Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
758 PM EDT Sun Oct 08 2023
Valid 12Z Thu Oct 12 2023 - 12Z Mon Oct 16 2023
...Deep storm likely to bring strong winds and high seas to the
Aleutians and surrounding waters late this week...
...Late week heavy precipitation threat over Southeast Alaska...
...Overview...
Today's guidance still differs some for what becomes of the upper
low initially over the western mainland but there is a general
theme of it eventually being displaced to the west or north as
some degree of upper ridging builds into the mainland from Canada.
As this evolution develops, one or more waves should lift
northward offshore the Panhandle into or near the Gulf of Alaska
late this week into the weekend. These waves should bring another
period of enhanced precipitation and winds to at least the
Panhandle, and possibly extending west into parts of coastal
Southcentral Alaska. Precipitation intensity should decline later
in the weekend into early next week. Farther west, still shows
very deep low pressure reaching the Aleutians/southern Bering Sea
by the start of the forecast early Thursday, bringing strong winds
and high seas to the region. The combination of this system and
possible interacting upstream energy should slowly weaken as the
overall area of low pressure ultimately settles to the south of
the eastern Aleutians/Alaska Peninsula. Late in the period there
is considerable uncertainty over the progression of tropical
cyclone Bolaven or its extratropical reflection across the
Pacific, with a track most likely to the south of the Aleutians.
...Guidance Evaluation and Preferences...
Differences in the first half of the period focus on the mainland
upper low, northeastern Pacific low pressure tracking into/near
the Gulf, and the deep Aleutians/Bering Sea storm. Latest
operational models seem to be trending toward less northward drift
of the initial mainland upper low (the northward scenario being
favored by recent days of ensemble mean runs with other models
varying widely), and instead having it get pushed more westward as
Canadian upper ridging builds into the mainland. The 12Z ECMWF
(and now the 12Z ECens mean) show some combination of initial
upper low energy and/or energy just to the south of the mainland
leading to a farther south overall low position by day 5 Friday.
This upper evolution corresponds to the 12Z ECMWF being on the
deep/western side for the surface wave that reaches near the
southern coast at that time. The majority of other guidance keeps
the surface low track a bit farther east, though hint at a
westward curl if timing is a little slower. Details for trailing
waviness become ambiguous, though with enough of a signal to
depict a weak wave into early day 6 Saturday. Meanwhile the past
couple ECMWF runs stray to the north and west side of the spread
for its Bering Sea low by Friday-Saturday. The GFS has been on
the deep side of the spread for this low. These guidance
comparisons led to a starting blend that incorporated the 12Z
models (somewhat more GFS/ECMWF than UKMET/CMC) on day 4 Thursday
and then switching the ECMWF component more to the 00Z run for
Northeast Pacific/Gulf considerations. Weight was sufficiently
modest to yield a non-ECMWF clustering for the Bering
Sea/Aleutians low.
By the latter half of the forecast, most guidance agreed with the
general idea of the upper pattern trending toward a north-south
trough axis aligned near or just west of the western coast of the
mainland while some ridging continues to extend westward into the
mainland from Canada. Individual runs show a lot of shortwave
uncertainties within the mean flow across the mainland though. At
the same time recent consensus has been showing a mean low
settling to the south of the Alaska Peninsula. However over the
past day a lot of guidance spread has developed with the eastward
progression across the Pacific of current Tropical Storm Bolaven,
forecast to strengthen to a typhoon before the start of the
extended forecast period. Much of this spread develops after
early day 5 Friday. 00Z/12Z ECMWF-CMC runs have been the fastest
and the GFS slowest. By the end of day 8 next Monday the 00Z
ECMWF and 06Z GFS were more than 40 degrees of longitude apart and
the overall possibilities of ensemble members are extremely
diverse as well. While guidance agrees on a track south of 50N
latitude regardless of timing, the 12Z ECMWF/ECMWF mean show
potential northern stream interaction that would add further
uncertainty to the forecast. A sufficiently fast timing of
Bolaven would either displace or feed into the mean low now
expected to the south of the Alaska Peninsula late in the period.
Given the uncertainties involved late in the forecast, preferences
transitioned toward the ensemble means and the 12Z GFS with the
weakening Bering Sea/Aleutians system settling south of the Alaska
Peninsula--more reflective of continuity. Hopefully another day
of guidance can provide a somewhat clearer picture of Bolaven's
role in the Pacific forecast.
...Weather and Hazards Highlights...
The forecast has been consistent in principle for the forecast of
deep low pressure expected to reach the Aleutians and southern
Bering Sea by the start of the forecast period early Thursday.
This system will likely bring strong winds and high seas to the
Aleutians and surrounding waters, with continued potential to
reach hazardous criteria at some locations. A broad shield of
focused rainfall will sweep across the Aleutians/Bering Sea as
well. After Thursday this storm should weaken somewhat as it
wobbles over the region for a couple days. This system and its
lingering lighter rainfall along with another possible feature
supported by upstream energy should ultimately track south of the
eastern Aleutians/Alaska Peninsula by next weekend. Then the
forecast for Bolaven will require monitoring. The majority of
guidance suggests that the track will be far enough south to keep
precipitation south of the Aleutians, with the faster/northern
side of the spread offering a period of easterly winds while a
slower/southward scenario would maintain more northerly winds
(currently the favored option).
With continued uncertainty in some important localized details,
the general theme persists for a period of enhanced precipitation
and winds over at least the Panhandle late this week as one or
more surface waves track northward over the northeastern Pacific
and into or near the Gulf. Specifics still have to be resolved
with respect to how far west of the Panhandle the moisture and
brisk winds may extend across southern parts of Southcentral.
Precipitation intensity should taper off going through the weekend
into early next week but some activity may persist, especially
over eastward/southeastward-facing terrain.
Expect a gradual warming trend over the course of the period.
Below normal temperatures, especially for highs, should be most
common late this week over the southeastern half or so of the
state. By early next week expect only some pockets of below
normal highs to remain while much of the state should see above
normal lows.
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