Prognostic Meteorological Discussion
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000
FXUS02 KWBC 051539
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1139 AM EDT SAT SEP 05 2015

VALID 12Z TUE SEP 08 2015 - 12Z SAT SEP 12 2015

...SYNOPSIS AND MODEL EVALUATION...

A DEEP LONGWAVE TROUGH WILL GRADUALLY SHIFT FROM THE NORTHWESTERN
QUARTER OF THE U.S. TOWARD THE NORTHEASTERN QUARTER OF THE COUNTRY
BY DAYS 6/7. ASSOCIATED WITH THIS TROUGH...A LEADING EDGE CYCLONE
THAT LIFTS THROUGH CANADA WILL INITIALLY INTRODUCE LOWER HEIGHTS
AND A SURFACE COLD FRONT INTO THE MIDWEST AND PLAINS. A SHORTWAVE
THEN DIGGING INTO THE BACK SIDE OF THE TROUGH WILL PRODUCE ANOTHER
LOW LEVEL CYCLONE...WHICH FORMS AS IT CROSSES THE GREAT LAKES ON
DAYS 5-7. THIS SYSTEM WILL PRODUCE A STRONGER PUSH ON THE FRONT IN
THE SOUTHERN PLAINS. BENEATH THE ACTIVE NORTHERN
STREAM...SUB-TROPICAL RIDGING OVER THE SOUTHERN PLAINS AND
SOUTHWEST STATES WILL RETROGRADE AND EXPAND INTO A DEVELOPING MEAN
RIDGE POSITION OFFSHORE OF THE WEST COAST...WHILE A SLOWLY
RETROGRADING AREA OF SLIGHTLY LOWER HEIGHTS ALONG THE GULF COAST
CONTINUES TO SEPARATE THIS RIDGE CENTER FROM THAT OVER THE WESTERN
ATLANTIC.

THE ENSEMBLE MEANS SHOW AGREEMENT AND RUN TO RUN CONTINUITY GIVES
US ABOVE AVERAGE CONFIDENCE IN THE LARGE SCALE FEATURES. THERE ARE
DIFFERENCES IN THE SPEED AT WHICH POST-TROPICAL IGNACIO APPROACHES
THE WEST COAST OF NORTH AMERICA...BUT THESE DIFFERENCES APPEAR NOT
TO FEEDBACK ONTO THE LARGER SCALE...AS THE CIRCULATION WEAKENS
WITHIN MEAN SHORTWAVE RIDGING. MODEL DIFFERENCES APPEAR MORE
GREATLY TIED TO HANDLING OF THE POLAR JET STREAM WITHIN FLOW THAT
IS INITIALLY PROGRESSIVE AND THEN AMPLIFIES ACROSS THE PACIFIC AND
NORTH AMERICA. AS NOTED IN THE PREVIOUS ISSUANCE...THE OPERATIONAL
ECMWF HAS BEEN ON THE FAST SIDE IN MOVING THE DEEP TROUGH THROUGH
THE NORTHERN U.S. THIS PERIOD. IT APPEARED TO BE TOO SLOW WITH THE
INITIAL CYCLONE LIFTING INTO CANADA...WHICH ALLOWED SOME OF THAT
ENERGY TO PHASE WITH THE UPSTREAM SHORTWAVE ARRIVING ON WEDNESDAY.
IT ALSO APPEARED TO BE AN OUTLIER OVER THE GULF OF ALASKA...TAKING
ONE SHORTWAVE INTO A NEGATIVE TILT AND SWINGING IT THROUGH A
LARGER SCALE POSITIVE TILT TROUGH VERY QUICKLY. THESE THINGS ADD
UP TO A FASTER SOLUTION OVERALL...AND ONE THAT BECOMES QUITE OUT
OF PHASE WITH THE MEANS OVER THE GULF OF ALASKA BY DAY 7.

WPC PREFERRED THE OPERATIONAL 00Z AND 06Z GFS...AND 06Z GEFS MEAN.
THE UKMET AND CANADIAN OFFERED VARYING DEGREES OF
SUPPORT...ALTHOUGH THEY WERE NOT USED TO DERIVE THE FORECAST
GRIDS...ESPECIALLY THE CANADIAN WITH ITS AGGRESSIVE BAJA
CALIFORNIA LANDFALL OF THE ANTICIPATED TROPICAL CYCLONE TO THE
SOUTH OF KEVIN IN THE EASTERN PACIFIC. TRACK GUIDANCE IS MORE IN
LINE WITH THE ECMWF/GFS AND ESPECIALLY THE GFS. AMONG THE GFS
RUNS...THE 06Z OPERATIONAL WAS SLIGHTLY PREFERRED OVER THE
CONUS...FOR BEING SOMEWHAT MORE PATIENT IN THE PROGRESSION OF THE
DEEP TROUGH...AND THE WPC GRIDS WERE DERIVED FROM WHAT AVERAGED
OUT TO BE A 40/30/20/10 BLEND OF THE TWO GFS RUNS...GEFS...AND 00Z
ECMWF ENSEMBLE MEAN.


...WEATHER HIGHLIGHTS...

THE SLOW MOVING FRONTAL ZONE IN THE CENTRAL AND EASTERN U.S. WILL
YIELD WIDESPREAD PRECIPITATION. PRECIPITABLE WATER VALUES LOOK TO
BE SOMEWHAT ABOVE NORMAL...AND RAIN MAY CERTAINLY BE HEAVY AT
TIMES...BUT THERE IS NO APPARENT LARGE SCALE HIGH IMPACT RAIN
EVENT IN THE CARDS JUST YET...AS A GRADUAL PROGRESSION IS FORECAST.

WITH HEIGHTS RISING OVER THE EAST PAC...AN AUTUMN-LIKE SURFACE
RIDGE WILL MIGRATE INTO THE WEST COAST...YIELDING AN OFFSHORE FLOW
EVENT. THE WPC GRIDS DO INCLUDE A FEW NEAR RECORD MAX TEMPERATURES
ON DAY 7 IN NORTHERN CA / SOUTHERN OR. VERY WARM WEATHER...PERHAPS
PUSHING RECORDS...WILL ALSO CONTINUE OVER THE MID ATLANTIC AND NEW
ENGLAND THROUGH WED OR THU...AHEAD OF THE COLD FRONT.

BURKE

$$




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