Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
549 PM EST Sat Nov 13 2021
Valid 12Z Wed Nov 17 2021 - 12Z Sun Nov 21 2021
...Much below normal temperatures likely across much of Alaska
through the medium range period...
...Overview and Guidance Evaluation/Preferences...
An upper level low, with troughing extending into the Gulf of
Alaska, will be present over southern Mainland Alaska to start the
period on Wednesday. The low should slowly retreat into Canada,
though some form of weak troughing may linger across the state
through next weekend. At the surface, a modestly deep cyclone will
be present in the Gulf, but should gradually weaken through the
period as the next system from the north east Pacific moves
towards the region. To the West, another deep upper low will drop
southward over/near the western Aleutians Thursday-Saturday, as
multiple perturbations in the base of the trough send a couple of
surface low pressure systems into the well south of the Aleutians,
and eventually into the southern Gulf by next Sunday.
For days 4-5/Wednesday-Thursday, the guidance shows reasonable
enough agreement to allow a general deterministic model blend
between the 12z GFS, ECMWF, and CMC. After this, differences and
uncertainties in the details quickly begin to arise especially
with respect to how much troughing gets left behind over the
Mainland the last half of the period and also with the next upper
low dropping into the western Aleutians and the shortwaves
rounding the base of that trough. As is typical for these long
range periods, the deterministic models get pretty chaotic south
of the Aleutians, that a blend towards the better agreeable
ensemble means seems to be the best approach to smooth out the
harder to resolve details. Some modest amounts of the 12z ECMWF
and CMC were maintained even through day 8, since those solutions
are closest to the ensembles and should give some additional
definition/depth to individual systems. This approach also
maintains good continuity from yesterdays forecast.
...Weather/Hazard Highlights...
Much of the precipitation over Alaska should be confined to the
Aleutians/Southern Coast/Panhandle, with much of the mainland
remaining very dry and cold, other than maybe a few snow showers.
The low pressure system in the Gulf on Wednesday-Thursday will
likely bring a period of moderate to locally heavy precipitation
to much of the Panhandle region with wet conditions, but lighter
in intensity, lingering the rest of the week associated with
maintained weak troughing across the Gulf. In addition, favorable
flow funneling through the mountain ranges of southern Alaska are
likely to result in the potential for localized but strong outflow
winds across downwind locations of southeast Alaska. Precipitation
chances again increase by next weekend across the Panhandle as the
next low pressure system approaches. A system into the Aleutians
late next week should bring unsettled conditions to the region
with light to moderate precipitation spreading towards the
Peninsula and Southern Coast as well.
Much below normal temperatures should prevail across much of
Alaska (with the exception of the North Slope region) through next
week as surface high pressure becomes established across much of
the state. The most pronounced anomalies should be over the
western to southern part of the state where daytime highs could be
as much as 20-30+ degrees below normal. Temperatures should remain
below normal into next weekend, but may moderate slightly as the
upper trough weakens and upper ridging attempts to sneak in from
the west.
Santorelli
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