The Weather Prediction Center

College Park, MD


Alaska Extended Forecast Discussion




Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
204 PM EST Mon Feb 19 2018

Valid 12Z Fri Feb 23 2018 - 12Z Tue Feb 27 2018

Models and ensemble continue to show a progressive pattern
continuing across Alaska during the medium range, north of a North
Pacific upper ridge. This will allow multiple upper shortwaves
originating in an area of negative height anomalies across
northeastern Asia to move east toward Alaska, amplifying as they
cross the Bering Sea, and eventually moving into mainland Alaska
and the Gulf. Predictability has perhaps improved somewhat
compared to recent days as models get a better grasp on the
progressive pattern. Uncertainties remain, however, largely due to
flow interactions between the stream of energy originating in
northeast Asia, and additional upper-level flow/smaller-scale
shortwave energy originating over the Arctic and affecting Alaska.

The forecast period begins early Fri with one upper shortwave
digging southeastward over the Gulf, and another crossing the
western Aleutians. Overall, the ECMWF and GFS showed similar
solutions with respect to the placement of these two features, but
the GFS has been fairly persistent with showing a greater degree
of amplification with the wave crossing the Gulf on Friday.
GFS/ECMWF continued to show similar solutions as shortwave
amplifies across the Bering/Aleutians Sat/Sat night. At the
surface, there is general consensus on one relatively deep surface
low lifting north across the Bering (although the GFS takes the
low farther west than the ECMWF), and a new triple point low
developing near the Aleutians on Sat, which then quickly moves
east into the Gulf on Sun. Yet another shortwave and surface low
cross the Bering Sun/Sun night, and spread among the
models/ensemble increases quite a bit by that time, with the ECMWF
more quickly bring the low east toward mainland AK, while the GFS
showed a deeper and slower solution. By the end of the period
(Tue) this system lifts north, either to the northern Bering or
perhaps northern AK, with an elongated occlusion extending into
the northern Gulf as it weakens. The WPC forecast was generally
based on a blend of the ECMWF/GFS along with the ECENS/GEFS
ensemble means. More weight was placed on the GFS early in the
period, as a somewhat more amplified wave crossing the Gulf Fri
was favored. Otherwise, ensemble weighting was gradually increased
through the period, with majority ECENS/GEFS weighting from Day 6
(Sun) onward.

In terms of sensible weather, this will continue to be a fairly
snowy pattern for much of Alaska, with periods of relatively high
pops even across interior areas of central/northern Alaska due to
the progressive flow with multiple streams affecting the state.
The Aleutians along with areas along the Gulf of Alaska coast in
southern/southeastern Alaska will see periodic rain/snow and gusty
winds as the series of systems traverse those areas. Temperatures
will be a bit colder than have been experienced recently, as the
weakened North Pacific ridge and the Arctic flow affecting much of
interior Alaska allow a colder air mass to settle in.

Ryan





Last Updated: 204 PM EST Mon Feb 19 2018