Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
756 PM EDT Sun Sep 18 2022
Valid 12Z Thu Sep 22 2022 - 12Z Mon Sep 26 2022
...Overview...
From late this week into the weekend expect an upper trough to
amplify over the eastern Bering Sea/western coast of the mainland
and into the North Pacific, with an anchoring upper low wobbling
southeastward from far eastern Siberia. Some Arctic energy could
feed into the circulation during the weekend and uncertainty in
how this will happen tempers confidence in details of the trough
and any remaining embedded low(s) by late in the period. The
initial eastern Siberia upper low will support a strong surface
system (but not nearly as deep as Merbok's remnants) that should
gradually weaken as it descends toward/along the western coast of
the mainland. Multiple progressive waves may track over the
southern coast of the mainland and/or Gulf of Alaska depending on
specifics of impulses carried along by the cyclonic flow aloft
while there is a signal for more stable low pressure near the Gulf
by early next week. This pattern will favor the most significant
precipitation focus along the southeastern coast and Panhandle.
...Guidance/Predictability Assessment...
Today's guidance preference is similar to yesterday, a 12Z
operational model blend for the first half of the period (Thursday
into early Saturday) followed by a transition toward 40 percent
ensemble means (split among the 12Z GEFS/CMCens and 00Z ECens) and
remaining input from the GFS/ECMWF/CMC by day 8 Monday. The GFS
component switched to the 06Z run by day 8 due to differences
described below. The result yielded only modest refinement of the
overall pattern from continuity, though unavoidable changes arose
with some lower-predictability wave/front details from the
southern coast southward into the Pacific after Friday.
Clustering and continuity are better than average during the first
half of the period as the Siberia system tracks southeast toward
the western coast of the mainland while the overall upper trough
amplifies. There has been some debate over the strength of the
frontal wave expected to be near the Kenai Peninsula as of early
Friday. Earlier GFS runs had been on the deep side, due in part
to a different handling of incorporating Typhoon Nanmadol into the
larger scale flow, but since the 06Z cycle the GFS has been weaker
and similar to the ECMWF. The 12Z CMC is now the deep extreme but
seemingly just from having a deeper/sharper small-scale shortwave.
The latest guidance composite is slightly deeper than was the
case 24 hours ago.
From the weekend into early next week the primary question for the
large scale evolution will be how retrograding energy over the
Arctic ultimately interacts with the upper trough that sets up
along the western coast of the mainland. The best clustering of
guidance suggests a decent proportion of this energy should feed
into the trough and possibly pull the overall upper low farther
south/southeast than advertised by the ensemble means by early
next week. 12Z/18Z GFS runs differ as they show greater
retrogression of this energy, keeping it more detached from the
initial trough. The 06Z GFS was closer to the majority in
principle. Recent deeper trends for the overall trough in the
ensemble means would favor somewhat of a tilt toward operational
models while a compromise for upper low position looks best as
solutions diverge and predictability decreases by day 8.
How this pattern evolves, as well as specifics of upper dynamics
emerging from eastern Asia, will affect the system that may reach
into the Aleutians by next Monday. Favoring depth a little closer
to the operational runs for the upper trough mentioned above
should in turn keep the Aleutians system's track more suppressed
than the CMCens/ECens (which are farthest north, across the
southern Bering Sea).
...Weather/Hazards Highlights...
The front extending south from strong low pressure initially over
eastern Siberia will spread an area of precipitation across the
western mainland on Thursday. Some of this activity may reach at
least moderate intensity over terrain-favored areas. Winds could
be brisk as well, especially earlier in the day. Meanwhile
precipitation will trend heavier along the southern coast and
Panhandle as low pressure evolves over the northern Gulf of Alaska
by early Friday. This system will quickly depart but the mean
flow (which may contain multiple waves/fronts) should maintain a
wet pattern with additional heavy amounts possible from near the
eastern side of the Kenai Peninsula through the Panhandle Friday
into the weekend. Confidence decreases regarding the coverage and
intensity of precipitation along the southern coast/Panhandle by
Sunday-Monday, with meaningful guidance differences over how low
pressure may track into or evolve over the northeastern
Pacific/Gulf of Alaska. The chilly mean trough aloft will support
multiple days of mostly light precipitation over other parts of
the mainland as well as the eastern Aleutians. The system
tracking out of eastern Asia may eventually bring some
precipitation into the Aleutians by early next week but with low
confidence in the details at this time.
The forecast pattern will support below average high temperatures
across the southern mainland and Panhandle, while central/northern
parts of the mainland should see at least a mix of above/below
normal highs if not more above normal readings. Morning lows will
be mostly above normal with the highest anomalies tending to be
over the North Slope. Any pockets of slightly below normal lows
should be over the far south and Panhandle.
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