Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
657 PM EDT Sat Apr 04 2020
Valid 12Z Wed Apr 08 2020 - 12Z Sun Apr 12 2020
...Overview...
The early part of the period will focus on a concentrated storm
system likely to reach the Gulf of Alaska by day 5 Wed, with
accompanying heavy precipitation and strong winds. After this
system departs expect upper ridging to progress across the North
Pacific/Bering Sea, and then build into the mainland once an
Arctic upper low and associated trough pass by. The ultimate
evolution/path of an upper low that starts the period over the
east-central mid-latitude Pacific continues to be somewhat of a
wild card for the latter half of the period. Farther west there
is general agreement that a system reaching the western Aleutians
mid-late week will be replaced by another one by next weekend.
There is some uncertainty over the eastward extent of the first
system though.
...Guidance/Predictability Assessment...
Over the past day the guidance has made some progress in narrowing
the spread for the strength and track of the system expected to
reach the Gulf of Alaska by Wed. Using an approach similar to
yesterday, a 12Z ECMWF/GFS blend tilted two-thirds toward the
ECMWF, maintains continuity for strength and yields perhaps a
slightly westward/slower adjustment for track. Latest ensemble
means support a similar path, between the farther west 12Z CMC and
southeast UKMET (which was the weakest/southeastward 24 hours ago).
Today's clustering appears better for the compact upper low that
crosses the Arctic and trough that extends southward to brush
northern parts of the mainland. Ensemble means have trended
noticeably more amplified with the trough over the past day or so,
adding support to an operational model blend.
Considerable differences arise with low pressure reaching the
western Aleutians mid-late week. The most prominent discrepancy
involves the latest GFS runs tracking fairly robust low pressure
all the way into the northern mainland, whereas nearly all other
guidance keeps the system(s) well westward. The 12Z GEFS mean has
a hint of the GFS idea but enough individual members hold it back
for the mean to bring a defined surface low center no farther east
than the west-central Bering Sea with only a front extending
northeastward. Preference remains with a non-GFS consensus for
this aspect of the forecast.
During the past 24 hours the ensemble means have trended stronger
for the system expected to reach the western Aleutians next
weekend. Operational runs generally support the existence of this
system but with typical differences for the details. This
supports an ensemble mean/consensus approach.
There is still a low-probability potential for what eventually
remains of the upper low and associated surface system over the
east-central Pacific as of Wed to have some influence on the
forecast later in the period. The majority of guidance maintains
the idea that it should remain fairly suppressed as mean ridging
aloft becomes more prevalent over the Northeast Pacific. Recent
ECMWF runs have been inconsistent though, with today's 12Z run
like yesterday bringing mid level energy and a surface reflection
well northward of most other solutions later in the period--while
00Z runs have been more suppressed. As a result prefer to lean
away from the 12Z ECMWF by next weekend.
Today's considerations led to starting day 4 Wed with the
aforementioned ECMWF/GFS blend for the Gulf system and other
features of note. Lack of confidence in the GFS upstream after
Wed led to quick phasing down of its weight (while adding some
GEFS), followed by further increases of GEFS/ECMWF mean input
along with transitioning 12Z ECMWF input toward the 00Z run
instead. By next weekend this led to 60-80 percent total ensemble
mean weight and a lingering ECMWF component of mostly just the 00Z
run.
...Weather/Threats Highlights...
The storm system reaching the Gulf of Alaska around midweek will
bring a brief period of heavy precipitation and strong winds.
Best confidence in highest precip totals is over the Panhandle and
possibly extreme southeast corner of the mainland. There is still
enough guidance spread in the track to support potential for a
farther northwest extent, or on the other hand confinement to the
Panhandle. Late week/weekend progression of Aleutians-Bering Sea
low pressure in the GFS would bring an area of meaningful
precipitation to the western/northern mainland but remaining
guidance is not enthusiastic about this scenario, keeping precip
lighter and more scattered. Agreement is better on moisture over
the western half of the Aleutians. Expect the North Slope region
to see above normal temperatures during the period while most
remaining areas should be below normal. Chilly surface high
pressure may drift far enough east by next weekend to support some
moderation in temperatures over the western part of the state.
Rausch
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