Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
655 PM EST Sun Jan 05 2025
Valid 12Z Thu Jan 09 2025 - 12Z Mon Jan 13 2025
...Continued stormy pattern this coming week through next weekend
with heavy precipitation and strong wind threats along the
southern mainland coast...
...Overview...
Latest guidance has changed little in terms of the large scale
pattern forecast. Expect a persistent Bering Sea into Pacific
upper trough and downstream ridge, with flow between the two
features directing a series of strong storm systems that will
focus heavy precipitation along the southern coast and produce
strong winds over the Northeast Pacific to the southern coast.
More localized areas of strong winds will be possible elsewhere.
Models/means still advertise a transition in storm track, with a
Thursday storm tracking into the Gulf of Alaska representing the
last in a series of south-to-north systems. Then a couple of
western-northern Pacific storms should track toward the
southwestern mainland, leading to broader westward extent of
significant precipitation. When including the shorter range, the
current five-day extended forecast period still represents just a
portion of a longer term period of anomalously high precipitation
totals along the southern coast. Expect above to well above
normal temperatures over a majority of the state during the
period, aside from some pockets of below normal readings over
parts of the North Slope late this week.
...Guidance Evaluation and Preferences...
During the first half of the period, a 12Z model blend consisting
of the ECMWF/GFS/CMC in order from more to less weight represented
the most common ideas of guidance for the storm tracking into the
Gulf of Alaska Thursday-Thursday night as well as the leading
North Pacific storm that should track from near the eastern
Aleutians into the southwestern mainland by Friday-Saturday.
Considerations for the second and likely stronger North Pacific
storm led to transitioning the guidance blend toward a split of
ECMWF input (00Z/12Z runs) and the GFS (more 06Z weight versus the
12Z run) with a little 12Z CMCens included (as it matched the 00Z
ECens mean track but was deeper) by day 7 Sunday. With increased
timing spread by day 8 Monday and Bering Sea through Pacific
details rapidly becoming more chaotic, the last part of the
forecast decreased 12Z ECMWF weight in favor of a little more 06Z
GFS while holding onto some CMCens input.
For the Thursday Gulf storm, guidance continues to show some
typical detail differences within a fairly consistent evolution
overall. Machine learning (ML) models have stabilized with the
depth of this system after nudging 1-2 mb stronger per day
recently, while the deeper side of the prior operational model
spread (into the 950s mb as of early Thursday) has risen into the
low 960s mb. Thus the manual forecast today has adjusted up a bit
into the lower half of the 960s mb. Recent UKMET runs have been
questionably weak though, favoring exclusion of its 12Z run from
the forecast. Meanwhile there continues to be some east-west track
spread into Thursday night, with GFS runs tending to lean a bit
west but acceptable as part of a multi-model compromise.
Guidance has continued a recent trend toward better
definition/longevity for the system forecast to track from near
the eastern Aleutians into the southwestern mainland during
Friday-Saturday, as previously advertised by the ECMWF/CMC and
their means along with the ML models. That said, the overwhelming
majority of model/ML runs say that the 12Z GFS becomes too deep by
early Saturday (just under 960 mb). Consensus shows a depth more
in the 970s to around 980 mb range. The new 18Z GFS has trended
back to or even a tad weaker than the majority cluster. There is
still a moderate amount of spread for the track by Saturday but
the favored blend is able to keep the manual forecast near
continuity.
Dynamical/ML models still generally show the second storm in the
series to be the stronger one, as upper dynamics become negatively
tilted and possibly close off an upper low for a time. Compared
to yesterday the ML models differ a bit more today for details but
still advertise a surface low reaching a depth around 960 mb by
early Sunday, which is notable considering that the ML models in
their relatively short history have tended to lean closer to
ensemble mean depth in principle. Speaking of the means, the
CMCens and ECens have been trending deeper over recent runs, with
the 12Z CMCens reaching into the 960s mb. ECMWF runs have
consistently been in the 950s mb while the 12Z ICON joined the
ECMWF. GFS runs had been weaker/more diffuse but latest runs have
trended deeper. As for timing, ML models remain somewhat slower
than the ECMWF/ECens/CMCens/ICON cluster, though not to an
exceptional degree relative to typical spread/error at 7-8 days
out in time. Ultimately the 06Z GFS provided the most coherent
somewhat slower compromise companion with the ECMWF cluster while
the 12Z and especially 18Z GFS strayed questionably slow. The
resulting blend still ended up closer to the ECMWF timing though.
Some future adjustments may well be needed but timing aside, the
majority of guidance offers average or better confidence in a
strong storm tracking into the southwestern/western mainland.
Individual model runs quickly become more chaotic with details
from the Bering Sea through the Pacific by day 8 Monday. There is
some signal for potential frontal waviness over the Pacific, while
there is less support for the 12Z ECMWF area of low pressure near
the western Aleutians by next Monday. This warranted a reduction
in 12Z ECMWF weight in the favored blend by that time.
...Weather/Hazards Highlights...
Heavy precipitation threats along the southern coast including the
Panhandle will continue, with periods of strong winds over the
northeastern Pacific/Gulf of Alaska toward the southern coast as
well. The last and potentially strongest in a series of
south-to-north Pacific storms should track into the Gulf of Alaska
by Thursday-Thursday night, producing one period of focused
rainfall and high elevation/inland snow. Then the best guidance
cluster still shows two North Pacific storms that may ultimately
track into the southwestern/western mainland by Saturday-Monday,
continuing the sequence of atmospheric river events aimed at the
southern coast. The transition in storm track will likely lead to
westward expansion of focused precipitation back to the Alaska
Peninsula, though still with lower amounts than expected between
the Kenai Peninsula and the Panhandle. Over the latter region,
latest guidance continues to show significant five-day QPF totals
(early Thursday to early Tuesday) with localized maxima of at
least 10-15 inches of liquid possible. Covering the period from
now into the first half of next week, forecast totals continue to
be particularly anomalous over the southeastern part of
Southcentral. Elsewhere, much more modest amounts of moisture may
occasionally spread areas of light snow farther north across the
mainland at times, with a southwest-northeast frontal boundary
providing some focus. Parts of the western mainland could see
somewhat heavier precipitation at times from the weekend into
early next week depending on the strength/track of arriving
storms. The mean southerly/southwesterly flow aloft combined with
the Saturday-Monday systems could produce periods of brisk to
strong winds over some areas of the mainland away from the
southern coast.
Above to well above normal temperatures will prevail over most of
the state during the period. Improving refinement of the two
potential systems tracking into the southwestern/western mainland
in the Saturday-Monday time frame have raised the potential for
greater persistence of very warm anomalies through the forecast
period versus previous forecast. A cooling trend may move in from
the southwest behind the second storm by next Monday. Parts of
the North Slope may see some below normal readings through the end
of the 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