Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
634 PM EDT Sun Aug 16 2020
Valid 12Z Thu Aug 20 2020 - 12Z Mon Aug 24 2020
...Guidance Evaluation and Preferences...
The amplified pattern which has shown a strong signal in the
extended guidance in recent days, appears likely to continue
through the end of next week. An upper-level low is expected to
persist across the Gulf of Alaska, with an upper ridge initially
over the eastern Bering Sea early in the extended gradually
shifting east across mainland Alaska through late in the week. By
next weekend, guidance shows some degree of weakening/flatting of
the ridge as strong westerlies and embedded shortwave energy erode
the northern/western side of the ridge. While the pattern will
briefly resemble a Rex Block, model/ensemble guidance suggest this
will not be a feature with much in the way of persistence given
the suggested relatively quick flattening of the ridge.
Models/ensembles continued to relatively good consensus from
recent days through the first part of the extended period (days
4-5), but showed more degradation compared to recent days later in
the forecast period. A blend of the 12Z GFS/ECMWF was used during
days 4-5 (Thu-Fri), with more weight on the GFS relative to the
ECMWF. The solutions were not fundamentally different, but the GFS
was perhaps a bit more well-centered within the overall consensus
for features through that time period. The primary point of
contention relates to timing of smaller-scale shortwave energy in
the westerlies across the Bering Sea, and how quickly this energy
erodes the ridge axis across mainland Alaska. While deterministic
solutions showed greater differences, a blend of ensemble means
likely represents the best consensus approach here by later in the
forecast period. Thus, starting on day 6 (Sat) and continuing
through day 8 (Mon), a gradual shift toward heavier use of
ECENS/GEFS ensemble means was shown.
...Weather Highlights/Threats...
A low pressure system is forecast to cross the southern Gulf of
Alaska through late this week. This system will keep widespread
precipitation in the picture across the Panhandle. As upper
ridging builds across the Bering Sea and western mainland Alaska,
drier conditions should prevail across much of mainland through
late in the week, with only the Panhandle/Southeast Alaska
remaining wet (on the eastern side of the persistent Gulf low).
Additionally, another frontal system is expected to cross the
Bering Sea late in the week, moving onshore in western mainland
Alaska Sun night into Mon, spreading showers inland with it. In
general, this pattern should favor temperatures several degrees
above average across much of the state through late in the week,
although temperatures across much of mainland/interior Alaska will
begin to decrease closer to seasonal norms by the weekend into
early next week as the incoming shortwave energy flattens the
ridge, along with increased clouds/precipitation. Relatively
widespread clouds/precipitation across the Panhandle will favor
below average temperatures across that area through the extended
forecast 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
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