Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
625 PM EST Fri Jan 31 2020
Valid 12Z Tue Feb 04 2020 - 12Z Sat Feb 08 2020
...Overview...
Today's guidance maintains better than average agreement for the
large scale pattern evolution through the period (Tue-Sat).
During the early-middle part of the forecast expect an Arctic
upper low to descend close to the northwest corner of the mainland
while initial Bering Sea low pressure/dynamics aloft weaken.
Progressive North Pacific-Aleutians flow will carry along multiple
waves/fronts. Then by Fri-Sat consensus brings a fairly strong
area of low pressure into the Aleutians/Bering Sea while the
associated upper trough promotes a rapidly strengthening
downstream ridge that should extend from the northeastern Pacific
into the mainland. This ridge will likely weaken/displace the
upper low lingering near the northwest corner of the mainland
through late week. Within the consensus pattern evolution there
are persistent differences for various details.
...Guidance Evaluation...
With the Arctic upper low and surrounding flow, guidance continues
to show enough spread to have a significant effect on the
northward extent of moisture over the mainland. Early in the
forecast GFS/GEFS mean runs remain on the eastern side of the
spread for the upper low while the CMC continues to define the
western side. Eventually the GFS/GEFS mean runs pull enough flow
from the western side of the feature to yield an upper low close
to the consensus position. The 00Z ECMWF was a bit west of the
guidance average while the 12Z run strays somewhat eastward after
early Thu. Ultimately prefer a centered solution as close to
continuity as possible, per a blend of the 00Z/12Z ECMWF runs and
12Z GFS/UKMET early and a model-mean blend (a little more 00Z
ECMWF mean than 12Z GEFS mean) thereafter.
Farther south the models are showing difficulty in resolving the
proportion of initial Bering Sea energy that retrogrades versus
progressing into the mainland with corresponding surface
differences. The 12Z ECMWF came in with an evolution that
reflected a variation of the 12Z GFS that holds back a western
surface low into Thu while some other 12Z solutions and 00Z ECMWF
are more consolidated/eastward with low pressure. The
aforementioned blend provides the best option for a single
deterministic forecast, with a hint of the western low.
Within the progressive North Pacific stream the operational models
offer a decent signal for separate waves reaching the Gulf of
Alaska by around early Tue and early Wed, albeit with typical
track/strength/timing differences. Not surprisingly these
features appear somewhat weaker in the means. A weakening front
should follow close behind the second wave. Guidance is now more
agreeable that the system nearing the western Aleutians by early
Wed will maintain definition into Thu before quickly weakening so
the forecast through Thu has been adjusted accordingly to reflect
a model blend.
For the larger scale system expected to track into the
Aleutians/Bering Sea by Fri-Sat, ensemble mean trends over the
past 1-2 days (GEFS slower, ECMWF mean faster) have brought those
solutions fairly close together. The main difference between the
models and means is that most models are suggesting that a leading
frontal wave should develop ahead of the parent low while the
means are more consolidated. Prefer about an even weight of the
models (00Z ECMWF and 12Z ECMWF/GFS) and ensemble means to resolve
this detail difference. The 12Z ECMWF mean has come in with some
support for this idea.
...Weather Highlights...
Highest five-day precipitation totals should exist from just east
of the Kenai Peninsula through the Panhandle. Some moisture will
extend over the western mainland with northward extent sensitive
to the ultimate evolution/position of the Arctic upper low, though
with some tendency to focus near a front draped over the
central/southwest mainland. Strong flow ahead of the system
reaching the Aleutians and Bering Sea Fri-Sat may enhance amounts
over the eastern Aleutians/Alaska Peninsula and push moisture into
the western mainland. Below normal temperatures should prevail
over northern areas with a slow southward push of colder readings
over the western mainland Tue-Fri, followed by a northward retreat
on Sat ahead of the aforementioned system. Above normal
temperatures will be more prevalent over southern/eastern areas.
Rausch
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Mon-Thu, Feb 3-Feb 6.
- Much below normal temperatures across portions of mainland
Alaska, Mon, Feb 3.
- Much below normal temperatures across portions of mainland
Alaska, Thu-Fri, Feb 6-Feb 7.
- High significant wave heights for coastal portions of mainland
Alaska and the Aleutians, Mon, Feb 3.
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can also 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