836  
FXUS02 KWBC 200659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 AM EST SUN JAN 20 2019  
 
VALID 12Z WED JAN 23 2019 - 12Z SUN JAN 27 2019  
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
CONFIDENCE REMAINS ABOVE AVERAGE WITH THE FORECAST OF A STRONGLY  
AMPLIFIED EASTERN PACIFIC/WEST COAST RIDGE AND EASTERN NORTH  
AMERICAN TROUGH ALOFT. EXPECT THIS PATTERN TO BRING SIGNIFICANT  
PRECIPITATION TO THE EASTERN THIRD OF THE COUNTRY AS THE MEAN  
PATTERN STARTS TO BECOME ESTABLISHED AND THEN BELOW TO MUCH BELOW  
NORMAL TEMPERATURES INTO PARTS OF THE EASTERN HALF OF THE U.S.  
FOR ABOUT THE FIRST THREE DAYS OF THE PERIOD (WED-FRI) MOST  
GUIDANCE DISPLAYS COMMON THEMES FOR SIGNIFICANT FEATURES ALBEIT  
WITH LINGERING DETAIL QUESTIONS. THEN FORECAST SPECIFICS RAPIDLY  
BECOME MORE UNCERTAIN REGARDING SHORTWAVES PROGRESSING AROUND THE  
TOP OF THE WEST COAST RIDGE AND THEN DROPPING INTO DOWNSTREAM MEAN  
TROUGH. THE MEDIUM TO SMALL SCALE NATURE OF THESE SHORTWAVES  
WOULD TEND TO YIELD LOW PREDICTABILITY.  
 
DURING THE FIRST HALF OF THE PERIOD WHEN FEATURES CLUSTER  
DECENTLY, THE MANUAL FORECAST STARTED WITH A 70-80 PERCENT  
OPERATIONAL MODEL BLEND WITH HIGHEST WEIGHTING ON THE 18Z GFS/12Z  
ECMWF. GREAT LAKES LOW PRESSURE AS OF EARLY WED STILL HAS  
UNUSUALLY HIGH SENSITIVITY TO EXACT DETAILS OF SHORTWAVE ENERGY  
ALOFT, SO THIS FEATURE'S EVOLUTION FROM WED INTO THU IS MORE  
AMBIGUOUS THAN WOULD TYPICALLY BE THE CASE FOR A 3-4 DAY FORECAST.  
SOLUTIONS ARE GRADUALLY BECOMING LESS DIVERSE FOR THE TRAILING  
FRONTAL WAVE TRACKING NORTHWARD ALONG THE EAST COAST AROUND THU  
BUT MEANINGFUL TIMING/TRACK/STRENGTH DIFFERENCES PERSIST. THE  
FORECAST FOR THIS SYSTEM ALSO DEPENDS ON SMALLER SCALE DETAIL  
ISSUES THAT CAN TAKE INTO THE SHORT RANGE TIME FRAME TO BE  
RESOLVED. A BLEND/ENSEMBLE MEAN APPROACH PROVIDES THE BEST OPTION  
FOR A SINGLE DETERMINISTIC FORECAST AND YIELDS REASONABLE  
CONTINUITY FROM PREVIOUS FORECAST. FARTHER TO THE WEST EXPECT A  
STRONG CANADIAN SHORTWAVE TO DROP INTO THE NORTHERN U.S. BY WED  
NIGHT-THU WITH A LEADING COLD FRONT PUSHING RAPIDLY SOUTH AND EAST  
FROM THE NORTHERN PLAINS. AN AVERAGE OF GUIDANCE PROVIDES A  
REASONABLE STARTING POINT FOR THIS FRONT, MINUS THE 12Z CMC WHICH  
WAS A SLOW EXTREME.  
 
