471  
FXUS02 KWBC 171608  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1107 AM EST MON DEC 17 2018  
 
VALID 12Z THU DEC 20 2018 - 12Z MON DEC 24 2018  
   
..PERIODS OF ACTIVE WEATHER EXPECTED OVER THE NORTHWEST
 
 
...WIDESPREAD PRECIPITATION AND WIND WITH LATE WEEK EASTERN  
SYSTEM....  
 
   
..OVERVIEW
 
 
THE FORECAST CONTINUES TO START WITH AN AMPLIFIED FLOW WITH A  
DIGGING TROUGH OVER THE MISSISSIPPI VALLEY, A STRONG UPPER RIDGE  
ACROSS THE WESTERN U.S. AND AN INCOMING TROUGH NEAR THE PACIFIC  
NORTHWEST. THE SOUTHERN PORTION OF THE EASTERN TROUGH IS STILL  
EXPECTED TO CLOSE OFF AND WILL TRACK NORTHEASTWARD OVER THE  
MID-ATLANTIC BY THE END OF THE WEEK AND TRAVERSE ACROSS THE  
NORTHEAST THROUGH THE WEEKEND. MEANWHILE, THE TROUGH IN THE  
PACIFIC WILL CROSS OVER THE INTERMOUNTAIN WEST ON THU/FRI, AND  
WILL MOVE OVER THE NORTHERN TIER STATES--WHICH WILL CAUSE A LESS  
AMPLIFIED MEAN FLOW OVER THE LOWER 48. BY MONDAY, CYCLONIC FLOW  
CAN BE EXPECTED OVER THE NORTHEASTERN U.S., WITH TROUGHING JUST  
OFF THE PACIFIC NORTHWEST ONCE AGAIN IN ADDITION TO TROUGHING  
ACROSS THE SOUTHWEST. OVERALL, OPERATIONAL MODELS AGREE WITH THE  
SYNOPTIC PATTERN BUT DIFFER WITHIN THE DETAILING OF DISTURBANCES  
FROM THE PACIFIC.  
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
OPERATIONAL MODELS WERE IN FAIR AGREEMENT IN THE BEGINNING OF THE  
MEDIUM RANGE PERIOD--WITH THE 00Z AND 06Z GFS BEING MORE  
AGGRESSIVE WITH THE SOUTHERN PORTION OF THE TROUGH BECOMING A  
CLOSED LOW. THE 00Z ECMWF WAS QUICKER TO HAVING THIS LOW BECOME  
AN OPEN WAVE BY FRIDAY. HOWEVER, THE 00Z CMC WAS MORE IN LINE  
WITH THE GFS--ALBEIT FASTER WITH THE PROGRESSION OF THE CLOSED  
LOW. THE UKMET AGREED WITH THE TIMING OF THE TROUGH, HOWEVER  
REMAINS AN OPEN LOW. MEANWHILE, OUT WEST, ALL THE OPERATIONAL  
MODELS WERE CLOSE WITH THE TIMING OF THE SYSTEM MOVING ACROSS THE  
PACIFIC NORTHWEST AND INTO THE INTERMOUNTAIN WEST. BECAUSE THERE  
WERE NOT LARGE DISCREPANCIES BETWEEN THE OPERATIONAL MODELS, USED  
A BLEND OF THE 06Z GFS/00Z ECMWF AND TO A LESSER WEIGHT THE 00Z  
CMC AND UKMET.  
 
