622  
FXUS02 KWBC 131959  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
358 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 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 SEVERE WEATHER  
POTENTIAL 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 (UP TO MINUS 10-20F ANOMALIES) OVER  
CALIFORNIA/WESTERN NEVADA ON DAY 3 THU WILL SPREAD NORTHEAST AND  
BY SAT-MON COVER AN AREA FROM THE WEST COAST INTO THE NORTHERN  
PLAINS. CLOUDS AND PRECIP SHOULD KEEP MORNING LOWS FROM BEING AS  
EXTREME. 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. BY SAT-MON TEMPERATURES OVER PARTS OF THE EAST ARE VERY  
UNCERTAIN GIVEN A WIDE RANGE OF POSSIBILITIES FOR FRONTAL POSITION  
AND STRENGTH. ENSEMBLE SPREAD FOR MAX TEMPERATURES AT SOME  
LOCATIONS ALONG THE NORTHERN/CENTRAL EAST COAST REACHES 30F OR SO  
BY THE START OF NEXT WEEK.  
 
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  
 
HAZARDS:  
- HEAVY PRECIPITATION ACROSS PORTIONS OF THE CENTRAL GREAT BASIN,  
CALIFORNIA, AND THE SOUTHWEST, THU-FRI, MAY 16-MAY 17AND SUN-MON,  
MAY 19-MAY 20.  
- HEAVY PRECIPITATION ACROSS PORTIONS OF THE NORTHERN PLAINS, THE  
NORTHERN ROCKIES, AND THE NORTHERN GREAT BASIN, THU-SAT, MAY  
16-MAY 18AND MON, MAY 20.  
- HEAVY RAIN ACROSS PORTIONS OF CALIFORNIA AND THE PACIFIC  
NORTHWEST, THU-FRI, MAY 16-MAY 17AND SUN-MON, MAY 19-MAY 20.  
- HEAVY RAIN ACROSS PORTIONS OF THE CENTRAL PLAINS, THE MIDDLE  
MISSISSIPPI VALLEY, THE LOWER MISSISSIPPI VALLEY, AND THE SOUTHERN  
PLAINS, SAT-MON, MAY 18-MAY 20.  
- HEAVY RAIN ACROSS PORTIONS OF THE CENTRAL PLAINS, THE UPPER  
MISSISSIPPI VALLEY, AND THE NORTHERN PLAINS, SAT-SUN, MAY 18-MAY  
19.  
- SEVERE WEATHER ACROSS PORTIONS OF THE CENTRAL PLAINS, THE  
SOUTHERN PLAINS, AND THE NORTHERN PLAINS, FRI, MAY 17.  
- SEVERE WEATHER ACROSS PORTIONS OF THE CENTRAL PLAINS AND THE  
SOUTHERN PLAINS, SAT, MAY 18.  
- FLOODING POSSIBLE ACROSS PORTIONS OF THE SOUTHEAST, THE LOWER  
MISSISSIPPI VALLEY, THE SOUTHERN APPALACHIANS, AND THE TENNESSEE  
VALLEY.  
- FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE CENTRAL  
PLAINS, THE LOWER MISSISSIPPI VALLEY, THE NORTHERN PLAINS, THE  
GREAT LAKES, THE MIDDLE MISSISSIPPI VALLEY, THE UPPER MISSISSIPPI  
VALLEY, THE SOUTHERN PLAINS, AND THE OHIO VALLEY.  
- FLOODING LIKELY ACROSS PORTIONS OF THE NORTHERN PLAINS, THE  
NORTHERN ROCKIES, AND THE NORTHERN GREAT BASIN.  
- MUCH ABOVE NORMAL TEMPERATURES ACROSS PORTIONS OF THE CENTRAL  
PLAINS AND THE MIDDLE MISSISSIPPI VALLEY, THU-FRI, MAY 16-MAY 17.  
 
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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