Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
335 PM EDT Sat Jul 07 2018

Valid 12Z Wed Jul 11 2018 - 12Z Sun Jul 15 2018

Relatively progressive flow is expected across Alaska during the
medium range forecast period, with a complex set of interactions
possible between North Pacific flow and Arctic flow. These
interactions introduce a high degree of uncertainty into the
forecast from late next week onward. As one shortwave and
weakening frontal system cross the Gulf on Wed, two additional
pieces of energy will come into focus west of Alaska, an upper low
nearing the western portion of the Aleutian chain, and an
energetic shortwave diving southeastward from eastern Russia into
the Bering Sea. The most significant aspect of the forecast is
whether these two systems will interact/merge, or remain separate.
Ensemble members have shown a notable trend over the past day
toward a slightly faster southern system which results in a
greater degree of interaction between the two, and the formation
of a deep upper low across the southern Bering Sea by late in next
week (the scenario shown by the ECMWF and CMC). The GFS, on the
other hand, has been persistent in keeping the southern system
slower, with no interaction between the two, causing the northern
stream energy to quickly move east and break down the ridge across
Southeast Alaska. These two scenarios are not really compatible in
terms of a blend, so the idea shown by the ECMWF/CMC was used, and
the GFS was rejected for this forecast package. A multi-model,
slight majority deterministic blend including the ECMWF/CMC/UKMET
formed the basis for the WPC forecast during days 4-5, with a
trend to majority ensemble (ECENS/NAEFS) weighting shown from day
6 onward. This blend depicts a scenario with more interaction
between the two aforementioned features, and a slower breakdown of
upper ridging across Southeast Alaska late next week.

In terms of sensible weather impacts, this pattern will result in
a period of generally near to below average temperatures along
with frequent precipitation for much of the state (especially
south of the Brooks Range). Rain could be heavy at times across
southern mainland as well as Southeast Alaska Wed into early Thu
in association with the passing upper shortwave and weakening
surface front.

Ryan


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