Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
655 PM EST Sun Feb 25 2024
Valid 12Z Thu Feb 29 2024 - 12Z Mon Mar 04 2024
...Overview...
Latest guidance is consistent in showing an amplified upper
pattern continuing into late this week, with a trough axis over
the eastern mainland and upstream ridge aligned from Aleutians
into Siberia. Farther west trough energy, supporting an
associated Bering Sea surface low/frontal system, should then push
into the ridge and promote lesser amplitude of flow over the
mainland. However there has been significant disagreement over
the details of this flatter flow from the weekend into early next
week--ranging between weak ridging or northwesterly/cyclonic flow.
High pressure crossing the mainland late this week will promote
cold temperatures, followed by gradual warming trend. Most
precipitation during the period will initially focus across the
Aleutians/Bering Sea and then spread into parts of the
southwestern mainland, possibly reaching the southern
coast/Panhandle.
...Guidance Evaluation and Preferences...
Most aspects of continuity have held up well for the first half of
the period from Thursday into early Saturday. There is some
refinement toward better defined low pressure reaching the central
Bering Sea by early day 6 Saturday, albeit from varying sources
(parent low per the GFS or a frontal wave tracking north from the
Pacific in the 12Z ECMWF/CMC/UKMET, the UKMET a southeast
extreme). An operational model blend consisting of the 12Z
ECMWF/GFS/CMC/UKMET in order from more to less weight provided a
reasonable starting point for this part of the forecast.
The latter half of the forecast has exhibited a lot of spread and
run-to-run variability for the mainland upper pattern and
resulting surface/precipitation forecasts, but latest trends seem
to be favoring an intermediate approach. The 00Z ECMWF and to
some degree ECMWF mean had advertised re-development of mainland
troughing that would support a dry and suppressed surface regime
while the 12Z runs backed off somewhat from that idea, bringing at
least some moisture into the southwestern mainland. On the other
hand the 12Z CMC brings the Saturday Bering Sea system into the
western/northwestern mainland with its moisture continuing
eastward with time. GFS runs have been less extreme with the
Saturday system but fairly aggressive with the northeastward
extent of another Bering Sea system and its moisture by day 8
Monday. The new 18Z GFS has trended slower with its day 8 Bering
Sea system, closer to the 12Z ECMWF/GEFS/CMCens in longitude.
ECens means have been farther west thus far.
Ultimately the most common theme after early Saturday is for
modest upper ridging to reach the southwestern half of the
mainland while the northeast may get brushed by some
northwesterly/cyclonic flow. The Saturday Bering Sea system
should weaken thereafter (while its leading front breaks off a
wave tracking south of the Alaska Peninsula/Kodiak Island and into
or south of the Gulf by Sunday-Monday) in favor of the next system
that may reach the central Bering Sea by day 8 Monday. This part
of the forecast transitioned to 30-70 percent ensemble means (12Z
GEFS/CMCens, 00Z ECens) with lingering input from the 12Z
GFS/ECMWF. Increasingly poor comparisons led to excluding the CMC
while blend weights helped to reduce ECens influence of its slower
day 8 Bering Sea system.
...Weather/Hazards Highlights...
Most of the mainland will see dry conditions late this week. A
system to the south of the Panhandle may generate some
precipitation over mainland the southern Panhandle. The tight
gradient between this departing storm and high pressure crossing
the mainland may produce strong winds, especially in areas of gap
flow, from the Alaska Peninsula eastward along the southern coast
to the northern Panhandle. Western Pacific into Bering Sea low
pressure and its leading frontal system will spread precipitation
of varying intensity across the Aleutians (mostly rain) and Bering
Sea through Thursday-Friday. Some of this moisture should extend
into the southwestern mainland to produce snow by the weekend.
There is still a fair degree of uncertainty over precise coverage
and amounts of precipitation though. A developing frontal wave
may track south of the mainland by Sunday-Monday, possibly
bringing some precipitation to the southern coast/Panhandle. How
much moisture reaches the southern coast will be very sensitive to
the exact low track. Another system tracking into the Bering Sea
by early next week may produce a second episode of
Aleutians/Bering Sea precipitation, with low confidence in whether
any of this moisture reaches the western mainland by next Monday.
Cold surface high pressure will bring well below normal
temperatures to much of the state late this week into Saturday,
with the North Slope being the one exception in seeing some above
normal readings. The typically coldest eastern interior locations
could see overnight lows reach as low as -40 degrees. Departure
of the surface high and flatter trend of the upper pattern should
bring a noticeable moderating trend for temperatures from west to
east heading into next week.
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