Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
758 PM EDT Sun Sep 03 2023
Valid 12Z Thu Sep 07 2023 - 12Z Mon Sep 11 2023
...Overview...
The ensemble means generally suggest a longer term upper trough
setting up over and south of the mainland, with the axis possibly
starting to drift westward a bit toward the end of the forecast
period and beyond. This should support a cool and unsettled
pattern with highest precipitation totals along the southeastern
coast and Panhandle. As for individual features, a leading trough
will amplify/sharpen over the northeastern Pacific after midweek
and direct some enhanced precipitation into the Panhandle and
vicinity while the means show a Friday-Saturday Bering Sea storm
ultimately tracking just south of the Alaska Peninsula/Kodiak
Island and toward the Gulf of Alaska by Sunday-Monday to bring
another episode of precipitation. Another area of low pressure
with a frontal system may cross the Bering Sea/Aleutians by early
next week. While most operational model runs conform to the
ensemble mean ideas late this week, they show increasing
divergence from each other and among consecutive runs for
important aspects of the forecast from the weekend into early next
week--thus lowering confidence for the time being.
...Guidance Evaluation/Preferences...
Recent days of guidance runs have been variable with the character
of the upper trough expected to amplify/sharpen over the
northeastern Pacific around Thursday-Saturday, complicating the
forecast along the Panhandle and southeastern coast. Today's runs
generally trend toward a deeper/sharper/slower trough that allows
for greater development of a frontal wave tracking in from the
southwest. This wave would reach a position just south of the
Panhandle by early Saturday and become better defined than initial
low pressure evolving over the Gulf of Alaska. The 12Z CMC is the
most extreme solution in terms of strength and north/northwest
track of the surface low after early Saturday, with the 12Z ECMWF
not as deep but with a fairly far northward track as well in
response to a more persistent upper low. On the other hand, the
12Z GFS is somewhat on the weak side of the envelope at the
surface. Current preference is to reflect some trending toward an
offshore wave but with intermediate strength and away from the 12Z
CMC/ECMWF specifics until supported by other guidance.
After a couple days of decent clustering (aside from a stray model
or two depending on the run) for the system forecast to track
across the Bering Sea and just south of the Alaska Peninsula and
Kodiak Island, the 12Z models have become much more divergent.
Latest GFS runs keep the parent low track well northward of
continuity, settling over the eastern Bering Sea, albeit with a
triple point wave whose track is somewhat closer to the parent low
of the means and still provides precipitation focus along the
southern coast. Meanwhile the 12Z CMC/ECMWF have shifted to a
more suppressed track south of the Alaska Peninsula, with the
ECMWF track so far south that no moisture reaches the southern
coast or northern Panhandle. Aside from the GEFS mean showing
some traits of the parent GFS runs very late in the period, the
GEFS/ECens/CMCens agree well on a track similar to
continuity--across the central Bering Sea and then just south of
the Alaska Peninsula/Kodiak Island. The only adjustment since
yesterday is slightly slower progression, while guidance as a
whole has trended a little weaker with this system. The divergent
nature of operational runs favors anchoring the forecast close to
the means to provide continuity while awaiting any pronounced
trends that can be accounted for with greater confidence.
Behind this system, signals exist for a general area of low
pressure/frontal system to be crossing the Bering Sea/Aleutians by
day 8 next Monday. However there has been a lot of spread and
variability for the details in the models, and individual ensemble
members are likewise sufficiently different to result in a diffuse
depiction in most means. Currently a blended approach yields a
weak system emerging over the northern Bering Sea with a frontal
system extending to the south.
An initial compact Arctic upper low continues to be a very
troublesome feature in the extended forecast, with guidance spread
as wide as ever. Northeastward departure (12Z CMC), settling over
the northeastern half of the mainland (12Z ECMWF), or
retrogression over Siberia (12Z GFS) appear to be among the less
likely scenarios based on recent days of guidance. Incorporation
into the larger scale mean trough to the south may have somewhat
better potential but still with rather low confidence.
Today's forecast employed a blended model approach early in the
period to reflect consensus ideas without giving too much weight
to extreme/low-confidence solutions over a particular area. This
composite used the 00Z/12Z ECMWF and 12Z GFS/CMC/UKMET.
Increasing preference for the ensemble means later in the forecast
favored 30 percent total input from the 12Z GEFS/CMCens and 00Z
ECens on day 6 Saturday and 60 percent means by days 7-8
Sunday-Monday. Lingering operational model components canceled
out where disagreeable and elsewhere added a little definition to
the means.
...Weather/Hazards Highlights...
The leading upper trough over the mainland into northeastern
Pacific will likely produce a broad area of precipitation over the
eastern mainland late this week. The Panhandle and parts of the
southeastern coast are likely see the highest totals in
association with an initial Gulf of Alaska surface low/frontal
system and then a possible frontal wave tracking into the
northeastern Pacific just offshore the Panhandle by Saturday. The
system forecast to track across the Bering Sea late in the week
will spread a broad area of precipitation and brisk to strong
winds (though perhaps not quite as strong as suggested by some
guidance in the past day or two) across the Bering Sea and
Aleutians at that time. However today's divergence in guidance
for surface low track by the weekend has lowered confidence in
precipitation coverage and amounts across the southern
coast/Panhandle mid-late period. Current preference for the
ensemble means that are close to continuity, showing a track just
south of the Alaska Peninsula/Kodiak Island, would maintain the
potential for meaningful precipitation along the southern coast
and then into the Panhandle. Coverage/amounts over the
southwestern mainland will also be sensitive to the storm track.
A weaker system may spread lighter rainfall across the Bering
Sea/Aleutians early next week.
Below normal high temperatures should prevail over most areas
during the period. Expect the coolest readings to be late in the
week when the deepest upper troughing should be over the state.
Highs will likely rebound some by Saturday-Monday. Low
temperatures will tend to average slightly warmer then normal,
though with some areas seeing modestly below normal readings on
one or more days--most likely over the southwest Thursday-Friday
and then progressing into the Interior Friday-Saturday.
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