Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
306 PM EDT Tue May 15 2018

Valid 12Z Sat May 19 2018 - 12Z Wed May 23 2018

Models continue to indicate a transition from a complex and blocky
flow pattern late this week to a more progressive pattern by
early-mid next week. An upper ridge axis is expected to initially
extend from western Canada into eastern/southeastern Alaska at the
start of the medium range (Sat), with a broad upper-level
circulation (and numerous embedded shortwaves) centered near the
Aleutians. Models/ensemble seem to have a much better handle over
the flow pattern evolution compared to this time yesterday,
including exactly how shortwave energy or Arctic origin interacts
with additional energy arriving in the westerlies - the end result
being consolidation of energy into a large upper low south of the
Alaska Peninsula by Sun night, and into the Gulf by Mon. Ensemble
spread surrounding the associated surface low nearing the Gulf by
Sun has reduced dramatically over the past 24 hours, and scatter
plots indicated good clustering around the means. The 12Z GFS/00Z
ECMWF fit well within this consensus, and a majority deterministic
blend including these solutions along with the 00Z ECENS/NAEFS
means served as a starting point for the WPC forecast during days
4-5.

By early to mid next week, models continue to show the upper jet
across the North Pacific becoming more active, with areas from the
Aleutians to the Gulf increasingly influenced by strengthening
upper-level westerly flow. As a result, models continue to show a
more progressive flow pattern evolving, and have trended toward
shifting the ridge axis eastward into the Canadian Rockies. This
flow pattern transition appears to have broad support based on
teleconnections associated with a few strong northern hemisphere
500 hPa height anomalies. As a result, heights are expected to
fall from Sun onward across Southeast Alaska, shortwave energy
rotating around the upper low likely to move ashore. By the middle
of next week, there general consensus that a vigorous shortwave in
the westerlies, and the associated surface low pressure system
should approach the Aleutians. Models show the typical
timing/strength differences with this system as would be expected
for days 7-8. The GFS has been a fast outlier with this system for
a few runs now, and the slower timing shown by the ECMWF and
ensemble means was preferred. Given these considerations, the WPC
forecast was gradually trended toward heavier ECENS/NAEFS
weighting from day 6 onward.

In terms of sensible weather, this will be a particularly wet
pattern through much of the period for southern mainland Alaska
and the Alaska Peninsula - the result of persistent and deep
southerly flow between the broad upper cyclone over the Bering and
the western North America ridge. By early next week, as confidence
is increasing in the upper ridge axis shifting east, expect an
increase in precipitation for Southeast Alaska as well. Meanwhile,
expect somewhat drier conditions across the interior, with
scattered and relatively light precipitation. Temperatures will be
near or even a bit above average (especially for areas across the
interior that are free of precipitation).

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
www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html