Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
755 PM EDT Sun May 21 2023
Valid 12Z Thu May 25 2023 - 12Z Mon May 29 2023
...Overview...
Latest guidance has provided some elements of continuity,
especially with the idea of an upper low and associated surface
system drifting across the Bering Sea. However there are
significant differences and changes from continuity for an upper
low forecast to be near the eastern Alaska Peninsula/Kodiak Island
to start the period early Thursday, while models/means have become
more mixed with respect to flow aloft over the mainland. The
combination of changes for the initial Kodiak Island upper low and
subtle adjustments for flow ahead of the Bering Sea system have
led to somewhat of an increase of five-day precipitation totals
across far southern areas where the pattern will be most favorable
for rain and high elevation snow.
...Guidance Evaluation/Preferences...
The 12Z cycle introduced a considerable amount of guidance
divergence and continuity change for the upper low now expected to
be near the eastern Alaska Peninsula/Kodiak Island as of the start
of the period early Thursday. Up to 12-24 hours ago there was a
decent clustering of operational models depicting a fairly
progressive scenario across or south of the Panhandle with
ECens/CMCens mean trends in that direction. Now the 12Z GFS is
the only progressive solution while the ECMWF shears it out as it
drifts into the southern mainland, the CMC maintains it for a
longer time over the mainland, and the UKMET actually wraps it
back into the northern periphery of the Bering Sea circulation.
The GEFS mean has in fact been the most stable solution, fairly
slow and weakening after the start of the period, while the 12Z
ECens/CMCens means track the feature into the Gulf by early Friday
before shearing out. Given the full array of model/ensemble
guidance, preference was to follow an intermediate solution
closest to the operational 12Z ECMWF run and 12Z GEFS/CMCens means
(the 00Z ECens mean available at forecast preparation time being
more reflective of the older progressive scenario).
In terms of a model/ensemble mean average, the forecast of the
Bering Sea system has at least been better behaved than the
leading system above. The 12Z ECMWF provides a very favorable
adjustment from a number of prior runs that had strayed to the
northern and/or western side of the envelope. Thus its solution
has the greatest individual weight in today's blend. On the other
hand the latest GFS runs and 12Z UKMET are somewhat on the
southern/southeastern side of the spread later in the period but
the 18Z GFS has nudged a little closer to the ensemble mean
average, with the 06Z GFS providing a good counterbalance to the
12Z run for the GFS portion of forecast input. CMC runs continue
to agree very poorly with other models/means for Bering Sea
through North Pacific details. The main adjustment in today's
forecast from continuity is slightly farther east progression and
a little more of a signal for modest triple point wave evolution
near Kodiak Island by next weekend.
With some potential dependence on surrounding features, today's
guidance is noticeably more mixed regarding flow aloft over the
mainland after day 4 Thursday when there is still reasonable
consensus for there to be a weakness over the mainland between one
ridge/high just north of eastern Siberia and a separate open ridge
over northwestern Canada. After that time the ensemble means had
been similar in depicting a weak ridge over the mainland bridging
the aforementioned two ridges, but the 12Z GFS and GEFS/CMCens
means maintain this weakness for the rest of the forecast. The
GFS/GEFS mean shift this weakness more to the western coast late
in the period, which the 12Z ECMWF also does after having more of
a ridge like ensemble mean continuity (and still in the 12Z
ECens). The favored blend yielded a tilt to continuity early-mid
period as an initial weakness gives way to some eastern-northern
mainland ridging around Friday-Saturday and then reflects the
emerging trend of a weakness over the west thereafter.
Based on the combined preferences for the above
systems/evolutions, today's forecast blend started with 40 percent
12Z ECMWF, 30 percent GFS (split between the 06Z and 12Z runs),
and 30 percent ensemble means through the period. The only slight
adjustment was around mid-period when the ensemble mean component
changed from just the 12Z GEFS/CMCens to include the 00Z ECens as
well after departure/shearing of the initial upper low near the
southern coast. The manual forecast incorporated some manual
enhancement to the initial blend output.
...Sensible Weather and Potential Hazards...
The upper low initially near the eastern Alaska Peninsula/Kodiak
Island and flow ahead of it should support areas of rain and high
elevation snow along and near the southern coast into the
Panhandle during the latter half of the week. Forecasts of this
system have been very inconsistent, keeping confidence below
average for resolving intensity and duration of precipitation. In
association with the Bering Sea system, expect a broad area of
rain to spread across the Bering Sea/Aleutians and into parts of
the western (especially southwestern) mainland late this week. A
potential triple point wave may help to focus precipitation along
the southern coast and possibly into parts of the Panhandle by
next weekend, while lighter activity lingers farther westward.
Uncertain flow details aloft over the mainland will determine the
distribution and amounts of precipitation away from the most
favored southern areas.
Expect high temperatures to show a cooling trend during the
period, with the majority of the state seeing below normal
readings and coolest anomalies tending to be over southern areas.
The far northwest and a few localized pockets elsewhere may see
above normal highs. Anomalies should be warmer for low
temperatures, with greater coverage of above normal but still some
areas being below normal.
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