523  
FXUS02 KWBC 290629  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
228 AM EDT SUN OCT 29 2017  
 
VALID 12Z WED NOV 01 2017 - 12Z SUN NOV 05 2017  
 
...COLDER AND SNOWIER FOR THE NORTHWEST INTO CALIFORNIA LATER THIS  
WEEK...  
 
   
..OVERVIEW, GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES
 
 
HIGHLY MERIDIONAL FLOW OVER THE PACIFIC OCEAN INTO WESTERN NORTH  
AMERICA SUPPORTS DIGGING TROUGHING INTO THE WEST LATER THIS WEEK  
WITH RATHER ZONAL FLOW IN THE CENTRAL/EASTERN STATES. THE RECENT  
18Z GFS/12Z ECMWF AND THEIR ENSEMBLE MEANS OFFERED A GOOD STARTING  
POINT FOR THE FORECAST BUT TRENDED TOWARD MORE ENSEMBLE MEAN  
INCLUSION BY THE END OF THE PERIOD. FORECAST REVOLVES AROUND TWO  
MAIN SYSTEMS: 1) NORTHERN STREAM LOW MOVING ALONG THE US/CANADIAN  
BORDER WED/THU 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 COLD FRONT SOUTHWARD AND  
THEN EASTWARD INTO OR/CA AND INTO THE INTERIOR WEST BY NEXT  
SUNDAY. FOR THE FORMER SYSTEM, DIFFERENCES IN THE LOCATION OF THE  
LOW PRESSURE AND FRONTAL BOUNDARY WERE EVIDENT RIGHT FROM THE  
START (WED) SO A BLEND OF THE GUIDANCE MAINTAINED CONTINUITY BEST  
WHILE TRENDING TOWARD THE MOST RECENT CONSENSUS. SURFACE FRONT  
WILL GET HUNG UP ALONG THE DIVIDE BUT EVENTUALLY SPILL OVER AS THE  
TROUGH DIGS INTO THE WEST. FOR THE LATTER SYSTEM, MODELS STILL  
FORECAST A DEEP TROUGH JUST OFF THE WEST COAST BUT THEN LIFTING  
NORTHEASTWARD NEXT SUNDAY. ECMWF WAS ON THE SLOWER/DEEPER SIDE  
WHILE THE GFS WAS A BIT QUICKER THAN THE ENSEMBLE MEANS, OF WHICH  
THEIR CONSENSUS WAS PREFERRED.  
 
   
..WEATHER THREATS/HIGHLIGHTS
 
 
MUCH COLDER AIR WILL SETTLE INTO 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  
WHEN SNOW LEVELS DROP TO ABOUT 2000-3000 FT. TEMPERATURES ABOUT  
10-20F BELOW AVERAGE ARE LIKELY FOR NORTHERN AND NW MONTANA  
WED-FRI AND THEN WESTWARD AND SOUTHWARD THROUGH OR/WA/ID AND  
CALIFORNIA NEXT WEEKEND. SNOW COULD BE HEAVY OVER NW MONTANA AT  
TIMES AS UPSLOPE FLOW STRENGTHENS BY FRIDAY WITH A 1036-1040MB  
HIGH OVER ALBERTA. THIS 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. EVEN SOUTHERN  
CALIFORNIA COULD SEE SOME PRECIPITATION NEXT WEEKEND DEPENDING ON  
THE DEPTH OF THE TROUGH.  
 
BACK TO THE EAST, OLD FRONTAL BOUNDARY WILL LINK INTO THE SYSTEM  
TO THE NORTH AND BRING A WARM FRONT NORTHEASTWARD THROUGH THE OHIO  
VALLEY WED-THU WITH AN AXIS OF LIGHT TO MODERATE RAIN FROM E TEXAS  
NORTHEASTWARD INTO THE EASTERN GREAT LAKES. SOUTHERN PART OF THE  
FRONT WILL LINGER THROUGH THE MID-MISSISSIPPI VALLEY WITH  
SCATTERED SHOWERS ALONG ITS BOUNDARY.  
 
FRACASSO  
 

 
 
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