Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
601 PM EST Fri Jan 17 2020
Valid 12Z Tue Jan 21 2020 - 12Z Sat Jan 25 2020
The large scale pattern across Alaska during the extended forecast
period is forecast to be strikingly different than that seen
during the short range. A mean trough/upper low is forecast to
develop/persist across the Arctic Ocean north of Alaska and
Siberia through the extended forecast period, with an active storm
track across the North Pacific, generally south of the Aleutians,
and into the Gulf of Alaska, and upper ridging shifting eastward
into west central Canada. Models/ensemble today have shown a trend
toward developing a negatively tilted upper ridge axis from the
Gulf of Alaska across the Alaska Peninsula and into the eastern
Bering Sea by late next week. If this occurs, it would allow for
the potential for shortwave energy originating in the arctic to
amplify and dive southeastward around the eastern side of the
ridge, potentially affecting eastern portions of Alaska.
A series of low pressure systems are forecast to track eastward
south of the Aleutians and toward the Gulf of Alaska through the
forecast period, as part of the active North Pacific storm track
mentioned above. Model spread with respect to these systems
increases gradually through time, with ensemble means offering a
more consistent picture especially toward the middle and end of
the forecast period. In general, this setup should keep
precipitation chances elevated across much of the Alaska
Panhandle/Southeast Alaska into much of next week, with heavy
precipitation possible early next week. Drier conditions should
prevail across much of the remainder of mainland Alaska. As the
negatively tilted upper ridge axis amplifies across the Alaska
Peninsula late next week, most deterministic solutions show the
potential for arctic shortwave energy to dig southward into
eastern portions of the Interior and perhaps even as far south as
the Panhandle. Solutions vary on the precise timing and structure
of such a feature, but this would result in the potential for
colder air to once again move into the North Slope and eastern
portions of the Interior. In contrast to yesterday, where the GFS
appeared to be quite well-centered within the ensemble spread, no
one piece of deterministic guidance today stood out as being more
well-centered than others within the ensemble spread. Thus, a
multi-model deterministic blend, including the 12Z ECMWF/GFS/CMC
was used as a forecast starting point early in the extended
period. Weight place on ensemble means was increased through time,
with a majority of the blend based on ensemble means starting on
day 6 (Thu) and onward.
Ryan
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
- Heavy precipitation across portions of the Alaska Panhandle,
Mon-Tue, Jan 20-Jan 21.
- Much below normal temperatures across portions of mainland
Alaska, Mon, Jan 20.
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