572  
FXUS02 KWBC 301554  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1154 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 WILL SUPPORT A TROUGH DIGGING INTO THE WEST LATER  
THIS WEEK THROUGH THE WEEKEND WITH ZONAL TO SOUTHWESTERLY FLOW  
ACROSS THE CENTRAL/EASTERN STATES. WITHIN THIS MEAN PATTERN THERE  
WILL BE TWO PRIMARY SYSTEMS OF INTEREST. FIRST IS A GREAT  
LAKES-EASTERN CANADA LOW THAT WILL BRING A COLD FRONT THROUGH THE  
EAST LATE THIS WEEK, WITH THE FRONT STALLING OVER  
CENTRAL/SOUTH-CENTRAL AREAS. MEANWHILE THE DIGGING WEST COAST  
TROUGH WILL EVENTUALLY EJECT ENERGY EASTWARD, PUSHING A STRONG  
COLD FRONT THROUGH THE WEST BY THE WEEKEND AND INTO THE  
PLAINS/GREAT LAKES SUN-MON AS PARENT LOW PRESSURE TRACKS FROM THE  
NORTHERN PLAINS INTO CANADA. THIS SYSTEM WILL PULL THE STALLED  
CENTRAL-EASTERN U.S. FRONT NORTH AS A WARM FRONT. BY DAY 7 MON  
UPSTREAM SHORTWAVE ENERGY WILL LIKELY DROP INTO THE GULF OF  
ALASKA/NORTHEAST PACIFIC AND COULD BEGIN TO INFLUENCE EJECTING  
WEST COAST TROUGH ENERGY.  
 
OVERALL THE GUIDANCE SHOWS BETTER THAN AVERAGE  
AGREEMENT/CONTINUITY, ALLOWING AN UPDATED CONSENSUS BLEND TO YIELD  
A FORECAST VERY SIMILAR TO PREVIOUS ISSUANCE. PRIMARY DETAIL  
DIFFERENCES OF NOTE INCLUDE 00Z/06Z GFS RUNS LEAVING THE STALLED  
EASTERN FRONT SOMEWHAT FARTHER NORTH THAN MOST OTHER SOLUTIONS  
AROUND DAY 5 SAT, THE 00Z GFS BECOMING A LITTLE SHARPER/MORE  
AMPLIFIED THAN THE MAJORITY WITH THE SOUTHERN HALF OF THE WEST  
COAST UPPER TROUGH AROUND LATE SAT-SUN (A LITTLE EXTREME BUT  
WITHIN THE FULL ENSEMBLE ENVELOPE), AND 00Z-06Z GEFS MEANS  
BECOMING SOMEWHAT WEAKER/SOUTHWARD THAN CONSENSUS WITH LOW  
PRESSURE FORECAST TO BE NEAR NORTHERN ONTARIO BY EARLY DAY 7 MON.  
THE GEFS MEANS ARE ALSO ON THE WEAK SIDE WITH THE LATE PERIOD  
ENERGY DROPPING INTO THE GULF OF ALASKA. ECMWF/ECMWF MEAN RUNS  
HAVE BEEN BETTER DEFINED WITH THIS ENERGY FOR MULTIPLE RUNS AND  
THE GFS STARTED LATCHING ONTO THE CURRENT CONSENSUS IDEA WITH THE  
12Z/29 RUN. OFF THE EAST COAST SUN-MON THE GFS AND TO SOME EXTENT  
GEFS MEAN ARE STRONGER THAN MOST OTHER GUIDANCE WITH ENERGY  
FEEDING INTO A MODEST WEAKNESS ALOFT AND AS A RESULT SHOW MORE  
INVERTED SURFACE TROUGHING/LOW PRESSURE.  
 
BASED ON THESE CONSIDERATIONS THE UPDATED FORECAST INCORPORATED  
ASPECTS OF THE 00Z-06Z GFS AND 00Z ECMWF/CMC FOR THE FIRST HALF OF  
THE PERIOD, FOLLOWED BY 60-70 PERCENT WEIGHTING OF THE 00Z  
ECMWF/ECMWF MEAN VERSUS THE 00Z-06Z GFS/06Z GEFS MEAN.  
 
   
..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 MUCH OF MONTANA AND SOME  
OF THE NORTHERN PLAINS AS OF THU-FRI WITH THE CHILLY AIR BLEEDING  
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 UP TO 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 UPPER TROUGH AND INFLUX OF MOISTURE.  
FARTHER INLAND ACTIVITY SHOULD REACH AT LEAST AS FAR SOUTH AS THE  
CENTRAL ROCKIES. REINFORCING HIGH PRESSURE TRACKING SOUTHEAST  
FROM ALASKA MAY BRING HIGHS DOWN TO 20-30F BELOW NORMAL OVER PARTS  
OF MONTANA BY SUN-MON.  
 
BACK TO THE EAST, A WARM FRONT WILL MOVE THROUGH THE NORTHEAST ON  
THU WITH AN AXIS OF LIGHT TO MODERATE RAIN ALONG/AHEAD OF THE COLD  
FRONT FROM EASTERN TEXAS NORTHEASTWARD THROUGH THE GREAT LAKES AND  
PARTS OF THE NORTHEAST. THE SOUTHERN PART OF THE FRONT WILL  
LINGER THROUGH THE MID-MISSISSIPPI VALLEY/TENNESSEE  
VALLEY/CAROLINAS WITH SCATTERED SHOWERS ALONG THE BOUNDARY FRI-SAT  
BEFORE LIFTING BACK TO THE NORTH AS THE WESTERN SYSTEM CROSSES THE  
NORTHERN PLAINS ON SUN. THE COLD FRONT TRAILING FROM THIS SYSTEM  
MAY GENERATE RAINFALL AS FAR SOUTH AS THE MID MS VALLEY DAYTIME  
MON WITH A FARTHER SOUTHWARD EXTENT SHORTLY THEREAFTER. SOUTHERN  
TIER STATES WILL SEE ABOVE NORMAL TEMPS THROUGH THE PERIOD WITH A  
FEW RECORD HIGHS POSSIBLE OVER TEXAS AND SCATTERED RECORD WARM  
LOWS ELSEWHERE. FRONTAL PROGRESSION WILL LEAD TO VARIABLE TEMPS  
OVER THE NORTHERN HALF OF THE EAST, WITH GREATEST ANOMALIES OF THE  
ABOVE NORMAL VARIETY EARLY AND LATE IN THE PERIOD.  
 
RAUSCH/FRACASSO  
 

 
 
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