564
FXAK02 KWNH 042353
PMDAK

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
652 PM EST Wed Dec 04 2024

Valid 12Z Sun Dec 08 2024 - 12Z Thu Dec 12 2024

...Heavy precipitation and gales expected along the southern coast
late this weekend through at least early next week...

...Overview...

The ensemble means remain consistent and agreeable in showing a
persistent upper ridge building northward over the northeastern
Pacific/mainland Alaska and slowly translating east through the
period. Multiple shortwaves/upper-lows to the west rotate around
persistent, mean low pressure over the Bering Sea in a gyre-like
pattern, with increasing uncertainty on the timing/evolution of
the shortwaves/upper-lows into next week. However, the overall
pattern featuring ridging along the Panhandle/eastern mainland and
approach of systems to the west will support very heavy
precipitation along the southern coast from Sunday through at
least Tuesday with a northward surge of Pacific moisture in strong
southerly flow. Confidence has also increased in the potential for
moderate to heavy precipitation for western portions of the
Mainland Sunday and Monday.

...Guidance Evaluation/Preferences...

Guidance remains in good agreement on the overall pattern that
will guide hazards and temperatures next week. An
upper-low/surface system lingering over the eastern Gulf and
Panhandle on Saturday will weaken/move inland as strong ridging
builds northward in its wake, setting up broad-scale mean ridging
over the Panhandle/eastern mainland/northwestern Canada that will
make slow eastward progression with persistent waves of low
pressure approaching from the west. Uncertainty grows with time on
the evolution of these waves, though the deterministic guidance
continues to show good agreement on the track and increasing
strength of an initial system lifting northward across the
northeastern Pacific and towards the Aleutians/Alaska Peninsula
early Sunday, passing into the Bering Sea Monday. This is quickly
followed by another surface system in its wake lifting towards the
Aleutians, though with some lingering uncertainty regarding the
initial consolidation of energies over the northern Pacific. The
GFS/ECMWF bring this system eastward faster compared to the CMC
and prior guidance. Then, all deterministic guidance diverges on
track Tuesday into Wednesday, as the CMC shows the system lifting
northward into the Bering similar to the prior system, the GFS
takes it into the southwest mainland, and the ECMWF shows a more
southerly track eastward into the Gulf of Alaska by Wednesday
morning. The ECens/GEFS mean guidance show stronger support for
upper-level troughing/surface low pressure into the Gulf of
Alaska, most similar to the ECMWF, and retains a pattern that was
similar to guidance from the prior day. Both the deterministic and
ensemble guidance agree that the strong upper-level ridging will
eventually weaken and shift eastward by later next week, allowing
for the potential for a more southerly track of additional
shortwave energy over the northeastern Pacific into the Gulf of
Alaska, though with higher uncertainty with respect to the
specifics.

The updated WPC forecast for Alaska begins with a composite blend
of the 12Z ECMWF/GFS/CMC for day 4 (Sunday) given the strong
agreement on the strength and location of the initial northeastern
Pacific low to track northward towards the Aleutians/Alaska
Peninsula. A small contribution from the 00Z ECens/12Z GEFS means
is added for day 5 (Monday) as uncertainty begins to grow with the
evolution/consolidation of the subsequent system in the wake of
the first approaching the western Aleutians. The contribution from
the ensemble means is further increased and the CMC removed during
the middle to late part of the forecast period day 6-7
(Tuesday-Wednesday) as track/timing differences with the second
system begin to grow, with the ECMWF/GFS favored for being closer
to the pattern in the ensemble means. The contribution from the
means is increased to 75% by the end of the period day 8
(Thursday) as the overall pattern shift featuring the eastward
moving ridge is generally well captured, but with differences in
the specifics in the deterministic guidance.

...Weather/Hazards Highlights...

Confidence continues to increase in a mid-latitude Pacific system
lifting northward over the eastern Aleutians/Alaska Peninsula and
into the Bering Sea as ridging builds northward to the east over
the Panhandle/eastern mainland, helping to enhance/focus moist
southerly Pacific flow along the southern coast and bring very
heavy precipitation to the region late this weekend and into early
next week. Reinforcement from a secondary system following in its
immediate wake looks to continue this threat now through at least
Tuesday, with a gradual shift in focus from west to east along the
southern coast and into the northern Panhandle with time. Expect
the upper-ridge to then shift far enough eastward to allow for the
precipitation focus to more quickly adjust southeastward into the
southern Panhandle and northwestern Canada by Wednesday. For the
period as a whole, expect mostly rain along coastal/low elevation
areas with snow elsewhere aside from the southern Panhandle where
milder conditions should promote broader rain coverage with higher
snow levels. Strong, gale force winds can also be expected with
the approach of the system from the eastern Aleutians east along
the Alaska Peninsula and southern coast Sunday through Monday.
Away from the southern mainland hazards, the details have come
into better focus regarding additional moderate to heavy
precipitation for the western mainland Sunday-Monday as the first
system tracks northward into the Bering Sea. Some areas of lighter
precipitation will likely linger beyond Monday, and also persist
to the north over the Brooks Range as well. Growing uncertainty in
the timing/evolution of subsequent systems from the northeastern
Pacific give low confidence to additional precipitation chances
for the Aleutians mid- to late next week. Expect above to well
above average temperatures to persist across the state through
most of the period, with those for morning lows even higher than
those for afternoon highs. Anomalies of 10-25 degrees can be
expected for afternoon highs with anomalies upwards of 20-35
degrees for morning lows. These highest anomalies will focus
across the interior mainland. Forecast highs early to mid-next
week generally range from the single digits to teens for the North
Slope, the teens and 20s for the interior mainland, 20s and 30s
along the western coast and for southwestern portions of the
state, and 30s and 40s along the southern coast. A pattern change
may bring cooler, more seasonable temperatures to the northern
half of the state by later next week.

Putnam

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

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