Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
330 PM EST Wed Nov 28 2018

Valid 12Z Sun Dec 02 2018 - 12Z Thu Dec 06 2018

The Alaska medium range period (Sun - Thur) features initial upper
level troughing on Day 4 over the Aleutians weakening and pushing
eastward into western Alaska mainland. After the weekend, upper
level ridging builds across the state and maintains itself through
the remainder of the period. Meanwhile, a closed upper level low
slides in south of the Aleutians on Monday, with a resulting
rather deep low at the surface (likely 970s or lower). The low may
not make it past the Aleutians before it weakens or get pushed
northward into the Bering Sea as it appears to be blocked by the
strong ridging over the mainland and panhandle regions.

There is relatively good agreement synoptically in the operational
models through the first half of the period between the 12z
GFS/00z ECMWF and the 06z parallel GFS (FV3) along with good
run-to-run continuity. This allowed for a majority operational
blend Days 4-6, with is consistent with the ensemble means as
well. After day 6, timing and differences in the details begins to
arise, and run-to-run continuity in the operational models is poor
as well. For day 7 and 8, preferred a blend of the ensemble means
(00z ECENS/06z GEFS/00z NAEFS) which also maintains good
continuity with yesterdays WPC progs synoptically, but with a
slightly farther south positioning of the upper low over the
Aleutians.

Sensible weather threats show a locally heavy rainfall threat
across southwestern Alaska (especially for coastal locations) as a
cold front across the peninsula gets stopped from moving eastward
by downstream ridging and eventually weakens all together this
weekend. This may set up a period of southerly flow into south
central Alaska with widespread precipitation, further enhanced by
the deep surface low drifting eastward south of the Aleutians
mid-period. With very good agreement in the operational models and
ensembles of the longwave pattern being blocked by upper ridging
near/along 140W, this would support a multi-day period of at least
locally heavy rainfall with in house QPF guidance suggesting
totals in excess of a couple inches. Above average temperatures
across Alaska will prevail through the period thanks to above
average heights and generally low-level southerly flow.

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