005  
FXUS02 KWBC 170700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
200 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 PERIOD WILL START WITH VERY AMPLIFIED FLOW, FEATURING  
A WESTERN RIDGE AND SHARPENING MISSISSIPPI VALLEY TROUGH. WITHIN  
THE SOUTHERN PART OF THE TROUGH AN UPPER LOW SHOULD CLOSE OFF AND  
THEN TRACK NORTHWARD OVER THE EASTERN U.S. INTO CANADA--POSSIBLY  
OPENING UP AT SOME POINT DUE TO INTERACTION WITH NORTHERN STREAM  
ENERGY. ASSOCIATED STRENGTHENING LOW PRESSURE SHOULD TRACK FROM  
THE LOWER MISSISSIPPI VALLEY THROUGH THE UPPER OHIO VALLEY AND  
EASTERN GREAT LAKES. MEANWHILE A FAIRLY STRONG SYSTEM TRACKING  
INTO WESTERN CANADA FROM JUST OFF THE PACIFIC NORTHWEST COAST WILL  
USHER IN A TRANSITION TOWARD LESS AMPLIFIED MEAN FLOW OVER THE  
LOWER 48. WITH A FAIR DEGREE OF UNCERTAINTY IN SHORTWAVE DETAILS  
THE OVERALL MEAN FLOW OVER THE WEST SHOULD TREND TOWARD SOME  
DEGREE OF MEAN TROUGHING FROM SUN ONWARD IN RESPONSE TO POSITIVE  
HEIGHT ANOMALIES SEEN OVER THE EAST-CENTRAL PACIFIC IN MOST D+8  
MULTI-DAY MEAN CHARTS FROM THE PAST COUPLE DAYS.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
THROUGH THE 12Z/18Z CYCLES THERE WAS A SOLID MODEL/ENSEMBLE MEAN  
CLUSTERING FOR MOST ASPECTS OF THE EASTERN U.S. STORM ASIDE FROM  
THE CMC BEGINNING TO STRAY ON THE FAST SIDE AFTER EARLY FRI, WHILE  
THE CMC MEAN STILL LOOKS EXCESSIVELY FAST IN GENERAL. NEW 00Z  
SOLUTIONS APPEAR TO ADD SOME FINER-SCALE DETAIL UNCERTAINTIES FROM  
LATE THU ONWARD. ON THE OPPOSITE SIDE OF THE COUNTRY GUIDANCE IS  
TAKING A LONG TIME TO RESOLVE DETAILS OF THE SYSTEM INITIALLY JUST  
OFF THE PACIFIC NORTHWEST COAST AND QUICKLY TRACKING INTO SOUTHERN  
CANADA, DUE TO SENSITIVITY IN SHORTWAVE SPECIFICS ALOFT. UKMET  
RUNS HAVE YET TO BE REMOTELY CLOSE TO CONSENSUS, WITH MOST BEING  
EXTREMELY FAST/SUPPRESSED AND THE NEW 00Z RUN NOW ON THE SLOW  
SIDE. BASED ON GUIDANCE COMPARISONS EARLY IN THE PERIOD, DAYS 3-4  
THU-FRI PLACED PRIMARY EMPHASIS ON THE 18Z GFS/12Z ECMWF WITH LESS  
WEIGHT ON THE CMC AND THE LEAST ON THE UKMET.  
 
BY MID-LATE PERIOD THE MOST PROMINENT FORECAST ISSUE FOR SENSIBLE  
WEATHER BECOMES THE PROGRESSION OF PACIFIC SHORTWAVES WHICH WILL  
BE DIFFICULT TO RESOLVE SEVERAL DAYS OUT IN TIME DUE TO BEING  
EMBEDDED WITHIN FAIRLY FAST AND LOW AMPLITUDE FLOW. IN THE  
SAT-MON PERIOD THE 12Z ECMWF DOES NOT COMPARE PARTICULARLY WELL TO  
OTHER GUIDANCE FOR A LEADING SHORTWAVE THAT IT HAS OVER THE PLAINS  
BY SUN, SO PREFER TO DOWNPLAY THAT ASPECT OF ITS FORECAST WITH  
SOME INPUT FROM THE PRIOR 00Z/16 RUN. FOR TRAILING  
SHORTWAVES/FRONTAL SYSTEMS REACHING THE WEST COAST, THE MOST  
COMMON THEME IN THE GUIDANCE IS FOR ONE TO ARRIVE BY DAY 6 SUN AND  
THEN ANOTHER TO APPROACH ON DAY 7 MON. RECENT GFS RUNS HAVE BEEN  
EITHER WEAK OR SLOW WITH THE LATTER WHILE THE FV3 GFS COMPARES  
BETTER TO THE MAJORITY CLUSTER. THIS DISTRIBUTION OF GUIDANCE  
FAVORS EMPHASIZING THE GEFS/ECMWF MEANS AND ECMWF RUNS FOR THIS  
ASPECT OF THE FORECAST.  
 
   
..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 LATE THIS WEEK. 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. SOME MOISTURE WILL  
REACH INTO THE NORTHERN ROCKIES. AFTER A BRIEF BREAK ANOTHER  
COUPLE SYSTEMS SHOULD BRING PERIODS OF PRECIPITATION TO THE  
NORTHERN HALF OR MORE OF THE WEST COAST FROM THE WEEKEND INTO  
EARLY NEXT WEEK WITH LIGHTER ACTIVITY STREAMING INTO THE INTERIOR  
WEST AND 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 MOISTURE OVER MOST OF THE EASTERN HALF OF THE LOWER 48  
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. GUIDANCE IS  
STABLE IN DEPICTING BEST POTENTIAL FOR HEAVIEST RAINFALL OVER THE  
FLORIDA PENINSULA AND NORTH CAROLINA COAST. 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. RAINFALL FROM THIS SYSTEM MAY BE  
PROBLEMATIC FOR AREAS THAT EXPERIENCED HEAVY RAIN IN THIS PAST  
WEEKEND'S EVENT. 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 15-25F ANOMALIES, WHILE AT THE SAME TIME 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 GENERALLY SOMEWHAT ABOVE NORMAL.  
 
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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