695  
FXUS02 KWBC 012045  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
445 PM EDT WED MAY 01 2019  
 
VALID 12Z SAT MAY 04 2019 - 12Z WED MAY 08 2019  
   
..OVERVIEW
 
 
UPPER RIDGING WILL REMAIN ANCHORED OVER THE NORTHEASTERN PACIFIC  
DURING THE MEDIUM RANGE PERIOD WHICH WILL DRIVE A SPLIT FLOW  
ACROSS THE LOWER 48. A CLOSED LOW IN THE SOUTHERN STREAM WILL MOVE  
INTO CALIFORNIA SUNDAY/MONDAY WHILE NORTHERN STREAM SYSTEMS SKIRT  
ALONG THE US/CANADIAN BORDER. RIDGING OFF THE SOUTHEAST COAST WILL  
WOBBLE IN PLACE BUT LIKELY BUILD NORTHWARD TOWARD THE END OF THE  
PERIOD NEXT WED. FRONTAL BOUNDARIES IN THE EAST WILL WAVER AS  
SURFACE WAVES MOVE THROUGH.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE MODELS/ENSEMBLES HAVE COME INTO BETTER AGREEMENT OFF THE WEST  
COAST WITH THE APPROACHING UPPER LOW BUT STILL EXHIBIT SPREAD IN  
TIMING WITH THE GFS GENERALLY QUICKER THAN THE ECMWF AND THE UKMET  
IN BETWEEN. TREND OVER THE PAST 24 HOURS HAS BEEN SLOWER IN THE  
GFS AND AGAIN PREFER A SOLUTION NEAR THE SLOWER HALF OF THE  
ENSEMBLES NEAR THE 00Z ECMWF ENSEMBLE MEAN BUT NOT AS SLOW AS THE  
DETERMINISTIC ECMWF. BY NEXT TUE/WED, TROUGH AXIS IS FORECAST TO  
MOVE THROUGH THE DESERTS WITH GOOD ENSEMBLE AGREEMENT. BLEND  
BETWEEN THE 06Z GEFS MEAN AND 00Z ECMWF ENSEMBLE MEAN BY THEN WAS  
A GOOD STARTING POINT.  
 
IN THE NORTHERN STREAM, A SURFACE LOW WILL MOVE THROUGH SOUTHERN  
CANADA THIS WEEKEND WITH A COLD FRONT ACROSS THE HIGH PLAINS AND  
UPPER MIDWEST THAT WILL PUSH THROUGH THE EASTERN GREAT LAKES AND  
NORTHEAST EARLY NEXT WEEK. DIFFERENCES IN TIMING REMAIN SO A  
BLENDED SOLUTION WAS PREFERRED AGAIN TODAY. TO ITS SOUTH, A WAVE  
OR TWO OF LOW PRESSURE WILL EXIT OFF THE MID-ATLANTIC COAST LATE  
THIS WEEKEND AND DROP ITS COLD FRONT INTO THE SOUTHEAST. BY NEXT  
TUE/WED, THAT BOUNDARY WILL DISSIPATE AS UPPER RIDGING INCREASES  
AND FORCES THE THERMAL GRADIENT TO THE FRONT TO THE NORTH. TAIL  
END OF THAT BOUNDARY WILL LINGER ACROSS THE CENTRAL PLAINS INTO  
THE CORN BELT AND IS STILL UNCLEAR IF A DEFINED SURFACE LOW WILL  
EXIT THROUGH THE GREAT LAKES OR JUST REMAIN A WAVY FRONT INTO NEXT  
WEEK. FOR NOW, OPTED TO MAINTAIN CONTINUITY WITH A WAVY FRONT  
RATHER THAN DEPICT A SURFACE LOW LIKE THE ECMWF.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
FRONT DRAPED ACROSS THE SOUTHERN APPALACHIANS BACK TO THE LOWER  
MISSISSIPPI VALLEY WILL BE A FOCUS FOR LOCALLY HEAVY  
RAIN/THUNDERSTORMS IN THE WARM SECTOR WITH ANOTHER AREA OF RAIN TO  
THE NORTH OF THE LOW ACROSS THE NORTHERN MID-ATLANTIC AND SOUTHERN  
NEW ENGLAND. THIS WILL KEEP TEMPERATURES BELOW AVERAGE ON SUNDAY  
ALONG THE PA/NY LINE EASTWARD THROUGH CT/MA/RI IN THE 50S OR LOW  
60S. COOLER THAN AVERAGE TEMPERATURES WILL REMAIN IN PLACE OVER  
THE HIGH PLAINS/UPPER MIDWEST NEXT WEEK BY 5-15 DEGREES F  
(TEMPERATURES ONLY IN THE 50S).  
 
