433  
FXUS02 KWBC 120717  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
217 AM EST WED JAN 12 2022  
 
VALID 12Z SAT JAN 15 2022 - 12Z WED JAN 19 2022  
 
...WINTRY WEATHER IS BECOMING INCREASINGLY LIKELY FOR PARTS OF THE  
APPALACHIANS AND EASTERN SEABOARD THIS WEEKEND INTO EARLY NEXT  
WEEK, BUT WITH AMPLE UNCERTAINTY...  
   
..OVERVIEW
 
 
THE MOST NOTABLE WEATHER FEATURE DURING THE MEDIUM RANGE PERIOD,  
AND ONE OF THE MOST UNCERTAIN, IS A DEVELOPING SURFACE LOW  
PRESSURE SYSTEM THAT SLIDES ACROSS THE SOUTHEAST AND THEN PIVOTS  
NORTHWARD NEAR THE EAST COAST AHEAD OF A POTENT MID-UPPER  
SHORTWAVE TROUGH. DETERMINISTIC MODELS TEND TO HAVE A  
WESTWARD/MORE INLAND TRACK WITH THE SURFACE LOW COMPARED TO THE  
ENSEMBLE GUIDANCE, SO UNCERTAINTY IS STILL AN ISSUE, BUT OVERALL  
GUIDANCE IS TRENDING TOWARD A LOW PRESSURE SYSTEM POSSIBLY INLAND  
OR AT LEAST NEAR ENOUGH TO THE EAST COAST TO CAUSE IMPACTFUL  
WINTRY WEATHER. MEANWHILE IN THE WEST, MAINLY DRY CONDITIONS ARE  
EXPECTED UNDERNEATH SURFACE HIGH PRESSURE, WITH THE EXCEPTION OF  
LIGHT PRECIPITATION IN THE PACIFIC NORTHWEST AND NORTHERN ROCKIES  
AS WEAK FRONTS TRACK THROUGH.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE LATEST MODEL GUIDANCE SHOWS FAIRLY GOOD AGREEMENT FOR THE  
LARGE-SCALE PATTERN, BUT WITH SOME LINGERING UNCERTAINTIES IN SOME  
IMPACTFUL DETAILS. MODELS AGREE THAT A STRONG SHORTWAVE WILL TRACK  
ACROSS THE CENTRAL PLAINS INTO THE SOUTHEAST THIS WEEKEND, CAUSING  
SURFACE LOW(S) TO DEVELOP AND STRENGTHEN, WITH SOME UNCERTAINTY IN  
THEIR PLACEMENT AND PHASING. BUT SUNDAY INTO MONDAY, GUIDANCE  
SHOWS GOOD CONSENSUS FOR SURFACE LOW PRESSURE TO CONSOLIDATE AND  
DEEPEN, BUT WITH VARIABILITY IN TERMS OF ITS TRACK. THE 12Z/18Z  
MODEL CYCLE, AND THE INCOMING 00Z MODEL CYCLE, CONTINUED TO SHOW A  
NOTABLE WESTWARD SHIFT COMPARED TO PREVIOUS GUIDANCE, WHICH WAS  
TRUE FOR BOTH DETERMINISTIC AND ENSEMBLE GUIDANCE. HOWEVER, THE  
DETERMINISTIC MODELS ARE GENERALLY EVEN FARTHER WEST THAN THEIR  
ENSEMBLE COUNTERPARTS--THE 12/18Z GFS RUNS AND THE 12Z CMC AND  
ECMWF (12Z UKMET WAS SOUTH/SUPPRESSED) CLUSTERED VERY WELL WITH AN  
ONSHORE TRACK OVER THE NORTHERN MID-ATLANTIC MONDAY MORNING, BUT  
IT IS POSSIBLE THEIR SUPPOSED AGREEMENT IS DECEPTIVE. WHEN LOOKING  
AT THE ENSEMBLE MEANS AND INDIVIDUAL ENSEMBLE MEMBERS, THE  
DETERMINISTIC GUIDANCE WAS WELL ON THE WESTERN SIDE OF THE SPREAD.  
FELT IT WAS PRUDENT AT THE TIME THE MEDIUM RANGE FORECAST WAS MADE  
TO TAKE A LOW TRACK IN BETWEEN THE 12Z DETERMINISTIC AND ENSEMBLE  
MEAN POSITIONS, WHICH LED TO A SURFACE LOW NEAR THE COAST OF  
DELAWARE/SOUTHERN NEW JERSEY BUT OFFSHORE IN THE WESTERN ATLANTIC  
ON 12Z MONDAY. THE INCOMING 00Z GUIDANCE HAS GENERALLY CONTINUED  
WITH A WESTWARD TREND, INCLUDING A GOOD PROPORTION OF 00Z GEFS  
MEMBERS AND THE 00Z CMC ENSEMBLE MEAN, SUGGESTING THE  
DETERMINISTIC GUIDANCE MAY HAVE BEEN LEADING THE ENSEMBLE MEANS IN  
A TRACK CONSENSUS. HOWEVER, IT SHOULD BE STRESSED THAT THE ENERGY  
LEADING TO THIS POTENT SHORTWAVE ARE STILL IN THE DATA-POOR  
PACIFIC OCEAN AT THIS TIME AND MODEL AND FORECAST CHANGES, PERHAPS  
LARGE, COULD STILL OCCUR.  
 
