Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
309 PM EST Wed Mar 06 2019
Valid 12Z Sun Mar 10 2019 - 12Z Thu Mar 14 2019
The models and ensemble means show better than average agreement
and continuity for the overall pattern during the days 4-8 period
(Sun-Thu). A strong but gradually weakening system near the
western coast of the mainland will establish a mean trough aloft
over 160-170W longitude. Trailing Aleutians/North Pacific trough
energy should take on a negative tilt by midweek with a closed or
nearly closed low possibly reaching near Kodiak Island by next
Thu. Farther north an Arctic upper low tracking along 80N
latitude should briefly extend a trough southwestward into the
North Slope region by early next week. Expect the high latitude
trough to retreat Wed-Thu as a western Canada into mainland Alaska
ridge builds ahead of the aforementioned negatively tilted trough.
The most prominent focus for precipitation during the period will
be along the southern coast and Panhandle while a couple episodes
of strong winds will be possible across and near the North Pacific.
Aside from minor detail and timing differences, the majority
cluster of guidance (00Z GFS/ECMWF/CMC and 06Z FV3 GFS) is
consistent and similar for significant features during the first
half of the period. Thus those models served as the basis for the
forecast early in the period. The 06Z GFS has some alternative
details for the strong Bering system nearing the southwestern part
of the mainland. Late arrival of the 12Z GFS prevented its
consideration for the forecast. Model consensus has also been
consistent for the system tracking along the Aleutians by Mon
though there is a hint of a slightly slower trend by Tue. The
ensemble means are starting to show better definition but not yet
quite to the level of an operational model average.
During the latter half of the period there is still a fair degree
of uncertainty over the exact evolution of upper trough energy
over/south of the Alaska Peninsula by Tue-Wed and how much
interaction may occur with the initially established trough to the
north--leading to differences in surface low track among the
guidance. The overall average of guidance for surface low
pressure has changed little over the past 24 hours. The ECMWF
mean is closest to the middle of the envelope and continuity with
a track just east of Kodiak Island by early Thu. The past couple
ECMWF runs are farther east while GFS runs have been waffling
between tracks to the east of Kodiak Island and farther southwest
like most GEFS mean runs thus far. An early look at the new 12Z
ECMWF reveals a track slightly west of the 00Z ECMWF mean. Based
on guidance through the 06Z cycle the preference is for an
intermediate approach that is closest to the ECMWF mean in
principle.
There is still considerable spread for details along and south of
the Aleutians during Tue-Thu. The most common theme is for a
progressive wave to track south of the Aleutians Wed into Thu,
with the ensemble means suggesting a hint of a feature in the
vicinity of operational runs. The 00Z CMC is faster and northward
of most other solutions.
A blend approach provides a reasonable starting point for the high
latitude upper high and trailing Arctic trough. There is still
some question over exactly what happens to the far southwest
periphery of the trough by midweek with minimal confidence in a
specific solution due to its small scale. At the surface expect a
front to settle toward the central part of the mainland as high
pressure extends farther eastward across the Arctic.
The typical increase of detail uncertainty and desire to maintain
continuity near the Gulf of Alaska favored incorporation of the
00Z ECMWF mean and 06Z GEFS means during the latter half of the
period, reaching 50-60 percent total weight by days 7-8 Wed-Thu
while holding onto some components of the 06Z FV3 GFS/00Z ECMWF.
Rausch
Bullet points from the WPC Days 3-7 Hazards chart:
https://www.wpc.ncep.noaa.gov/threats/threats.php
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Sun-Tue, Mar 10-Mar 12.
- High winds across portions of the Alaska Panhandle and mainland
Alaska, Sun-Mon, Mar 10-Mar 11, and Wed, Mar 13.
- High winds across portions of mainland Alaska, Sat-Sun, Mar
9-Mar 10.
- High winds across portions of the Aleutians, Sat-Mon, Mar 9-Mar
11.
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:
https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html