330  
FXUS02 KWBC 131600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1200 PM EDT MON MAY 13 2019  
 
VALID 12Z THU MAY 16 2019 - 12Z MON MAY 20 2019  
   
..OVERVIEW
 
 
MEAN TROUGHING ALOFT WILL BRING VERY ACTIVE WEATHER TO A LARGE  
PORTION OF THE WESTERN AND CENTRAL U.S. DURING THE THU-MON PERIOD.  
ONE OR MORE PERIODS OF HEAVY PRECIPITATION (INCLUDING SOME HIGH  
ELEVATION SNOW) WILL BE POSSIBLE FROM THE WEST COAST STATES  
THROUGH THE NORTHERN 3/4 OF THE INTERIOR WEST/ROCKIES, WITH  
EPISODES OF HEAVY RAINFALL AND SEVERE WEATHER POSSIBLE OVER THE  
CENTRAL U.S. MEANWHILE GUIDANCE SPREAD YIELDS VERY LOW CONFIDENCE  
IN SPECIFICS OVER THE EAST, ASIDE FROM LIKELY EXISTENCE OF A  
STRONG FRONT THAT WILL SEPARATE CHILLY AIR TO THE NORTH FROM VERY  
WARM AIR FARTHER SOUTH.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
GUIDANCE IS STEADILY TRENDING TOWARD RESOLVING THE LEADING STRONG  
TROUGH REACHING THE WEST COAST BY DAY 3 THU AND PROGRESSING INLAND  
THEREAFTER. ECMWF/ECMWF MEAN RUNS WERE EARLIER TO CATCH ONTO THE  
SEPARATED NATURE OF THE TROUGH WITH EMBEDDED UPPER LOW DURING  
THU-FRI. BY DAY 5 SAT OPERATIONAL GFS RUNS BECOME QUESTIONABLE  
WITH THEIR SPEED OF HEIGHT FALLS CROSSING THE NORTHERN HALF OF THE  
PLAINS. THE 06Z FV3 GFS COMPARES BETTER TO REMAINING CONSENSUS.  
SOME SOLUTIONS INCLUDING THE ECMWF/UKMET/FV3 SUGGEST THAT SOUTHERN  
TROUGH ENERGY (WHETHER FROM THE INITIAL UPPER LOW OR OTHERWISE)  
COULD FORM ANOTHER UPPER LOW THAT REACHES THE PLAINS LATE SAT-SUN.  
 
IN RELATIVE TERMS THERE IS GOOD AGREEMENT THAT UPSTREAM FLOW WILL  
BRING ANOTHER ROUND OF STRONG HEIGHT FALLS TO THE WEST COAST BY  
DAYS 6-7 SUN-MON. INTERESTINGLY, HERE THE ECMWF/ECMWF MEAN HAVE  
SHOWN A BIT OF A TREND TOWARD THE GFS/GEFS MEAN THAT HAVE BEEN  
DEEPER OVER THE PAST DAY OR SO.  
 
MODELS/ENSEMBLES INCREASINGLY DIVERGE FOR EASTERN U.S. FLOW  
DETAILS, HIGHLIGHTED BY A DEEP NEW ENGLAND/MID-ATLANTIC TROUGH  
ALOFT IN THE 00Z ECMWF VERSUS A STRONG RIDGE IN GFS RUNS. THE 00Z  
ECMWF IS FAIRLY EXTREME RELATIVE TO ITS ENSEMBLES BUT NOT AN  
OUTLIER, WHILE THE PREVIOUS ECMWF RUN WAS LESS EXTREME AND CLOSER  
TO THE CURRENT ECMWF MEAN. REMAINING SOLUTIONS SHOW FLAT TO  
SLIGHTLY CYCLONIC FLOW. TELECONNECTIONS RELATIVE TO THE NEGATIVE  
HEIGHT ANOMALY CENTER ASSOCIATED WITH THE WEST COAST MEAN TROUGH  
FAVOR MODEST CYCLONIC FLOW ALOFT--BUT ONE CANNOT RULE OUT A  
TRANSIENT RIDGE THAT COULD STILL YIELD SLIGHT TROUGHING OVER A  
MULTI-DAY TIME FRAME. THUS PREFERENCES LEAN ONLY SLIGHTLY TOWARD  
THE ECMWF MEAN SCENARIO RELATIVE TO GEFS MEAN LATE IN THE PERIOD.  
 
