212  
FXUS02 KWBC 281505  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1104 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
 
 
15 UPDATE...  
 
NO SIGNIFICANT CHANGES WERE MADE TO THE FORECAST FROM OVERNIGHT.  
CONFIDENCE IS A BIT ABOVE AVERAGE WITH RESPECT TO THE LARGE SCALE  
FLOW PATTERN ACROSS NORTH AMERICA THROUGH THE MEDIUM RANGE. REX  
BLOCK OVER THE CENTRAL PACIFIC IS FORECAST TO BECOME BETTER  
ESTABLISHED BY LATE NEXT WEEK. TELECONNECTIONS ASSOCIATED WITH THE  
POSITIVE HEIGHT ANOMALIES OVER THE ALEUTIANS AND THE NEGATIVE  
HEIGHT ANOMALIES OVER THE CENTRAL PACIFIC BOTH FAVOR TROUGHING  
DIGGING INTO THE PACIFIC NORTHWEST WITH ZONAL TO SLIGHTLY  
ANTICYCLONIC FLOW ALONG THE EASTERN SEABOARD. THESE ANOMALY  
CENTERS HAVE A FAIRLY STRONG CORRELATION TO ARCTIC AIR SPILLING  
SOUTH ALONG THE EASTERN SLOPES OF THE NORTHERN ROCKIES AS WELL AS  
THE NORTHERN PLAINS, AND PERHAPS CROSSING THE CONTINENTAL DIVIDE  
AND REACHING THE NORTHWESTERN STATES. A CLIPPER-TYPE SYSTEM MOVING  
FROM BRITISH COLUMBIA/ALBERTA INTO THE MIDWEST AND GREAT LAKES  
DAYS 3-5 WILL BE THE LEADING EDGE OF COLDER AIR. A BLEND LARGELY  
BASED ON THE 00Z ECMWF/GFS SERVED AS A STARTING POINT FOR THE  
FORECAST FOR DAYS 3-4 (TUE-WED). AFTER THAT TIME, 00Z ECENS/NAEFS  
ENSEMBLE WEIGHTING WAS BOOSTED, FAIRLY SUBSTANTIALLY BY DAYS 6-7  
(FRI-SAT). GREATEST AREA OF UNCERTAINTY SURROUNDS INTENSE  
SHORTWAVE ENERGY DIGGING INTO THE PAC NW BY DAYS 6-7, AND TO WHAT  
DEGREE THIS ENERGY UNDERCUTS THE STRONG NORTH PAC RIDGE. 00Z ECMWF  
WAS THE MOST AGGRESSIVE WITH UNDERCUTTING THE RIDGE AND LOWERING  
HEIGHTS WEST INTO THE PAC, WHILE THE 00Z/06Z GFS ALONG WITH THE  
ENSEMBLE MEANS WERE LESS AGGRESSIVE. TELECONNECTIONS SUGGEST THAT  
MOST SIGNIFICANT HEIGHT FALLS SHOULD NOT PROGRESS TOO FAR WEST  
BENEATH THE RIDGE, SO THE 00Z ECMWF SOLUTION WAS DEEMPHASIZED  
QUICKER RELATIVE TO THE GFS BY DAYS 6-7.  
 
RYAN  
 
PREVIOUS DISCUSSION...  
   
..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  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page