305  
FXUS02 KWBC 180657  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
256 AM EDT MON OCT 18 2021  
 
VALID 12Z THU OCT 21 2021 - 12Z MON OCT 25 2021  
 
***HEAVY RAINFALL POTENTIAL NEXT WEEKEND FROM CENTRAL CALIFORNIA  
TO WESTERN WASHINGTON***  
   
..OVERVIEW
 
 
AN ACTIVE WEATHER PATTERN WILL BE DEVELOPING ACROSS THE WESTERN  
U.S. BY THE END OF THE WEEK AS A SERIES OF STRONG STORM SYSTEMS  
OFF THE WEST COAST PIVOT AROUND A RATHER PRONOUNCED TROUGH OVER  
THE NORTHEAST PACIFIC. THIS WILL RESULT IN A COUPLE OF COLD  
FRONTAL PASSAGES THAT WILL PROGRESS INLAND ACROSS THE  
INTERMOUNTAIN WEST WITH HEAVY PRECIPITATION AND GUSTY WINDS.  
MEANWHILE ACROSS THE EASTERN U.S., A COLD FRONT AND UPPER LEVEL  
TROUGH WILL BE EXITING THE EAST COAST BY FRIDAY WITH A CANADIAN  
SURFACE HIGH SETTLING SOUTHWARD ACROSS THE PLAINS AND MIDWEST.  
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODEL GUIDANCE FOR THE 00Z CYCLE IS IN DECENT AGREEMENT WITH THE  
OVERALL SYNOPTIC SCALE PATTERN THROUGH FRIDAY NIGHT, PARTICULARLY  
WITH THE TIMING OF THE COLD FRONT REACHING THE EAST COAST AND THE  
FIRST PACIFIC COLD FRONT AFFECTING THE WEST COAST REGION, SO A  
GFS/ECMWF/UKMET MODEL BLEND SUFFICED AS A STARTING POINT IN THE  
FORECAST PROCESS. THE 12Z CMC STRAYED FROM THE MODEL CONSENSUS  
WITH AN EMBEDDED CLOSED LOW CROSSING THE NORTHERN ROCKIES, BUT IT  
WAS OTHERWISE CLOSE TO THE OTHER GUIDANCE.  
 
FOR THE STORM SYSTEM REACHING THE WEST COAST SATURDAY NIGHT, THE  
GFS IS A BIT MORE AMPLIFIED AND FASTER, WITH HEIGHT FALLS  
SPREADING ACROSS THE INTERMOUNTAIN WEST SOONER THAT THE CMC/ECMWF.  
BY THE END OF THE FORECAST PERIOD, THE CMC IS STRONGER WITH A  
SHORTWAVE TROUGH CROSSING THE CENTRAL PLAINS, WHEREAS THE ENSEMBLE  
MEANS ARE SUGGESTING MORE OF AN UPPER RIDGE AXIS HERE. FOR DAYS 6  
AND 7, THE WPC FORECAST INCORPORATED MORE OF THE EC AND GEFS  
MEANS, WHILE STILL MAINTAINING SOME OF THE OPERATIONAL GUIDANCE.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
ACROSS THE WEST COAST REGION, MULTIPLE ROUNDS OF SHOWERS ARE  
EXPECTED FROM NORTHERN CALIFORNIA TO WESTERN WASHINGTON, AND SOME  
OF THESE WILL LIKELY BE MODERATE TO HEAVY AT TIMES WITH MOIST  
ONSHORE FLOW AHEAD OF MULTIPLE PACIFIC STORM SYSTEMS. SOME  
LOCATIONS MAY GET ON THE ORDER OF 3 TO 6 INCHES OF RAIN OVER THE  
ENTIRE FORECAST PERIOD, PARTICULARLY ACROSS WINDWARD TERRAIN.  
SNOW IS LIKELY FOR THE HIGHEST MOUNTAINS OF THE CASCADES AND  
SIERRA NEVADA, MAINLY ABOVE PASS LEVEL. THERE IS SOME SIGNAL IN  
THE MODEL GUIDANCE FOR AN ATMOSPHERIC RIVER EVENT NEXT WEEKEND  
AHEAD OF AN ENERGETIC PACIFIC STORM SYSTEM ACROSS THE COASTAL  
RANGES OF NORTHERN CALIFORNIA AND SOUTHWEST OREGON. STRONG WINDS  
ARE ALSO POSSIBLE NEAR THE COAST. BY SUNDAY AND INTO MONDAY, SOME  
OF THIS MOISTURE IS EXPECTED TO REACH THE INTERMOUNTAIN WEST AND  
THE CENTRAL/NORTHERN ROCKIES. ACROSS THE EASTERN U.S., A BROKEN  
SWATH OF SHOWERS AND PERHAPS SOME THUNDERSTORMS CAN BE EXPECTED  
AHEAD OF THE ADVANCING COLD FRONT, BUT THE RELATIVE DEARTH OF DEEP  
MOISTURE AHEAD OF IT WILL SERVE TO LIMIT THE HEAVY RAINFALL  
POTENTIAL.  
 
IN TERMS OF TEMPERATURES, IT WILL FEEL MORE LIKE MID NOVEMBER  
ACROSS MUCH OF THE NORTHERN PLAINS IN THE WAKE OF THE DEPARTING  
STORM SYSTEM ON THURSDAY, WITH HIGH TEMPERATURES RUNNING ABOUT 5  
TO 15 DEGREES BELOW AVERAGE. THERE SHOULD BE A GRADUAL MODERATION  
IN TEMPERATURES GOING THROUGH THE END OF THE WORKWEEK AND INTO THE  
WEEKEND AS AN UPPER-LEVEL RIDGE BEGINS TO BUILD IN, AND THEN  
READINGS UP TO 10 DEGREES ABOVE AVERAGE FOR THE HIGH PLAINS BY  
NEXT SUNDAY. GENERALLY COOLER THAN NORMAL TEMPERATURES ARE  
FORECAST FOR THE WEST COAST, INFLUENCED BY THE UPPER TROUGH AND  
MAINLY OVERCAST SKIES. AHEAD OF THE COLD FRONT, IT WILL LIKELY  
FEEL MORE LIKE LATE SEPTEMBER ACROSS THE EAST COAST WITH HIGHS ON  
THE ORDER OF 5-10 DEGREES ABOVE AVERAGE FOR THURSDAY, ALTHOUGH  
HUMIDITY LEVELS SHOULD REMAIN IN CHECK. SLIGHTLY BELOW NORMAL  
HIGHS ARE THEN EXPECTED BEHIND THE COLD FRONT.  
 
HAMRICK  
 
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDICES ARE AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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