IN THE WEST, RIDGING IS LIKELY TO HAVE HOLD OF THE REGION THROUGH  
THE WEEKEND, OTHER THAN A SOUTHERN STREAM UPPER LOW CENTERED  
OFFSHORE OF CALIFORNIA MEANDERING. THIS FEATURE IS FORECAST TO  
TRACK EASTWARD EARLY IN THE WORKWEEK AND JOIN WITH THE MAIN FLOW,  
AND POSSIBLY INTERACT WITH NORTHERN STREAM ENERGY ENTERING THE  
PACIFIC NORTHWEST AROUND MONDAY. MODEL AGREEMENT FOR THIS OVERALL  
PATTERN WAS FAIRLY GOOD, BUT WITH INCREASING DIFFERENCES  
ESPECIALLY WITH EMBEDDED SHORTWAVES AS THE PERIOD PROGRESSED.  
 
THE WPC FRONTS/PRESSURES FORECAST WAS BASED ON A BLEND OF THE 12Z  
DETERMINISTIC GUIDANCE AND A SMALL PROPORTION OF THE GEFS AND EC  
ENSEMBLE MEAN GUIDANCE, ELIMINATING THE UKMET AND INCREASING THE  
PROPORTION OF ENSEMBLE MEAN GUIDANCE TO 40% BY DAY 5/MONDAY AND  
HOLDING THAT BLEND THROUGH DAY 7. THIS LED TO A MORE INLAND TRACK  
OF THE LOW PRESSURE SYSTEM AND ITS ASSOCIATED PRECIPITATION/WINTER  
WEATHER IMPACTS COMPARED TO THE PREVIOUS FORECAST.  
   
..WEATHER HIGHLIGHTS
 
 
MODERATE TO POSSIBLY LOCALLY HEAVY RAIN COULD MOVE ACROSS THE  
SOUTHEAST QUICKLY THIS WEEKEND GIVEN THE LOW PRESSURE AND FRONTAL  
SYSTEMS IN THE VICINITY. ON THE NORTHERN SIDE OF THE PRECIPITATION  
SHIELD, THE LOW PRESSURE SYSTEM COULD CREATE SOME POTENTIAL FOR  
WINTRY WEATHER ACROSS PORTIONS OF THE MIDDLE/LOWER MISSISSIPPI  
VALLEY INTO TENNESSEE AND KENTUCKY ON SATURDAY, WHICH SHOULD BE  
MAINLY LIGHT, BUT CONFIDENCE IS LOW FOR AMOUNTS THERE ESPECIALLY  
GIVEN THE SURFACE LOW WILL NOT HAVE CONSOLIDATED YET. BETTER  
CHANCES FOR HEAVIER SNOW ARE LIKELY IN THE SOUTHERN APPALACHIANS  
BEGINNING SATURDAY, AND SNOW IS CURRENTLY EXPECTED TO SPREAD  
NORTHEASTWARD ALONG/ON THE BACKSIDE OF THE DEEPENING LOW TRACK  
OVER THE EASTERN SEABOARD THIS WEEKEND INTO MONDAY. THERE IS STILL  
UNCERTAINTY FOR THE DETAILS OF THIS WINTRY WEATHER THAT WILL NEED  
TO BE RESOLVED IN FUTURE FORECASTS, BUT THE TREND CONTINUES FOR  
IMPACTFUL WEATHER FROM THE SYSTEM.  
 
ELSEWHERE, AFTER THE LOW EXITS NORTHEAST OF THE U.S. ON TUESDAY,  
WESTERLY FLOW AND ANOTHER SHORTWAVE SHOULD CAUSE GOOD CONDITIONS  
FOR LAKE EFFECT/ENHANCED SNOW OVER THE GREAT LAKES. THE WEST COULD  
SEE SOME LIGHT PRECIPITATION DURING THE PERIOD, PARTICULARLY OVER  
THE PACIFIC NORTHWEST AND INTO THE NORTHERN ROCKIES EARLY NEXT  
WEEK, WITH MAINLY DRY CONDITIONS ELSEWHERE EXCEPT POSSIBLY  
SOUTHERN ARIZONA/NEW MEXICO SATURDAY. RAIN COULD DEVELOP ACROSS  
THE LOWER MISSISSIPPI VALLEY BEGINNING TUESDAY WITH RETURN FLOW  
FROM THE GULF OF MEXICO.  
 
TEMPERATURE-WISE, CHILLY WEATHER WITH MUCH BELOW NORMAL  
TEMPERATURES BY 15-30 DEGREES IS FORECAST FOR THE NORTHEAST OVER  
THE WEEKEND, WITH SLIGHTLY COOLER THAN AVERAGE WEATHER THROUGH  
EARLY NEXT WEEK FOR THE SOUTHEASTERN QUADRANT OF THE COUNTRY.  
TEMPERATURES SHOULD GENERALLY BE ABOVE AVERAGE IN THE PLAINS  
PARTICULARLY FROM SUNDAY ONWARD AFTER A COOLER DAY SATURDAY. THE  
WEST SHOULD SEE TEMPERATURES WITHIN ABOUT 5-10 DEGREES OF NORMAL,  
WITH A TENDENCY TOWARD SLIGHTLY WARMER THAN AVERAGE CONDITIONS  
ESPECIALLY FOR LOWS.  
 
TATE  
 
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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