Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
655 PM EST Fri Feb 18 2022
Valid 12Z Tue Feb 22 2022 - 12Z Sat Feb 26 2022
...Heavy precipitation likely along the southern coast of the
mainland next week...
...Overview...
The majority of guidance has been fairly consistent with the
forecast of the overall pattern evolution next Tuesday-Saturday.
A weak shortwave (composed of multiple pieces of energy) crossing
the mainland around Tuesday will temporarily dampen eastern
Pacific into Alaska/western Canada mean ridging aloft. Then the
ridge should strongly rebuild, amplifying as far north as the
Arctic, while its axis gradually progresses eastward across the
mainland. At the same time one or more potentially wavy fronts
will extend southeastward from deep low pressure migrating from
the northwestern Pacific into the western Bering Sea. The
deep-layer flow of moisture between the mean ridge and the fronts
to the west will likely favor a couple episodes of significant
precipitation focused particularly from the Alaska Peninsula
through the Kenai Peninsula/Prince William Sound. Lesser amounts
should extend as far east as the Panhandle.
...Guidance/Predictability Assessment...
The most noteworthy aspect of the 12Z guidance cycle is the fact
that 12Z ECMWF trends have significantly improved operational
model agreement for the large-scale forecast. To start the period
on Tuesday, the ECMWF trended slower to other guidance for the
overall shortwave crossing the mainland along with the associated
surface reflection and precipitation shield along the southern
coast. Models have been variable with the
fine-scale/lower-predictability details so additional adjustments
may be possible. Then later in the week the 12Z ECMWF switched to
an open ridge that has been promoted by recent runs of the GFS/CMC
and GEFS/ECens/CMCens means. This is in contrast to the prior
couple ECMWF runs that had closed off an upper high in such a
position as to hold moisture well westward of other guidance
including the corresponding ECMWF means. Some GFS runs, and even
momentarily the new 12Z ECMWF mean, still show potential for an
upper high to close off but at a sufficiently high latitude that
it would not have much influence on the forecast over the Gulf of
Alaska and vicinity. Guidance is starting to shuffle around
somewhat on exact strength of the upper ridge. Recent GFS/GEFS
runs have trended a bit weaker from continuity though the 18Z GFS
has rebounded a bit.
Latest trends have contributed to an increase in confidence for
the larger pattern but confidence and predictability are still
quite low for exact details of one or more surface lows that are
likely to develop and track over an area encompassing the North
Pacific into the Bering Sea and Gulf of Alaska. Resolving the
forecast for these lows will be important for refining the best
precipitation and wind focus. Starting with the forecast
preference of a 12Z operational model blend Tuesday into early
Thursday followed by gradually increasing input of 12Z GEFS/00Z
ECens thereafter (reaching 40 percent total weight by day 8
Saturday) yielded the best hint of low pressure south of the
eastern Aleutians/Alaska Peninsula to near Kodiak Island in the
Thursday-Saturday time frame.
Finally, guidance has been variable for strength/track/timing
details of low pressure tracking across the Arctic around
midweek--corresponding to differences in dynamics aloft. Latest
model average has trended toward better definition of a front that
may drop into the North Slope and linger for a period of time.
...Weather/Hazard Highlights...
The forecast is consistent in showing two primary episodes of
significant precipitation next week. The first will be a
continuation of an event affecting the Alaska Peninsula and
southern coast (and nearby parts of the mainland) starting Monday,
associated with the upper shortwave crossing the mainland on
Tuesday. Best focus for precipitation by Tuesday should reach
areas from the eastern Peninsula and Kodiak Island to near Prince
William Sound. Somewhat lower totals will be possible along the
southeastern coast and Panhandle Tuesday into early Wednesday.
Then the fairly strong gradient ahead of an initial front crossing
the Bering Sea/Aleutians, and possibly another front, should
settle over the northeastern Pacific for a while. The resulting
flow of Pacific moisture would support another episode of
significant precipitation most likely to focus from the Alaska
Peninsula though and a little east of the Kenai Peninsula. Some
moisture should extend along the southeastern coast into the
Panhandle but with lesser totals. While confidence has improved
for precipitation coverage and general focus for highest totals
during the latter half of the week, there is still considerable
uncertainty over details of individual surface waves that will
play an important role in refining the strongest emphasis for
precipitation and wind. Both events next week may spread moisture
over broader portions of the mainland, though confined more to
western areas in the second one.
Most of the state should see above to well above normal
temperatures during Tuesday-Saturday. The main exceptions will be
the central latitudes of the mainland where some pockets of
moderately below readings may exist around midweek after the
passage of an upper shortwave/surface front, as well as the
southern two-thirds of the Panhandle which should be below normal
for highs through the period. In both areas the lows are likely
to have somewhat warmer anomalies.
Rausch
Hazards:
- Heavy precipitation across portions of mainland Alaska, Mon, Feb
21 and Thu, Feb 24.
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