Prognostic Meteorological Discussion
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2
000
FXAK02 KWNH 221917
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
316 PM EDT MON MAY 22 2017

VALID 12Z FRI MAY 26 2017 - 12Z TUE MAY 30 2017

A PATTERN SHIFT WILL OCCUR ACROSS ALASKA DURING DAYS 4-8, WITH A
NEGATIVE HEIGHT ANOMALY INITIALLY CENTERED OVER WESTERN AK
EXPECTED TO GRADUALLY RETROGRADE. MEANWHILE, ANOMALOUS RIDGING
WILL BUILD FROM THE WESTERN CONUS/CANADA NORTHWARD INTO AK. THIS
WILL ALLOW AN AREA OF PERSISTENT WEAKLY NEGATIVE HEIGHT ANOMALIES
TO DEVELOP ACROSS THE ALEUTIANS. MODELS ARE IN GENERAL AGREEMENT
ON THIS LARGE-SCALE PATTERN EVOLUTION, BUT SHOW SOME RELATIVELY
SIGNIFICANT DIFFERENCES WITH RESPECT TO THE DETAILS. THE ECMWF/EC
ENS MEAN HAVE SHOWN SOMEWHAT BETTER RUN-TO-RUN CONSISTENCY
RELATIVE TO THE GFS. THE 00Z AND 06Z RUNS OF THE GFS WERE MORE
AMPLIFIED WITH SHORTWAVE ENERGY ORIGINATING NEAR THE KAMCHATKA
PENINSULA ON DAY 4, WHICH QUICKLY PHASED WITH ADDITIONAL SHORTWAVE
ENERGY RESULTING IN A DEEP CLOSED UPPER LOW ACROSS THE BERING SEA
BY DAY 5. THE 00Z AND 12Z ECMWF AS WELL AS THE 00Z EC MEAN SHOWED
A WEAKER SHORTWAVE ON DAY 4, WHICH MOVES MORE QUICKLY
SOUTHEASTWARD WITH PHASING NOT OCCURRING UNTIL 24-48 HOURS LATER
THAN THE GFS, RESULTING IN A CLOSED UPPER LOW AND ASSOCIATED
SURFACE CYCLONE NEAR OR SOUTH OF THE ALEUTIANS BY DAYS 6-7. THE
EC/EC MEAN SOLUTION IS ALSO SUPPORTED BY THE 12Z GEM GLOBAL AND
THE 00Z CMC ENSEMBLE MEAN. THE 12Z GFS MADE A FAIRLY SIGNIFICANT
CHANGE AND NOW RESEMBLES THE ECMWF, ALTHOUGH IT IS NOW LIKELY A
BIT TOO FAST WITH THE SHORTWAVE BY DAY 5-6. GIVEN THE
INCONSISTENCY SHOWN IN THE GFS, THE WPC FORECAST LEANED HEAVILY ON
A BLEND OF THE ECMWF/EC ENSEMBLE MEAN, WITH A SMALLER COMPONENT OF
THE 00Z NAEFS INCLUDED DUE TO ITS INCLUSION OF CMC ENSEMBLE
MEMBERS.

IN TERMS OF SENSIBLE WEATHER IMPACTS, THE BUILDING RIDGE ACROSS AK
WILL RESULT IN GRADUALLY RISING TEMPERATURES TO ABOVE AVERAGE
VALUES, PARTICULARLY ACROSS EASTERN AND CENTRAL AK WHERE THE RIDGE
WILL BE MORE PROMINENT. WHILE SCATTERED PRECIP WILL OCCUR ACROSS
MUCH OF AK EARLY IN THE MEDIUM RANGE (THU-FRI), PRECIP CHANGES
WILL GRADUALLY DECREASE FROM EAST TO WEST AS THE RIDGE BUILDS IN
FROM SAT-TUES, WITH THE BEST CHANCES FOR PRECIP ACROSS THE EASTERN
ALEUTIANS AND THE SOUTHWESTERN COAST OF MAINLAND AK, WHERE
SOUTHERLY FLOW AHEAD OF THE BROAD TROUGH WILL KEEP SUFFICIENT
MOISTURE IN PLACE FOR PRECIP.

RYAN

$$




USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.