Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
641 PM EST Wed Nov 13 2024
Valid 12Z Sun Nov 17 2024 - 12Z Thu Nov 21 2024
...Potential for strong winds over parts of northwestern Mainland
Alaska around the middle of next week...
...Overview...
Mainland upper troughing from the weekend into early next week
(including an upper low hovering north-northeast of the mainland
and another one that may drift from the southwestern mainland into
the Gulf of Alaska) will promote a brief period of well below
normal temperatures, with coldest anomalies likely to be during
the first half of next week. The trough will give way to a strong
ridge building into the Aleutians/Bering Sea and then moving over
the mainland Tuesday-Wednesday. The gradient between the ridge
and a strong system lifting northward through Siberia will bring
an axis of strong winds and moisture from the western Aleutians to
the northwestern mainland during Tuesday through midweek, though
with some uncertainty over wind/snowfall details over the
mainland. Meanwhile, a modest system potentially brushing the
Aleutians around Sunday should briefly drop southeastward as
Pacific upper troughing amplifies, and then settle over the
northeastern Pacific around 50N latitude. Flow around this system
may bring some light to moderate precipitation to at least the
southern Panhandle.
...Guidance Evaluation/Preferences...
Within the larger scale trough initially covering the mainland,
some detail uncertainties remain with southwestern mainland
dynamics that should progress southeastward into the Gulf of
Alaska. The GFS/ECMWF have relatively better ensemble mean
support for a slow translation of energy into the Gulf Sunday into
Monday while the CMC/UKMET offer potential for somewhat faster
progression. Farther north, guidance continues to waffle for the
position of the upper low wobbling to the north-northeast of the
mainland through the first half of next week. The current
model/mean average provides a reasonable intermediate solution,
compared to the 12Z/12 ECMWF/CMC runs that were well north and
some other runs that were at least as far south as the northern
coast of the mainland. Machine learning (ML) models have been
keeping the upper low a fair distance north of the mainland near
140W or even a little farther eastward.
Over the past couple days the dynamical guidance has shown a
notable trend toward the ML models that have generally been on the
weaker side of the envelope for the system forecast to brush the
Aleutians around Sunday. In addition, today's trend in the
dynamical models has been a modest southward nudge. This
southward trend continues into early next week, followed by
relative continuity toward the system settling in the vicinity of
47-52N/135-145W during Tuesday-Wednesday, followed by weakening.
Some CMC/CMCens mean runs have been on the northern/northeastern
side of the spread, though a couple ML models offer potential for
low pressure to get a little closer to the southern Panhandle than
the majority cluster. There is still a fair degree of detail
uncertainty by Tuesday onward, as supporting dynamics may come
from a combination of the initial North Pacific shortwave, the
upper low reaching the Gulf, and energy between the two features.
Both dynamical and ML guidance show considerable spread and
run-to-run variability for exactly how far eastward the tight
gradient and moisture extend eastward across the northwestern
mainland. Of particular note, 12Z ECMWF runs from the past two
days have been on the slow side (ECens means favoring a compromise
somewhat closer to the 00Z run) while a number of GFS runs have
been on the aggressive side with the eastward extent of winds and
for QPF totals. However, a couple ML models suggest even broader
extent of fast flow. Preference sides with an intermediate
solution given the spread/variability. The new 18Z GFS has come
in closer to this idea.
Guidance considerations led to incorporating a split of the
12Z/00Z ECMWF runs for 40 percent total weight of the forecast
through the period, with the 12Z GFS comprising 30 percent through
Wednesday (20 percent Thursday). With the 00Z ECens mean
providing the best match to preference over the Pacific, that
solution increased to 15-25 percent late in the period to pick up
after the end of the UKMET run and to offset some GFS details late.
...Weather/Hazards Highlights...
The southern coast and Panhandle may see some lingering mostly
light precipitation as low pressure persists over the Gulf of
Alaska into Monday. A modest system brushing the Aleutians around
Sunday should bring a brief period of moderate totals there before
dropping southeastward into the east-central Pacific as it
develops into a larger scale low. The low will likely remain far
enough south to keep most of the surrounding flow of moisture from
reaching much farther north than the southern Panhandle which may
see some moderate amounts. Lighter/more scattered activity may
reach into the northern Panhandle/Southcentral coast. Some
locations near the Alaska Peninsula may see brisk winds into
Sunday, behind the Gulf low. Winds may strengthen along the
southern coast/Panhandle early-mid week within the tightening
gradient between Mainland Alaska/western Canada high pressure and
the Pacific low. By the middle of next week expect flow ahead of
a cold front pushing across the Bering Sea to bring strong
southerly winds and snow (enhanced over windward terrain) into the
northwestern mainland, with some uncertainty over the eastward
extent and amounts. There is at least enough confidence in the
wind threat to depict a High Winds area in the Days 3-7 Hazards
Outlook for parts of the far northwestern mainland valid next
Wednesday. Arrival of the cold front next Thursday should begin
to temper the wind/snow somewhat.
A cold front dropping south through the mainland as an Arctic
upper low hovers just to the north/northeast, along with trailing
surface high pressure building across the mainland next
Monday-Tuesday, will bring a pronounced colder trend from Sunday
into the first half of the next work week. Coldest anomalies
should be during Monday-Tuesday and persisting into Wednesday
morning over the eastern Interior. Interior locations could reach
at least as cold as 20-30F below zero. Temperatures should trend
warmer Wednesday-Thursday as high pressure moves into Canada and a
front approaches/reaches the western mainland.
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