776  
FXUS02 KWBC 031522  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1021 AM EST SUN DEC 03 2017  
 
VALID 12Z WED DEC 06 2017 - 12Z SUN DEC 10 2017  
   
..MILD AND DRY IN THE WEST BUT COLD AND WINTRY IN THE EAST
 
 
   
..PATTERN OVERVIEW/GUIDANCE EVALUATION/PREFERENCES
 
 
HIGH AMPLITUDE FLOW WILL DOMINATE THE MEDIUM RANGE PERIOD WITH A  
STAGNANT/PERSISTENT UPPER RIDGE HOLDING STRONG ACROSS THE WESTERN  
U.S. THE 00Z GFS 500-MB HEIGHT ANOMALIES SHOW DEPARTURES FROM  
CLIMATOLOGY EXCEEDING 3 STANDARD DEVIATIONS AT TIMES OVER BRITISH  
COLUMBIA/ALBERTA. REFLECTED AT THE SURFACE WILL BE A ROBUST AREA  
OF HIGH PRESSURE ACROSS THE INTERMOUNTAIN WEST WHICH WILL ENSURE  
OFFSHORE GRADIENTS ALONG THE WEST COAST. THE COUNTERPART TO THIS  
RIDGE WILL BE A LONGWAVE TROUGH EXTENDING FROM THE GREAT PLAINS  
EASTWARD WITH MULTIPLE REINFORCING CLIPPER SYSTEMS HELPING  
MAINTAIN THIS NEGATIVE HEIGHT ANOMALY. A PAIR OF EMBEDDED  
SHORTWAVES ARE WORTH NOTING AS THEY EACH DEVELOP WAVES OF LOW  
PRESSURE WHICH COULD BE TRIGGERS FOR WINTRY PRECIPITATION. THE  
INITIAL BATCH OF HEIGHT FALLS TRAVERSING THROUGH THE  
MID-SOUTH/SOUTHEASTERN U.S. ON FRIDAY MORNING WILL FORM A SURFACE  
LOW OFF THE CAROLINA COAST WHICH HAS TRENDED WESTWARD IN RECENT  
MODEL CYCLES. ADDITIONALLY...
A STRONG CLIPPER SYSTEM SWINGING  
THROUGH THE GREAT LAKES/OH RIVER VALLEY LATE FRIDAY WILL BRING A  
REINFORCING SHOT OF ARCTIC AIR TO THE REGION FOR NEXT WEEKEND. A  
MILLER-B TYPE SETUP MAY UNFOLD AS A DEEP SURFACE LOW TRAVERSING  
THE GREAT LAKES MAY GIVE WAY TO COASTAL WAVE DEVELOPMENT OFF THE  
MID-ATLANTIC COAST ON SATURDAY.  
 
GIVEN THE HIGHLY AMPLIFIED REGIME IN STORE...CONFIDENCE IN THE  
FORECAST SOLUTIONS IS ABOVE AVERAGE AT LEAST THROUGH THURSDAY. THE  
ONLY TRUE DIFFERENCE IS WITH REGARDS TO A WEAK ELONGATED BAND OF  
VORTICITY STRETCHING ALONG THE SOUTHWESTERN U.S. BORDER WITH MX  
WHICH THE 00Z CMC SHOWS DOES NOT EXIST. SHIFTING FOCUS TO THE  
INITIAL WAVE DEVELOPMENT OFF THE SOUTHEASTERN U.S.  
COAST...MULTI-DAY ENSEMBLE LOW PLOTS HAVE SHOWN A WESTWARD SHIFT  
IN TIME. A VAST MAJORITY OF THE 00Z ENSEMBLE MEMBERS FAVOR A  
POSITION A COUPLE HUNDRED MILES EAST OF THE CAROLINAS ON 08/1200Z.  
THE 00Z CMC IS WELL TO THE EAST WHICH SITS ON THE FAR EASTERN EDGE  
OF THE SPREAD. THE 06Z/00Z GFS AND 00Z ECMWF AGREE ON LIFTING THE  
CYCLONE JUST EAST OF THE 40N/70W BENCHMARK ON FRIDAY EVENING  
ALTHOUGH THE 06Z GFS SITS TO THE WEST AND IS INCREDIBLY DEEP.  
WHILE A POSSIBILITY...IT DOES APPEAR TO BE A REMOTE OUTCOME AT  
THIS TIME. THE ROUTE DEPICTED BY THE 00Z GFS/ECMWF IS THE  
PREFERRED SOLUTION IN THIS CASE. REGARDING THE TRAILING...YET MORE  
POTENT WAVE CROSSING THE GREAT LAKES...THE 06Z/00Z GFS CONTINUE TO  
BE MORE AMPLIFIED THAN THE CONSENSUS. THE ONLY SUPPORT THEY HAVE  
IS THE 00Z UKMET WHICH FAVORS A SECOND SURFACE LOW PUSHING THROUGH  
THE OH VALLEY ON 09/0000Z. ON THE CONTRARY...THE PREVIOUS TWO  
ECMWF RUNS AS WELL AS THE 00Z CMC ARE SOMEWHAT FLATTER AND PUT  
GREATER EMPHASIS ON THE NORTHERN MOST WAVE. ENSEMBLE SPAGHETTI  
PLOTS SHOW THE 00Z GFS IS A SOUTHERN OUTLIER REGARDING THE  
AMPLITUDE OF THE TROUGH WITH VERY FEW OF ITS OWN MEMBERS  
SUPPORTING SUCH A SOLUTION. SOME HINT COASTAL DEVELOPMENT IS  
POSSIBLE DURING NEXT WEEKEND BUT UNCERTAINTY IS QUITE GREAT AT  
THIS TIME SCALE. LOOKING TO THE WEST...THE RIDGE REMAINS A FIXTURE  
THROUGHOUT THE FORECAST WITH SOME OPERATIONAL MODELS SHOWING A  
SHORTWAVE PUSHING INTO BRITISH COLUMBIA ON SATURDAY. GIVEN THE  
STOUT NATURE OF THIS RIDGE...ANY INFLUENCES FROM THIS WAVE WILL  
LIKELY BE MINIMAL.  
 
