266  
FXUS02 KWBC 231545  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1045 AM EST THU NOV 23 2017  
 
VALID 12Z SUN NOV 26 2017 - 12Z THU NOV 30 2017  
   
..PATTERN ASSESSMENT AND MODEL CHOICES
 
 
IN THE NEAR TERM AND TO BEGIN THE MEDIUM RANGE WE SEE A  
CONTINUATION OF THE PATTERN FEATURING UNSETTLED WEATHER OVER THE  
PACIFIC NORTHWEST...AN ANOMALOUSLY WARM RIDGE FROM THE SOUTHWEST  
TO THE GREAT PLAINS...AND TROUGHING ALONG/OFF THE EAST COAST.  
AFTER DAY 5...TUESDAY...THERE IS A CHANGE IN THE  
PATTERN...ALTHOUGH IT IS SOMEWHAT SUBTLE. FORCING MECHANISMS / A  
BULK OF COLD AIR GATHERING OFF THE ASIAN CONTINENT ACROSS THE  
NORTH PACIFIC LEADS TO A MORE PROGRESSIVE WAVE TRAIN...AND THE LOW  
OVER NORTHEAST CANADA BEGINS TO RETROGRADE TO LESSEN THE HEIGHT  
ANOMALIES ALONG THE EAST COAST OF NORTH AMERICA AND SUPPORT THE  
DEVELOPING ZONAL FLOW AND WAVE TRAIN STRETCHING FROM THE PACIFIC  
ALL THE WAY ACROSS THE U.S.  
 
THROUGH DAY 4 AND EARLY DAY 5 WE SEE ENOUGH AGREEMENT BETWEEN THE  
00Z ECMWF...UKMET...ECMWF ENSEMBLE MEAN...AND 06Z GEFS MEAN...FOR  
DECENT CONFIDENCE IN THE FORECAST DETAILS. THE GFS DISPLAYED RUN  
TO RUN DISCONTINUITIES BETWEEN THE 00Z AND 06Z RUN...AND BOTH WERE  
MUCH FASTER THAN THE GEFS MEAN...WHICH THROWS UP A RED FLAG...AND  
WE DID NOT INCORPORATE THE OPERATIONAL GFS IN OUR BLENDS. FARTHER  
IN TIME THE SPAGHETTI PLOTS START TO BREAK DOWN AND UNCERTAINTY  
GROWS AS THE FLOW REGIME COULD SUPPORT ANY NUMBER OF SHORT TO  
MEDIUM WAVELENGTH SYSTEMS MOVING IN RAPID SUCCESSION. IT IS  
ENCOURAGING...HOWEVER...THAT THE ECWMF ENSEMBLE MEAN AND GEFS MEAN  
ARE VERY SIMILAR EVEN OUT THROUGH DAY 7...SO THAT AT LEAST THERE  
IS AN EASY COURSE OF ACTION FOR DERIVING THE FORECAST. THE NAEFS  
MEAN...INFLUENCED BY THE CANADIAN ENSEMBLE...GOES SOMEWHAT OUT OF  
PHASE WITH THE OTHER TWO MEANS ON DAY 5-6...MAINLY OVER THE  
WESTERN U.S...BUT THEN COMES BACK INTO SYNC BY DAY 7...WHICH  
FURTHER BOOSTS CONFIDENCE IN THE LARGE SCALE PATTERN.  
 
SO IN FORMULATING OUR BLENDS WE INCORPORATED SOME ECMWF/UKMET  
EARLY ON...THEN WENT TO A 50/50 BLEND OF THE TWO MAJOR MEANS LATE  
DAY 5 ONWARD. THIS KEPT PRETTY GOOD CONTINUITY AS WELL...THE MAIN  
DIFFERENCE BEING MORE DEFINITION TO A LOW SPINNING UP ON THE  
CANADIAN PRAIRIES DAY 4-5.  
 
   
..IMPACTS
 
 
TELECONNECTIONS USING A PERSISTENT POSITIVE HEIGHT ANOMALY OVER  
THE NORTH ATLANTIC PROVIDE A PATTERN THAT BEARS STRONG RESEMBLANCE  
TO THE GEFS/EC MEAN BLEND...AND WOULD SUGGEST CONTINUED WET  
WEATHER IN THE PACIFIC NORTHWEST...A SECONDARY AXIS OF HEAVY RAIN  
CROSSING THE FLORIDA PENINSULA...AS WELL AS ABOVE AVERAGE  
TEMPERATURES FOR MUCH OF THE CONUS. RECORD WARMTH OVER THE WEST /  
SOUTHWEST AND PLAINS WILL LIKELY PERSIST THROUGH SUNDAY AND MONDAY  
WITH HIGHS IN THE 60S...70S...AND 80S.  
 
BURKE  
 

 
 
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