Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
752 PM EDT Thu May 20 2021
Valid 12Z Mon May 24 2021 - 12Z Fri May 28 2021
...Overview...
The best clustering of latest guidance continues to show that a
mean low over the Aleutians/Bering Sea and downstream ridge over
parts of the mainland (also building into the northeastern Pacific
around midweek or so) will be the dominant upper features during
the period, but some important specifics vary. A persistent
uncertainty is what role individual waves emerging from the
western Pacific will play--including whether they merge with the
mean low over the Aleutians/Bering or remain separate. This
pattern will tend to focus episodes of precipitation from the
Aleutians into the Alaska Peninsula and eventually farther east
along the southern coast.
...Guidance/Predictability Assessment...
The most notable adjustment in most guidance compared to 24 hours
ago is a farther westward trend with the mean low over the
Aleutians/Bering Sea from about Tuesday onward. This trend
corresponds to earlier incorporation of a leading wave that tracks
out of the western Pacific, and among the means was first
suggested in Wednesday's 12Z ECens. Today's 12Z GFS appears to
have been a relative hiccup relative to surrounding runs. It
pulls the initial upper low well eastward of other models/means
while the 06Z and new 18Z versions compare better to consensus.
Later in the period another Pacific wave provides some divergence
in solutions, with the 12Z ECMWF particularly aggressive in
bringing a strong low into the Alaska Peninsula and beyond on day
8 Friday. Other models offer different varieties of
track/timing/strength but with the prevailing theme of being much
less extreme.
Short-range guidance has finally become more convergent for
compact upper low/shortwave energy reaching the western mainland,
ultimately feeding into the overall Gulf/Panhandle upper trough by
early Monday. This evolution ended up being somewhat of a
compromise among the wide range of prior solutions--closer to
earlier GFS runs in terms of dropping it into the trough versus
lingering as an upper low over the mainland per some ECMWF runs,
but with a track adjusted somewhat northeast of GFS runs from a
day or so ago. Some details of flow over the mainland are still
uncertain in the extended period with models differing in
specifics of Arctic flow that could reach into at least northern
areas. A blended approach that holds onto general mean ridging
over the mainland provides the best consistency for the time
being, while awaiting better agreement for low-predictability
shortwaves.
Based on guidance comparisons through arrival of the 12Z ECMWF,
today's forecast started with the 12Z ECMWF/CMC/UKMET and 06Z GFS
for the early part of next week. Then the forecast trended to
40-50 percent total ensemble (12Z GEFS/00Z ECens) input for
Thursday-Friday with lingering minority contributions from the 06Z
GFS, 12Z CMC, and last two ECMWF runs (to temper the 12Z run's
Alaska Peninsula low by Friday).
...Weather/Threats Highlights...
Initial low pressure and associated frontal system over the
Aleutians/Bering Sea as well as one or more trailing systems that
may merge into the overall mean low should bring periods of
unsettled weather to areas from the Aleutians through at least the
Alaska Peninsula and eventually southern coast. Some moisture may
extend into parts of the western and southern mainland as well.
Fronts/embedded waves may provide localized enhancement of
precipitation and winds at times. The best signal for highest
totals during the five-day period exists from the Alaska Peninsula
to Kenai Peninsula and over favored terrain just north of Bristol
Bay.
The Panhandle and portions of the southern mainland as well as
parts of the North Slope will tend to see below normal high
temperatures during the period. Other locations will likely
experience above normal highs with the greatest anomalies over the
west, though some moderation may occur after midweek. Above
normal readings will be more prevalent for lows, with the
Panhandle and parts of the eastern mainland tending to be the
primary exception.
Rausch
Hazards:
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 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