Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
655 PM EST Wed Mar 10 2021
Valid 12Z Sun Mar 14 2021 - 12Z Thu Mar 18 2021
...Overview and Guidance/Uncertainty Assessment...
After some notable trends leading into the previous forecast, the
past day of model and ensemble guidance has maintained fairly good
continuity and clustering with larger-scale aspects of the
pattern. Consensus shows an upper low near the Panhandle early
Sunday lifting up to the north before shearing/departing
underneath an upper high that closes off just north of the
mainland and continues into the Arctic. Meanwhile elongating
Bering Sea/Aleutians upper trough energy should develop a wave
near/east of the Alaska Peninsula late weekend into early next
week. Strong flow on the southwest side of the trough will likely
pull Siberia/Bering energy southeastward help form an upper low
over the Peninsula and southwestern mainland by early Tuesday.
The upper low should then quickly drop southward into the Pacific
while an upper ridge builds over the Bering Sea and Aleutians.
Also by Wednesday-Thursday guidance is suggesting that flow around
the northern side of this ridge should become more progressive
with a fair degree of uncertainty regarding the shape of flow over
portions of the mainland--but averaging out to some degree of mean
troughing.
For about the first half of the period (Sunday-early Tuesday) a
blend of 12Z operational models provided a good representation of
significant features while downplaying less confident details of
any particular solution. Lingering small scale and thus low
predictability detail differences exist for low pressure that may
be near the southern Panhandle into Sunday, favoring an
intermediate approach close to continuity. Meanwhile there are
some position differences for the Alaska Peninsula wave with the
12Z GFS a bit on the slower/southwestern side of the spread for a
time.
After early day 6 Tuesday guidance rapidly strays apart from each
other and continuity for the specifics of individual surface lows
embedded within and around the upper low circulation that descends
into the Pacific. This spread and variability favor a transition
to a model and ensemble mean (12Z GEFS/00Z ECens) mix for the rest
of the period, including a split of ECMWF input between the past
two runs. This solution keeps the main area of low pressure well
south of the mainland late in the period but some individual runs
do offer the potential for one or more waves to reach farther
north and have some influence over the Gulf of Alaska. The blend
also provides a good conservative starting point for the shape of
flow over the mainland by late in the period. The most common
theme has been for some degree of upper troughing but with
variance in depth/position, and there have been occasional
solutions that even build in some ridging. The progressive flow
around the ridge that builds into the Bering Sea may bring a front
into the mainland around or shortly after midweek.
...Weather/Hazards Highlights...
Any precipitation affecting the Panhandle should be tapering off
Sunday as low pressure weakens and departs. Continuity is fairly
good in the forecast of a frontal wave developing near the Alaska
Peninsula around Sunday and then drifting eastward. Expect this
wave to focus precipitation over the eastern Peninsula and Kodiak
Island initially and then farther east to the southeastern coast
and Panhandle. Some activity could be locally moderate to heavy.
From about Tuesday onward precipitation should trend lighter and
more scattered across the southern coast and Panhandle as the
large scale upper low descends into the Pacific. However there is
the possibility that one or more waves could track around the
northern side of the circulation and bring an episode of locally
enhanced precipitation and winds to the Gulf of Alaska region.
Persistent precipitation over the Aleutians should eventually
trend lighter with time.
The North Slope will see the most consistently above normal
temperatures during the period albeit with a gradual cooling trend
over the course of next week. Farther south, the expected warming
trend from west to east looks somewhat more modest compared to
yesterday. In general there is better potential for min
temperatures than daytime highs to rise to above normal levels.
Still expect the coldest anomalies over eastern areas and the
Panhandle to be on Sunday followed by some moderation but again
tending to stay below normal especially for lows.
Rausch
Hazards:
- High winds across portions of Coastal Alaska, Sat, Mar 13.
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards 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