413  
FXUS02 KWBC 270659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 AM EST WED DEC 27 2017  
 
VALID 12Z SAT DEC 30 2017 - 12Z WED JAN 03 2018  
 
...ARCTIC AIR CONTINUE ITS DOMINANCE EAST OF THE ROCKIES INTO THE  
NEW YEAR...  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT..
 
 
MODEL AND ENSEMBLE FORECAST SPREAD REMAINS WELL ABOVE NORMAL WITH  
BASICALLY TWO MAIN FORECAST CAMPS. GUIDANCE VARIANCE SEEMS TO  
MAINLY STEM FROM THE UNCERTAIN HANDLING OF MID-LOWER LATITUDE  
SYSTEMS EMINATING FROM THE E-CENTRAL PACIFIC TO THE WRN US. RECENT  
ECMWF AND TO A LESSER EXTENT ECMWF ENSEMBLE RUNS PROGRESS TROUGH  
ENERGIES INTO THE WEST COAST AND BUILD LEAD RIDGING OVER THE  
INTERMOUNTAIN WEST...SHUNTING COLD NRN STREAM TROUGH DIGGING  
INCREASINGLY WITH TIME FROM THE CENTRAL TO THE EAST-CENTRAL US  
NEXT WEEK. RECENT GFS AND TO A LESSER EXTENT GEFS RUNS DELAY  
PACIFIC MID-LOWER LATTITUDE ENERGY INSURGENCE INTO THE WRN US AND  
HOLD MORE RIDGING OFFSHORE. THIS ALLOWS ROOM FOR MUCH MORE COLD  
NRN STREAM DIGGING/AMPLITUDE THROUGH THE W-CENTRAL THEN  
EAST-CENTRAL US DAYS 4-7 THAN ECMWF/ECMWF ENSEMBLES.  
 
WHILE EITHER FORECAST CAMP IS PLAUSIBLE...IT IS DISTURBING THAT  
BOTH CAMPS OF GUIDANCE ARE WELL CLUSTERED WITHIN  
THEMSELVES...SUGGESTING ABOVE NORMAL PREDICTABILITY IF YOU DON'T  
CONSIDER THE OTHER SEEMINGLY OPPOSITE AND MUTUALLY EXCLUSIVE CAMP.  
ENSEMBLE MEMBERSHIP WOULD BETTER ILLISTRATE FORECAST  
SPREAD/UNCERAINTY IT DESIGNED LESS DIRECTLY TIED TO THE  
INITIALIZATION AND PHYSICS OF THE PARENT MODEL. AM LEARY TO  
SIGNIFICANTLTY REDUCE THE EFFECTS OF NRN STREAM COOLING INTO THE  
CENTRAL AND ERN US GIVEN RECENT FLOW GIVEN UNCERTAIN INFLUX OF  
PACIFIC ENERGIES INTO THE WEST WHERE THE ECMWF SEEMS  
OVERDONE...BUT SOME GRADUAL MODIFICATION SEEMS PRUDENT GIVEN BOTH  
GEFS AND ECMWF ENSEMBLE MEANS OFFER LESS NRN STREAM AMPLITUDE THAN  
RECENT GFS RUNS. OVERALL...THE WPC MEDIUM RANGE PRODUCT SUITE WAS  
PRIMARILY DERIVED WITH THIS FORECAST THOUGHT PROCESS...IN NO MANS  
LAND BETWEEN THE TWO CAMPS. THE 00 UTC GFS HAS BACKED OFF SOME  
AMPLITUDE AS WELL...BUT THE 00 UTC ECMWF IS DELAYED.  
 
   
..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 SRN AND ERN US OVER THE NEXT WEEK AS COLD HIGH  
PRESSURE SETTLES TO THE EAST OF A SHARP QUASI-STATIONARY FRONT  
BANKING UP AGAINST THE ROCKIES. DOWNSTREAM WEEKEND WRN ATLANTIC  
COASTAL LOW DEVELOPMENT WOULD EFFECT MARITIME INTERESTS AND OFFER  
SOME IMPACT FOR THE ERN 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 WOULD BRING A WINTERY OVERRUNNING PRECIPITATION  
THREAT.  
 
LOCATIONS OUT WEST WILL GENERALLY STAY ON THE MILD SIDE...BUT  
WEEKEND SHORTWAVE TROUGH PASSAGE FAVORS A SWATH OF COOLED PCPN  
FROM THE NWRN US ONWARD OUT THROUGH THE NRN ROCKIES AND N-CENTRAL  
US. THE POTENTIAL FOR ORGANIZED INTO THE NEW YEAR ALSO REMAINS FOR  
CA AND TO A LESSER EXTENT THE INTERMOUNTAIN WEST THEN SWRN US/SRN  
ROCKIES ASSUMING MID-LOWER LATITUDE PACIFIC TROUGH ENERGIES WORK  
INLAND MORE IN LINE WITH THE ECMWF THAN THE MUCH DRIER GFS.  
 
SCHICHTEL  
 

 
 
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