Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
704 PM EDT Sat Aug 15 2020
Valid 12Z Wed Aug 19 2020 - 12Z Sun Aug 23 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, with an upper low persisting across
the Gulf of Alaska, and an upper ridge initially building across
the Bering Sea by the middle of next week, spreading east across
mainland Alaska late in the week. The combination of the ridge
spread into the Interior, and the persistent upper low over the
Gulf begins to resemble a Rex Block in ensemble guidance by next
weekend, which may shift the pattern toward a less progressive
state by the end of the forecast period and beyond.
Model/ensemble guidance showed near to above average consensus at
the large scale through the extended forecast period. A blend of
the 12Z ECMWF/GFS was used heavily as a forecast starting point
initially on days 4-5 (Wed-Thu). A gradual trend toward increased
weight placed on ensemble means (ECENS/GEFS) was shown through
time after Thu, with majority ensembles in the blend during days
7-8 (Sat-Sun). Relatively good consensus among the ensemble means
by later in the forecast period helped to smooth out any
differences on the details of specific features among the
deterministic guidance.
...Weather Highlights/Threats...
A low pressure system is forecast to cross the southern Gulf of
Alaska through the middle/end of next week. This system will keep
widespread precipitation in the picture across the Panhandle.
Scattered rain/mountain snow showers are possible across southern
mainland Alaska on Wed as a weakening frontal boundary and
upper-level shortwave traverse the region. By later next week, as
upper ridging builds across the Bering Sea and western mainland
Alaska, drier conditions should prevail across much of mainland,
with only the Panhandle/Southeast Alaska remaining wet, on the
eastern side of the persistent Gulf low. Additionally, late next
week, another frontal system is expected to cross the Bering Sea,
and approach western mainland Alaska, spreading at least scattered
showers inland with it. In general, this pattern should favor
temperatures several degrees above average across much of the
state. The North Slope will see somewhat cooler temperatures north
of the weakening frontal boundary through the middle of next week,
with high temperatures expected to be a few degrees below average.
Widespread clouds and precipitation across the Panhandle will keep
temperatures there several degrees 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