Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
403 PM EDT Sun Apr 29 2018

Valid 12Z Thu May 03 2018 - 12Z Mon May 07 2018

...Guidance and Predictability Assessment...

Forecast confidence remains below average across Alaska during the
medium range, with some significant model differences and
run-to-run continuity issues. The ECMWF showed significant
continuity issues even early in the period (Thu-Fri), with the 00Z
run quickly moving the Bering Sea low into interior Alaska, while
the GFS/CMC represented a slower consensus, and a similar
evolution of a triple point low that quickly occludes near the
Alaska Peninsula Fri night into Sat. The WPC forecast leaned
toward this GFS/CMC consensus (the 12Z ECMWF also looked much
closer to this consensus), and away from the 00Z ECMWF.

Models show general agreement that the large scale pattern will
become increasingly amplified by late in the week in to the
weekend, with ridging building northward along the North American
west coast into Southeast/East Central Alaska. Meanwhile,
additional shortwave energy reinforces the large scale persistent
trough south of Alaska. The result will be an elongated area of
deep southerly flow ahead of the trough, directed into southern
Alaska and the Gulf. The amplified flow regime will also increase
the potential for interaction/absorption of Arctic shortwave
energy into the westerlies, enhancing the uncertainty surrounding
the forecast specifics late in the period. Nonetheless,
models/ensembles show some consensus that a trough/upper low may
amplify across the western Bering Sea by late in the weekend, with
another potential surface cyclone moving eastward as well.
Confidence in the placement or strength of this system is very low
at this time, given the aforementioned factors.

The WPC forecast was based on a blend of continuity along with the
06Z GFS/00Z CMC as well as the 00Z ECENS/06Z GEFS ensemble means.
Greater weight was placed toward ensemble means later in the
forecast period.


...Pattern Highlights/Threats...

An active pattern is anticipated for Alaska and vicinity late this
week into early next week. The southern Bering Sea low by late in
the week may threaten maritime interests with heavy seas/winds, as
well as increasing rain/snow coverage and intensity across the
Alaska Peninsula and southern mainland Alaska. The amplification
of a full latitude trough from the Arctic to the north central
Pacific by next weekend will support aforementioned deep southerly
flow into southern/southeastern Alaska. GEFS standardized
anomalies suggest precipitable water values may reach +2 to +3
standard deviations across portions of southern/southeastern
Alaska by next weekend, which may support areas of heavy
precipitation, especially in favored windward terrain areas. A
strengthening pressure gradient between low pressure near the
Alaska Peninsula and high pressure over British Columbia may
support gusty maritime conditions across the Gulf by late week
into next weekend as well. Meanwhile interior areas of Alaska will
see some potential for precipitation early in the period as the
Bering low moves inland and fills, but somewhat drier conditions
should prevail by late week into next weekend as flow becomes more
southerly.

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