Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
624 PM EST Mon Dec 30 2019
Valid 12Z Fri Jan 03 2020 - 12Z Tue Jan 07 2020
Guidance has continued to provide a fair amount of spread and some
changes/trends over portions of the forecast domain but at least
as of the 12Z/30 cycle there seems to be a little more clustering
in principle. Latest consensus has adjusted somewhat westward for
an initial upper low now expected to be near the eastern Alaska
Peninsula/Kodiak Island as of early Fri. This feature should
progress east-southeast while energy remaining over the southern
mainland forms another upper low a little north of the first one.
Overall upper trough/low emphasis is likely to shift northward to
cover more of the mainland by the latter half of the period in
response to westward elongation of flow to the south of an upper
ridge that should cover areas from Siberia into the Arctic north
of the mainland. There is still some uncertainty over the shape
of this ridge though, affecting details over the mainland and
Bering Sea. Guidance agrees fairly well that progression of
energy within a western Pacific trough aloft will bring height
falls into the western Aleutians by early next week, building a
ridge into the Aleutians/Bering Sea--providing added support to
maintain troughing over the mainland. Forecast evolution should
lead to a colder trend over a majority of the mainland late this
week into the weekend with temperatures stabilizing early next
week.
Perhaps with some help from the westward trend of the upper low
initially near the Peninsula/Kodiak Island, a number of models
have adjusted northwestward for a wave forecast to take shape over
the central Pacific Tue and then track northeastward to or near
the Alaska Panhandle by around Fri. This trend seemed to begin
with the 12Z/29 GFS. Supporting energy is elongated and ensemble
spread is considerable so confidence is below average for a system
this early in the extended period. The upper dynamics/surface low
to the west provide additional potential complications in the
forecast.
Meanwhile a decent majority of guidance (ECMWF/CMC and their means
among other solutions) suggests that initial upper ridging over
Siberia will close off to a greater extent than forecast by the
operational GFS, leading to some degree of east-west weakness over
the Bering in contrast to the nearly north-south ridge in the GFS.
The GEFS mean is not much weaker with the ridge than the GFS.
The ECMWF/CMC means are not yet confident in the depth of the
weakness, so leaning toward the means appears best at this time.
Guidance is starting to become a little more similar in principle
with a system forecast to track south of the Aleutians during the
weekend and then continue into the northeastern Pacific/Gulf of
Alaska. An average of operational runs is not far from an average
of the GEFS/ECMWF means, though the 00Z ECMWF mean was quite weak
so more 12Z GEFS input was preferred among the ensemble solutions.
The new 12Z ECMWF mean trended stronger.
By early next week there is decent agreement/continuity with the
idea of a strong surface gradient and associated brisk winds
moving into the Aleutians and southern-western Bering ahead of a
front that may reach the Aleutians late in the period. Overall
consensus having trended a bit slower with the front versus 24
hours ago would favor phasing out the faster 12Z CMC as part of
the blend in this time frame.
Forecast preferences led to starting with a nearly even
GFS/ECMWF/CMC blend early in the period with operational input
still at 80 percent into day 6 Sun. This blend provided good
detail for the system forecast to track toward the Panhandle and
upper features over/near the Gulf, while yielding a pattern aloft
close to the ECMWF mean over the Bering Sea. The forecast
transitioned toward slightly more than half ensemble weight by day
8 Tue with more GEFS than ECMWF mean due to the GEFS being
stronger with the North Pacific system. Elsewhere the means are
either more comparable or an average produces a good intermediate
result when included with the operational runs.
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
No significant hazards are expected over Alaska during this
forecast period.
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