257  
FXUS02 KWNH 030716  
PREEPD  
 
PRELIMINARY EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
315 AM EDT THU MAY 3 2018  
 
VALID 12Z SUN MAY 6 2018 - 12Z THU MAY 10 2018  
   
..PATTERN OVERVIEW
 
 
THE FORECAST PERIOD BEGINS ON SUNDAY WITH A BROAD UPPER LEVEL  
TROUGH OVER EASTERN NORTH AMERICA, A RIDGE OVER THE INTERMOUNTAIN  
WEST, AND A SMALLER SCALE TROUGH NEAR THE WEST COAST. THE FLOW  
PATTERN BECOMES QUASI-ZONAL ACROSS MOST OF THE CONTINENTAL U.S.  
GOING INTO THE FIRST HALF OF NEXT WEEK AS THE EASTERN U.S. TROUGH  
LIFTS OUT SOME AND THE WESTERN U.S. RIDGE BECOMES LESS AMPLIFIED  
WITH TIME. THE FASTER FLOW ALOFT SHOULD BE MOSTLY RELEGATED TO  
CANADA AS THE POLAR JET STREAM LIFTS NORTH. BY THE END OF THE  
FORECAST PERIOD WEDNESDAY NIGHT, A STRONGER TROUGH IS LIKELY TO  
AFFECT THE WESTERN U.S.  
 
   
..GUIDANCE AND UNCERTAINTY ASSESSMENT
 
 
MODEL GUIDANCE IS IN RELATIVELY DECENT AGREEMENT AT THE BEGINNING  
OF THE FORECAST PERIOD. HOWEVER, GIVEN THE LOWER AMPLITUDE FLOW  
ALOFT, THE TIMING AND MAGNITUDE OF INDIVIDUAL SHORTWAVE  
DISTURBANCES AND SURFACE LOWS BECOMES LESS CERTAIN GOING INTO  
EARLY NEXT WEEK. THE CMC BECOMES MORE AMPLIFIED WITH A TROUGH  
OVER THE SOUTHEAST U.S. AND OVER THE NORTHERN PLAINS BY TUESDAY.  
THE ECMWF IS STRONGER WITH THE NEXT PACIFIC STORM SYSTEM REACHING  
THE WEST COAST BY THE MIDDLE OF THE WEEK, WITH THE GFS TRACKING IT  
FARTHER NORTH. THE WPC FORECAST WAS BASED MAINLY ON THE  
DETERMINISTIC GFS/ECMWF/UKMET ALONG WITH SOME PREVIOUS WPC  
CONTINUITY THROUGH TUESDAY, AND THEN INCREASING PERCENTAGES OF THE  
EC AND NAEFS MEANS FOR THE FINAL TWO DAYS.  
 
   
..SENSIBLE WEATHER
 
 
SHOWERS AND STORMS ARE LIKELY FOR PORTIONS OF THE EAST COAST FOR  
SUNDAY WITH THE FRONTAL BOUNDARY SLOWLY EXITING THE COAST. THE  
HEAVIEST RAINFALL APPEARS LIKELY OFFSHORE FOR NOW, BUT ANY TREND  
FARTHER TO THE WEST WILL NEED TO BE MONITORED. THE OTHER AREA  
THAT WILL HAVE INCREASED CHANCES FOR RAIN AND THUNDERSTORM  
ACTIVITY WILL BE THE NORTHERN PLAINS TO THE UPPER MIDWEST TOWARDS  
THE MIDDLE OF NEXT WEEK AS A SURFACE LOW DEVELOPS AND TRACKS  
ACROSS THE REGION. ELSEWHERE, PRECIPITATION APPEARS TO BE  
ISOLATED TO SCATTERED AT BEST.  
 
IN TERMS OF TEMPERATURES, EXPECT READINGS TO GENERALLY BE ABOVE  
AVERAGE FOR THE PLAINS TO THE INTERMOUNTAIN WEST, INCLUDING THE  
DESERT SOUTHWEST WHERE TRIPLE DIGIT HIGHS ARE LIKELY TO BE  
REALIZED FOR THE LOWER ELEVATIONS. NEAR NORMAL TEMPERATURES ARE  
EXPECTED TO RETURN TO THE EAST COAST REGION AFTER SEVERAL DAYS OF  
UNSEASONABLY WARM CONDITIONS. SOME AREAS OF BELOW NORMAL  
TEMPERATURES ARE LIKELY NEAR THE GREAT LAKES OWING TO THE TROUGH  
OVER SOUTHEAST CANADA AND THE COLD LAKE TEMPERATURES.  
 
D. HAMRICK  
----------  
ASSOCIATED WPC MEDIUM RANGE FORECASTS OF 500 MB HEIGHTS, SURFACE  
SYSTEMS, WEATHER GRIDS, QUANTITATIVE PRECIPITATION FORECASTS, AND  
WINTER WEATHER OUTLOOK SNOW/SLEET PROBABILITIES CAN BE FOUND AT...  
 
HTTP://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTP://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTP://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTP://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTP://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
 

 
 
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