Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
206 PM EST Fri Dec 14 2018

Valid 12Z Tue Dec 18 2018 - 12Z Sat Dec 22 2018

The Alaska medium range (Tues-Sat) upper level pattern begins with
general elongated troughing over mainland Alaska, with a couple of
shortwaves moving through in the base of the trough into the Gulf.
Troughing over Alaska should retrograde westward as ridging builds
across western Canada into Eastern Alaska, and a deep broad upper
low drops southward across the western Aleutians. At the surface,
this would send a low pressure system and associated fronts into
or just south of the Aleutians, while a couple of surface lows
lift into the Gulf and weaken as they approach the coast of
southern Alaska/the Panhandle. Models and ensembles show good
clustering in the day 4-5 period to warrant a mostly deterministic
based solution. Uncertainty in the details and timing of
individual shortwaves/systems arises after this as indicated by
noisy and chaotic spaghetti plots of individual ensemble members.
Despite some minor timing differences, there is decent agreement
on the overall synoptic pattern in the ensemble means (between the
GEFS/ECENS/NAEFS) so increasing weighting of the means was used in
the blend for days 6 and onward. This maintains good continuity
with yesterdays forecast.

Regarding sensible weather threats, widespread rains along coastal
regions before the medium range period begins should decrease in
intensity by early next week, with some showers or locally heavy
rainfall continuing for the Panhandle region as a surface low
lifts northward offshore. A deep surface low over or just south of
the Aleutians will bring periods of rain to the island chain, with
heavy rains again moving back into the Peninsula/Southern
Coast/Panhandle by the end of the week as its front approaches the
coast. Gusty winds may accompany these systems as well, with
general cloudy and showery conditions extending farther inland
across interior portions of the state. Temperature wise, below to
well below normal temps dominate the beginning periods, with a
general warming expected as next week progresses with upper
ridging beginning to build back in.

Santorelli

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