AS PREVIOUSLY STATED, IN REGARDS TO THE LATER HALF OF THE MEDIUM  
RANGE PERIOD, THE OVERALL PATTERN IS ESTABLISHED BUT THE DETAILING  
IS STILL UNCERTAIN. SPECIFICALLY, THE PACIFIC NORTHWEST SHORTWAVES  
ARE STILL DIFFICULT TO RESOLVE AS THE FLOW BECOMES MORE ZONAL/LESS  
AMPLIFIED. MODELS ARE STILL SHOWING SHORT WAVES MOVING ACROSS THE  
WEST COAST ON DAY 6/SUN AND DAY 7/MON. WITHIN THE OPERATIONAL  
MODELS, THE TIMING AND EVOLUTION OF THESE SHORT WAVES HAVE A LARGE  
SPREAD. THE 06Z GEFS MEAN AND 00Z ECMWF ENSEMBLE MEAN WERE USED  
MUCH MORE DURING THE DAY 5-7/SAT-MON TIME FRAME SINCE THEY WERE  
COMPARABLE BUT STILL USED SOME OF THEIR RESPECTIVE OPERATIONAL  
MODELS.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE SYSTEM TRACKING FROM OFF THE PACIFIC NORTHWEST COAST INTO  
CANADA WILL BRING A PERIOD OF LOCALLY ENHANCED RAIN/HIGHER  
ELEVATION SNOW TO NORTHWESTERN AREAS ON THURSDAY AND INTO FRIDAY.  
THE OLYMPICS/VANCOUVER ISLAND AND NORTHERN CASCADES SHOULD SEE THE  
HIGHEST TOTALS WITH THIS SYSTEM WHILE STILL MEANINGFUL AMOUNTS  
EXTEND AS FAR SOUTH AS NORTHERN CALIFORNIA. PRECIPITATION IS  
EXPECTED TO INCREASE ACROSS THE NORTHERN ROCKIES ESPECIALLY BY  
FRIDAY. LIGHT PRECIPITATION MAY MOVE THROUGH THE PACIFIC NORTHWEST  
REGION BY THE BEGINNING OF THE WEEKEND, BUT WILL INCREASE  
SIGNIFICANTLY BY SUNDAY AND WILL CONTINUE INTO MONDAY FROM THE  
PACIFIC NORTHWEST TO NORTHERN CALIFORNIA IN ADDITION TO THE  
NORTHERN ROCKIES. RAIN/SNOW WILL BE OF VARYING INTENSITY WITH SOME  
LOCALLY ENHANCED AREAS POSSIBLE DEPENDING ON EXACT DETAILS OF EACH  
SYSTEM. SIMILAR TO EARLIER IN THE PERIOD, HIGHEST ARE MOST LIKELY  
TO BE OVER THE PACIFIC NORTHWEST AND NORTHERN CALIFORNIA.  
 
THE STORM SYSTEM TRACKING THROUGH THE EAST WILL SPREAD A BROAD  
AREA OF PRECIPITATION OVER MOST OF THE EASTERN HALF OF THE U.S.  
DURING THE LATTER HALF OF THE WEEK. A MAJORITY OF PRECIPITATION  
WITH THIS STORM WILL BE IN THE FORM OF RAIN BUT CHILLY CYCLONIC  
FLOW IN ITS WAKE WILL SUPPORT AREAS OF LAKE  
EFFECT/TERRAIN-ENHANCED SNOW DURING THE WEEKEND. THE HEAVIEST  
RAINFALL COULD POTENTIALLY FALL FROM THE EASTERN TENNESSEE  
VALLEY/WESTERN NORTH CAROLINA ALONG WITH COASTAL NORTH CAROLINA.  
OTHER AREAS OF FOCUS FOR SIGNIFICANT TOTALS WILL BE OVER EASTERN  
NEW ENGLAND IN ASSOCIATION WITH A PERIOD OF ATLANTIC INFLOW, AND  
LOCATIONS NORTHWEST OF THE STORM TRACK. BECAUSE OF SATURATED SOIL  
FROM THE MOST RECENT MULTI-DAY RAINFALL EVENT, ADDITIONAL RAINFALL  
FROM THIS SYSTEM MAY BE PROBLEMATIC FOR THESE AREAS. SOME AREAS  
COULD SEE STRONG WINDS WITH THIS SYSTEM AS WELL.  
 
EXPECT VERY WARM TEMPERATURES OVER AND NEAR THE PLAINS LATE THIS  
WEEK WITH SOME PLUS 20F OR GREATER ANOMALIES OVER NORTHERN AREAS  
ESPECIALLY FOR MORNING LOWS. THE EASTERN U.S. SYSTEM WILL ALSO  
BRING A BRIEF SURGE OF WARMTH INTO THE MID-ATLANTIC/NEW ENGLAND  
WITH SOME PLUS 15-20F ANOMALIES, WHILE SIMULTANEOUSLY PULLING  
CHILLY AIR INTO THE SOUTHEAST. FROM THE WEEKEND INTO EARLY NEXT  
WEEK THE TREND TOWARD LESS AMPLIFIED MEAN FLOW ALOFT WILL PROMOTE  
HIGHS WITHIN A FEW DEGREES OF NORMAL OVER MOST AREAS AND MORNING  
LOWS MAY BE SLIGHTLY ABOVE NORMAL.  
 
REINHART/RAUSCH  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND 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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