534  
FXUS02 KWNH 280629  
PREEPD  
 
PRELIMINARY EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
229 AM EDT SUN MAY 28 2017  
 
VALID 12Z WED MAY 31 2017 - 12Z SUN JUN 04 2017  
 
PATTERN OVERVIEW  
~~~~~~~~~~~~~~~~  
TROUGHING IS EXPECTED IN THE NORTHWEST AND NORTHEAST THIS PERIOD  
WITH RIDGING IN BETWEEN. SYSTEMS MOVING THROUGH THE  
SOUTHWEST/SOUTHERN ROCKIES SHOULD MOVE EASTWARD DUE TO A KICKER  
SHORTWAVE MOVING TOWARDS SOUTHERN CA, MAINTAINING A BREAK IN THE  
SUBTROPICAL RIDGE IN THE WESTERN GULF. THE DEGREE OF THE  
TROUGHING IS SHOWING INCREASING VARIABILITY IN BOTH THE NORTHEAST  
AND NORTHWEST. IN THE CASE OF THE NORTHEAST, THIS CASTS DOUBT ON  
MOISTURE RETURN INTO THE REGION NORTH OF THE MID-ATLANTIC STATES.  
 
MODEL PREFERENCES  
~~~~~~~~~~~~~~~~~  
THE MODEL GUIDANCE DURING THE PERIOD IS BEGINNING TO SQUABBLE ON  
DETAILS LARGE AND SMALL, SHOWING LARGER DIFFERENCES WITH EACH  
SUCCESSIVE MODEL RUN WITH THE POSITION OF THE DEEP CYCLONE IN  
SOUTHEAST CANADA AND WITH THE DEPTH OF TROUGHING MOVING INTO THE  
NORTHWEST NEXT WEEKEND. TO HELP ACCOUNT FOR UNCERTAINTY, THE  
PRESSURES/500 HPA HEIGHTS/WIND GRIDS STARTED OUT WITH AN EVEN  
SPLIT OF THE 12Z CANADIAN/12Z UKMET/12Z ECMWF/18Z GFS BEFORE USING  
INCREASING AMOUNTS OF THE 12Z ECMWF/12Z NAEFS ENSEMBLE MEANS  
(WHICH REACHES OVER 50% ON DAY 8). THE TEMPERATURES, RAIN  
CHANCES, CLOUDS, DEW POINTS, AND WEATHER GRIDS WERE HALF 12Z  
ECMWF/12Z NAEFS MEAN AND HALF 12Z CANADIAN/12Z ECMWF/18Z GFS.  
 
THE DAYS 4-7 PRECIPITATION FORECAST WAS MORE CHALLENGING THAN EVER  
DUE TO AN ECMWF WARM CORE CYCLONE DEVELOPMENT IN THE NORTHWEST  
GULF EMANATING FROM NORTHEAST MEXICO LIKELY CAUSING ISSUES ACROSS  
THE SOUTHERN PLAINS IN ITS QPF DISTRIBUTION AND THE EVER CHANGING  
GFS GUIDANCE OVER THE LOWER 48, WHICH MIGHT BE TOO WET ACROSS  
SECTIONS OF THE UPPER OHIO VALLEY AND NEW ENGLAND ON ITS 00Z RUN  
DUE TO QUESTIONABLE POSITIONING OF ITS SOUTHEAST CANADA DEEP  
CYCLONE. CHOSE TO COMPROMISE BETWEEN THE LAST TWO RUNS OF THE  
IN-HOUSE PSEUDO ENSEMBLE BIAS-CORRECTED QPF, WHICH IS A ROUGH  
COMPROMISE OF THE 18Z GFS, 00Z GFS, AND 12Z ECMWF AND SHOULD  
ACCOUNT REASONABLY FOR THE UNCERTAINTY.  
 
WEATHER HIGHLIGHTS  
~~~~~~~~~~~~~~~~~~  
COOLER THAN NORMAL TEMPERATURES ARE EXPECTED TO CONTINUE INTO NEXT  
WEEK ACROSS THE GREAT LAKES AND NORTHEAST DUE TO LINGERING UPPER  
LEVEL TROUGHING. MEANWHILE, WARM CONDITIONS ARE EXPECTED ACROSS  
MUCH OF INTERMOUNTAIN WEST...WITH AN APPROACHING COLD FRONT  
BRINGING SOME ASSOCIATED LATE-SEASON PRECIPITATION.  
 
UNSTABLE SHOWERS ARE EXPECTED ACROSS THE GREAT LAKES DUE TO COLD  
AIR ALOFT AND ACROSS FL AND THE LOWER ATLANTIC SEABOARD DUE TO  
LOW-LEVEL HEAT AND MOISTURE AVAILABILITY DURING DAYTIME HEATING  
NEAR AN AGING STATIONARY FRONT -- THE MODELS SHOW THEIR BEST  
AGREEMENT IN THESE AREAS FROM A QPF PERSPECTIVE. LOCALLY HEAVY  
RAINS ARE POSSIBLE ACROSS PORTIONS OF THE SOUTHERN PLAINS AND  
MID-MISSISSIPPI VALLEY AHEAD OF A PAIR OF UPPER LEVEL SYSTEMS  
EJECTING OUT OF THE SOUTHWEST AND SOUTHERN ROCKIES, BUT ACROSS THE  
UPPER OH VALLEY AND NORTHEAST SIGNIFICANT MODEL TO MODEL AND RUN  
TO RUN DIFFERENCES ARE CLOUDING THE PICTURE AND LOWERING  
CONFIDENCE.  
 
ROTH/PETERSEN  

 
 
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