Prognostic Meteorological Discussion
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FXAK02 KWNH 271802
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
202 PM EDT THU APR 27 2017

VALID 12Z MON MAY 01 2017 - 12Z FRI MAY 05 2017

A WEAK TO MODERATELY ACTIVE STORM TRACK IS DEPICTED BY MOST
GUIDANCE TO REMAIN BETWEEN 45-55N THROUGH THE PERIOD BEFORE THE
LOWS ARRIVE IN THE GULF OF ALASKA AND WEAKEN WHILE THE NORTHERN
TIER REMAINS UNDER THE INFLUENCE OF NEAR TO ABOVE NORMAL MID-LEVEL
HEIGHTS AND THUS MORE TRANQUIL CONDITIONS. WHILE THE 12Z GFS HAS
MOVED INTO VERY GOOD AGREEMENT WITH THE 00Z ECMWF REGARDING THE
CYCLONE SURGING AHEAD OF THE CLOSED LOW AND EVENTUALLY MERGING
WITH IT IN THE GULF OF ALAKSA BY DAY 6/12Z WED, THERE REMAINS A
LOT OF SPREAD AND MODEST RUN-TO-RUN DISCONTINUITY. THUS, PREFER
THEIR AGREEMENT FOR HELPING TO DEPICT A MODESTLY STRONG LOW, BUT
LESS THAN THE DETERMINISTIC SOLUTIONS SHOW, BEFORE QUICKLY
WEAKENING IT WITHIN THE LARGER OCCLUDED LOW, BY AVERAGING THE 12Z
GFS/00Z ECMWF WITH ABOUT 1/2 OF THE GLOBAL ENSEMBLE MEANS. THERE
IS EVEN MORE SPREAD AND UNCERTAINTY WITH THE NEXT POTENTIAL
CYCLONE REACHING THE WESTERN ALEUTIANS BY DAY 7/12Z THU, WITH THE
SPREAD DIFFICULT OF RESOLVE DUE TO LARGE DIFFERENCES IN THE EXTENT
AND STRENGTH OF PHASING OF MULTIPLE UPSTREAM SHORTWAVE TROUGHS.
THUS, FOR THIS SYSTEM, ONLY PREFER ABOUT 10 TO 20 PERCENT OF THE
12Z GFS/00Z ECMWF FOR A TOTAL DETERMINISTIC CONTRIBUTION, WITH THE
REMAINING CONTRIBUTION COMING FROM EQUAL AMOUNTS OF THE GLOBAL
ENSEMBLE MEANS. THE END RESULT SUPPORTS NEAR TO ABOVE AVERAGE
PRECIPITATION FOR THE SOUTHERN MAINLAND AND SOUTHEAST, NEAR TO
ABOVE NORMAL TEMPERATURES FOR WESTERN ALASKA, AND BELOW NORMAL
TEMPERATURES ACROSS THE NORTH.

JAMES

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