467  
FXUS02 KWBC 111538  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1038 AM EST MON DEC 11 2017  
 
VALID 12Z THU DEC 14 2017 - 12Z MON DEC 18 2017  
   
..OVERVIEW
 
 
UPPER RIDGING IN THE WEST WILL FINALLY BE KNOCKED DOWN LATE THIS  
WEEK BUT BROAD CYCLONIC FLOW WILL STILL DOMINATE THE EAST. THIS  
WILL GUIDE A SERIES OF MAINLY PACIFIC SYSTEMS ACROSS THE LOWER 48  
AS THE ARCTIC BOUNDARY WOBBLES NEAR THE US/CANADIAN BORDER. A SLOW  
MODERATING TREND WILL UNFOLD ACROSS THE EAST THROUGH THE WEEKEND  
WHILE THE WEST WILL SEE NEAR TO ABOVE AVERAGE TEMPERATURES.  
PRECIPITATION WILL FINALLY COME BACK TO THE PACIFIC NORTHWEST BY  
FRIDAY BUT LIMITED TO WA/OR.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
LATEST 00Z GUIDANCE SUITE OFFERED A REASONABLE STARTING POINT TO  
THE FORECAST FOR THU-FRI (BLEND OF THE GFS/ECMWF/UKMET/CANADIAN).  
00Z ECMWF/CANADIAN WERE IN THE MIDDLE OF THE SPREAD ACROSS THE  
LOWER MS VALLEY AS A POSITIVELY TILTED TROUGH SWINGS THROUGH THE  
REGION (GFS QUICKER, UKMET SLOWER) WHICH FIT BEST WITH THE  
ENSEMBLE MEANS. TO THE NORTHWEST THERE WAS MUCH MORE DISAGREEMENT  
ON TIMING OF THE SHORTWAVE INTO WESTERN WASHINGTON FRIDAY THOUGH  
THE ECMWF AND ITS ENSEMBLES HAVE BEEN MORE CONSISTENT THAN THE  
GFS/GEFS. FOR NEXT SUN/MON, UPPER TROUGH WILL PROGRESS EASTWARD  
OUT OF THE CENTRAL PLAINS AND CARRY THAT FRONT TO THE EAST COAST  
WHILE ANOTHER FLOWS THROUGH THE NORTHWEST INTO THE HIGH PLAINS.  
AGAIN, TIMING DIFFERENCES EXPAND WITH TIME PER THE DETERMINISTIC  
MODELS AS THEY SPLIT THE TROUGH AND EITHER FOCUS ON THE NORTHERN  
OR SOUTHERN PORTION. A CONSENSUS BLEND SERVED TO MINIMIZE CHANGES  
FROM OVERNIGHT. THIS SUGGESTED LEAVING ANOTHER SOUTHERN POTENTIAL  
VORTICITY TAIL JUST WEST OF BAJA CALIFORNIA AS HEIGHTS BUILD  
THROUGH THE WESTERN CARIBBEAN.  
 
FRACASSO  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THERE WILL ONLY BE A BRIEF PERIOD OF BELOW AVERAGE TEMPERATURES  
AND THIS APPEARS TO BE CONFINED TO LOCATIONS EAST OF THE MS RIVER  
ON THURSDAY/FRIDAY. THE FINAL ARCTIC SURGE SHOULD BRING A  
REINFORCEMENT OF COLD AIR TO THE REGION WITH THE MOST ANOMALOUS  
TEMPERATURES BEING OVER NEW ENGLAND WITH ANOMALIES IN THE 10 TO 20  
DEGREE BELOW CLIMATOLOGY RANGE. THIS SUGGESTS HIGHS ON THURSDAY  
SHOULD STRUGGLE TO GET OUT OF THE UPPER SINGLE DIGITS AND TEENS  
OVER INTERIOR NEW ENGLAND. OTHERWISE...MUCH OF THE COUNTRY SHOULD  
SEE ABOVE AVERAGE TEMPERATURES AS THE FLOW FLATTENS AND BECOMES  
MORE PACIFIC DOMINANT. DOWNSLOPING FLOW IN THE LEE OF THE ROCKIES  
WILL RAISE TEMPERATURES INTO THE 50S AS FAR NORTH AS EASTERN MT ON  
FRIDAY. THIS IS EASILY 15 TO 20 DEGREES ABOVE AVERAGE.  
 
LOOKING AT PRECIPITATION DURING THE PERIOD...SOME LAKE EFFECT  
ACTIVITY IS POSSIBLE ALTHOUGH IT SHOULD BE MORE TAME GIVEN A  
CHANGING SYNOPTIC-SCALE PATTERN. WHILE PRECIPITATION CHANCES WILL  
LOOM THROUGHOUT THE PERIOD...IT SHOULD BE WARM ADVECTION DRIVEN  
VERSUS MESOSCALE BANDING RELATED. OUT WEST...CONDITIONS WILL BE  
MUCH MORE ACTIVE THAN THEY HAVE BEEN THIS MONTH GIVEN THE  
FLATTENING OF THE RIDGE. WIDESPREAD RAIN/SNOW SHOULD BLANKET THE  
PACIFIC NORTHWEST AND LOCATIONS DOWNSTREAM INTO THE CENTRAL GREAT  
BASIN/UPPER INTERMOUNTAIN WEST/NORTHERN ROCKIES. GIVEN A LACK OF  
COLD AIR...SNOWFALL SHOULD BE RELEGATED TO THE HIGHER ELEVATIONS.  
THIS THREAT COULD CONTINUE THROUGH DAY 7/DECEMBER 18 GIVEN THE  
SEMI-ZONAL FLOW IN PLACE. ELSEWHERE...WARM ADVECTION SHOWERS ARE  
POSSIBLE FROM THE WESTERN GULF COAST TOWARD THE TN  
VALLEY/SOUTHEASTERN U.S. THIS WEEKEND ALTHOUGH MODEL/ENSEMBLE  
SPREAD OF QPF IS HIGH.  
 
RUBIN-OSTER  
 

 
 
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