Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
756 PM EDT Sun Oct 03 2021
Valid 12Z Thu Oct 07 2021 - 12Z Mon Oct 11 2021
...Confidence Increasing for a Strong Storm to Reach the Southwest
Mainland Late This Week...
...Pattern Overview and Guidance/Predictability Assessment...
Guidance maintains the progressive and energetic flow that will
carry along a series of storm systems from the short term into the
extended range period. The strongest extended-range storm should
track from the western Aleutians toward/into the southwestern
mainland during the latter half of the week. A somewhat weaker
system may track across the Bering Sea around Sunday-Monday.
Surface lows will also periodically reach/evolve over the Gulf of
Alaska in response to the passing dynamics aloft. Expect areas of
strong winds and enhanced precipitation to accompany the series of
storms.
For the western Aleutians to southern mainland system late this
week, stronger/northward ECMWF trends over the past day have
provided improved guidance clustering compared to previous days.
There is still some shuffling among the models for details though.
Now the ECMWF/CMC/UKMET are all quite deep, similar to what some
earlier GFS runs were showing, and with a track close to Nunivak
Island. The ensemble means are reasonably close to this track.
On the other hand the latest GFS runs have trended a tad
weaker/south versus yesterday's 12Z run. Enough GEFS members are
deeper than the operational run to suggest a solution somewhat
deeper than the GFS. A slightly enhanced operational model
composite yields a surface low depth around 970 mb valid 12Z
Friday. The forecast evolution is essentially a deeper version of
continuity from the past couple days.
The system forecast to track over the Bering Sea by next
Sunday-Monday generally shows better than average agreement in
principle relative to typical forecasts 7-8 days out in time.
There are still various track and detail differences though. The
12Z CMC/GFS and an average of the past two ECMWF runs are
generally not far from the ensemble means, so by days 7-8 the
forecast blend tempers their solutions with only a modest 30
percent total input of the 12Z GEFS/00Z ECMWF means.
The operational model to model/mean blend provides reasonable
continuity over the Gulf of Alaska, with one wave early Thursday
from leading dynamics and the next one Saturday-Sunday in
association with the energy that originally supports the late week
Bering Sea storm.
...Weather/Hazard Highlights...
The Panhandle may see some lingering precipitation on Thursday
with a wave that will track out of the Gulf of Alaska. Some
specifics are still a question mark but confidence is gradually
increasing for a vigorous storm that should track from the western
Aleutians into the eastern Bering Sea/western mainland late in the
week. This storm will likely produce a period of strong winds
across the Aleutians/Bering Sea and into parts of the mainland,
along with a broad precipitation shield covering the Aleutians and
approximately the southwest half of the mainland. Some pockets of
locally heavy totals are possible. A Gulf wave during the weekend
will also focus precipitation along the southeastern coast and
Panhandle. Expect a somewhat weaker system to track across the
Bering Sea next Sunday-Monday with another episode of
precipitation and brisk to strong winds. It is a little early to
determine how this system may ultimately affect the mainland.
Below normal high temperatures will prevail over the southern
mainland and Panhandle through the period. Above normal highs
over the northern half to two-thirds of the mainland late this
week should steadily trend cooler to yield a mix of above/below
normal readings during the weekend and possibly more below normal
coverage by next Monday. Low temperatures will be more above
normal in general. Northern/northwestern locations may see well
above normal lows Thursday into Friday or Saturday with some
moderation thereafter. Below normal lows should be confined
mostly to portions of the Panhandle but could expand into
Southcentral by the start of next week.
Rausch
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