THE FORECAST APPROACH FEATURED A SIGNIFICANT CHUNK OF THE 00Z  
GFS/ECMWF THROUGH DAY 4/THURSDAY WITH SMALLER CONTRIBUTIONS FROM  
THEIR CORRESPONDING ENSEMBLE MEANS. BY DAY 5/FRIDAY...STARTED TO  
PULL AWAY FROM THE 00Z GFS ACROSS THE GREAT LAKES AND LEANED MORE  
ON THE 00Z ECMWF/ECMWF ENSEMBLE MEAN AND 06Z GEFS MEAN. WHILE  
UTILIZING AN ENSEMBLE-BASED APPROACH INTO NEXT WEEKEND...DID KEEP  
A SMALL FRACTION OF THE 00Z ECMWF IN THE BLEND GIVEN CERTAINTY ON  
THE SYNOPTIC SCALE.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
GENERALLY SPEAKING...COLDER THAN NORMAL TEMPERATURES WILL PREVAIL  
FROM THE CENTER OF THE COUNTRY EASTWARD. THE EXCEPTION WILL BE  
FROM SOUTHERN NM EASTWARD ALONG THE RIO GRANDE RIVER GIVEN ACTIVE  
PRECIPITATION EXPECTED. HIGHS ON WEDNESDAY/THURSDAY SHOULD STAY IN  
THE 40S TO LOW/MID 50S WHICH IS AROUND 10 TO 15 DEGREES BELOW  
CLIMATOLOGY. WITHIN THE LONGWAVE TROUGH ITSELF...WHILE COLD  
TEMPERATURES ARE EXPECTED...THEY WILL NOT BE EXTREME BY ANY NATURE  
GIVEN IT IS THE EARLY PART OF DECEMBER. IT WILL BRING A  
SIGNIFICANT SHOT OF COLD AIR TO THE SOUTHERN U.S. WHICH WILL ALLOW  
FOR BELOW FREEZING OVERNIGHT TEMPERATURES AS FAR SOUTH AS CENTRAL  
TX EASTWARD TO JUST NORTH OF THE GULF COAST FROM FRIDAY ONWARD.  
MEANWHILE...TEMPERATURES BELOW ZERO ARE POSSIBLE OVER SECTIONS OF  
THE UPPER MIDWEST.  
 
REGARDING PRECIPITATION...MODERATE TO HEAVY RAINFALL WILL BE  
POSSIBLE ALONG THE GULF COAST ON THURSDAY ALONG THE NEXT FRONTAL  
SURGE WHICH EVENTUALLY DEVELOPS THE MENTIONED AREA OF LOW PRESSURE  
OFF THE SOUTHEASTERN U.S. COAST. THE DEGREE OF DEVELOPMENT AS WELL  
AS PROXIMITY TO THE EASTERN SEABOARD WILL PLAY A ROLE IN HOW MUCH  
PRECIPITATION WILL LIFT NORTHWARD FROM THURSDAY-SATURDAY. GIVEN  
THE MULTITUDE OF ARCTIC SURGES...TEMPERATURES SHOULD BE COLD  
ENOUGH FOR SNOW WEST OF THE I-95 CORRIDOR INTO THE  
CENTRAL/NORTHERN APPALACHIANS. IT REMAINS TO BE SEEN IF THIS WILL  
EXTEND INTO THE ADJACENT LOWER ELEVATIONS. MEANWHILE...PERSISTENT  
CYCLONIC FLOW OVER THE GREAT LAKES WILL KEEP THE LAKE EFFECT TRAIN  
GOING THROUGHOUT THE FORECAST PERIOD. SOME FAIRLY IMPRESSIVE  
MULTI-DAY TOTALS WILL BE POSSIBLE GIVEN STEEP LAPSE RATES OVER THE  
RATHER MILD LAKE SURFACES. ELSEWHERE...THE PERSISTENT OFFSHORE  
FLOW ALONG THE WEST COAST WILL SUPPORT DRY/BREEZY CONDITIONS. THIS  
COMBINATION SHOULD ENHANCE THE THREAT FOR WILDFIRES ACROSS  
SOUTHERN CA AS MENTIONED IN THE STORM PREDICTION CENTER FIRE  
WEATHER OUTLOOK.  
 
RUBIN-OSTER  
 

 
 
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