Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
753 PM EDT Sat Mar 30 2024
Valid 12Z Wed Apr 03 2024 - 12Z Sun Apr 07 2024
...Overview...
Latest guidance still shows a moderately progressive pattern that
will bring periods of active weather, though in most cases likely
not reaching hazardous criteria, from the Bering Sea/Aleutians
into the western and southern mainland. A strong Pacific upper
ridge will extend its northern periphery into the mainland around
Wednesday-Thursday as one system tracks over the far northern
Bering Sea and a potential separate wave tracks northeastward to
its south. Then diffuse upper troughing that crosses over and
south of the mainland should support a Gulf of Alaska wave late in
the week. Then expect a fairly vigorous storm system to track
over the south-central Bering Sea by next weekend. By late week
into the weekend, models and ensembles differ considerably for
upper flow details over the Arctic and southward into parts of the
mainland.
...Guidance Evaluation and Preferences...
Models and ensembles are still having some difficulty resolving
details of the system over the northern Bering Sea at the start of
the forecast early Wednesday. In contrast to 24-36 hours ago when
there was a pronounced faster operational model cluster and slower
ensemble mean/ECMWF-initialized machine learning model (ML)
cluster, there is now less pronounced grouping of solutions and
some moderating of the extreme sides of the prior spread. Latest
GFS runs have strayed to the slow side of the full envelope for
the parent low and supporting upper dynamics. Otherwise, in
general the ensembles and MLs have trended somewhat faster than
yesterday and the ECMWF/UKMET/CMC have adjusted moderately slower.
In addition to the surface reflection depending on timing of the
upper system, it will also be sensitive to exactly when the
initial upper low opens. Thus predictability appears somewhat
below average for the time frame involved. Farther south, there
is a better signal today for a trailing wave to race northeastward
from the North Pacific toward the western coast of the mainland
underneath the northern system. These features may ultimately
consolidate into a single surface system near the northern coast
of the mainland by early Friday.
Behind these systems, there is reasonable agreement toward a
diffuse upper trough drifting across and south of the mainland,
with some potential sharpening south of the mainland. Guidance
has been consistent so far in depicting an associated modest Gulf
of Alaska surface wave during Friday-Saturday. On the other hand,
by late week through the weekend a lot of spread and variability
develop for upper flow details across the Arctic and into the
mainland. An upper low may exist north of 70N latitude but with a
position/path anywhere between north of Siberia or north of the
mainland. The new 18Z GFS reaches closest to the northwestern
mainland while the ensemble means suggest the upper low should
remain well off to the northwest. The most likely scenario is for
some degree of upper troughing to persist over the northwestern
mainland through the weekend.
The overall forecast of the system most likely to track across the
south-central Bering Sea around next weekend has held up fairly
well over the past day. Earlier mixed signals in the MLs have
trended strongly toward the operational model/ensemble mean
scenario, and for the time being the most pronounced spread and
variability involve system timing. GFS/ECMWF runs have been
waffling between faster and slower solutions (12Z ECMWF slower
than the 00Z run, 12Z/18Z GFS runs faster than the 00Z/06Z runs)
while the 12Z CMCens leans to the fast side of the spread. An
overall average among recent runs (with the CMC included) provides
a fairly stable forecast aside from possibly being a tad slower
than 24 hours ago.
Today's guidance comparisons led to starting the first half of the
forecast with 40 percent of the 12Z ECMWF and 20 percent each of
the 12Z GFS/UKMET/CMC, in order to give less than usual weight to
the GFS for the early period evolution from the Bering Sea into
the mainland. After early Friday the blend replaced the
terminating UKMET slot with the 12Z GEFS/00Z ECens means. However
the remaining 80 percent maintained operational model input (with
the only tweak being to split ECMWF input between the 12Z/00Z runs
given timing/detail differences), for days 7-8 next weekend in
light of relative support in principle from the means/MLs.
...Weather/Hazards Highlights...
The system forecast to track across the northern Bering Sea around
midweek, plus a potentially fast-moving wave tracking
northeastward from the North Pacific into the western mainland,
will likely spread a broad area of precipitation and brisk winds
across the Aleutians/Bering Sea and into the mainland. There is
still enough uncertainty in system specifics to temper confidence
in the ultimate magnitude of winds and precipitation. However
there is a more persistent general signal for relatively higher
precipitation totals over the southwestern half of the mainland
with best emphasis over windward terrain. Precipitation focus
should also extend to the southeastern coast/Panhandle for a
couple days late in the week, aided by a surface wave evolving
over the Gulf of Alaska. Then the system forecast to track across
the south-central Bering Sea should spread another broad area of
precipitation and enhanced winds across that region late in the
week into next weekend. Farther eastward, moisture from this
system should be confined more to the far southwestern mainland
and southern coast by next weekend.
Below normal temperatures will linger over interior and eastern
portions of the state into Wednesday. Much warmer air will move
in from the west as upper ridging builds overhead around
Wednesday-Thursday, with the North Slope likely to see the
greatest departures above normal. Flattening of the upper
flow/weak troughing toward the end of the week into next weekend
should lead to more moderate positive temperature anomalies over
the North Slope and a mix of above/below normal highs farther
south. Morning lows should remain above normal over most of the
state though. The Panhandle and areas near the Southcentral coast
may see highs remain somewhat below normal through next weekend.
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