278  
FXUS02 KWBC 271605  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1104 AM EST WED DEC 27 2017  
 
VALID 12Z SAT DEC 30 2017 - 12Z WED JAN 03 2018  
 
...ARCTIC AIR WILL CONTINUE ITS DOMINANCE EAST OF THE ROCKIES INTO  
THE NEW YEAR...  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT..
 
 
THE MAIN FEATURE OF THE MEDIUM RANGE PERIOD IS MULTIPLE WAVES OF  
SHORTWAVE ENERGY DROPPING THROUGH THE WESTERN U.S. WHICH SHOULD  
ACT TO MAINTAIN TROUGHING ACROSS THE EASTERN U.S. AS WE MOVE INTO  
THE NEW YEAR. THE 00Z ECMWF RUN SHOWS MUCH STRONGER RIDGING OVER  
THE WEST COAST THAN ITS PREVIOUS FEW RUNS WHICH ALLOWS FOR MORE  
AMPLIFICATION OF TROUGHING ACROSS THE EAST. THOUGH STILL NOT AS  
AMPLIFIED/DEEP AS THE GFS...THE LASTEST RUN OF THE ECMWF AT LEAST  
SUGGESTS A POSSIBLE TREND IN THIS DIRECTION. AMPLIFIED TROUGHING  
IS ALSO SHOWN BY THE ENSEMBLE MEANS BUT WITH WEAKER RIDGING OUT  
WEST DUE TO SIGNIFICANT UNCERTAINTY WITH A SHORTWAVE PUSHING  
EASTWARD THROUGH THE EAST PAC AND POSSIBLY IMPACTING THE SOUTHWEST  
U.S. FOR THE EASTERN U.S. TROUGH...THE WPC PROGS FAVOR AN EQUAL  
BLEND OF THE 00Z ECMWF/06Z GFS WITH THEIR RESPECTIVE ENSEMBLE  
MEANS.  
 
AS MENTIONED PREVIOUSLY...ANOTHER HIGHLY UNCERTAIN FEATURE OF THE  
MEDIUM RANGE PERIOD IS THE POSITIONING OF A SHORTWAVE  
TROUGH/CLOSED LOW ACROSS THE EAST PAC AND WHETHER OR NOT THIS  
TROUGH MOVES INLAND ACROSS THE SOUTHWEST U.S. BY DAYS 6 AND 7. BIG  
SHIFT IN CONTINUITY WITH THE 00Z ECMWF WHICH NOW SUGGESTS THE LOW  
STAYS WELL OFFSHORE...WHICH IS IN MUCH CLOSER AGREEMENT TO THE  
PREVIOUS FEW RUNS OF THE GFS. HOWEVER...THERE ARE STILL MANY EC  
AND GEFS ENSEMBLE MEMBERS SUGGESTING SOUTHWEST U.S. TROUGHING AND  
THEREFORE THE MEANS ALSO SHOW WEAK TROUGHING MOVING ONSHORE. GIVEN  
THE SPREAD...ITS HARD TO LATCH ONTO ON SOLUTION OVER THE OTHER AND  
THUS THE WPC PROGS FAVORED AN EQUAL BLEND OF THE OPERATIONAL AND  
ENSEMBLE MEANS. COMPARED TO THE PREVIOUS SHIFT...THIS WOULD MEAN  
STRONGER RIDGING ACROSS THE WESTERN U.S. WITH A MUCH WEAKER  
SEMBLANCE OF TROUGHING ACROSS THE SOUTHWEST...WITH THE STRONGER  
TROUGHING/LOW REMAINING OFFSHORE. AGAIN...THIS PARTICULAR SOLUTION  
IS WITH VERY LOW CONFIDENCE GIVEN THE CONTINUED SPREAD.  
 
   
..POTENTIAL WEATHER/THREATS HIGHLIGHTS
 
 
OVERALL...IT SEEMS QUITE LIKELY THAT MULTIPLE SHOTS OF REINFORCING  
ARCTIC AIR WILL ENSURE FRIGID TEMPERATURES OVER THE NATION FROM  
THE ROCKIES TO THE SOUTHERN AND EASTERN U.S. OVER THE NEXT WEEK AS  
COLD HIGH PRESSURE SETTLES TO THE EAST OF A SHARP QUASI-STATIONARY  
FRONT BANKING UP AGAINST THE ROCKIES. A DOWNSTREAM WEEKEND WESTERN  
ATLANTIC COASTAL LOW DEVELOPMENT WOULD EFFECT MARITIME INTERESTS  
AND OFFER SOME IMPACT FOR THE EASTERN SEABOARD...BUT THE FORECAST  
TREND IS OFFSHORE. PERIODIC SURGES OF COLD CYCLONIC FLOW OVER THE  
GREAT LAKES FAVORS ADDITIONAL LAKE EFFECT SNOWS...PROLONGING AN  
ACTIVE RECENT PATTERN. ACROSS THE GULF COAST...ANY MOISTURE RETURN  
WITH IMPULSE PASSAGES COULD BRING A WINTERY OVERRUNNING  
PRECIPITATION THREAT.  
 
LOCATIONS OUT WEST WILL GENERALLY STAY ON THE MILD SIDE...BUT A  
WEEKEND SHORTWAVE TROUGH PASSAGE WOULD FAVOR A SWATH OF COOLED  
PRECIPITATION FROM THE NORTHWESTERN U.S. INTO THE NORTHERN ROCKIES  
AND NORTH-CENTRAL STATES. PRECIPITATION IN THE SOUTHWEST TO START  
THE NEW YEAR IS HIGHLY DEPENDENT ON HOW THE TROUGHING ACROSS THE  
EAST PAC PLAYS OUT. OF COURSE...A MORE OFFSHORE SOLUTION WOULD  
FAVOR DRYER CONDITIONS (LIKE THAT OF THE GFS AND TO SOME EXTENT  
THE 00Z ECMWF)...BUT ANY ENERGY WHICH DOES MAKE IT ONSHORE MAY  
MEAN ORGANIZED PRECIPITATION ACROSS PORTIONS OF CALIFORNIA AND THE  
SOUTHWEST.  
 
SANTORELLI  
 

 
 
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