640  
FXUS02 KWBC 300623  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
223 AM EDT MON OCT 30 2017  
 
VALID 12Z THU NOV 02 2017 - 12Z MON NOV 06 2017  
 
...COLDER AND STORMY PATTERN FOR THE WEST LATER THIS WEEK INTO  
NEXT WEEK...  
 
   
..OVERVIEW, GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES
 
 
AMPLIFIED FLOW OVER THE PACIFIC OCEAN AND ALASKA INTO WESTERN  
NORTH AMERICA SUPPORTS DIGGING TROUGHING INTO THE WEST LATER THIS  
WEEK THROUGH THE WEEKEND WITH ZONAL TO SW FLOW IN THE  
CENTRAL/EASTERN STATES. THE RECENT GFS/ECMWF/CANADIAN OFFERED A  
GOOD STARTING POINT FOR THE FORECAST THU-SAT AS AN UPPER LOW DROPS  
SOUTHWARD OUT OF BRITISH COLUMBIA IN THE PAC NW. FOR SUN/MON,  
TRENDED TOWARD THE GEFS/ECMWF ENSEMBLE MEANS WITH LESS DETAIL FROM  
THE GFS/ECMWF AS THEIR REMAINS DIFFERENCES IN HOW THE TROUGHING  
EJECTS INTO THE PLAINS. TREND HAS BEEN A BIT QUICKER OVERALL WHICH  
FAVORED THE GFS OVER THE ECMWF IN PREVIOUS RUNS BUT THE ENSEMBLE  
CONSENSUS REMAINED THE BEST GUIDE.  
 
FORECAST REVOLVES AROUND TWO MAIN SYSTEMS: 1) NORTHERN STREAM LOW  
MOVING OUT OF THE GREAT LAKES THURSDAY AND DRAGGING ITS COLD FRONT  
OFF THE NORTHEAST/MID-ATLANTIC COAST FRIDAY AND 2) DIGGING UPPER  
TROUGHING ALONG THE WEST COAST WHICH WILL PUSH A STRONG COLD FRONT  
SOUTHWARD/EASTWARD THROUGH THE WEST, OUT INTO THE PLAINS SUNDAY  
AND THEN THROUGH THE GREAT LAKES NEXT MONDAY.  
 
   
..WEATHER THREATS/HIGHLIGHTS
 
 
MUCH COLDER AIR WILL SETTLE INTO MUCH OF MONTANA 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 WHEN SNOW  
LEVELS DROP TO ABOUT 2000-3000 FT IN THE PAC NW. TEMPERATURES  
ABOUT 10-20F BELOW AVERAGE ARE LIKELY FOR NORTHERN AND NW MONTANA  
WHICH WILL BLEED WESTWARD AND SOUTHWARD THROUGH OR/WA/ID AND  
CALIFORNIA THIS WEEKEND. SNOW IS LIKELY TO BE HEAVY OVER NW  
MONTANA AT TIMES AS UPSLOPE FLOW STRENGTHENS BY FRIDAY WITH A  
1036-1040MB HIGH OVER ALBERTA. PRECIPITATION WILL EXPAND THROUGH  
THE WEST/INTERMOUNTAIN WEST AS THE UPPER TROUGH AND PACIFIC FRONT  
WORK INLAND. BY NEXT WEEKEND, SEVERAL INCHES OF PRECIPITATION ARE  
LIKELY FOR MUCH OF NORTHERN CALIFORNIA INTO SW OREGON, ESPECIALLY  
OVER THE COASTAL RANGES, KLAMATH MOUNTAINS, NORTHERN SIERRAS, ETC.  
WITH SEVERAL FEET OF SNOW POSSIBLE FOR THE FAVORED HIGHER TERRAIN.  
EVEN SOUTHERN CALIFORNIA COULD SEE SOME PRECIPITATION NEXT WEEKEND  
DEPENDING ON THE DEPTH OF THE TROUGH AND INFLUX OF MOISTURE.  
 
BACK TO THE EAST, A WARM FRONT WILL MOVE THROUGH THE NORTHEAST ON  
THURSDAY WITH AN AXIS OF LIGHT TO MODERATE RAIN ALONG/AHEAD OF THE  
COLD FRONT FROM E TEXAS NORTHEASTWARD INTO THE EASTERN GREAT  
LAKES. SOUTHERN PART OF THE FRONT WILL LINGER THROUGH THE  
MID-MISSISSIPPI VALLEY/TENNESSEE VALLEY/CAROLINAS WITH SCATTERED  
SHOWERS ALONG ITS BOUNDARY FRI-SAT BEFORE LIFTING BACK TO THE  
NORTH AS THE WESTERN SYSTEM MOVES THROUGH THE HIGH PLAINS SUNDAY.  
 
FRACASSO  
 

 
 
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