102  
FXUS02 KWBC 180658  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
158 AM EST FRI JAN 18 2019  
 
VALID 12Z MON JAN 21 2019 - 12Z FRI JAN 25 2019  
   
..OVERVIEW
 
 
GUIDANCE AGREES QUITE WELL UPON A SIGNIFICANT PATTERN TRANSITION,  
FROM ONE GENERALLY FEATURING A PROGRESSION OF PACIFIC SYSTEMS  
TOWARD AN AMPLIFIED FLOW REGIME CONSISTING OF A BUILDING EASTERN  
PACIFIC/WEST COAST RIDGE (EXTENDING INTO NORTHWESTERN NORTH  
AMERICA) AND CORRESPONDING MEAN TROUGH COVERING THE EASTERN HALF  
OF THE CONTINENT. THE INCREASINGLY LARGE SCALE OF THE EVENTUAL  
MEAN PATTERN SHOULD LEAD TO BETTER THAN AVERAGE PREDICTABILITY  
OVERALL BUT EMBEDDED SHORTWAVES WILL BE SMALLER IN SCALE AND STILL  
PROVIDE MEANINGFUL UNCERTAINTY FOR SOME SENSIBLE WEATHER DETAILS.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
THE STRONG STORM SYSTEM AFFECTING THE LOWER 48 IN THE SHORT RANGE  
TIME FRAME SHOULD REACH THE CANADIAN MARITIMES BY EARLY DAY 3 MON  
AND CONTINUE NORTHEASTWARD. WITH THE DEPARTURE OF THIS SYSTEM THE  
NEXT FOCUS WILL BE ON A FAIRLY VIGOROUS WESTERN U.S. SHORTWAVE  
WHICH ALONG WITH INTERACTING NORTHERN STREAM FLOW WILL MOST LIKELY  
SUPPORT PLAINS THROUGH GREAT LAKES LOW PRESSURE OVER THE COURSE OF  
NEXT WEEK. SOME OF THE WESTERN ENERGY MAY ALSO PRODUCE WAVINESS  
ALONG THE TRAILING FRONT AS IT CROSSES THE EAST. THE ENSEMBLE  
MEANS UP TO THIS POINT SEEM TO BE IN AN ONGOING PROCESS OF  
BECOMING BETTER DEFINED WITH THIS SYSTEM BUT AN AVERAGE OF THEIR  
TIMING (ECMWF MEAN SOMEWHAT SLOWER THAN THE GEFS MEAN) PROVIDES A  
STABLE ANCHORING POINT FOR THE FORECAST. RECENT ECMWF RUNS HAVE  
BEEN SHOWING A PRONOUNCED FASTER TREND TOWARD THIS INTERMEDIATE  
SOLUTION AND IS STILL SOMEWHAT SLOWER THAN ITS MEAN IN THE 12Z  
CYCLE. GFS RUNS HAVE BEEN CONSISTENTLY IN THE FASTER HALF OF THE  
SPREAD WITH LATEST RUNS A BIT FASTER THAN THOSE FROM 24-36 HOURS  
AGO. CONFIDENCE IN ANY SOLUTION IS STILL ONLY MODERATE AS  
INDIVIDUAL ENSEMBLE MEMBERS AND OTHER MODELS PROVIDE OTHER  
ALTERNATIVES FOR SPECIFICS--WHICH STILL APPEAR FAIRLY SENSITIVE TO  
STREAM INTERACTION AND EXACT SMALL-SCALE DETAILS OF SHORTWAVE  
ENERGY. THERE IS EVEN MORE UNCERTAINTY REGARDING WHAT WAVINESS  
MAY EXIST ON THE TRAILING FRONT, WITH DEPENDENCE ON HOW INITIAL  
WESTERN ENERGY EVOLVES AND EVENTUALLY SOME INDIRECT INFLUENCE FROM  
UPSTREAM ENERGY REACHING WESTERN NORTH AMERICA BY TUE-WED. CMC  
RUNS ARE QUITE SLOW VERSUS CONSENSUS FOR THE OVERALL WESTERN  
TROUGH WHILE THE NEW 00Z GFS/GEFS MEAN/UKMET IN THEIR OWN WAYS ARE  
RAISING THE POSSIBILITY OF A SLOWER AND/OR STRONGER TRAILING WAVE.  
 
