Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
635 PM EST Thu Jan 14 2021
Valid 12Z Mon Jan 18 2021 - 12Z Fri Jan 22 2021
...Active Gulf of Alaska early next week followed by a shift in
focus to the Bering Sea...
...Overview and Guidance/Uncertainty Assessment...
A fairly progressive pattern will become more amplified after
midweek as a strong system tracks through the western
Aleutians/Bering Sea, promoting an upper ridge that builds over
the Northeast Pacific and mainland. Within the leading
progressive pattern expect a fairly vigorous storm to track into
the Gulf of Alaska on Mon with accompanying areas of heavy
precipitation and strong winds. Then a more modest feature should
break off from a Bering Sea system and track across the southern
mainland/Gulf by around Wed. A deep system tracking through the
western Aleutians/Bering Sea mid-late week will likely bring its
associated front into the southwestern mainland by next Fri.
Model/ensemble guidance generally agrees well for the overall
pattern and the most significant features. There are still some
detail questions though. For the Gulf storm early in the week the
main consideration was to use the 06Z GFS instead of the 12Z run
for the GFS component of the blend since the new run was somewhat
on the weak/fast side versus other solutions. Otherwise there was
good clustering and deeper ensemble mean trends over recent days
supported the operational consensus. Confidence is lower for the
details of the trailing weaker system (as it is pushing through a
larger scale mean ridge aloft) expected to reach the mainland/Gulf
by Wed, with individual model runs not very consistent for the
track of the best-defined surface low. GFS/ECMWF runs and the
ensemble means show better than average agreement and consistency
for the strong low tracking through the western Aleutians/Bering
Sea during the latter half of the period. However after day 7 Thu
guidance starts to diverge for the shortwave details of North
Pacific flow. This affects timing of the front extending out from
the western Bering low and any potential embedded waviness.
The aforementioned operational model blend of the 06Z GFS and
other 12Z runs on Mon reverted to 12Z GFS inclusion soon
thereafter once the initial Gulf system departs. Agreement was
sufficiently good to allow for 70 percent operational model weight
into early day 7 Thu, though complementing ECMWF input with some
of its old 00Z run as well. Increasing divergence for details
favored half total 12Z GFS/12Z-00Z ECMWF and the other half 12Z
GEFS/00Z ECMWF means by day 8 Fri. The new 12Z ECMWF mean
maintained support for this general approach.
...Weather/Hazards Highlights...
The strong system tracking into the Gulf early in the week will
bring areas of heavy precipitation to the southern coast and
Panhandle. Winds to gale or storm force will accompany the area
of low pressure before the system moves inland and weakens by
early Tue. Consult Ocean Prediction Center products for more
information. An upstream frontal system with one or more possible
waves should bring less intense precipitation across the Aleutians
and then into parts of the mainland (most likely extreme south)
and Panhandle early-mid week. Deep western Aleutians/Bering Sea
low pressure Wed onward and its leading front will bring
precipitation and locally strong winds across the Aleutians, with
moisture likely reaching the southern/western mainland late next
week. Low-confidence waviness along the front could help to
enhance precip intensity over some areas. The evolving pattern
will promote above to well above normal temperatures over most
areas during the period along with a gradual shift in greatest
anomalies from the eastern mainland to western areas. Modestly
below normal anomalies for highs may be possible on Mon over the
Alaska Peninsula and parts of the western coast.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Sun-Mon, Jan 17-Jan 18.
- High winds across portions of the Alaska Panhandle and mainland
Alaska, Mon, Jan 18.
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
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