Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
339 PM EDT Sat Apr 28 2018

Valid 12Z Wed May 02 2018 - 12Z Sun May 06 2018

...Guidance and Predictability Assessment...

Forecast confidence remains a bit below average during the medium
range across Alaska, with some significant model differences even
early in the period. GFS and ECMWF continued to show some
significant differences in the amplitude of a shortwave expected
to be crossing the Alaska Peninsula on Wed, but the CMC was the
biggest outlier, showing significantly different timing resulting
in a solution almost completely out of phase with the GFS/ECMWF.
By late in the week, models show a general consensus that an upper
low should deepen across the Aleutians or perhaps the southern
Bering Sea, with the potential for a moderately deep surface
cyclone. Pattern becomes increasingly complex by next weekend as a
larger scale trough amplifies from the Arctic to the north central
Pacific, introducing complexities of interactions between
shortwaves of Arctic origin and polar/subtropical energy farther
south.

The WPC forecast was based on a blend of continuity along with the
00Z ECMWF/ECENS and 12Z GFS/06Z GEFS. Based on Alaska/WPC
coordination, the forecast showed a slight preference toward the
EC/ECENS solutions. Greater weight was placed toward ensemble
means later in the forecast period.


...Pattern Highlights/Threats...


An active pattern is anticipated for Alaska and vicinity next
week. The Aleutian/Bering Sea storm development by mid to late in
the week may threaten maritime interests with heavy seas/winds,
with an increase in rain/snow coverage across the Aleutians as
well as southern/southeastern Alaska. The amplification of a full
latitude trough from the Arctic to the north central Pacific by
next weekend will support continued deep southerly flow and
widespread precipitation across southern/southeastern Alaska, with
the potential for gusty winds as well in association with any
additional surface lows that develop (confidence in the specifics
by next weekend is low).

WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:

http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html

Ryan