Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
636 PM EDT Tue May 26 2020
Valid 12Z Sat May 30 2020 - 12Z Wed Jun 03 2020
...Overview and Weather Hazards/Threats...
A mid/upper-level ridge centered across the arctic just north of
Alaska is expected to slowly weaken through the extended forecast
period. The ridge will promote well above average temperatures for
much of mainland Alaska (especially western mainland and the
Interior) over the weekend, persisting into early next week - with
forecast highs ranging from 5 to as much as 20 deg F above
average. Temperatures may approach 80 deg at a few locations
across the Interior. Meanwhile, an active and somewhat blocked
pattern farther south will keep persistent negative height
anomalies in place across the North Pacific from near/south of the
Aleutians east into the Gulf of Alaska. A pattern with some degree
of blocking across the northern hemisphere should favor a
slower/less progressive evolution of features across the North
Pacific and Alaska, and this concept guided, to some extent, the
model preference today (see below).
An initially deep barotropic low in the Gulf of Alaska on Sat is
expected to gradually fill/weaken through the weekend as another
low pressure system slowly traverses the Aleutians, with a new
triple point low forecast to develop in the Gulf of Alaska by Mon
or Tue. The leading system in the Gulf of Alaska will keep onshore
flow in place, with scattered areas of precipitation (more
widespread in favored upslope areas) in southern mainland and
Southeast Alaska. Areas of showers will also cross the Aleutians
over the weekend in association with the next system. As the
triple point low develops in the Gulf by early next week,
precipitation across southern mainland Alaska could become more
widespread, with moisture (and scattered precipitation) eventually
spreading north into portions of the Interior toward the middle of
next week.
...Guidance/Predictability Assessment...
A blend of the 12Z ECMWF/CMC was used as a basis for the forecast
during days 4-5 (Sat-Sun). The GFS was excluded from the blend for
this forecast cycle, as it seemed to be an outlier with moving a
deep upper low east into the Gulf of Alaska much more quickly than
the overall model/ensemble consensus. Additionally, the GFS
weakened the arctic ridge more quickly than the other guidance,
opening the Interior up to stronger easterly flow sooner than the
overall consensus. Starting on day 6 (Mon) and continuing through
day 8 (Wed), weighting of ensemble means (ECENS and NAEFS) was
gradually increased through time, with majority weight on ensemble
means by days 7-8.
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:
- Much above normal temperatures for portions of mainland Alaska,
Fri-Tue, May 29-Jun 2.
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