Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
758 PM EDT Fri Oct 06 2023
Valid 12Z Tue Oct 10 2023 - 12Z Sat Oct 14 2023
...Deep storm likely to bring strong winds to the Aleutians and
surrounding waters by next Thursday...
...Overview...
In most respects the latest guidance agrees fairly well with the
forecast pattern and significant features for about the first half
of the period, Tuesday into early Thursday. A leading
northeastern Pacific wave should reach near the southern coast of
the mainland as of early Tuesday, with a lighter trend for
precipitation over the Southeast into midweek. Meanwhile an upper
low over the western mainland may begin to drift northward after
Wednesday while a strongly deepening western Pacific storm should
arrive into the Aleutians/southern Bering Sea by late
Wednesday/Thursday with accompanying strong winds. Then guidance
diverges for various aspects of the forecast from late Thursday
into Saturday. These include the ultimate track of the
Bering/Aleutians storm, how quickly the mainland upper low departs
and character of ridging to its west, and finally the details of
northward-tracking low pressure offshore the Panhandle by around
Friday. This system may bring another episode of enhanced
precipitation and winds to the Panhandle.
...Guidance Evaluation and Preferences...
Looking at the first half of the period, one discrepancy of note
among the guidance is that latest GFS/GEFS runs are a slow extreme
with the system expected by consensus to reach near the southern
coast of the mainland by the start of the forecast 12Z Tuesday.
GFS runs have been on the deeper side of the spread as well,
though the new 18Z run has trended weaker. However the GFS runs
join other solutions with a surface low position just east of the
Kenai Peninsula by early day 5 Wednesday. Recent CMC runs have
tended to be a little south/southeast with the mainland upper low,
with the 12Z CMC most pronounced in that regard around Wednesday.
For a day 6 forecast (valid early Thursday), current guidance
shows better than average clustering position-wise for the deep
storm reaching near the central Aleutians. The 12Z GFS got into
the 940s mb for depth but the 18Z GFS raised its central pressure
into the 950s, closer to other models that have tended to be in
the 950s-low 960s. A deterministic central pressure in the upper
950s appears reasonable at this time.
Solutions rapidly diverge (with significant run-to-run variability
as well) in varying ways by the latter half of the period through
Saturday. Recent GFS/ECMWF runs have been dramatically waffling
with respect to what happens to the initial mainland upper low and
ridging to the west. The 00Z ECMWF had depicted a rapid upper low
departure with a ridge building into the mainland, reflective of
some earlier GFS runs, while the 12Z ECMWF and latest CMC runs
hold onto a mainland trough/upper low through Friday, then nudge
it west some by Saturday as a downstream ridge pokes into the
eastern mainland. 12Z/18Z GFS runs are closer to the
north-northeastward drift seen in the past couple days of the more
consistent GEFS/ECens/CMCens mean runs, though the 12Z GFS shows
an eastward ridge displacement relative the means that generally
suggest it will be over eastern Siberia.
As for other late-period features, solutions for the
Aleutians/Bering Sea storm range among southeastward progression
(most pronounced in the CMC), moderate eastward motion perhaps
with some brief wobbling (the slight majority including the GFS
GEFS/CMCens means with the 12z ECens trending that way), or even
looping back to the north/west (ECMWF runs and to some degree the
00Z ECens mean). Meanwhile there is still a decent signal for low
pressure to be offshore the Panhandle by early Friday but the 12Z
ECMWF has split the upper flow so that loses this wave while
emphasizing one farther south. Ensemble means maintain general
support of this wave, with the GFS/CMC and 00Z ECMWF in that
neighborhood aside from some timing/track differences. The
average track appears to have trended a bit westward from 24 hours
ago but the main moisture axis still appears to be positioned
farther east than with the atmospheric river event taking place
before the start of the extended forecast.
Based on guidance comparisons and continuity, the first half of
the forecast emphasized a 12Z operational model composite, with
somewhat less GFS input than usual on day 4 Tuesday due to its
slower wave tracking into the Gulf. The forecast quickly
incorporated more 12Z GEFS/CMCens and 00Z ECens input later in the
period, reaching 70 percent total weight by day 8 Saturday, with
lingering minority contributions from the 12Z GFS/ECMWF.
...Weather and Hazards Highlights...
Confidence continues to increase with respect to the deep low
pressure system forecast to reach the central Aleutians/southern
Bering Sea by no later than Thursday. Expect this system to bring
strong winds to the Aleutians and surrounding waters, with
increasing potential for reaching hazardous criteria. A broad
shield of focused rainfall will sweep across the Aleutians/Bering
Sea as well. After Thursday this storm should weaken somewhat but
with decreasing confidence regarding its track and how quickly it
will weaken. Farther east, some precipitation along the
Panhandle/southeastern coast should continue into Tuesday but with
a lighter trend versus prior days, with coverage continuing to
decrease through midweek. Another wave tracking northward
offshore the Panhandle late in the week may bring another episode
of precipitation whose axis should be oriented a little farther
east than with the previous wave reaching near the southern coast
by the start of Tuesday, along with enhanced winds. This late
week wave should focus highest totals over portions of the
Panhandle. There is still a decent level of uncertainty over
exact details of this system, so continue to monitor forecasts
with respect to coverage and amounts over the Panhandle (with
higher totals most likely farther south). Locations over and near
the North Slope may see some light snow around Tuesday.
High temperatures should be below normal over at least the
southern two-thirds of the state for most of the period, with
areas near the northern coast seeing the best potential for some
above normal anomalies. Readings may moderate a little toward the
end of the week as the western mainland upper low potentially
lifts northward. Low temperatures will tend to have warmer
anomalies, leading to a mix of above and below normal readings.
Below normal lows are most likely over some western/southwestern
areas and into parts of the Interior.
Rausch
Hazards:
- Heavy rain across portions of the Alaska Panhandle, Thu-Fri, Oct
12-Oct 13.
- High winds across portions of the Aleutians, Thu-Fri, Oct 12-Oct
13.
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