Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
649 PM EDT Tue Jul 13 2021
Valid 12Z Sat Jul 17 2021 - 12Z Wed Jul 21 2021
...Overview...
Blocky pattern will start to break down and move east during the
medium range, but with continued troughing into the Bering. An
upper low near Haida Gwaii will very slowly move into British
Columbia with weakening ridging over the Interior that will move
into the Yukon. This will keep northwestern areas rather unsettled
as a lead and secondary frontal system move into the region.
...Guidance/Predictability Evaluation...
The ensembles had a good handle on the overall longwave pattern
through the period, but with less predictability over the Bering
by next week. The trend has been a bit quicker overall with the
pattern breakdown/shift roughly west of 150W for the weekend
period but an unclear trend for next week. For the lead system,
the 12Z GFS/ECMWF paired rather well with the means as the upper
and surface system move into western Alaska before
weakening/dissipating around next Monday as it bumps into ridging.
Used a large deterministic component given the good agreement,
though the UKMET and Canadian were not quite as agreeable. Around
the Panhandle, the upper low will only slowly move eastward into
British Columbia, perhaps after a wobble or two as a shortwave
rounds the base later this weekend.
For the next Bering system, models diverge on which incoming
shortwave may take precedence and such a determination is not yet
discernible. Blended in an increased weighting of the ensemble
means to account for the uncertainty. The ECMWF was quicker than
its mean but the GFS was the opposite, so a middle ground solution
was preferred for now. Across the North Slope, flatter flow will
support a wavy boundary as a couple areas of low pressure traverse
the Beaufort/Arctic.
...Weather/Threats Highlights...
Precipitation will be focused over western/northwestern areas
during the period under southwest flow, as well as across the
Aleutians along the tail-end of the frontal boundaries. Widespread
tenth to a quarter inch with focused half to one inch rainfall
(locally more) is possible over northwestern areas over a 1-2 day
period with each system. Lighter precipitation will overspread
parts of the North Slope/Brooks Range as well, especially by next
week.
Portions of the North Slope/Brooks Range and Panhandle/southeast
corner of the mainland should see the most persistent below normal
highs during the period, though northern areas may see some
day-to-day variability depending on frontal position right along
the coast. Moisture arriving from the west during the weekend
into next week will bring a cooling trend from west to east, with
increasing coverage of below normal highs. On the other hand low
temperatures will tend to be more above normal with only smaller
areas of below normal anomalies.
Fracasso
No significant hazards are expected over Alaska during this
forecast period.
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