BY DAYS 6-7 SAT-SUN THE BLEND RAPIDLY SHIFTED TO 70-100 PERCENT  
18Z GEFS/12Z ECMWF MEAN WEIGHTING. THUS FAR OPERATIONAL MODEL  
RUNS HAVE BEEN QUITE VARIED WITH THE EXPECTED SURFACE PATTERN IN  
RESPONSE TO DIFFERENCES IN SHORTWAVE EVOLUTION TO THE EAST OF THE  
ROCKIES AS WELL AS INFLUENCE FROM UPSTREAM ENERGY ROUNDING THE  
RIDGE. AS A RESULT CONFIDENCE IN ANY SPECIFIC OPERATIONAL  
SCENARIO IS VERY LOW AT THIS TIME. THE ONE GENERAL SIGNAL THAT  
THE GUIDANCE PROVIDES IS FOR SOME TYPE OF SURFACE SYSTEM TO AFFECT  
THE CENTRAL/EASTERN STATES, PRODUCING AN AREA OF PRECIPITATION OF  
UNCERTAIN COVERAGE AND INTENSITY.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
PRIMARILY DURING WED-THU THE FRONT TRAILING FROM INITIAL GREAT  
LAKES LOW PRESSURE, AND THEN A WAVE DEVELOPING ALONG THE SOUTHERN  
PART OF THE FRONT AND TRACKING NORTHWARD, WILL PROVIDE A FOCUS FOR  
WIDESPREAD PRECIPITATION FROM NEAR THE MISSISSIPPI VALLEY TO EAST  
COAST. EXPECT HIGHEST TOTALS WITHIN AN AREA FROM THE GULF COAST  
REGION AND SOUTHERN APPALACHIANS INTO SOUTHERN NEW ENGLAND. THERE  
IS STILL UNCERTAINTY IN STRENGTH/TIMING/TRACK OF THE FRONTAL WAVE  
BUT CURRENTLY THE BEST POTENTIAL FOR AT LEAST SOME WINTRY WEATHER  
EXTENDS FROM WEST OF THE APPALACHIANS NORTHEASTWARD INTO NEW  
ENGLAND.  
 
BEHIND THIS SYSTEM THE GREAT LAKES WILL LIKELY SEE PERIODS OF LAKE  
EFFECT/ENHANCED SNOW OF VARYING INTENSITY. LOW PRESSURE AND A  
STRONG COLD FRONT PASSING THROUGH THE REGION MAY BRING A PERIOD OF  
SYNOPTIC SCALE SNOW DURING THE LATTER HALF OF THE WEEK. ANOTHER  
FEATURE COULD ENHANCE SNOWFALL FOR A TIME DURING THE WEEKEND.  
 
A FOCUSED BAND OF MOISTURE WILL PRODUCE SNOW AND LOW ELEVATION  
RAIN FROM THE PACIFIC NORTHWEST INTO NORTHERN ROCKIES DURING  
WED-THU WITH ACTIVITY TAPERING OFF FROM WEST TO EAST. LOCATIONS  
ALONG THE EASTERN SLOPES OF THE NORTHERN HALF OF THE ROCKIES MAY  
SEE ADDITIONAL PERIODS OF LIGHT SNOW DEPENDING ON HOW SHORTWAVES  
EVOLVE DOWNSTREAM FROM THE WEST COAST RIDGE ALOFT. THIS SHORTWAVE  
ENERGY AND ASSOCIATED SURFACE REFLECTION MAY THEN PRODUCE AN AREA  
OF PRECIPITATION SPREADING ACROSS THE EASTERN HALF OF THE LOWER 48  
NEXT WEEKEND. AT THIS TIME CONFIDENCE IN COVERAGE/INTENSITY OF  
PRECIP IS VERY LOW. WITHIN THE MOISTURE SHIELD THE RAIN-SNOW LINE  
MAY REACH FAIRLY FAR SOUTH GIVEN THE COLD PATTERN.  
 
THE WAVY FRONT HEADING INTO THE EAST WILL BRING A BRIEF SURGE OF  
WARMTH AHEAD OF IT WED-THU WITH ANOMALIES EXCEEDING PLUS 10F  
ESPECIALLY FOR MORNING LOWS. A STRONG COLD FRONT PUSHING  
SOUTHEAST FROM THE NORTHERN PLAINS DURING THE LATTER HALF OF THE  
WEEK WILL EXPAND COVERAGE OF BELOW TO MUCH BELOW NORMAL  
TEMPERATURES OVER THE EASTERN HALF OF THE COUNTRY BY FRI-SUN. THE  
MOST PERSISTENT CORE OF COLDEST ANOMALIES (15-25F BELOW NORMAL)  
SHOULD EXIST OVER THE MIDWEST/UPPER MISSISSIPPI VALLEY. THE UPPER  
RIDGE SETTLING NEAR THE WEST COAST WILL SUPPORT PLUS 5-10F OR SO  
ANOMALIES IN ITS VICINITY. WARM ANOMALIES MAY EXTEND INTO THE  
NORTHERN ROCKIES AND HIGH PLAINS LOCATIONS THAT REMAIN WEST OF A  
MEAN FRONTAL BOUNDARY FORECAST TO MEANDER OVER OR NEAR THE HIGH  
PLAINS FOR MULTIPLE DAYS.  
 
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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