Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
752 PM EDT Sun Aug 14 2022
Valid 12Z Thu Aug 18 2022 - 12Z Mon Aug 22 2022
...Continued threat for heavy precipitation along/near the
southern coast and possibly into the northern Panhandle...
...Overview...
Latest guidance shows a combination of themes from continuity and
some refinements/changes. There is still a good signal for some
type of interaction between a mean upper low over the northern
Bering Sea/western mainland (perhaps split into two low centers)
and a system tracking along the Aleutians, yielding an upper low
most likely to take a Bristol Bay-Kodiak Island track during the
weekend. Recent trends have been stronger with this system,
leading to more precipitation spreading across the Aleutians early
in the period and a broader field of brisk to strong winds across
the Bering Sea through North Pacific. With some adjustments in
specifics, the overall evolution continues to favor a multi-day
period for some areas along and near the southern coast to see
significant precipitation. On the other hand, recent
model/ensemble runs have been split and variable for flow details
from the Arctic into the mainland from the weekend into early next
week. This issue may begin to have an effect on the primary
system of interest late in the period.
...Guidance/Predictability Assessment...
As happens at times, it appears that latest operational model runs
may be leading the ensemble means a bit on resolving some details
during the first half of the period. Models as a whole suggest
that the mean low over the northern Bering Sea/western mainland as
of the start of the forecast early day 4 Thursday may be elongated
or split among two centers, with the southern one weakening or
progressing onward while the northern one potentially interacts
with or merges with the Aleutians system (which is deeper in
today's runs) around day 5 Friday. Over the past day the ensemble
means have trended somewhat southwestward with the overall upper
low around Friday, hinting at the operational model evolution.
While predictability for some of these details is in the lower
half of the spectrum, one positive is that by days 6-7
Saturday-Sunday the model clustering is better than it was from
yesterday's 12Z cycle and the model/mean consensus for the upper
low tracking over Bristol Bay/Kodiak Island is similar to prior
forecast. One concern after early Sunday is that the 12Z and 18Z
GFS runs hold the upper low farther back than the remaining
models/means.
Meanwhile confidence is lower for basic aspects of flow over the
Arctic and into the mainland from the weekend into early next
week. The past two ECMWF runs through the 12Z cycle have brought
an Arctic upper low down into the mainland with the 12Z CMC adding
to this scenario. On the other extreme, some solutions suggest
this upper low could progress eastward across the Canadian
Archipelago instead, leaving the northern mainland under somewhat
of a col region (12Z CMCens/00Z ECens means). The 12Z GEFS mean
keeps the northwest mainland under weak troughing. Preferences
for the storm to the south led to some reflection of the ECMWF/CMC
scenario but in weaker form, and the new 12Z ECMWF mean has
adjusted toward more troughing to hint at this idea.
The operational model blend for the first half of the period
tilted more to the 12Z GFS/ECMWF relative to the CMC/UKMET based
on comparison of details for individual features among each other
and the means. Trending this evolution toward an even
model/ensemble mean mix by day 8 Monday again provided the best
balance of maintaining some operational model detail while
incorporating the agreeable large scale pattern of the ensemble
means. The GFS component quickly transitioned from the 12Z run to
the 06Z version after early Sunday since the 06Z GFS compared
better to other models/means.
...Weather/Hazard Highlights...
Latest model trends are suggesting increasing potential for a
fairly strong storm to track along the Aleutians Thursday-Friday,
followed by a track through Bristol Bay and Kodiak Island during
the weekend as it gradually begins to weaken. The surface low
should continue to weaken early next week but may linger just off
Kodiak Island. This system will bring a brief period of focused
rainfall across the Aleutians during the latter half of the week
and should produce a broad field of brisk to strong winds across
the Bering Sea through North Pacific from Thursday into the
weekend. Consensus now shows the Panhandle seeing a break in
significant precipitation after an episode of heavy activity
through Wednesday (in the short range), but with some locally
heavy precipitation persisting into Thursday over areas near
Prince William Sound. Then moisture ahead of the Aleutians storm
should quickly spread across the Alaska Peninsula and southern
coast as well as some of the favored terrain over the far southern
mainland, reaching the northern Panhandle during the weekend.
Relatively slow progression of the storm during the
Saturday-Monday period should maintain the potential for some
areas of heavy precipitation along and near the southern coast,
possibly extending into the northern Panhandle. Recent trends
have been lighter for totals over the southern Panhandle. At
least several inches of total precipitation are possible over some
locations during the course of the period. Expect less extreme
amounts of rain and high elevation snow across the rest of the
state, with coverage and amounts still to be determined by
uncertain specifics of flow aloft.
The temperature forecast looks fairly consistent from yesterday,
with most of the state seeing below average highs and the greatest
negative anomalies likely to be over the North Slope and the far
south/Panhandle. Pockets of slightly above normal highs could
exist over central latitudes of the mainland and the far southern
Panhandle. Lows will tend to be below normal over western areas
and somewhat above normal over the east and the Panhandle.
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