCLONE AND  
ALSO THE EAST PACIFIC CYCLONE, A PATTERN WHICH HAS CONTINUED FOR  
THE 00Z RUN. THIS MAKES IT DIFFICULT TO HAVE CONFIDENCE IN ANY  
ONE RUN OF THE GFS.  
 
MODELS HAVE TYPICAL TIMING/AMPLITUDE DIFFERENCES WITH NORTHERN  
STREAM ENERGY CROSSING THE CENTRAL APPALACHIANS/MID  
ATLANTIC/COASTAL WATERS THU, WITH POTENTIAL LOW PRESSURE  
DEVELOPMENT OFF THE COAST FRI 17 MAY. THERE REMAINS LATITUDINAL  
SPREAD AS TO WHERE LOW PRESSURE DEVELOPS ON THE SURFACE FRONT,  
ALONG WITH TYPICAL TIMING DIFFERENCES, AS THE ENSEMBLE MEANS ARE A  
FEW HOURS SLOWER WITH THE FRONTAL PROGRESSION ACROSS MAINE. A  
BLEND OF THE 18Z GEFS/12Z ECMWF ENSEMBLE MEANS WITH LESS WEIGHT ON  
THE 12Z CANADIAN/ECMWF MODELS WAS USED UNTIL BETTER AGREEMENT  
DEVELOPS.  
 
AGREEMENT AMONGST THE MODELS/ENSEMBLES SUPPORTS AN AMPLIFIED UPPER  
TROUGH REACHING THE WEST COAST THU 16 MAY, THE SOUTHERN STREAM  
PORTION OF WHICH THEN MOVES INLAND INTO THE GREAT BASIN FRI 17 MAY  
AND THEN EJECTS ON TO THE HIGH PLAINS SAT 18 MAY. MODELS DIFFER ON  
THE EXACT TIMING AND PHASING OF THE TROUGH. BIG DIFFERENCES  
EXIST ON THE INTENSITY OF THE POSSIBLE EMBEDDED CLOSED LOW SAT 18  
MAY, AS THE GFS/ECMWF HAVE SHOWN CONSIDERABLE RUN TO RUN CHANGES.  
AS A A RESULT, GREATER WEIGHTING IN THE FORECASTS WERE GIVEN TO  
THE ENSEMBLE MEANS, INCLUDING THE 12Z NAEFS MEAN.  
 
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 SLOWLY EAST NORTHEAST TOWARDS THE UPPER MS  
VALLEY SUN 19 MAY. GOOD CLUSTERING AMONG THE 12Z ECMWF/18Z GEFS  
MEAN/12Z ECMWF ENSEMBLE MEAN/12Z CANADIAN GLOBAL LED TO A  
CONSENSUS OF THESE FORECASTS. THE 00Z GFS TRENDED FASTER AND MOVED  
THE LOW WELL UP INTO CANADA BY 12Z MON 20 MAY, MAKING IT APPEAR TO  
BE A LOW PROBABILITY SOLUTION THAT DIFFERS FORM THE 12-18Z RUNS,  
OTHER MODELS, AND ITS ENSEMBLE MEAN.  
 
ANOTHER LOW IS FORECAST TO DEVELOP ON THE HIGH PLAINS AS THE NEXT  
WAVE WITHIN THE UPPER TROUGH CROSSES THE GREAT BASIN TO THE CO/NM  
ROCKIES MON 20 MAY.  
   
..WEATHER HIGHLIGHTS/THREATS
 
 
A SURFACE WAVE AND COLD FRONT SHOULD PRODUCE SHOWERS OVER THE  
NORTHEAST THU BEFORE MOVING OFF THE COAST ON FRI. 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 SHOULD BRING INCREASED CLOUDS/PRECIPITATION TO  
THE PACIFIC NORTHWEST AND CA, 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 AND EASTERN TEXAS.  
 
TEMPERATURE-WISE, HIGHS/LOWS OF 10-15 F ABOVE NORMAL ARE FORECAST  
OVER THE NORTHERN TO CENTRAL PLAINS THU, PROGRESSING INTO THE MID  
MS VALLEY FRI, OHIO VALLEY SAT/SUN, AND CENTRAL APPALACHIANS NEXT  
MON 20 MAY.  
WITH THE APPROACHING TROUGH, CA/OR/WA WILL SEE A REVERSAL TO  
COOLER THAN NORMAL TEMPERATURES IN THE MIDDLE TO LATTER PORTIONS  
OF THE WEEK.  
TEMPERATURES SEVERAL DEGREES ABOVE NORMAL DEVELOP OVER THE  
SOUTHEAST LATE IN THE WEEK AND CONTINUE THROUGH THE WEEKEND AND  
NEXT MON 20 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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