Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
218 PM EST Thu Dec 20 2018

Valid 12Z Mon Dec 24 2018 - 12Z Fri Dec 28 2018

...A Widespread Heavy Precipitation Threat for the Alaskan
Southern Tier...

...Guidance/Predictability Assessment and Preference...

Ensemble means offer a reasonably similar larger scale pattern
evolution for much of the next week, but deterministic guidance
still differs significantly with the amplitude and timing of
embedded small-mid scale systems days 4-8. Accordingly decided
after coordination with Alaskan NWS weather forecast offices to
primarily derive the WPC medium range Alaskan forecast from a
blend of compatable GEFS and ECMWF ensemble means along with some
input from the National Blend of Models. Leaned weighting in this
blend strongly toward the ECMWF ensemble mean to maintain max WPC
continuity.

...Weather/Threats Highlights...

It remains the case that the main storm track into next week
includes a series of organized low pressure systems and frontal
systems that track along/south of the Aleutians and the Alaska
Peninsula then into the Gulf of Alaska and southern Bering Sea.
These lows present both a maritime hazard and a periodic heavy
precipitation threat throughout the next week for the
Aleutians/AKpen/Southwest Alaska and southern/southeastern Alaska.
Individual system timing and strength remains quite variable in
guidance, but the overall storm track seems reasonable. In this
pattern, a lead system exiting the eastern Gulf of Alaska Monday
is replaced upstream with a set of lows from the Aleutians that
are supported by an amplifying mid-upper level mean trough. The
lows are expected to increasingly effect the full southern tier of
Alaska and offshore into the Bering Sea as well as the Gulf of
Alaska next week. Activity may increasing over time next week with
stormy conditions including widespread heavy to excessive
precipitation channel into the state between the aforementioned
supporting mean trough aloft and a building downstream upper
ridge. The inland threat would be enhanced into coastal
communities and over favored southern tier terrain. 

Overtop, forecast guidance remains uncertain but has become a bit
better clustered overall over the past two days with the depiction
of the pattern over the North Slope/Interior and Bering Strait
days 4-8. Shortwave trough/closed low energy aloft tends to favor
a weakness aloft that may support some limited light snows.   

Schichtel


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