Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
753 PM EDT Fri Sep 03 2021
Valid 12Z Tue Sep 07 2021 - 12Z Sat Sep 11 2021
...Pattern Overview and Model Guidance...
The main large-scale upper-level feature affecting Alaska through
the period will be a broad upper low spinning west of the state
and dropping slowly southward from the Chukchi Sea Tue into the
Bering Sea by the latter part of the week. While this feature's
existence has good agreement among model/ensemble guidance,
shortwave troughs and thus surface low pressure systems moving
around this broad gyre are less consistent. The main shortwave of
concern rounds south of the upper low, but there is uncertainty
with whether it tracks south of the Aleutians into the
northeastern Pacific/Gulf of Alaska or north into the Bering
Sea/possibly southwestern mainland. For this forecast cycle, the
bulk of the model guidance took a more southerly track with the
shortwave and surface low, which was a change from yesterday's
guidance and WPC's previous forecast. At the time of forecast
preparation, 12Z GFS was the main player taking the features
north, so leaned away from this solution in favor of a blend of
the 06Z GFS and 12Z CMC/UKMET/ECMWF early in the period, and
trended toward an even mix of the deterministic guidance and GEFS
and EC mean solutions later in the period. Noted that the 18Z GFS
came in with a southerly track of the low as well, so hopefully
this is the best direction to trend, but changes in future
forecast issuances remain quite possible.
Additionally, there remains good model agreement for a fairly deep
surface low pressure system in conjunction with a shortwave in the
northeastern Pacific Tue into midweek, which is forecast to weaken
Wed and especially by Thu as it approaches the Panhandle.
...Sensible Weather...
A wet pattern is likely to remain across the Panhandle Tue and Wed
given moist inflow ahead of the potent surface low moving toward
the region, and rainfall totals could be modest to locally heavy
there. Precipitation is also possible in the northwest part of the
state through around Wed, with higher totals in terrain enhanced
areas, some of which could fall as snow. Then, rainfall along with
gusty winds are expected with the surface low currently expected
to track across the northeast Pacific for the latter half of the
week. The uncertainty with the low track makes it difficult to
pinpoint exactly where, but generally, the Aleutians, Alaska
Peninsula, Southcentral Alaska, and eventually the Panhandle could
see enhanced rainfall with this feature.
With general southerly moist flow across the state, cloudy
conditions are likely. High temperatures are expected to be around
or below normal across all but the eastern part of the mainland
especially for the early part of the week. Meanwhile, low
temperatures are likely to be around or slightly above average for
most of Alaska through much of the period.
Tate
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 outlook 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