Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
302 PM EST Tue Nov 27 2018

Valid 12Z Sat Dec 01 2018 - 12Z Wed Dec 05 2018

...Significant storm for the Bering Sea and eastern Aleutians
Saturday...

A trough/ridge pattern will mostly hold in place over the Bering
Sea/eastern Alaska next week as upstream ridging east of Japan and
near the New Siberian Islands maintains additional troughing over
China/Mongolia. With an amplified pattern downstream across the
rest of North America as well, the atmosphere will be primed to
spin up at least one powerful system later this week (Fri-Sat)
with another one potentially next week Mon-Tue.

For the Sat-Sun period, the models have trended toward a much
stronger system headed into the central Bering Sea with its front
moving into western/southwestern Alaska before the medium range
period. Blend of the 00Z ECMWF/GFS and 06Z parallel GFS (FV3)
offered good clustering with the ensemble means (06Z GEFS and 00Z
ECMWF ensemble mean) with a central pressure below 960mb. Strong
winds (especially along the coast) and modest to locally heavy
rain/snow are likely in southwestern Alaska (Kuskokwim Mountains).
Front will weaken inland and get stopped from moving eastward
altogether due to downstream ridging. This may set up a couple day
period of southerly flow into Southcentral with widespread but
decreasing rain/snow.

For Mon-Wed next week, 00Z ECMWF/GFS and 06Z parallel GFS again
offered good clustering with the next system creeping over the
Aleutians but the ECMWF was much different east of 160W than the
others as it was most aggressive in flow separation south of the
Gulf which allowed a surface low to lift northward unlike the
ensemble consensus. Preferred to have the deep low (likely 960s mb
or lower) Monday be the main system that will move its front
through the Aleutians and again into southwestern Alaska. However,
with a moisture plume just ahead of the system, precipitation may
be focused into Kodiak and the southern/eastern Kenai peninsula.
In-house QPF guidance shows in excess of several inches liquid
equivalent with a secondary max along the western Alaska Range
(west side of Cook Inlet up to Denali). Ensembles were in very
good agreement next Tue/Wed on the longwave pattern getting at
least temporarily blocked by upper ridging along 140-150W and saw
no reason to deviate which will likely stall the front somewhere
near Kodiak (supporting the possible heavy precipitation).

Temperatures will be above average outside the Panhandle thanks to
above average heights and generally southerly flow at the lower
levels.


Fracasso


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