Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
621 PM EST Thu Nov 19 2020
Valid 12Z Mon Nov 23 2020 - 12Z Fri Nov 27 2020
...Strong system near Kodiak and into the Gulf Monday...
...Overview and Guidance/Predictability Assessment...
Active pattern will see two robust systems of different flavors
impact Alaska. Lead system will deepen to around 960mb near Kodiak
by 12Z Mon before slipping eastward through the southern Gulf then
across the southern Panhandle into Canada. The GFS finally came
around to the other solutions yet still was displaced a bit to the
south. Blend of the 12Z ECMWF/UKMET/Canadian offered a good
starting point with that system but the GFS was acceptable
elsewhere. Next system will move through the Bering from the east,
with added southern stream energy. GFS was again perhaps too
aggressive with this influx from the south, and took that low as
the new dominant low toward Kodiak into the Gulf. However, ongoing
model/ensemble consensus is to maintain that low as only a weaker
feature, allowing it to be absorbed into the larger Bering system.
That has repercussions on the flow over northern Alaska where the
GFS would allow ridging to maintain itself whereas the other
guidance shows lower heights chipping away at the ridge (which
will be already weakening anyway). Therefore, given the recent
poor track record of the GFS opted to stay with the larger
ECMWF/Canadian-led cluster which was supported by at least some of
the GEFS members (and mostly ECMWF members) and continues
continuity as well. This would take a triple point low into the
Gulf around next Wed/Thu as the old parent low dissipates (slowly)
in the central then eastern Bering. A weaker front or two may
rotate around the larger Bering circulation as well, again
depicted on the surface progs. Trended toward more ensemble
weighting by next Wed/Thu (about 80%) to account for detail
disagreements in the ECMWF/Canadian and their recent runs.
...Weather/Threats Highlights...
Mon-Tue system has the potential for widespread modest rain/snow
along the eastern Aleutians, southwest coastal mainland, and over
Kodiak to the southern Kenai along with breezy/windy conditions
(40kts+). Storm track may force much of the precipitation to just
the southern Panhandle to British Columbia, though it will likely
churn up some large waves over the Gulf (see the Ocean Prediction
Center over the next few days for more information). Next system
in the Bering may have a less favorable moisture source but could
still bring some rain/snow to the island chain on southwesterly
flow ahead of the front. Some precipitation is forecast to expand
over western/southwestern areas and Southcentral next Wednesday
but at this point it is unclear how much enhancement there will be
based on southern moisture influx. This should spread eastward
across the Gulf into the Panhandle around next Thursday.
Temperatures will remain well above normal along northern areas
thanks to upper ridging and above normal lower level temperatures,
though they should trend a bit back toward normal later next week.
Southeastern third of the mainland into the Panhandle will see
near to below normal temperatures.
Fracasso
Hazards:
- Heavy precipitation across portions of mainland Alaska, Wed, Nov
25.
- Heavy precipitation across portions of the Alaska Panhandle,
Mon, Nov 23.
- High winds across portions of the Alaska Panhandle and mainland
Alaska, Mon-Tue, Nov 23-Nov 24.
- High significant wave heights for coastal portions of the Alaska
Panhandle, Tue, Nov 24.
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