186  
FXUS02 KWBC 200630  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
129 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 AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MUCH TO TALK ABOUT TONIGHT - HIGHLY AMPLIFIED UPPER-LEVEL PATTERN  
IS EXPECTED TO BE IN PLACE ACROSS NORTH AMERICA BY DAY 3 (WED).  
THIS PATTERN WILL CONSIST OF AN ELONGATED RIDGE AXIS EXTENDING  
FROM THE EAST-CENTRAL SUBTROPICAL PACIFIC NORTHWARD ALONG THE  
NORTH AMERICAN WEST COAST AND NEARLY TO THE NORTH POLE. THE  
STRONGEST HEIGHT ANOMALIES ASSOCIATED WITH THE RIDGE (+3 STANDARD  
DEVIATIONS) WILL INITIALLY BE CENTERED ACROSS NORTHWESTERN CANADA.  
THE RIDGE WILL REMAIN PRESENT THROUGH THE PERIOD, ALTHOUGH THERE  
ARE SIGNS AMONG THE GUIDANCE THAT THE SOUTHERN PORTION OF THE  
RIDGE AXIS COULD SHIFT WESTWARD BY THE MIDDLE OF NEXT WEEK,  
ALLOWING HEIGHTS TO FALL ALONG THE WEST COAST. DOWNSTREAM OF THE  
RIDGE, A BROAD AND DEEP UPPER TROUGH WILL BE AMPLIFYING ACROSS THE  
CENTRAL U.S. AT THE START OF THE FORECAST PERIOD AS A DEEP UPPER  
LOW MOVES SOUTHWARD FROM NEAR HUDSON BAY INTO ONTARIO. THE  
CONSENSUS IS THAT THE TROUGH AXIS WILL GRADUALLY SHIFT EASTWARD  
INTO NEXT WEEK, ALTHOUGH QUESTIONS REMAIN SURROUNDING EXACTLY HOW  
QUICKLY THIS OCCURS. THE FLOW BETWEEN THE RIDGE AXIS AND THE DEEP  
UPPER LOW WILL BE ORIENTED NEARLY NORTH TO SOUTH FROM THE HIGH  
LATITUDES INTO THE CENTRAL U.S., ALLOWING ABUNDANT ARCTIC AIR TO  
SPILL SOUTHWARD INTO THE NORTHERN ROCKIES, PLAINS, AND MIDWEST.  
WHILE MODELS SHOW GENERAL CONSENSUS ON THE LARGE SCALE PATTERN,  
DIFFERENCES WITH RESPECT TO THE SPECIFICS OF THE PATTERN EVOLUTION  
AND THE DETAILS OF INDIVIDUAL SYSTEMS ARE RATHER LARGE.  
 
A SURFACE LOW/FRONT TRAVERSING THE OH/TN VALLEYS AND EASTERN U.S.  
WED-THU WILL BE REINFORCED BY THE ARRIVAL/MERGER OF A POLAR FRONT,  
SETTING UP A CONTRAST BETWEEN WARM SUBTROPICAL AIR EAST OF THE  
BOUNDARY AND CHILLY POLAR AIR WEST OF THE BOUNDARY. MODELS STILL  
SHOW A RATHER LARGE AMOUNT OF SPREAD AROUND THIS SURFACE LOW ON  
DAY 3, WITH THE ECMWF MUCH SLOWER/FARTHER SOUTH RELATIVE TO THE  
GFS. THE ECENS/GEFS MEANS WERE LARGELY IN THE MIDDLE, AND THERE  
SEEMED TO BE DECENT ENSEMBLE MEMBER CLUSTERING AROUND THESE MEANS  
DESPITE THE FACT THAT NO DETERMINISTIC SOLUTION RESEMBLES THIS  
POSITION. THUS, THE DAY 3-4 FORECAST PROGS HAD A SOMEWHAT LARGER  
AMOUNT OF ECENS/GEFS MEAN (ABOUT 50 PERCENT) THAN WOULD NORMALLY  
BE THE CASE, WITH THE OTHER HALF CONSISTING OF THE DETERMINISTIC  
ECMWF/GFS. THE FRONT SHOULD STALL BRIEFLY ACROSS THE SOUTHEAST AS  
A WAVE OF LOW PRESSURE DEVELOPS ALONG IT, WHICH IS THEN EXPECTED  
TO MOVE NORTHEASTWARD ALONG/JUST OFF THE EASTERN SEABOARD. THE GFS  
HAS CONSISTENTLY BEEN ON THE FAST SIDE WITH MOVING THIS SURFACE  
LOW NORTHWARD ON CHRISTMAS DAY (DAY 5), WHILE THE 12Z ECMWF WAS  
MUCH CLOSER TO THE ECENS/GEFS MEAN CONSENSUS, WHICH IS SUPPORTED  
BY A DECENT CLUSTERING OF MEMBERS FROM BOTH ENSEMBLES. THUS, THE  
ECMWF IS GENERALLY THE PREFERRED DETERMINISTIC SOLUTION FOR  
HANDLING OF THE EAST COAST SURFACE LOW DAYS 5-6. THE GFS WAS  
REMOVED ENTIRELY FROM THE FORECAST BLEND STARTING ON DAY 5, IN  
FAVOR OF INCREASED ENSEMBLE MEAN WEIGHTING ALONG THE OPERATIONAL  
ECMWF.  
 
BY DAYS 6-7 THE UPPER TROUGH SHOULD BE MOVING EASTWARD, SPREADING  
THE ARCTIC FRONT AND SOME AMOUNT OF MODIFIED ARCTIC AIR EAST WITH  
IT. AS MENTIONED, SOME UNCERTAINTY REMAINS REGARDING HOW QUICKLY  
THIS OCCURS. THE GFS HAS BEEN PERSISTENT IN MOVING THE TROUGH AXIS  
AND UPPER LOW EASTWARD MUCH MORE QUICKLY THAN THE ECMWF AND THE  
ENSEMBLE MEANS. THUS, AS WITH DAY 5, A BLEND OF THE ECMWF ALONG  
WITH THE ECENS/GEFS MEANS WAS THE PREFERRED APPROACH FOR DAYS 6-7.  
THIS WOULD BRING THE ARCTIC FRONT ACROSS THE APPALACHIANS ON DAY 6  
(TUE), WITH THE MODIFIED ARCTIC AIR MASS SPREADING ACROSS MUCH OF  
THE EASTERN U.S. BY DAY 7 (WED).  
 
   
..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 10-20 DEG F ABOVE AVERAGE FOR THE  
SOUTHEAST, OH VALLEY, AND 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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