Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
757 PM EDT Wed Aug 16 2023
Valid 12Z Sun Aug 20 2023 - 12Z Thu Aug 24 2023
...Overview...
Early in the period, the primary focus will be on a wave forecast
to track through the western/northern Bering Sea and into the
northwestern mainland, to the south of a deep Arctic upper low.
The system of interest should be composed of Tropical Storm Lan
(still tracking near Japan) and some interacting shortwave energy
in the short range time frame, with leading moisture potentially
interacting with a front initially draped over the northern
mainland as well as favored windward terrain. Most guidance
continues to show a mean trough aloft persisting west of the
mainland through at least the middle of next week, possibly
supporting low pressure somewhere between the North Pacific and
Bering Sea by then. Farther east there is still considerable
uncertainty regarding specifics of a couple potential waves that
may track over the North Pacific/Gulf of Alaska Sunday-Tuesday.
...Guidance Evaluation/Preferences...
Recent trends toward a faster and weaker depiction of the
western/northern Bering Sea into northwestern mainland system have
continued into today's 12Z guidance. The continuation of this
trend has most recently been led by the GFS/UKMET which 24 hours
ago were the first to suggest that some of the supporting
shortwave energy may pull off to the south by around the start of
the extended period to yield a faster/less amplified shortwave
that continues across the Bering Sea. This general idea is now
nearly unanimous in the operational models. The GFS/ECMWF/CMC
offer the most compatible clustering among the 12Z runs and look
like the best option for representing the consensus faster trend.
The GFS is a bit stronger than other solutions but remains a good
contributor to the blend. The UKMET strays to the faster/sheared
side of the spread but cannot be discounted given the prevailing
trends.
Guidance has been having a very difficult time with tracking the
Arctic upper low after its starting position to the north of the
western mainland. Yesterday the prevailing scenario was for some
degree of northward/eastward progression aside from a stray GFS
run eventually wobbling back closer to the northwestern mainland.
Latest runs have shifted dramatically westward to north of Siberia
by early-mid week, with the ensemble means generally gravitating
toward that idea as well. Now the 12Z CMC is on its own with a
path over the northwest corner of the mainland by late next
Wednesday-Thursday (after taking it into the Canadian Archipelago
in the prior run). Prefer a model blend early given the consensus
trends, followed by a transition toward the means that temper the
trend a bit.
Farther south, the 12Z UKMET is the slow extreme (like yesterday)
for the leading wave that may be offshore the southeastern
coast/Panhandle by early Sunday while other guidance has improved
its clustering. A separate shortwave/upper low may produce a weak
wave that could track into the Gulf of Alaska by early next week,
but there is still a lot of spread for specifics and
predictability is low due to the feature's small scale. The 12Z
ECMWF evolution brings more moisture to the southern coast than
other guidance. Meanwhile the relative majority of guidance
suggests that some combination of the energy forecast to drop
south of the Aleutians and upstream flow should yield an area of
low pressure over the Aleutians/Bering Sea by next
Tuesday-Thursday. The ensemble means generally favor a track
through the central Aleutians/southern-eastern Bering Sea, with
the 06Z/12Z GFS and farther south 00Z/12Z ECMWF a bit faster. The
12Z CMC and new 18Z GFS are a little more suppressed though.
Based on combined preferences early in the period, the starting
blend used a 40/35/25 weight of the 12Z ECMWF/GFS/CMC. Then the
forecast incorporated increasing 12Z GEFS/CMCens and 00Z ECens
weight, reaching 70 percent total by day 8 Thursday, while phasing
out the CMC due to its lower confidence solutions for the Arctic
upper low and North Pacific system.
...Weather/Hazards Highlights...
Moisture ahead of the system expected to reach the northwestern
mainland by early Monday may produce some enhanced rainfall over
western/northern parts of the state Sunday into early Monday. A
front draped across the northern tier may provide some added
focus, and favored windward terrain may see relatively higher
totals as well. Continued system progression should lead to a
lighter precipitation trend after Monday. While this event should
produce meaningful rainfall, totals are currently not expected to
reach hazardous thresholds. There may be a brief period of brisk
winds over some areas given the faster trends for timing but
weaker trends for system strength may be an offsetting factor.
There may be a short break in precipitation behind this system,
perhaps followed by another area of precipitation that may move in
from the west by midweek or so depending on uncertain late-period
Pacific/Bering Sea low pressure details. There is still a lot of
uncertainty with rainfall specifics along the southern
coast/Panhandle from the weekend into early next week. A wave
initially offshore the Panhandle may produce a little light rain
on Sunday. A trailing wave may bring some moisture to areas from
the Alaska Peninsula to the Panhandle but specifics will be very
sensitive to the exact low track.
The northern mainland should see the greatest negative anomalies
for high temperatures during the period, while other parts of the
west and south should generally see below normal highs as well.
The eastern interior and southern Panhandle will see the best
potential for above normal highs. Low temperatures should be more
above normal, with just scattered pockets of modestly below normal
anomalies.
Rausch
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