Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
718 PM EDT Wed Jul 14 2021
Valid 12Z Sun Jul 18 2021 - 12Z Thu Jul 22 2021
...Overview...
Blocky pattern will start to break down and move east during the
medium range, but with continued/renewed troughing into the Bering
and Bering Strait. An upper low initially near Haida Gwaii Sunday
will very slowly move into British Columbia as weakening ridging
over the Interior/Southcentral moves into the Yukon as the pattern
relaxes. This will keep western areas rather unsettled as a lead
and secondary frontal system move into the region.
...Guidance/Predictability Evaluation...
The ensembles continued to have 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 tad slower
(after being a bit quicker yesterday) overall with the pattern
breakdown/shift including the upper low to the south of the
Panhandle. For the lead system Sunday, the 12Z
GFS/ECMWF/UKMET/Canadian 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 to start given the good
agreement with expected differences in timing/strength that were
within the ensemble spread. 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 which may yield a surface cold front south of the region.
For the next Bering system, models diverged on how quickly to send
the system northeastward with no clear trend the past day or two.
Blended in an increased weighting of the ensemble means to account
for the uncertainty. Focus for precipitation also differed along
the western coastline between the GFS (a bit farther south) and
the ECMWF (spread out more to the north). Blended solution was
preferred. Across the North Slope, flatter flow will support a
wavy boundary as a couple areas of low pressure traverse the
Beaufort/Arctic but eventually weaken.
...Weather/Threats Highlights...
Precipitation will be focused over western 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 these areas over a 1-2 day period with each system.
Lighter precipitation will overspread parts of the North
Slope/Brooks Range as well.
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. In addition, Bering systems will bring a cooling trend
from west to east, with increasing coverage of below normal highs,
as heights fall. On the other hand low temperatures will tend to
be more above normal with only smaller areas of below normal
anomalies over western areas.
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