236
FXAK02 KWNH 122334
PMDAK

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
633 PM EST Tue Nov 12 2024

Valid 12Z Sat Nov 16 2024 - 12Z Wed Nov 20 2024

...Heavy precipitation over the Southcentral coast and Panhandle
Friday into Saturday...

...Overview...

Today`s models/ensembles continue to show upper troughing over the
mainland from the weekend into the first part of next week while
progressive North Pacific/Aleutians energy and associated low
pressure settle into the east-central Pacific.  This will be part
of a more amplified regime by next Tuesday-Wednesday, supported by
a large scale trough/upper low emerging from eastern Asia and
ridge that builds into the Aleutians/Bering Sea and then the
mainland.  The mainland upper trough will bring a pronounced
colder trend for the first half of next week.  Low pressure
nearing the southwestern coast of the mainland by Friday and
transitioning to the Gulf thereafter will support enhanced
precipitation along the southeastern coast/Panhandle
Friday-Saturday.  Aleutians low pressure will be accompanied by an
area of rainfall, but it is uncertain how much of this system`s
moisture may eventually reach the Panhandle by midweek.  The
strong gradient between the upstream ridge/trough early-mid week
should progress into the western Aleutians and much of the Bering
Sea, but again with some uncertainty over how much wind/moisture
reaches the western mainland by next Wednesday.

...Guidance Evaluation/Preferences...

Agreement and continuity have been fairly good with the surface
low forecast to be over the Gulf of Alaska in the Friday-Monday
period.  There is still some modest timing uncertainty regarding
the transition of Friday (late short-term) western coast low
pressure to the Gulf, with a majority of solutions holding onto
slightly more western low pressure by 12Z Saturday compared to 24
hours ago.  Some differences involve smaller scale aspects of
dynamics aloft with low predictability for multiple days out in
time.

Guidance clustering has improved considerably over the past day
for the system forecast to track along the Aleutians this weekend.
 Previously slow/deep ECMWF and fast GFS runs have trended closer
to each other and in fact to the point of flipping, such that the
12Z ECMWF is faster/weaker than the 12Z GFS (while the 18Z GFS has
adjusted slower/weaker).  Overall the ECens mean has been the most
consistent recently for track/timing, with the 12Z GEFS mean
having trended to that solution.  Now the 12Z UKMET strays a bit
to the northeast side of the spread.  Machine learning (ML) models
continue to show some spread as well, but again are closer
together than before.  ML runs still lean to the weaker side of
the dynamical model spread.

As this system moves into the east-central Pacific
Monday-Wednesday, there are various detail uncertainties that will
affect specifics within a more agreeable area of mean low
pressure.  These include potential incorporation of some flow from
the western side of the mainland upper trough along with the
possibility of a developing frontal wave from the south becoming
absorbed into the overall system.  Trends over the past couple
days have generally been a little farther south, with the 12Z
operational model runs a little south of the ensemble means.  The
new 12Z ECens mean adjusted slightly southward from the 00Z run.
These trends lead to later arrival/reduced northward extent of
surrounding moisture into the Panhandle and vicinity.  Some ML
models have not reflected this southward/southwestward adjustment,
and in fact the new 18Z GFS has reverted to pulling low pressure
farther north by next Wednesday.

Guidance exhibits a fair amount of variability for the Arctic
upper low that may have at least a peripheral influence on the
northern mainland.  The 12Z ECMWF/CMC have made the most
pronounced changes compared to yesterday, keeping the feature a
lot farther north than before.  The 00Z ECens mean got close to
the northern coast but otherwise the ensemble means have kept it
somewhat northward, while ML runs favor a northward position too
(and possibly as far east as 140W or so, offering support for at
least a compromise involving the 12Z ECMWF/CMC.  A model/ensemble
mean composite provides reasonable continuity or a slight
northward adjustment depending on the day.

By Wednesday the dynamical and ML models show some spread and
run-to-run variability regarding how quickly the moisture and
strong gradient pushing across the Bering Sea reach the western
(especially northwestern) mainland.  Overall, recent GFS runs lean
on the fast side while the 12Z ECMWF is on the slow side.
Preference is for an intermediate approach until better
clustering/continuity develop.

Today`s forecast started with primary emphasis on the 12Z
GFS/ECMWF and minority 12Z CMC input early in the period,
splitting ECMWF input between the 12Z/00Z runs by Day 6 Monday as
more detail differences develop.  Days 7-8 Tuesday-Wednesday
incorporated 30-40 percent total weight of the 12Z GEFS/00Z ECens
means while maintaining some influence from the aforementioned
model runs.  This provided good continuity with the weekend
Aleutians system and led to a slight southward trend as this low
pressure reaches farther eastward.

...Weather/Hazards Highlights...

Flow ahead of low pressure becoming established over the Gulf of
Alaska will bring a plume of Pacific moisture into Southcentral
and the Panhandle during the late week/early weekend time frame.
This will likely produce some areas of heavy precipitation in
these regions, especially along the coastal terrain.  The Days 3-7
Hazards Outlook continues to reflect this threat during the
Friday-Saturday period.  The Alaska Peninsula and vicinity may see
a period of brisk to strong winds on the back side of the Gulf low
during the weekend.  The next system should track farther south,
potentially bringing a period of rain and some increased winds
this weekend.  Recent trends have been indicating that this system
may have moderate strength at most, however.  This storm`s
moisture should stay south of the Alaska Peninsula, while guidance
spread farther east tempers confidence regarding how much moisture
may reach the Panhandle and vicinity by next Tuesday-Wednesday.
On a relative basis, the southern Panhandle has a better chance to
see some of this moisture.  Also during that time frame, southern
tier areas may see stronger winds due to the tightening gradient
between this low and high pressure that builds across the northern
mainland.  A tight gradient ahead of a frontal system pushing into
the western Aleutians/Bering Sea and Siberia early-mid week will
support stronger winds and precipitation.  Some of the brisk winds
and moisture may reach the western/northwestern mainland by next
Wednesday.

Into Saturday expect a mix of below/above normal temperatures with
a tendency for more above normal min temperatures especially near
Gulf of Alaska low pressure.  A cold front dropping south through
the mainland as an Arctic upper low hovers north of the mainland,
along with trailing high pressure building into the northern
mainland by next Tuesday, will bring a pronounced colder trend
from Sunday into the first half of the next work week.  Coldest
anomalies should be over Interior/southwestern areas around
Monday-Tuesday.  Temperatures should begin to trend warmer on
Wednesday as high pressure moves eastward and southerly flow
pushes into the far west.

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

$$