Prognostic Meteorological Discussion
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FXAK02 KWNH 231916
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
315 PM EDT WED AUG 23 2017

VALID 12Z SUN AUG 27 2017 - 12Z THU AUG 31 2017

MODELS/ENSEMBLES SHOW A GENERALLY SIMILAR LARGE SCALE FLOW PATTERN
EVOLUTION ACROSS ALASKA DURING DAYS 4-8 (SUN-THU), BUT SIGNIFICANT
DIFFERENCES EXIST WITH RESPECT TO THE DETAILS OF INDIVIDUAL
FEATURE EVOLUTION. MID/UPPER-LEVEL RIDGING IS FORECAST TO PERSIST
ACROSS FAR EASTERN RUSSIA, WITH VARYING SOLUTIONS AMONG THE
GUIDANCE AS TO THE STRENGTH OF THE RIDGE. FARTHER EAST, A BROAD
AREA OF NEGATIVE HEIGHT ANOMALIES IS EXPECTED TO PERSIST FROM THE
NORTH PACIFIC ACROSS THE ALEUTIANS AND WESTERN AND SOUTHERN
MAINLAND ALASKA. UPPER-LEVEL ENERGY IS FORECAST TO UNDERCUT THE
EASTERN RUSSIA RIDGE ON MON BEFORE PHASING TO SOME DEGREE WITH
ENERGY TRAVERSING THE NORTH PACIFIC/ALEUTIAN UPPER LOW AND
AMPLIFYING. THIS PROCESS IS PROBABLY THE LARGEST SOURCE OF
UNCERTAINTY IN THE FORECAST, AS THE OUTCOME ALOFT AFFECTS THE
POSITION AND DEPTH OF A SURFACE LOW EITHER IN THE SOUTHEASTERN
BERING SEA OR THE GULF OF ALASKA. THE 00Z ECMWF SHOWED THE
STRONGEST UPPER LOW BY DAYS 5-7, WHICH RESULTS IN A SIGNIFICANTLY
DEEPER SURFACE LOW THAT IS FARTHER NORTH AND WEST RELATIVE TO
OTHER GUIDANCE, IN THE SOUTHEASTERN BERING SEA. THE 12Z ECMWF HAS
BACKED OFF OF THIS SUBSTANTIALLY, AND NOW SHOWS A SOLUTION CLOSER
TO THE GFS, ALBEIT STILL A BIT DEEPER AND FARTHER NORTHWEST.
REGARDLESS, OF THE EXACT EVOLUTION, THIS SYSTEM WILL HAVE THE
POTENTIAL TO BRING SIGNIFICANT PRECIPITATION TO AREAS ALONG THE
SOUTHERN COAST, WITH THE POTENTIAL FOR STRONG WINDS ACROSS THE
GULF OF AK.

CONVERSELY, FARTHER NORTH THE ECMWF HAS BEEN CONSISTENT WITH
SHOWING AN AREA OF VIGOROUS UPPER-LEVEL ENERGY ORIGINATING IN THE
HIGH ARCTIC MOVING SOUTHEASTWARD AND AFFECTING PORTIONS OF
NORTHERN ALASKA SUNDAY AND MONDAY. THE GFS SHOWS A SIMILAR
FEATURE, ALTHOUGH IT HAS CONSISTENTLY BEEN WEAKER AND MORE
PROGRESSIVE. AT THIS TIME, BASED ON DISCUSSION WITH WFO FAIRBANKS,
THE ECMWF IS PREFERRED FOR DAYS 4-5 ACROSS NORTHERN AK. AFTER THAT
TIME THE ECMWF WAS DEEMPHASIZED IN THE FORECAST BLEND BASED ON ITS
00Z RUN AND RATHER LARGE RUN-TO-RUN INCONSISTENCIES WITH RESPECT
TO THE LARGER-SCALE FLOW EVOLUTION AND THE RESULTING GULF OF
AK/BERING SEA CYCLONE. OTHERWISE, THE WPC FORECAST WAS BASED ON A
BLEND OF THE GFS ALONG WITH THE GEFS/NAEFS/ECENS MEANS, WITH A
GRADUAL TREND TOWARD HEAVIER ENSEMBLE WEIGHTING LATER IN THE
FORECAST PERIOD.

RYAN

$$





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