Prognostic Meteorological Discussion
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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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