974  
FXUS02 KWBC 201554  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1054 AM EST WED DEC 20 2017  
 
VALID 12Z SAT DEC 23 2017 - 12Z WED DEC 27 2017  
   
..ARCTIC AIR TO PLUNGE INTO THE CENTRAL U.S. BY THE WEEKEND
 
 
...WINTRY WEATHER POSSIBLE ON CHRISTMAS DAY FOR PORTIONS OF THE  
APPALACHIANS AND NORTHEAST...  
 
   
..OVERVIEW
 
 
ACTIVE PATTERN TO START THE MEDIUM RANGE PERIOD WILL COINCIDE WITH  
A BUSY COUPLE OF TRAVEL DAYS LEADING UP TO THE CHRISTMAS HOLIDAY.  
STRONG RIDGING UP INTO ALASKA WILL HELP DRIVE ARCTIC AIR INTO MUCH  
OF THE LOWER 48 THIS WEEKEND INTO NEXT WEEK, EVENTUALLY LEADING TO  
A COLD AND RATHER DRY PATTERN BY THE END OF THE FORECAST. BEFORE  
THEN, A FRONTAL SYSTEM WILL SLOWLY PUSH OFFSHORE THE EAST COAST  
BUT A WAVE OF LOW PRESSURE IS FORECAST TO RIDE ALONG THE BOUNDARY  
SUN/MON OFF THE NEW ENGLAND COAST. COMBINED WITH AN INITIAL ARCTIC  
FRONT AND A REINFORCING SHOT IN ITS WAKE, THIS MAY SET THE STAGE  
FOR A MODEST WINTRY EVENT FOR AT LEAST THE INTERIOR OF NEW  
ENGLAND/NORTHEAST SAT-MON. BY NEXT TUE/WED, COLD ARCTIC AIR  
(THOUGH MODIFIED) WILL DOMINATE THE CONUS EAST OF THE ROCKIES AS A  
PACIFIC SYSTEM PUSHES INTO WA/OR AND THE INTERMOUNTAIN WEST.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE SYNOPTIC PATTERN AND EVOLUTION THROUGH NEXT WEEK REMAINS  
RATHER WELL-FORECAST, BUT SIGNIFICANT DETAIL DIFFERENCES LINGER  
AMONG THE DETERMINISTIC/ENSEMBLE GUIDANCE THAT IMPACT THE  
FORECAST. OPTED TO STAY NEAR AN ENSEMBLE/CONTINUITY CONSENSUS AND  
ELIMINATE THE LOWEST PROBABILITY SOLUTIONS (THAT BEING THE 00Z  
CANADIAN AND SOMEWHAT THE 00Z GFS), MEANING THE FORECAST WAS BASED  
ON A LARGE BLEND OF THE 06Z GFS/GEFS, 00Z ECMWF/ECENS MEAN, AND  
00Z UKMET ALONG WITH CONTINUITY. COULD NOT FAVOR ONE SOLUTION OVER  
THE OTHER GIVEN THE CHANGES SEEN RIGHT FROM SATURDAY (DAY 3) AS  
WELL AS THE INCONSISTENCY IN HOW THE HANDLE THE UPPER HIGH  
BREAKDOWN IN NW CANADA AND RESULTANT UNDERCUTTING INTO THE PAC NW  
NEXT WEEK. THE GUIDANCE TYPICALLY DISAPPOINT IN THESE PATTERN  
BREAKDOWNS IN THE NORTHWEST. REGARDING THE BROAD CYCLONIC FLOW IN  
THE EAST, LITTLE PREDICTABILITY IS SEEN IN SMALL/WEAKER SHORTWAVES  
RACING THROUGH THE FLOW AS THE MODELS HAVE NOT BEEN CONSISTENT ON  
TIMING/AMPLITUDE. A CONSENSUS APPROACH WAS THE PATH OF LEAST  
REGRET.  
 
IN THE EAST, TREND TOWARD A WEAKER LEAD LOW INTO THE EASTERN GREAT  
LAKES (W PA/NY) SATURDAY CONTINUES IN THE ENSEMBLES AND MOSTLY THE  
DETERMINISTIC RUNS. BY CHRISTMAS EVE INTO CHRISTMAS MORNING,  
ENSEMBLES AND MOST DETERMINISTIC GUIDANCE STILL DEVELOP LOW  
PRESSURE ALONG THE FRONT IN THE SOUTHEAST AND TAKE IT  
NORTHEASTWARD TO A POSITION NEAR SE MASSACHUSETTS WITH THE 00Z GFS  
FARTHEST EAST (AND OMITTED FROM THE CONSENSUS BLEND). THIS WAS A  
NUDGE EASTWARD FROM 24 HRS AGO BUT THE THREAT OF A SNOWY/WHITE  
CHRISTMAS REMAINS FOR AT LEAST THE INTERIOR OF THE NORTHEAST BUT  
PERHAPS TO THE COAST DEPENDING ON THE TEMPERATURE PROFILE AND  
AVAILABLE PRECIPITATION. REINFORCING ARCTIC BOUNDARY MAY MOVE  
THROUGH NEXT TUESDAY WHICH WILL SET OFF THE LAKE EFFECT MACHINE  
AGAIN.  
 
