Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
654 PM EDT Tue Aug 18 2020
Valid 12Z Sat Aug 22 2020 - 12Z Wed Aug 26 2020
...Overview and Guidance Evaluation/Preferences...
The pattern across Alaska will undergo some degree of change
during the extended forecast period. An upper-level ridge axis
initially across mainland Alaska will be broken down over time as
strong westerly flow and embedded shortwaves fairly quickly erode
the ridge. Models differ somewhat on the speed with which the
ridge breaks down, but differ more so with respect to the timing
of individual shortwaves traversing the relatively strong jet.
Consensus among the 12Z ECMWF/GFS was sufficient during days 4-5
(Sat-Sun) to use a blend of these two solutions as a basis for the
WPC forecast. After that, differences among deterministic guidance
quickly increase. The GFS timing with shortwaves crossing the
Bering Sea toward the Gulf of Alaska strays from the overall
consensus, with the GFS on the fast side. In general, this is a
scenario with below average predictability when it comes to the
details, so ensemble use was quickly increased through time during
days 6-8 (Mon-Wed). The ECMWF solution remained at least somewhat
more in line with ensemble means, and thus a minority component of
the ECMWF was used through day 8, while the GFS was phased out
during that time frame. There is relatively good large scale
consensus that by next Mon-Tue, the ridge should be sufficiently
weakened to open mainland Alaska up to a barrage of shortwaves
associated with the active North Pacific jet, resulting in a
transition to a stormier pattern for Alaska by early next week.
...Weather Highlights/Threats...
Relatively dry conditions and warm temperatures will be in place
across much of interior Alaska on Sat, before the upper-level
ridge breaks down. A frontal system across the Bering Sea will
begin to affect the west coast of Alaska on Sat with scattered
showers, before spreading into much of the Interior on Sun, as
energy transfers to a new triple point low in the Gulf of Alaska.
This system may produce heavy rains across portions of southern
mainland Alaska, especially the Alaska Range, Sun-Mon. Into next
week, expect scattered to numerous showers to persist across much
of mainland Alaska as the ridge weakens and shortwaves traverse
the region through midweek. Dropping temperatures will support
snows at the higher elevations of the Alaska and Brooks Ranges.
High temperatures Sat are forecast to be 5 to 15 deg F above
average. From Sun onward, as shortwaves break down the ridge and
heights fall, expect temperatures to decrease to near or even
several degrees below normal.
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 rain across portions of mainland Alaska, Sun-Mon, Aug
23-Aug 24.
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