085  
FXUS02 KWBC 280624  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
223 AM EDT SAT OCT 28 2017  
 
VALID 12Z TUE OCT 31 2017 - 12Z SAT NOV 04 2017  
   
..COLDER AND SNOWIER FOR THE NORTHWEST NEXT WEEK
 
 
   
..OVERVIEW
 
 
MAIN DRIVER TO THE CONUS PATTERN WILL BE IMPRESSIVELY STRONG UPPER  
RIDGING OVER WESTERN ALASKA WHICH FAVORS TROUGHING DIGGING  
SOUTHWARD INTO THE PACIFIC NORTHWEST AND RATHER ZONAL FLOW ACROSS  
THE CENTRAL/EASTERN STATES. A STORM SYSTEM WILL TREK ACROSS THE  
US/CANADIAN BORDER WED/THU AND DRAG ITS COLD FRONT OFF THE  
NORTHEAST AND MID-ATLANTIC COASTS FRIDAY. TAIL END OF THIS FRONT  
WILL TRY TO OOZE OVER THE DIVIDE AS ARCTIC HIGH PRESSURE SLIDES  
SOUTHEASTWARD OUT OF FAR NW CANADA THROUGH ALBERTA/SASKATCHEWAN.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES
 
 
12/18Z GFS AND 12Z ECMWF OFFERED GOOD CLUSTERING NEAR THEIR  
ENSEMBLE MEANS THROUGH THE ENTIRE PERIOD OVER MOST OF THE CONUS. A  
BLEND OF THE DETERMINISTIC MODELS EARLY YIELDED TO ABOUT A 60/40  
SPLIT BY NEXT FRI/SAT BETWEEN THE ENSEMBLES AND GFS/ECMWF,  
RESPECTIVELY. ONE AREA OF LESS CONFIDENCE INCLUDED OFF THE WEST  
COAST WHERE AN UPPER LOW WILL PAUSE AROUND 32N/124W BEFORE SLIDING  
EASTWARD AND WEAKENING. THE ECMWF WAS SLOWER WITH THIS PROGRESSION  
THAN THE 12Z GFS/UKMET/CANADIAN WITH THE ECMWF ENSEMBLE MEAN  
MOSTLY IN BETWEEN. OPTED TO RELY ON THE ECMWF EPS AS A MIDDLE  
GROUND SOLUTION. THOUGH THE ENSEMBLES WERE RATHER ALIGNED OFF THE  
WEST COAST WITH THE TROUGH AXIS, THE STRENGTH OF THIS LIKELY UPPER  
LOW IS UNCLEAR. STRENGTH OF THE UPSTREAM UPPER RIDGE OVER ALASKA  
(+3 TO +3.5 STND DEV 500MB HEIGHTS EVEN IN THE ENSEMBLE MEANS)  
SUGGESTS THE POTENTIAL FOR A DEEP SYSTEM NOT UNLIKE THE  
GFS/CANADIAN/ECMWF.  
 
   
..WEATHER THREATS/HIGHLIGHTS
 
 
UPPER PATTERN WILL BRING MUCH COLDER AIR TO THE NORTHERN ROCKIES  
AND EVENTUALLY WA/OR AS THE COLD FRONT SLOWLY SPILLS OVER THE  
DIVIDE.RAIN AND SNOW WILL BE ON THE INCREASE ESPECIALLY BY NEXT  
WEEKEND. TEMPERATURES WILL TREND COLDER, LIKELY 10-20F BELOW  
AVERAGE FOR NORTHERN MONTANA. BACK TO THE EAST, WEAKENING COLD  
FRONTAL BOUNDARY ON TUESDAY COMBINED WITH AN UPPER LEVEL SHORTWAVE  
WILL ACT TO EXPAND SHOWERS INTO/THROUGH THE LOWER/MID-MS VALLEY  
INTO THE OHIO VALLEY AHEAD OF THE NEXT FRONT. THIS WILL PUSH  
EASTWARD FRI/SAT INTO THE I-95 CORRIDOR.  
 
FRACASSO  
 

 
 
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