Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
659 PM EST Mon Jan 09 2023
Valid 12Z Fri Jan 13 2023 - 12Z Tue Jan 17 2023
...Overview...
Guidance remains fairly agreeable for the large scale pattern
evolution during the period. A compact upper low forecast to be
just north of the Bering Strait as of early Friday should progress
east-northeast and leave behind a persistent upper weakness near
the northern coast of the mainland, while an upper ridge prevails
from western Canada into most of the mainland. This ridge will
likely stay in place through the period but gradually weaken with
time. Elsewhere, as an initially deep Aleutians storm weakens,
guidance suggests potential for various random surface lows across
a broad area covering the Bering Sea/Aleutians/North Pacific into
the northeastern Pacific/Gulf of Alaska. The upper pattern should
trend toward elongated upper troughing from the Bering Sea through
northeastern Pacific by early next week as a strong zonal jet
becomes established over the Pacific. The forecast pattern will
tend to maintain episodes of precipitation along parts of the
southern coast, and on average with lighter amounts over the
Panhandle and Aleutians. At the surface the mainland will remain
in a gradient between strong high pressure over higher latitudes
of the Arctic and the various surface lows to the south/southwest.
...Guidance/Predictability Assessment...
Overall the ensemble means and ECMWF runs have recently been the
most consistent with the depiction of mainland ridging and the
weakness expected near the northern coast. GFS runs have been a
little more variable, with at least half fitting into the
consensus scenario reasonably well but stray runs occasionally
showing up--such as the one from 00Z/09 closing off a deep low
over the mainland early next week. CMC/UKMET runs have been more
erratic, pushing the ridge farther north yesterday and then
suppressing it a little more today (though just early in the
period). A greater weight of the 12Z ECMWF/GFS relative to the
UKMET/CMC keeps the general pattern similar to continuity, with
the only minor trend being for upper heights within the ridge to
be slightly lower today.
Beyond the initial high-confidence forecast for the deep Aleutians
storm on Thursday (still hanging on to a depth in the low 960's mb
at that time, after possibly being in the 950's late in the short
range), guidance continues to show a lot of uncertainty for
specifics of individual surface lows across the Pacific and Bering
Sea. 00Z/06Z GFS runs are on their own with the extreme depth of
a low that tracks northward from the Pacific into the Bering Sea
during the weekend. However there has been somewhat of a signal
that a low could follow that general evolution albeit weaker, so a
moderate input of GFS guidance reflects such a possibility.
Various possibilities for individual lows exist farther eastward
as well, with some potential influence on areas between the Alaska
Peninsula and the Panhandle. In a broad sense a moderate majority
of guidance (including the 12Z ECMWF and latest ECMWF/CMC means)
suggests that Bering Sea low pressure may migrate eastward toward
the end of the period next Tuesday while a separate low gravitates
toward or consolidates just east of Kodiak Island.
Forecast considerations led to a starting blend consisting of 70
percent total 12Z GFS/ECMWF runs and the rest 12Z UKMET/CMC for
days 4-5 Friday-Saturday. Then the blend began to split GFS/ECMWF
input between their last two runs to tone down the influence of
any particular pronounced but low-confidence surface low, while
raising 12Z GEFS/00Z ECMWF mean influence up to 40-50% total by
days 7-8 Monday-Tuesday.
...Weather/Hazards Highlights...
The primary focus for meaningful precipitation will likely
continue to extend across the Alaska Peninsula, southern coast,
and Panhandle at varying times. The relatively higher totals on a
multi-day basis should be over the Kenai Peninsula and parts of
the Alaska Peninsula given fairly persistent easterly low level
flow. Most activity should be in the light-moderate range but a
sufficiently close track of any low-predictability surface low
could produce locally heavier precipitation for a time. The
general pattern may also lift some moisture northward across the
Panhandle but again with precipitation tending to be in the
lighter half of the spectrum. The compact upper low/shortwave
initially brushing the northern mainland may produce light snow in
its vicinity late this week. Otherwise most of the mainland away
from the southern coast should stay fairly dry. The Aleutians
should see unsettled conditions through the period, but with low
confidence for any individual lows that could focus
precipitation/wind beyond the initial deep low over the area on
Friday. The gradual weakening of upper ridging over the state
over the course of the period should promote a colder trend for
temperatures. Lows may may still be above normal over most
locations by early next week after starting out well above normal
on Friday, but gradually increasing coverage of below normal highs
will be possible over the interior and North Slope.
Rausch
No significant hazards are expected over Alaska during this
forecast period.
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