Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
751 PM EDT Thu Aug 31 2023
Valid 12Z Mon Sep 04 2023 - 12Z Fri Sep 08 2023
...Overview...
Guidance generally shows a progressive pattern containing
shortwaves of varying strength/amplitude, supporting individual
surface systems and episodes of precipitation. Leading shortwaves
over and south of the mainland early next week should be fairly
modest while the models are converging on a stronger system
arriving from the west by mid-late week, bringing more enhanced
precipitation to the western/southern mainland and then the
Panhandle. There is greater uncertainty about another system that
could reach the Bering Sea/Aleutians toward the end of the week.
This pattern will tend to favor below average high temperatures
over a majority of the state.
...Guidance Evaluation/Preferences...
Regarding leading features, a model consensus looks good for the
shortwave crossing the Gulf of Alaska/northeastern Pacific. The
main difference of note with the surface low is that latest
GFS/GEFS mean runs are a little more offshore the Panhandle versus
most other models and means, favoring a forecast tilt toward the
non-GFS majority. A separate weak northern stream shortwave
should quickly cross the mainland but there continues to be a lot
of spread and variability for the anchoring upper low farther
north. It may progress eastward per the 12Z ECMWF or linger
farther west (06Z/12Z GFS) and perhaps even get incorporated into
the next system that arrives by midweek (12Z UKMET). At the very
least, the 00Z/31 GFS that tracked the upper low over the northern
mainland appears to be a very low probability scenario. Current
preference blends out this feature given the relatively greater
importance of other synoptic scale systems.
Widely diverging solutions in some earlier runs for the mid-late
week time frame over and south of the mainland have quickly come
into better relative agreement in the 12Z model cycle. Currently
the primary cluster shows a fairly vigorous trough reaching the
western mainland by Wednesday and continuing eastward in fairly
amplified form through late week. GFS runs were a little earlier
to advertise the amplified nature of this shortwave (in contrast
to the flat/progressive ECMWF) but were on the extreme side in
dropping an upper low southward into the northeastern Pacific.
The 12Z GFS and new 18Z run have backed off from the Pacific upper
low, generally keeping the core of the upper trough over the
mainland. Ensemble means have been trending sharper and more
amplified with the trough over the past day (versus much flatter
flow), with 12Z ECens trends reinforcing this theme. A 12Z
operational model cluster is reasonable at this time for the upper
trough and the associated surface system expected to track into
the northwestern mainland by early Wednesday. Then the model/mean
consensus indicates a triple point wave settling over the Gulf of
Alaska by Thursday-Friday. In spite of the current clustering
mid-late week, prior guidance behavior and spread temper
confidence somewhat for the time being.
There is greater uncertainty with a possible system tracking over
the Aleutians/Bering Sea by late in the week. A slight majority
of guidance (GFS/CMC runs and their ensemble means) shows a phased
trough (with or without an embedded closed low) that would lead to
a Bering Sea surface low track, with the operational runs fairly
strong. On the other hand latest ECMWF runs have been pulling off
an upper low over the western Pacific, keeping the dominant
surface low there while higher surface pressure prevails over the
Bering Sea--though with a weak leading wave crossing the
Aleutians. The 00Z ECens mean tilted toward the operational
scenario but the 12Z version has nudged somewhat northeastward
(Aleutians into the Bering Sea during next Friday). Even the
ensemble means have not been particularly consistent over the past
couple days for the general surface pattern, reinforcing the low
confidence in this aspect of the forecast. Current preference
leans to the relative majority of guidance that has the
best-defined system reaching the Bering Sea by Friday, with the
resulting surface blend closest to a stronger version of the 12Z
GEFS mean.
Guidance considerations led to a starting blend consisting of the
ECMWF/GFS/CMC/UKMET in order of more to less weight from day 4
Monday into early day 6 Wednesday, with 40-55 percent total weight
of the 12Z GEFS/00Z ECens included along with lingering input from
the ECMWF/GFS/CMC by days 7-8 Thursday-Friday.
...Weather/Hazards Highlights...
Leading shortwave energy crossing the mainland on Monday may
produce some light precipitation, while a wave expected to track
to the west/south of the Panhandle early in the week may produce
somewhat higher totals (but still not heavy) over that region. A
stronger system anchored by a surface low arriving into the
northwestern mainland by early Wednesday should spread a broad
area of precipitation across at least the western half of the
mainland Tuesday through early Wednesday with moisture continuing
eastward thereafter. Some terrain enhancement will be possible
over western and southern areas and latest guidance consensus for
system evolution (including a Gulf of Alaska triple point wave
after midweek) yields higher rain/high elevation snow totals along
the southeastern coast and Panhandle during the Wednesday-Thursday
time frame. Currently expect this activity to remain below
hazardous criteria though. The system forecast to reach the
Bering Sea by next Friday would spread an area of precipitation
and brisk winds across the Aleutians/Bering Sea late in the week
but confidence is below average for details at this time. The
forecast pattern will support below normal high temperatures over
much of the state next week. Low temperatures will tend to
average slightly warmer then normal, though with some areas seeing
modestly below normal readings on one or more days.
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 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