CLOSED LOW INTO CA/SOUTHWEST EARLY NEXT WEEK WILL SUPPORT  
SCATTERED SHOWERS AND SOME CONVECTION FROM INTO THE GREAT  
BASIN/ROCKIES AS THE HEIGHT FALLS MOVE EASTWARD. PRECIPITATION  
(GENERALLY ALL RAIN WITH SNOW LEVELS QUITE HIGH) WILL MOSTLY FOCUS  
ON HIGHER ELEVATIONS BUT WITH SOME RAIN INTO THE VALLEYS,  
DEPENDENT ON MESOSCALE DETAILS. MOISTURE INFLUX INTO THE REGION  
APPEARS TO BE MODEST ACROSS SOUTHERN CA/NV/AZ BUT DRIER MID-LEVELS  
MAY INHIBIT THAT POTENTIAL RAINFALL, AT LEAST AT THE ONSET. WINDS  
WILL INCREASE IN TANDEM WITH HEIGHT FALLS OVER ESPECIALLY HIGHER  
TERRAIN AS TEMPERATURES TREND COOLER THAN AVERAGE. PRECIPITATION  
MAY EXPAND OVER THE ROCKIES AROUND THE MIDDLE OF NEXT WEEK AS THE  
UPPER LOW CONTINUES EASTWARD. SOUTHWEST TO WESTERLY FLOW ALOFT  
ACROSS THE PLAINS NEXT TUE-WED WILL SUPPORT SHOWERS AND STORMS  
ALONG THE NORTHERN STATIONARY FRONT TO THE NORTH AND NEAR THE GULF  
COAST AS THE SOUTHERN STATIONARY FRONT MOVES NORTHWARD.  
 
FRACASSO  
 
HAZARDS:  
- HEAVY PRECIPITATION ACROSS PORTIONS OF THE NORTHERN PLAINS AND  
THE NORTHERN ROCKIES, MON-TUE, MAY  
6-MAY 7.  
- HEAVY RAIN ACROSS PORTIONS OF THE CENTRAL PLAINS, THE LOWER  
MISSISSIPPI VALLEY, THE GREAT LAKES,  
THE MIDDLE MISSISSIPPI VALLEY, THE UPPER MISSISSIPPI VALLEY, THE  
SOUTHERN PLAINS, AND THE OHIO  
VALLEY, MON-TUE, MAY 6-MAY 7.  
- HEAVY RAIN ACROSS PORTIONS OF THE CENTRAL PLAINS, THE LOWER  
MISSISSIPPI VALLEY, THE TENNESSEE  
VALLEY, THE CENTRAL ROCKIES, THE MIDDLE MISSISSIPPI VALLEY, THE  
SOUTHEAST, THE SOUTHERN PLAINS, AND  
THE OHIO VALLEY, WED, MAY 8.  
- HEAVY RAIN ACROSS PORTIONS OF THE MID-ATLANTIC, THE NORTHEAST,  
AND THE CENTRAL APPALACHIANS,  
SAT-SUN, MAY 4-MAY 5.  
- FLOODING POSSIBLE ACROSS PORTIONS OF THE CENTRAL PLAINS, THE  
MIDDLE MISSISSIPPI VALLEY, THE GREAT  
LAKES, THE OHIO VALLEY, AND THE UPPER MISSISSIPPI VALLEY.  
- FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE LOWER  
MISSISSIPPI VALLEY, THE NORTHERN  
PLAINS, THE GREAT LAKES, THE NORTHERN GREAT BASIN, THE MIDDLE  
MISSISSIPPI VALLEY, THE UPPER  
MISSISSIPPI VALLEY, THE SOUTHERN PLAINS, AND THE OHIO VALLEY.  
- FLOODING LIKELY ACROSS PORTIONS OF THE CENTRAL PLAINS, THE  
MIDDLE MISSISSIPPI VALLEY, THE LOWER  
MISSISSIPPI VALLEY, AND THE SOUTHERN PLAINS.  
 
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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