Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
215 PM EST Sun Nov 04 2018

Valid 12Z Thu Nov 08 2018 - 12Z Mon Nov 12 2018

By the mid-late part of the forecast period (Sat-Mon) recent
ECMWF/GFS runs and their ensemble means as a whole show a
persistence or furthering of recent trends toward a sharper and
more amplified trough-ridge pattern--fed by shortwave energy that
emerges from eastern Asia late this week.  Specifics at the
surface still appear to be highly uncertain given ongoing
model/ensemble spread and run to run changes.  However low
confidence may be in the exact surface evolution, the consensus
upper pattern should eventually favor an increasing flow of
moisture into the Gulf of Alaska and Southeast Coast/Panhandle. 
00Z CMC ensembles show an alternative scenario that is less
amplified, but such a flatter forecast by the CMC mean tends to be
less correct when the GEFS/ECMWF means advertise a more amplified
regime.

Guidance is showing steady improvement in clustering for the early
part of the forecast, days 4-5 Thu-Fri.  This is especially the
case for the system expected to be over the Gulf of Alaska as of
early Thu.  There are still moderate differences in timing and
latitude for the system over the Bering Sea but in a broad sense
the guidance has stabilized considerably relative to what was the
case before yesterday.  The 00Z ECMWF begins to lag most other
guidance including its ensemble mean by early Fri, though partial
inclusion within a blend is still reasonable as movement may
become sensitive to the exact shape/position of a small upper
ridge that may build to the north.  Guidance continues to show an
area of strong winds to the south of the Bering low with the axis
of the low level jet likely remaining just north of the Aleutians.

During the weekend recent ECMWF runs and GEFS/ECMWF means offer
the most common idea of surface low emphasis transferring from the
Bering system to a farther south low perhaps a little south of the
eastern Aleutians/Alaska Peninsula/Kodiak Island.  Latest GFS runs
have been pulling the southern feature much farther southward. 
The 06Z GFS compares most closely to the above cluster aloft
though its surface pattern is less similar.  The ongoing
sharper/more amplified trend has led to a consensus Sun-Mon
surface low position somewhat westward of yesterday's forecast.

Ahead of this evolution the operational runs have become more
dispersed regarding the ultimate track of a developing wave
initially over the middle latitudes of the east-central Pacific. 
Most solutions currently range anywhere from the Gulf of Alaska
(00Z GFS) to just south of the Panhandle (00Z ECMWF) but the full
ensemble envelope is even wider than that.  There is good
agreement against flow being as progressive as in the 00Z CMC but
still there seems to be a little more of a hint in today's
guidance that the northern periphery of the building eastern
Pacific ridge may delay its strengthening a bit to allow for a
farther eastward track of the initial mid-latitude wave than seen
in most models yesterday.  This aspect of the forecast continues
to have below average confidence and thus merits a conservative
approach in the deterministic forecast until better agreement in
the guidance.  Even if not from this wave specifically, as
mentioned above the consensus pattern evolution should ultimately
bring some increase of moisture/precipitation into the Gulf and
most likely areas from the Kenai Peninsula through the Panhandle.

Based on early period considerations the forecast started with a
blend of the 06Z GFS and 00Z ECMWF/UKMET for days 4-5 Thu-Fri. 
Rapidly decreasing confidence in details led to introducing 40
percent total 06Z GEFS/00Z ECMWF ensemble mean weight for day 6
Sat and then 60-70 percent thereafter.  The blend held on to a
lingering minority weight of the 06Z GFS/00Z ECMWF to incorporate
some operational model signal in the forecast.

Rausch


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