381  
FXUS02 KWBC 181603  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1102 AM EST MON DEC 18 2017  
 
VALID 12Z THU DEC 21 2017 - 12Z MON DEC 25 2017  
 
...ABOVE AVERAGE CONFIDENCE IN THE POTENTIAL FOR AN OUTBREAK OF  
ARCTIC AIR ACROSS THE CENTRAL U.S. BY THE WEEKEND...  
 
   
..OVERVIEW
 
 
MODELS CONTINUE TO LOCK ON TO A SIGNIFICANT AMPLIFICATION OF THE  
FLOW PATTERN THROUGH CHRISTMAS MORNING - RIDGE EXTENDING FROM THE  
WEST COAST TO THE ARCTIC AND A MEAN TROUGH FROM THE HUDSON BAY  
AREA TO THE DESERT SOUTHWEST/MEXICO. THIS SETUP WILL FAVOR COLD  
AIR TO SURGE SOUTHWARD... WHICH WILL CREATE A SHARP THERMAL  
GRADIENT ACROSS THE SOUTHERN TIER STATES AS WARM, MOIST AIR LIFTS  
NORTHWARD (THANKS TO A STRENGTHENING SUB-TROPICAL RIDGE OFFSHORE  
THE SOUTHEAST COAST). THIS BOUNDARY WILL MORE OR LESS REMAIN  
ACROSS THE SOUTHERN PLAINS/GULF STATES THROUGH THE EXTENDED PERIOD  
AND WILL ACT AS A FOCUS FOR WIDESPREAD PRECIPITATION... WITH  
OCCASIONAL WINTER WEATHER NORTH OF THE BOUNDARY AND A LONG  
DURATION RAIN ALONG AND SOUTH OF THE FRONT. BRIEF PERIODS OF  
HEAVIER RAINFALL MAY BE POSSIBLE. SHORTWAVE ENERGY PASSING THROUGH  
SOUTHERN CANADA AND THE NORTHERN U.S. WILL KEEP THE UPPER  
MISSISSIPPI VALLEY TO THE NORTHEAST COLD AND SNOWY. NORTHWESTERLY  
WINDS WILL LIKELY INDUCE LAKE ENHANCED SNOW FOR THE FAVORED  
UPSLOPE REGIONS IN NEW YORK AND MAINE.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
OVERALL THE DETERMINISTIC AND ENSEMBLE MEANS ARE IN STRONG  
AGREEMENT THAT A LARGE SCALE CHANGE WILL OCCUR OVER THE NEXT  
WEEK...ALTHOUGH DIFFERING IN REGARDS TO THE TIMING AND LOCATIONS  
OF INDIVIDUAL SHORTWAVES. THE ECWMF AND THE GFS HAS CONTINUED TO  
TREND FASTER WITH THE EASTWARD PROGRESSION OF THE TROUGH. THE CMC  
WAS REMAINED SLOW WHICH APPEARED TO BE AN OUTLIER. GUIDANCE BEGAN  
AS A BLEND OF THE 00Z ECWMF AND 06 GFS...AS THE TROUGH AMPLIFIES  
THE DEGREE OF SPREAD AMONGST GUIDANCE WIDENED AS WELL. FOR  
CHRISTMAS EVE AND CHRISTMAS THE BLEND HAD A HEAVIER WEIGHTING OF  
THE 06Z ENSEMBLE MEAN AND LESSER WEIGHTING OF THE EC/GFS.  
   
..WEATHER HIGHLIGHTS/THREATS
 
 
A BAND OF SNOW IS FORECAST TO TRACK FROM THE CENTRAL PLAINS TO  
THE NORTHEAST AS A LOW PRESSURE PASSING THROUGH. THE HIGHEST  
ACCUMULATIONS ARE FAVORING THE UPPER MISSISSIPPI VALLEY/UPPER  
GREAT LAKES. SNOW WILL ALSO BE POSSIBLE OVER NEW ENGLAND FRI/SAT  
WHERE WARM ADVECTION WILL BE THE LIKELY MECHANISM. THERE REMAINS  
SOME SPREAD OVER EXACTLY HOW STRONG THIS SYSTEM WILL BE, AND THUS  
FOR PRECISE INTENSITY OF SNOW/WINDS.  
 
THE AREA EXTENDING FROM FROM LOWER MS/TN VALLEYS INTO THE SOUTHERN  
APPALACHIANS HAS THE HIGHEST PROBABILITY OF SEEING THE HEAVIEST  
RAINFALL DURING THE MEDIUM RANGE. THIS WILL BE FROM A COMBINATION  
OF THE COLD FRONT CROSSING THE REGION LATER IN THE WEEK, AND THEN  
WITH OVERRUNNING PRECIP NORTH OF THE FRONT THAT SHOULD SETTLE NEAR  
THE GULF/CAROLINA COASTS BY THE WEEKEND.  
 
THE CENTRAL U.S. WILL HAVE A SIGNIFICANT DROP IN TEMPERATURES  
LATER IN THE WEEK. A VAST AREA WILL HAVE MAXIMUM TEMPERATURES  
FALLING FROM 10-20 DEG F ABOVE NORMAL TO 20-30 DEG F BELOW NORMAL  
BY NEXT WEEKEND. RELATIVE WARMTH OVER THE EAST WITH SOME ANOMALIES  
EXCEEDING +10 DEG F SHOULD EXTEND THROUGH SAT BEFORE BEGINNING TO  
FADE BY SUN-MON WITH FRONTAL PASSAGE.  
 
CAMPBELL  
 

 
 
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