Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
654 PM EST Fri Dec 30 2022
Valid 12Z Tue Jan 03 2023 - 12Z Sat Jan 07 2023
...Effects from the major New Year's weekend storm likely to be
tapering off by the start of the forecast period Tuesday...
...Overview and Guidance/Predictability Assessment...
Today's guidance maintains a similar theme for the large scale
pattern but a lot of spread and run-to-run variability for most of
the details. As of Tuesday, consensus indicates continued
weakening of the short-range storm near the Alaska Peninsula.
Otherwise the ensemble means agree upon mean ridging aloft over
western Canada, extending into Mainland Alaska at times, while a
broad upper low drifts from the Sea of Okhotsk into the Aleutians.
The means suggest the overall low could open up by the end of the
period next Saturday. Overall the operational models fit this
general pattern but both they and individual ensemble members
suggest a very chaotic regime in terms of resolving individual
shortwaves and associated surface lows. This applies to the
northeastern Pacific where the pattern will tend to support
north/northwest motion of systems, as well as to the broad area of
mean low pressure tracking near the Aleutians.
Over the northeastern Pacific there is a general signal for two
primary systems. The first may be a rather compact low that
approaches the Gulf of Alaska from the south around Wednesday and
possibly linger somewhere near the southern coast/Kodiak Island
into Thursday. If anything, guidance has diverged somewhat over
the past day thus keeping confidence low. In order to keep some
degree of continuity given the uncertainty, preference was to
emphasize a compromise among the 12Z/00Z ECMWF runs and 12Z UKMET
with the 00Z ECens/12Z CMCens means offering some support for this
scenario as well. Latest GFS runs are somewhat on the slow side
though not completely out of the realm of possibility. The second
system could be somewhat larger in scale and have some influence
on the southern Panhandle as it passes by. CMC/GFS runs have been
somewhat consistent in showing this system, while the ensemble
means offer some support as well into day 6 Thursday (when it is
still well south) before losing it. The 12Z ECMWF is a fast
northward extreme, bringing it into the Panhandle by early
Thursday. The new 12Z ECMWF mean maintains the consensus idea
that this system will most likely remain offshore, while the
latest CMC runs and 00Z ECMWF at least wait until Friday to have
it reach the Panhandle. Preference here is to include some 12Z
GFS/CMC with the 00Z ECMWF to add some definition to the means
while keeping the low track offshore per the overall pattern in
the means.
Guidance spread over the Aleutians, and for that matter much of
the Pacific and Bering Sea, becomes increasingly pronounced after
Tuesday. The means provide a reasonable anchor for the forecast
given their broad agreement, with best emphasis for low pressure
near the Aleutians. However individual model runs/ensemble
members show varying numbers of individual surface lows/frontal
systems and offer potential for at least one of these to stray
well into the Bering Sea or remain suppressed over the Pacific.
Based on preferences for areas of interest, the start of the
forecast began with a 00Z/12Z ECMWF and 12Z UKMET composite
followed by some inclusion of the 12Z CMCens/00Z ECens by day 5
Wednesday to account for Aleutians detail uncertainties and
closest agreement with that model cluster for the Gulf of Alaska
and vicinity. Lack of relative support for the 12Z ECMWF and
better comparisons for the GFS/CMC for the second East Pacific
system led to removal of the 12Z ECMWF and modest introduction of
the 12Z GFS/CMC by the latter half of the period. Total weight of
the 12Z GEFS/CMCens and 00Z ECens means reached 50-60 percent by
days 6-8 Thursday-Saturday, helping to temper the individual model
spread over and near the Aleutians.
...Weather/Hazards Highlights...
The overall pattern should promote periods of precipitation along
the southern coast and Panhandle through the Tuesday-Saturday
period. Depending on specifics of individual systems, there could
be some pockets of locally heavy activity at times. However
general amounts should be more moderate than expected in the short
range (through Monday) but may be meaningful from a multi-day
total perspective. Currently the most likely systems of note may
bring the best precipitation focus to areas near Kodiak Island and
Kenai Peninsula around Wednesday-Thursday and perhaps the southern
Panhandle late in the week. Otherwise most of the mainland will
likely experience mostly dry conditions during the period. Expect
the Aleutians to see an unsettled pattern as a leading
wave/frontal system pushes into the region Tuesday-Wednesday with
additional systems bringing various emphasis for
precipitation/wind. Confidence in specifics over the Aleutians
beyond Tuesday remains quite low. Much of the state should see
above normal temperatures through the end of the week, though
there may be a very gradual moderating trend with time. Anomalies
for lows should generally be greater than those for max
temperatures. The Aleutians may see the best potential for below
normal readings.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Mon, Jan 2.
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