BASED ON FORECAST CONSIDERATIONS THE UPDATED FORECAST BLEND  
INCORPORATED AN OPERATIONAL MODEL BLEND (PLUS THE FV3 GFS) EARLY  
IN THE PERIOD AND THEN RAPIDLY INCREASED 00Z ECMWF MEAN/06Z GEFS  
MEAN INPUT WITH TIME.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
OVER THE WEST EXPECT TWO PRIMARY EPISODES OF ENHANCED  
PRECIPITATION, ONE WITH A SYSTEM REACHING NEAR THE COAST AT THE  
START OF THE PERIOD THU AND THEN ANOTHER AFFECTING THE WEST COAST  
AND POINTS INLAND SAT ONWARD. GREATEST 5-DAY TOTALS SHOULD EXTEND  
FROM FAVORED TERRAIN IN NORTHERN-CENTRAL CALIFORNIA INTO THE  
NORTHERN ROCKIES--AND AT SOME LOCATIONS COULD BE QUITE HIGH FOR SO  
LATE IN THE WET SEASON. BEST POTENTIAL FOR HIGH ELEVATION SNOW  
SHOULD BE EARLY AND LATE PERIOD IN THE SIERRA NEVADA AND FRI OR  
FRI NIGHT ONWARD IN THE ROCKIES.  
 
PROGRESSION OF THE INITIAL WESTERN STORM WILL GENERATE AREAS OF  
HEAVY RAIN/THUNDERSTORMS OVER THE CENTRAL U.S. BY THE WEEKEND,  
WITH THE STORM PREDICTION CENTER ALSO INDICATING POTENTIAL FOR  
SEVERE WEATHER FOR AT LEAST A COUPLE DAYS. AHEAD OF THIS  
ACTIVITY, A WAVY FRONT DRAPED OVER THE MIDWEST COULD SERVE AS A  
FOCUS FOR LOCALLY HEAVY RAINFALL LATE THIS WEEK.  
 
OVER THE EAST, A LINGERING LOW LATITUDE FRONT MAY BRING PERIODS OF  
RAIN TO THE SOUTHERN FLORIDA PENINSULA. FARTHER NORTH ONE OR MORE  
FRONTS MAY BRING SOME RAINFALL BUT CONFIDENCE IS LOW IN SPECIFICS.  
 
AS THE WEST COAST MEAN TROUGH ALOFT BECOMES ESTABLISHED, WELL  
BELOW NORMAL HIGHS OVER CALIFORNIA/WESTERN NEVADA ON DAY 3 THU  
(MINUS 10-20F ANOMALIES) WILL SPREAD NORTHEAST WITH TIME AND BY  
SAT-MON COVER AN AREA FROM THE WEST COAST INTO THE NORTHERN  
PLAINS. CLOUDS AND PRECIP WILL KEEP MORNING LOWS CLOSER TO NORMAL  
OVER THIS AREA. LEADING WARMTH WILL PROGRESS FROM THE  
ROCKIES/PLAINS AND LIKELY MODERATE GRADUALLY AS IT REACHES THE  
EAST. SOME LOCATIONS OVER THE NORTH-CENTRAL PLAINS MAY SEE HIGHS  
10-20F ABOVE NORMAL THU-FRI. TEMPERATURE DISTRIBUTION OVER PARTS  
OF THE EAST IS VERY UNCERTAIN GIVEN A WIDE RANGE OF POSSIBLE  
FRONTAL LOCATION NEAR THE EAST COAST. ENSEMBLE SPREAD REACHES AT  
LEAST 30F FOR HIGHS AT SOME LOCATIONS, DEPENDING ON WHETHER THE  
FRONT IS TO THE NORTH OR SOUTH.  
 
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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