Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
753 PM EDT Mon Oct 04 2021
Valid 12Z Fri Oct 08 2021 - 12Z Tue Oct 12 2021
...Strong Storm to Reach the Southwest Mainland Late This Week...
...Pattern Overview and Guidance/Predictability Assessment...
Most of today's models and ensembles maintain the theme of
progressive and energetic flow that will carry a series of storm
systems across the Bering Sea then into the southwestern mainland
and Gulf of Alaska. Areas of enhanced precipitation and/or strong
winds will likely accompany each system from the Aleutians/Bering
Sea into parts of the southern mainland and Panhandle. The
leading storm expected to reach the southwestern mainland late
this week should be the strongest of the forecast period. The
next one may reach the eastern Bering Sea by Monday. Expect the
associated dynamics from these systems to support Gulf of Alaska
waves during the weekend and again around Tuesday. A third system
could emerge over the Bering Sea by next Tuesday.
Models show a gradual convergence for details of the
Friday-Saturday storm. The 12Z ECMWF/UKMET/CMC have generally
nudged slightly weaker/southward from yesterday in a partial nod
to recent GFS runs. Meanwhile by early Saturday the 12Z/18Z GFS
runs have trended toward maintaining the parent low for a longer
time over the southwestern mainland as the other models have been
showing. Continuity looks good for low pressure along the
southeastern coast during the weekend.
Into early Sunday the model/ensemble mean consensus has held up
fairly well over the past couple days for the system tracking into
the Bering Sea during the weekend. However after that time
guidance begins to go astray for various important details. GFS
runs have been more eager than other guidance to lift the initial
low well north into the eastern tip of Siberia or to weaken it
while increasing emphasis on North Pacific low pressure. The
ECMWF/CMC along with the ensemble means generally favor having the
main system continue into the eastern Bering Monday and then
possibly the southwestern mainland followed by Gulf development,
in a manner similar to the previous system. The end result by
Tuesday would be a Gulf system farther north than in the GFS.
There are still significant differences among the non-GFS
majority, as the CMC leans a bit to the slower side (along with
some low-confidence embedded details) but the 12Z ECMWF adjusted
noticeably slower than the 00Z run. The new 12Z ECMWF mean
trended a bit slower by the end of the period as well.
Thus far guidance has displayed better than average agreement in
principle relative to typical late-period forecasts for the next
system currently expected to emerge into the western Bering Sea by
next Tuesday.
A blend of 12Z operational models represented latest consensus
well into early Sunday. After that time the starting point
transitioned toward a combination of half models (12Z/00Z ECMWF
and 06Z GFS) and half means (12Z GEFS/00Z ECens) by day 8 Tuesday.
The 06Z GFS was somewhat less disagreeable relative to the
preferred majority versus the 12Z run after Sunday, while the
model-mean approach helped to resolve the gradually widening
timing/detail differences late in the period.
...Weather/Hazard Highlights...
Additional fine-tuning will be likely in the coming days but the
general forecast is consistent for a vigorous storm to reach the
eastern Bering Sea and southwestern mainland late this week. This
storm along with subsequent Gulf wave should produce a period of
strong winds across the Aleutians/Bering Sea and Alaska Peninsula
into the Northeast Pacific, with some brisk winds possibly
extending into the southern mainland. Expect the broad moisture
shield to bring highest rain/snow totals to areas along the
southern coast and Panhandle, aided by the Gulf wave. Meaningful
amounts are also likely over the southwestern mainland/Peninsula.
The next system in the series may spread precipitation and
enhanced winds across the Aleutians and Bering Sea during the
weekend. The majority scenario would have some moderate to
locally heavy precipitation extending into the southwestern
mainland and then the southern coast/Panhandle around
Monday-Tuesday, while a lower-probability suppressed evolution
would be considerably drier. Yet another system with
precipitation and brisk winds may reach the western Bering Sea by
next Tuesday.
The southern mainland and Panhandle should see below normal high
temperatures through the period. The northern two-thirds of the
mainland will see mostly above normal highs on Friday followed by
a modest cooling trend to yield a mix of above/below normal
readings from the weekend into early next week. Most of the state
will see above normal low temperatures with warmest anomalies over
northern areas. The main exception will be the southern
Panhandle, with below normal lows possibly extending through the
rest of the Panhandle into parts of Southcentral by next Monday.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Fri, Oct 8.
- High winds across portions of mainland Alaska and the Aleutians,
Thu-Sat, Oct 7-Oct 9.
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook 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