Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
601 PM EST Tue Dec 08 2020
Valid 12Z Sat Dec 12 2020 - 12Z Wed Dec 16 2020
...Guidance and Uncertainty Assessment...
Upper flow will be complicated by a retrograding upper low across
the Interior, beneath a rather stable upper high over Wrangel
Island. This supports a strong surface high pressure over the
Beaufort Sea/Arctic Ocean and a storm track along the Aleutians
and into the Gulf. The models and ensembles were in decent
agreement at best which was not unexpected in a rather complicated
pattern evolution. The ensembles were mostly aligned but differed
on timing and somewhat the track of the systems in the flow.
Overall preferred the ECMWF and its ensembles especially later in
the period, but otherwise the 12Z GFS/UKMET/Canadian offered
reasonable clustering for the Sat-Mon period.
For the weekend, split in the guidance on whether or not to take
low pressure across the Peninsula into Bristol Bay (GFS/UKMET) or
have it linger just to the east (ECMWF and continuity). Opted to
forge a middle ground on top of the island chain with no clear
trend or preference. Thereafter, still expect a system to wrap up
into the Gulf Sunday as it sheds its fronts to the east. By then,
upper low will rotate southwestward as another Pacific shortwave
rounds the bend underneath it, allowing it to deepen and lift into
the Gulf. The GFS never allows the Pacific energy to organize and
shears out the system but the ECMWF and many ensemble members
still show a deeper solution, taking it into the southern Gulf
next Tue and weakening next Wed. With good ensemble support,
preferred this scenario though tempered by a majority ensemble
weighting. With the upper high remaining to the north, flow next
week could open up from the west between 50-60N, realized by the
next system near the western Aleutians.
...Weather/Hazards Highlights...
Strong high pressure to the north and storminess in the Gulf will
lead to windy conditions over the Interior early next week, though
rather dry. Systems in the Gulf or nearby areas will focus
precipitation over coastal areas of the AKPen and Kenai peninsula
on southeasterly flow as well as the western side of Cook Inlet.
Parts of the Panhandle may see at least modest precipitation,
though the proximity of the systems to the land areas remains in
question. As a result, models solutions vary on intensity there.
Temperatures will be near to above normal over northern areas of
the mainland, but near to below normal over southwestern areas
(under the cold upper low) and much of the Panhandle with offshore
flow at the surface.
Fracasso
Hazards:
- High winds across portions of interior Alaska, Sun-Tue, Dec
13-Dec 15.
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards 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