Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
747 PM EDT Fri Oct 01 2021
Valid 12Z Tue Oct 05 2021 - 12Z Sat Oct 09 2021
...Pattern Overview and Guidance/Predictability Assessment...
Expect a progressive and active weather pattern that will bring
episodes of significant precipitation to western/southern parts of
Mainland Alaska as well as the Panhandle. Individual storms will
likely produce areas of strong winds as well. Upper dynamics and
possible low pressure initially over the western Aleutians/Bering
Sea as of early Tuesday should catch up to and merge with a
leading northern Bering storm (extratropical reflection of Typhoon
Mindulle) by early Wednesday, followed by development of a Gulf
wave by Thursday. Another system should reach near the western
Aleutians by Thursday and continue to the east or northeast
thereafter, with another wave over the Gulf by next
Saturday--whether from triple point development or progression of
the initial low.
Aside from finer-scale details that have low predictability in the
extended time frame, guidance is fairly well behaved for the
general scenario of low pressure consolidating over the Bering Sea
Tuesday into Wednesday followed by a wave reaching the Gulf of
Alaska by early Thursday. A 12Z operational model blend represents
consensus well for this part of the forecast.
The trailing system that may reach the western Aleutians by early
Thursday is more problematic. On average guidance has trended
somewhat east/southeast for the late week low track over the past
24-36 hours of runs. However the GFS, 12Z CMC/UKMET, and latest
ensemble means (whether explicitly or implied) suggest the past
couple ECMWF runs may have become too suppressed. The
GFS/CMC/UKMET are all quite strong too. Interestingly the 00Z CMC
was similar to the 00Z ECMWF before adjusting to a somewhat faster
version of the GFS in the 12Z run. At least the 12Z ECMWF
adjusted slower from its 00Z run, narrowing the timing spread
among the models. Following a model/ensemble mean blend later in
the period yielded a low that was closest to the 12Z CMC in
principle (though weaker given the uncertainty) through at least
day 7 Friday. Solutions converge for low pressure over the Gulf
by early day 8 Saturday.
...Weather/Hazard Highlights...
Bering Sea low pressure will spread a broad area of precipitation
into the western mainland on Tuesday. Moisture should continue
streaming into the west on Wednesday while expanding across the
southern coast into the Panhandle Wednesday-Thursday as low
pressure develops in the Gulf of Alaska. Highest totals with this
event should be over and near the southern coast/Panhandle.
Strong winds will be most likely across the Bering Sea/Aleutians
and into the far western mainland, with a period of brisk to
strong winds also possible around the Gulf system. Latest
guidance consensus would have another vigorous storm tracking from
the western Aleutians into the western mainland late in the week
and low pressure reaching the Gulf by Saturday, producing
additional precipitation and strong winds. There is still a
lingering lower probability for a more southern track that would
keep the significant moisture/wind to the south of the mainland,
so confidence is not yet very high regarding specific effects over
the southwestern mainland.
The forecast pattern should favor below normal high temperatures
over the southern mainland and Panhandle through the period.
Above normal highs will spread eastward/northeastward across the
northern two-thirds of the mainland. Clouds and precipitation
will promote warmer anomalies for morning lows, with most
locations aside from the Panhandle trending above normal after
midweek.
Rausch
Hazards:
- Heavy precipitation across portions of mainland Alaska, Wed, Oct
6.
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Wed-Fri, Oct 6-Oct 8.
- High winds across portions of mainland Alaska, Mon-Wed, Oct
4-Oct 6.
- High winds across portions of mainland Alaska and the Aleutians,
Mon-Fri, Oct 4-Oct 8.
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