AS FOR THE SHORTWAVE(S) REACHING THE WEST BY MID-LATE WEEK,  
TELECONNECTIONS RELATIVE TO THE EAST PACIFIC RIDGE OFFER POTENTIAL  
FOR SOME SOUTHWESTWARD ELONGATION WHILE TELECONNECTIONS RELATIVE  
TO ATLANTIC RIDGING SUGGEST MORE PROGRESSION. THIS LEAVES A  
MODEL/MEAN BLEND AS A REASONABLE STARTING POINT TO RESOLVE THIS  
ASPECT OF THE FORECAST.  
 
BASED ON GUIDANCE THROUGH 12Z/18Z CYCLES, FORECAST PREFERENCES LED  
TO USING A BLEND PRIMARILY EMPHASIZING OPERATIONAL GUIDANCE EARLY  
FOLLOWED BY GRADUAL INCREASE OF ENSEMBLE MEAN WEIGHT (18Z GEFS AND  
12Z ECMWF MEAN) FROM 20 PERCENT EARLY TO 70 PERCENT LATE IN THE  
PERIOD.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE STRONG STORM TRACKING AWAY FROM NEW ENGLAND WILL LEAVE IN ITS  
WAKE VERY COLD AIR OVER THE EAST EARLY IN THE WEEK, ALONG WITH  
WINDY CONDITIONS AND LOW WIND CHILL VALUES OVER THE NORTHEAST.  
ANOMALIES MAY REACH 15-30F BELOW NORMAL FOR MIN/MAX READINGS MON  
INTO EARLY TUE FROM PORTIONS OF THE OHIO VALLEY/GREAT LAKES INTO  
THE MID-ATLANTIC AND NORTHEAST. CURRENTLY THE BEST POTENTIAL FOR  
ANY DAILY RECORDS TO BE CHALLENGED WOULD BE FOR COLD HIGHS OVER  
THE NORTHERN MID-ATLANTIC/SOUTHERN NEW ENGLAND ON MON.  
 
EXPECT THE SYSTEM EMERGING FROM THE WEST TO SPREAD MOSTLY SNOW  
ACROSS PARTS OF THE GREAT BASIN AND ROCKIES AS WELL AS NORTHERN  
PLAINS ON MON. AMPLIFICATION OF THE SYSTEM AS IT PROGRESSES  
EASTWARD SHOULD THEN PROMOTE A PERIOD OF GULF MOISTURE INFLOW AND  
HEAVY RAINFALL POTENTIAL FROM THE LOWER HALF OF THE MISSISSIPPI  
VALLEY EASTWARD/NORTHEASTWARD FROM TUE ONWARD. BEYOND THE UPPER  
MISSISSIPPI VALLEY THE HIGHEST PROBABILITY FOR SIGNIFICANT  
SNOWFALL SHOULD EXTEND ACROSS THE GREAT LAKES AND NORTHEAST WITH A  
POSSIBLE TRANSITION OF WINTRY MIX BETWEEN THIS AREA AND PLAIN RAIN  
TO THE SOUTH. ALSO NOTE THAT SNOW POTENTIAL COULD EXTEND FARTHER  
SOUTH DEPENDING ON HOW A POSSIBLE FRONTAL WAVE EVOLVES.  
PROGRESSION OF THIS SYSTEM WILL BRING A BRIEF SURGE OF WARMTH  
ACROSS THE CENTRAL-EASTERN STATES TUE-THU (PLUS 10-20F ANOMALIES)  
FOLLOWED BY A RETURN TO BELOW NORMAL TEMPERATURES.  
 
A FOCUSED BAND OF SHORTWAVE ENERGY AND MOISTURE WILL BRING A  
PERIOD OF ENHANCED SNOW AND LOW ELEVATION RAIN TO AREAS FROM THE  
PACIFIC NORTHWEST INTO THE NORTHERN-CENTRAL ROCKIES, MOSTLY IN THE  
TUE-WED TIME FRAME. ADDITIONAL SHORTWAVES COULD PRODUCE LIGHT  
SNOW OVER/NEAR THE ROCKIES LATER IN THE WEEK BUT WITH LOW  
CONFIDENCE IN LOCATION AND TIMING. APPROACH OF THE BUILDING  
EASTERN PACIFIC RIDGE ALOFT WILL LIKELY LEAD TO MODERATELY ABOVE  
NORMAL TEMPERATURES OVER THE WEST COAST STATES BY MID-LATE WEEK  
BUT CENTRAL-SOUTHERN ROCKIES SHOULD REMAIN SOMEWHAT BELOW NORMAL  
THROUGH FRI.  
 
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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