Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
756 PM EDT Mon Apr 21 2025

Valid 12Z Fri 25 Apr 2025 - 12Z Tue 29 Apr 2025

...Overview...

The consensus forecast from the Arctic into parts of the mainland
has remained fairly consistent, with an Arctic ridge to the 
northwest of the mainland and a central-northern mainland weakness
aloft tending to lift toward the northern coast as ridging builds
in from the southeast. At the surface expect a persistent 
gradient between a wavy southern mainland front and Arctic high 
pressure. Meanwhile there is continued guidance spread and run-to-
run variability for systems across the North Pacific and into the
Aleutians/southern Bering Sea. One or more of these systems may 
bring some focused precipitation to locations between the 
Aleutians and the southern coast/Panhandle during the period but 
with continued below average confidence for specifics. 
Temperatures should remain below average over the north while 
above average readings prevail over most southern areas.

...Guidance/Predictability Assessment...

For the system initially over the northeastern Pacific/eastern 
Gulf of Alaska as of early Friday, yesterday's notable shift 
toward the northern part of the prior envelope has generally held 
up. GFS/GEFS mean runs continue to be south of most other 
solutions aside from the 12Z ICON. Latest machine learning (ML) 
models favor a solution most similar to the ECMWF cluster. The new
18Z GFS has nudged somewhat northward and the 18Z ICON has 
shifted close to the ECMWF cluster too, further improving 
confidence.

Behind this system, there is initially some degree of clustering 
and continuity with low pressure tracking over the 
Aleutians/southern Bering Sea around the end of the week while the
leading wavy frontal system continues eastward to the south of 
the Panhandle. However guidance then diverges for the ultimate 
evolution/progression of the Aleutians/Bering Sea low and a 
potential mid-latitude Pacific system. Dynamical model trends over
the past couple days and 00Z ML models have been suggesting more 
emphasis on the Pacific low versus some ensemble means that merge 
these systems in some fashion or even emphasize Bering Sea low 
pressure as in the 12Z ICONens. Some CMCens/ECens guidance hints 
at both features, while CMC runs have been leaning on the 
progressive side overall. An added wild card is upper low energy 
initially over Siberia. The 12Z ECMWF brings it eastward across 
the Bering Sea toward the western coast of the mainland along with
some surface low reflection. Some earlier CMC runs brought the 
upper low eastward as well but most other guidance including ML 
models are not enthusiastic about such eastward progression of the
Siberia upper low. Preference adjusted to reflect a little more 
Pacific low emphasis versus continuity after Saturday, yielding 
somewhat more easterly flow over the Alaska Peninsula and toward 
the Kenai Peninsula. New 12Z ML models highlight the uncertainty 
with this part of the forecast as they are generally less defined 
with the primary features.

Farther west the ensemble means and ML models have shown better 
agreement and continuity over the past couple days for a system 
that will most likely track near or somewhat south of the 
Aleutians by next Monday-Tuesday. Operational models vary a lot 
more, ranging between having it become overwhelmed by a persistent
leading system by Tuesday versus the slower/larger-scale 
evolution of the 12Z CMC. Latest GFS runs and the 00Z ECMWF end up
closest to the ensemble means in principle by the end of the 
period even if they may be a tad east and with lower confidence in
their way of getting there.

Adding together the above considerations, today's forecast for 
Friday-Saturday started with a split of 12Z/00Z ECMWF runs and 
parts of the 12Z GFS/UKMET/CMC. This reflected non-GFS preferences
over/near the eastern Gulf and a model composite elsewhere. Then 
the forecast removed the 12Z ECMWF due to its questionable Bering 
Sea evolution while increasing 12Z GEFS/00Z ECens input. Monday- 
Tuesday started with 60 percent means and the 40 percent total 12Z
GFS/00Z ECMWF. The ensemble mean weight helped to maintain 
reasonable continuity for the northern mainland upper weakness and
the ridge building to its south.

...Weather/Hazards Highlights...

The upper low forecast to linger over the eastern Gulf into 
Friday may still produce some light rain and higher elevation snow
over parts of the Panhandle before it finally moves inland by the
weekend. Upstream low pressure over the Aleutians and southern 
Bering Sea will bring some light rainfall to that region on Friday
while a leading front may provide somewhat more precipitation 
focus (but still light to locally moderate) over the Alaska 
Peninsula. From Saturday onward confidence decreases with respect 
to the eastward extent of precipitation along the southern coast 
as well as for amounts. The favored model/ensemble ultimately 
brings moderate totals across most of the southern coast with 
potential for some enhancement over eastward-facing terrain. An 
upstream system tracking a little south of the Aleutians may bring
another episode of rain early next week. Some of this moisture 
could reach as far eastward as the Alaska Peninsula. Lighter 
precipitation could extend farther north over southern/western 
areas at times. Deep-layer ridging over higher latitudes of the 
Arctic will likely support northeasterly or easterly winds of 
varying strength through the period. 

The northern half to third of the mainland should see below 
normal temperatures due to persistent flow around southeast 
periphery of the Arctic ridge. Most southern areas will see above 
normal readings, though clouds/precipitation should support below 
average daytime highs along the southeastern coast into at least 
Friday. Anomalies will generally be warmer for morning lows 
relative to those for daytime highs. Also expect a gradual warming
trend that will slowly reduce the coverage of below normal 
temperatures by next 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