TO THE WEST, THOUGH THE ARCTIC AIR WILL MOSTLY FOCUS WEST OF THE  
ROCKIES, LEADING BOUNDARY SHOULD PUSH THROUGH WA/OR/ID THIS  
WEEKEND AS HEIGHT RISE ON SUNDAY, WEAKENING THE FRONT. ENSEMBLES  
SHOW A PACIFIC SYSTEM COMING ASHORE ON MONDAY BUT THE N-S SPREAD  
REMAINS HIGH -- CONSENSUS ARGUES FOR A SOMEWHAT WEAK SYSTEM MOVING  
INTO OREGON WITH AN INCREASE IN RAIN/SNOW TO THE NORTHWEST AND  
INTERIOR.  
 
FRACASSO  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE SURFACE LOW TRACKING ACROSS THE OH VALLEY/GREAT LAKES SAT WILL  
BE ACCOMPANIED BY A BAND OF SNOW ON THE NORTH SIDE OF THE SYSTEM.  
SNOW WILL ALSO BE POSSIBLE OVER NEW ENGLAND WHERE WARM ADVECTION  
WILL BE THE LIKELY MECHANISM. HEAVY RAIN MAY BE ONGOING AT THE  
START OF THE PERIOD ACROSS PORTIONS OF THE TN/OH VALLEYS, AND THE  
THREAT FOR HEAVY RAIN MAY CONTINUE AT LEAST EARLY INTO THE MEDIUM  
RANGE UNTIL THE SURFACE COLD FRONT CLEARS THOSE AREAS SOMETIME ON  
SAT. SURFACE FRONTAL WAVE DEVELOPMENT ALONG THE  
SOUTHEAST/MID-ATLANTIC COASTLINE SUN-MON SHOULD SPREAD AN  
ORGANIZED ROUND OF PRECIP ACROSS AREAS FROM THE CAROLINAS  
NORTHWARD. WHILE LOW-LEVEL THERMAL FIELDS REMAIN SOMEWHAT  
UNCERTAIN, SOME POTENTIAL FOR WINTER WEATHER EXISTS, ESPECIALLY  
FOR THE APPALACHIANS AND INTERIOR MID-ATLANTIC/NORTHEAST. THE  
CURRENT FORECAST WOULD LIKELY MEAN A RAIN/SNOW LINE SITUATED A BIT  
WEST OF THE I-95 CORRIDOR, AND A COLD RAIN FOR THE MAJOR CITIES,  
GIVEN THAT THE ARCTIC FRONT WOULD STILL BE TO THE WEST OF THE  
REGION AND NO OBVIOUS SOURCE OF LOW-LEVEL COLD AIR IS EVIDENT.  
CHRISTMAS EVE/DAY SNOW WOULD BE POSSIBLE, HOWEVER, FROM THE  
APPALACHIANS INTO THE INTERIOR NORTHEAST. THERE WOULD BE SOME  
POTENTIAL FOR A CHANGEOVER TO SNOW LATE ON CHRISTMAS DAY CLOSER TO  
THE NORTHEAST U.S. COAST, DEPENDING ON THE EXACT STRENGTH AND  
TRACK OF THE LOW.  
 
TEMPERATURES ACROSS THE CENTRAL U.S. WILL QUICKLY DECREASE SAT-MON  
AS A COUPLE SURGES OF ARCTIC AIR SPREAD INCREASINGLY COLD  
TEMPERATURES SOUTHWARD. THE COLDEST TEMP ANOMALIES WILL INITIALLY  
BE JUST EAST OF THE ROCKIES SAT-SUN, WITH HIGH TEMPS 15 TO 30 DEG  
F BELOW AVERAGE. BY MON-TUE, THE COLDEST ANOMALIES, ALSO 15 TO 30  
DEG F BELOW AVERAGE, WILL SHIFT TO THE NORTHERN PLAINS AND UPPER  
MIDWEST, WHERE HIGH TEMPERATURES FOR SOME AREAS WILL REMAIN BELOW  
ZERO, AND LOWS MAY SURPASS -20 DEG F. RECORDS LOWS ARE POSSIBLE AT  
SOME LOCATIONS. MEANWHILE, ABOVE AVERAGE TEMPS WILL PERSIST ACROSS  
THE EASTERN U.S. UNTIL THE FIRST COLD FRONT IS ABLE TO PASS. HIGH  
TEMPS SAT ARE FORECAST TO BE 10-20 DEG F ABOVE AVERAGE FOR THE  
SOUTHEAST, OH VALLEY, AND MID-ATLANTIC (70S IN THE SOUTHEAST AND  
NEAR 60 IN THE MID-ATLANTIC). TEMPERATURES ACROSS THESE AREAS WILL  
DECREASE TO 5 TO 10 DEG F BELOW AVERAGE BY MON, WITH 10-20 DEG F  
BELOW AVERAGE HIGHS POSSIBLE BY THE MIDDLE OF NEXT WEEK WITH THE  
ARRIVAL OF THE MODIFIED ARCTIC AIR.  
 
RYAN  
 

 
 
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