Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
753 PM EDT Sat Oct 02 2021
Valid 12Z Wed Oct 06 2021 - 12Z Sun Oct 10 2021
...Pattern Overview and Guidance/Predictability Assessment...
Compared to yesterday there is not much change in the forecast
pattern and embedded uncertainties. Progressive flow will carry
along a series of storm systems that will bring episodes of
precipitation and strong winds to areas from the Aleutians/Bering
Sea into the western and southern mainland as well as the
Panhandle and Gulf of Alaska. During Wednesday-Thursday initial
northern Bering Sea low pressure should track northeastward and
weaken while a wave develops over the Gulf by early Thursday.
There is still a fair degree of uncertainty with the next storm
which for now is more likely to track from the western Aleutians
into the eastern Bering Sea/western mainland around the end of the
week followed by another wave evolving over the Gulf by early
Saturday. Then another system may reach the western
Aleutians/Bering Sea by next Sunday.
An early-period model blend offers good continuity for the low
over the northern Bering Sea as of early Wednesday. At the same
time this blend (with slight manual adjustment) reflects a
northern solution closer to the ensemble means for the Gulf wave
as of early Thursday, in contrast to the the latest GFS/CMC which
pull the feature farther south.
Model/ensemble clustering and spread have stayed remarkably
similar over the past 24 hours for the system expected to emerge
from the western Aleutians Thursday. GFS/CMC/UKMET runs continue
to show varying degrees of strong development with a northeastward
track into the eastern Bering Sea by early Friday, while a number
of recent ECMWF runs have been weaker and farther south. Latest
GEFS means reflect the GFS cluster. CMC/ECens members are more
diverse, leading to a weaker depiction in their respective means
but those means at least imply a low track similar to the majority
cluster. Also of note, operational GFS runs are within the GEFS
spread for their very deep low but other models/ensembles suggest
the upper dynamics and surface low depth may not be quite as
extreme. Minimal change of the guidance distribution compared to
yesterday likewise favors maintaining a forecast close to
continuity by way of a model blend that leans more toward the
majority--albeit a tad weaker given the ongoing uncertainty.
Strength of the system will be very sensitive to exactly how the
upper support evolves and the medium to smaller scale of the
shortwave keeps predictability for several days out in time lower
than desired. The aforementioned spread of CMC/ECens members
would suggest that potential still exists for a significant shift
in the guidance majority at some point.
By next weekend the majority cluster maintains continuity for the
wave expected to be over the Gulf as of Saturday. Models/means
offer a decent signal for another system reaching the western
Aleutians/Bering Sea by Sunday, with guidance spread fairly
typical for the time frame. A transition to a model/mean blend
for this part of the forecast represents the most common guidance
ideas and intermediate approach for systems of interest.
...Weather/Hazard Highlights...
Initial low pressure over the far northern Bering Sea will
maintain a flow of moisture into the western mainland through
Wednesday, while precipitation will increase over the southern
mainland into the Panhandle as a Gulf of Alaska wave likely
develops by early Thursday. The highest totals with this event
should be over and near the southern coast/Panhandle but isolated
significant totals may also exist over favored terrain in the
western mainland. Brisk to strong winds will be possible over the
Bering Sea and western mainland and perhaps to a lesser degree
over the Gulf with the Thursday wave. Confidence remains only
moderate for another vigorous storm which in the more likely
scenario would track from the western Aleutians into the eastern
Bering Sea/western mainland late in the week followed by low
pressure reaching the Gulf by Saturday. This solution would bring
meaningful precipitation and/or strong winds across the
Aleutians/Bering Sea and into the western mainland, and then bring
another period of enhanced precipitation to the southern
coast/Panhandle. There is still a lingering lower probability for
a weaker and more southern system that would keep any significant
moisture/wind to the south of the mainland. Yet another low may
reach the western Aleutians/Bering Sea next weekend with
accompanying precipitation and enhanced winds.
Most of the period will feature below normal high temperatures
over the southern mainland and Panhandle while above normal
readings prevail over the northern two-thirds of the mainland.
The episodes of unsettled weather will lead to warmer anomalies
for morning lows, with most locations aside from the Panhandle
trending above normal. The greatest warm anomalies for lows
should be over the northern mainland late in the week.
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