Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
617 PM EDT Wed Aug 19 2020
Valid 12Z Sun Aug 23 2020 - 12Z Thu Aug 27 2020
...Overview and Guidance Evaluation/Preferences...
Over the course of the extended period, the primary feature of
interest will be a developing and stalling upper level low across
the eastern portions of the Bering Sea. At the surface, initial
low pressure will skirt across the Aluetions and southern
mainland, occluding and stalling a bit before becoming absorbed
within the developing large scale low. For the most part, the
synoptic pattern showed fairly good agreement between the various
deterministic models, at least through day 5/6. Beyond that,
typical model biases were seen where the 12Z GFS was faster and
ahead of the 12Z ECMWF/CMC. The ensemble means were useful by day
6/7 and were considered a compromise solution given the
deterministic model differences later in the period. As such, the
WPC blend started off with a near equal weight of the 12Z
ECMWF/GFS/CMC then transitioned to higher weights of the ECENS and
GEFS means as forecast consistency and confidence lowered.
...Weather Highlights/Threats...
Relatively dry conditions and warm temperatures will be in place
across much of interior Alaska late in the weekend, before the
upper ridge breaks down and transitions to the large upper low
across the eastern Bering Sea. 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/Taylor
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, Monday 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