Prognostic Meteorological Discussion
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FXAK02 KWNH 181825
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
224 PM EDT SAT APR 18 2015

VALID 12Z WED APR 22 2015 - 12Z SUN APR 26 2015

FLOW IS FCST TO REMAIN FAIRLY STABLE FROM THE MULTI-DAY MEAN
PERSPECTIVE WITH A LONG TERM TROUGH POSITIONED JUST OFF THE WRN
COAST OF THE MAINLAND EXTENDING INTO THE NRN/NERN PACIFIC WHILE A
MEAN RIDGE SHOULD HAVE ITS AXIS OVER W-CNTRL CANADA.  SOME
CHALLENGES REMAIN FOR FEATURES EMBEDDED WITHIN THE MEAN FLOW...
ESPECIALLY ACROSS THE NRN/NERN PAC AND BERING SEA.

OVER THE PAST 2-3 DAYS BOTH THE OPERATIONAL MODELS AND ENSEMBLES
HAVE BEEN ERRATIC WITH THE HANDLING OF PROGRESSIVE NRN PAC ENERGY
AND ITS INTERACTION WITH A PORTION OF SHORT RANGE CNTRL PAC
ENERGY/SFC LOW PRES.  IN CONTRAST TO WEAK RUNS FROM 24 HRS AGO
THERE IS NOW A DECENT CONSENSUS FOR A FAIRLY STG SFC LOW TO BE S
OF KODIAK ISLAND AS OF THE START OF THE FCST PERIOD 12Z WED.  FROM
THAT TIME ONWARD INDIVIDUAL MODEL/ENSEMBLE SOLNS DIVERGE
CONSIDERABLY WITH THE TRACK.  AT THE VERY LEAST WOULD FAVOR
LEANING AWAY FROM THE NRN SIDE OF THE ENVELOPE... OCCUPIED BY THE
06Z-12Z GFS/06Z GEFS MEAN AT DIFFERING FCST HRS... AS THE RECENT
TENDENCY IN GUIDANCE HAS BEEN TOWARD PULLING AT LEAST SOME ENERGY
TOWARD A MEAN TROUGH OVER THE WEST COAST OF THE CONUS.  THE NWWD
PROGRESSION SEEN IN THE 12Z GFS BY FRI-SAT IS NOT WELL SUPPORTED
THUS FAR.

THE PAST COUPLE RUNS OF THE ECMWF MEAN ARE FARTHER NWD THAN THE
00Z/17 VERSION FOR THE SYSTEM TRACKING EWD FROM KAMCHATKA.  THIS
ADJUSTMENT REFLECTS LESS TROUGH AMPLITUDE ALOFT VERSUS THAT
EARLIER RUN AND LEADS TO MORE SIMILAR TRACKS AMONG THE ENSEMBLE
MEANS.  HOWEVER OPERATIONAL MODEL RUNS THROUGH THE 00Z/06Z CYCLE
AND 12Z GFS ARE STILL SHOWING POTENTIAL FOR A SHARPER/MORE
AMPLIFIED UPR TROUGH AND SFC LOW TRACK A LITTLE SWD OF THE MEANS.
AT LEAST A COMPROMISE BTWN THE MEANS AND 00Z ECMWF/GFS SEEMS
REASONABLE AT LEAST INTO DAY 6 FRI.  BY DAY 7 SAT RECENT ECMWF
MEAN SOLNS END UP CONVERGING UPON A SFC LOW TO THE SW OF KODIAK
ISLAND... WITH THE 00Z NAEFS MEAN HOLDING THE SFC LOW JUST A
LITTLE WWD OF THE 00Z ECMWF MEAN POSN SAT-SUN.  THE 06Z GEFS MEAN
IS FASTER THAN OTHER SOLNS.

THE NEXT SYSTEM IN THE SERIES SHOULD COME INTO THE PICTURE NEAR
THE WRN ALEUTIANS BY DAY 8 SUN.  THE 00Z ECMWF IS THE FAST EXTREME
FOR THIS FEATURE... RUNNING ABOUT A DAY AHEAD OF MOST OTHER SOLNS.
 TYPICAL UNCERTAINTY WITH DISTRIBUTION/SHAPE OF MID LVL ENERGY AT
THE DAY 8 TIME FRAME RECOMMENDS THE ENSEMBLE MEANS AS A STARTING
POINT BUT WITH THE UNDERSTANDING THAT OTHER SOLNS SUCH AS STRONGER
GFS RUNS ARE PSBL AS WELL.

MEDIUM TO SMALLER SCALE DETAILS OF FLOW OVER THE MAINLAND LATE IN
THE PERIOD ARE STILL FAIRLY UNCERTAIN BUT THE ENSEMBLE MEANS HAVE
BEEN THE MOST CONSISTENT IN DEPICTING SOME RISE IN HGTS ALOFT...
THOUGH NOT YET SUPPORTING A STRONG AND/OR CLOSED RIDGE THAT HAS
SHOWN UP IN A SMALL NUMBER OF OPERATIONAL RUNS SUCH AS THE 06Z
GFS.. 12Z/17 GFS.. OR 00Z/17 ECMWF.  TELECONS RELATIVE TO THE CORE
OF POSITIVE HGT ANOMALIES OVER NERN CANADA IN D+8 CHARTS ALSO
SUPPORT THE FLAT RIDGING THAT EVENTUALLY DEVELOPS IN THE ENSEMBLE
MEANS.

ACCOUNTING FOR THE SPREAD AMONG THE MORE LIKELY SOLNS AMONG LATEST
GUIDANCE... DAYS 4-6 WED-FRI INCORPORATE A BLEND OF THE 00Z
ECMWF/GFS AND ECMWF/NAEFS MEANS WITH MORE WEIGHTING TO THE
OPERATIONAL RUNS TO MAINTAIN DETAIL WHILE DAYS 6-7 REFLECT A 60/40
BLEND OF THE ECMWF MEAN/NAEFS MEAN AS CONFIDENCE IN OPERATIONAL
RUNS DECREASES.

RAUSCH

$$




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