Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
556 PM EST Wed Feb 19 2020
Valid 12Z Sun Feb 23 2020 - 12Z Thu Feb 27 2020
The large scale flow pattern across Alaska and the North Pacific
will undergo some degree of modulation during the extended
forecast period, transitioning from a relatively progressive
pattern to one with at least some degree of blocking. Increased
atmospheric blocking around the northern hemisphere next week is
expected to result in a relatively strong positive height anomaly
center across northeast Asia, and a negative height anomaly center
across northern Europe. Teleconnections associated with both of
these strong hemispheric anomalies support development and
persistence of a mean upper-level trough across the
Aleutians/Bering Sea, and some degree of ridging across western
Canada. This setup is in fact what appears in the ensemble means
as we look forward to next week, despite quite a bit of
variability on the details of individual low pressure systems.
A blend of the 12Z ECMWF/06Z GFS was heavily used as a basis for
the forecast during days 4-5 (Sun-Mon), as these solutions showed
relatively good consensus with each other and with ensemble means.
A compact low pressure system is forecast to cross the
southeastern Gulf of Alaska and move inland across British
Columbia on Sun, potentially bringing a period of enhanced
precipitation to portions of Southeast Alaska. As upper-level
ridging begins to build by Sun-Mon, heights should briefly rise
across much of mainland Alaska before the ridge axis shifts east
into its favored position across western Canada and the
aforementioned mean trough/upper low sets up across the Aleutians.
A weakening low pressure/frontal system is forecast to approach
the Alaska Peninsula and southern mainland Alaska on Tue, bringing
a round of potentially heavy precipitation and high winds, with
another system perhaps approaching by Thu. Deterministic solutions
differed by the middle of next week as to the strength of the
ridge across western Canada, with the GFS allowing a strong North
Pacific jet to break down the ridge, and the ECMWF keeping a
stronger ridge in place. Ensemble means support some moderate
degree of ridge persisting, which as mentioned is also supported
by hemispheric teleconnections. Additionally, rapidly increasing
spread among deterministic solutions by the early to middle
portion of next week with respect to the track/intensity of
individual low pressure systems warranted increasingly heavy use
of ECENS/GEFS ensemble means during the day 6-8 (Tue-Thu) time
period.
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
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Tue-Wed, Feb 25-Feb 26.
- High winds across portions of mainland Alaska, Tue-Wed, Feb
25-Feb 26.
- Much below normal temperatures across portions of mainland
Alaska, Sat, Feb 22.
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