Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
624 PM EDT Wed Oct 21 2020
Valid 12Z Sun Oct 25 2020 - 12Z Thu Oct 29 2020
...Heavy precipitation possible for the Panhandle and coastal
Southcentral areas late weekend/early next week...
...Guidance/Predictability Assessment...
Troughing is forecast to settle and be reinforced over
Southcentral next week as upper ridging south of the Gulf moves
very slowly eastward. Additional ridging over the central Pacific
will flatten and allow quickly-moving systems into the mean trough
axis along 160W. This setup will maintain a wet southwesterly flow
into the Panhandle and portions of coastal southeastern
Southcentral (Kenai eastward) as the surface boundary may be
blocked from moving east by the upper ridge.
The 12Z models were in general agreement on the synoptic scale but
varied on system timing/track/intensity within the longwave
pattern. Ensemble means also differed but were within tolerance.
The 12Z GFS was quite different with the handling of a surface
wave toward Kodiak vs the ECMWF/Canadian (west vs east), which did
not fit the consensus nor continuity. Opted to rely mostly on the
12Z ECMWF/ECMWF ensemble mean with some added details of the
Canadian despite its offset system timing. By later in the week,
nearly zonal flow east of the Dateline suggest low confidence in
details but this should keep the storm track south of the
Aleutians regardless. To the north, height falls will usher in
colder temperatures for the North Slope and much of the Interior
north of the Alaska Range. This cold front may make its way toward
Denali and Southcentral but should weaken as its support moves
into Canada.
...Weather/Threats Highlights..
Expect the heaviest precipitation during the period to be over the
far southern/southeastern mainland and Panhandle as a series of
frontal waves move through the Gulf and into the Panhandle or
southeast mainland. Several inches of rainfall (perhaps several
feet of snow at higher elevations) are probable, with localized
areas of focused activity. Lingering surface/upper low in the
Bering will promote showery weather early in the period before a
drier trend develops.
Above to much above normal temperatures should prevail over a
majority of the mainland during the period, especially north of
the Alaska Range. The North Slope and western portions of the
mainland should see the greatest positive anomalies ahead of the
upper trough/cold front dropping into the state from the northwest
late Monday. This will bring temperatures back toward more
seasonable levels but still above normal over the North Slope. The
Panhandle and parts of the far southeastern mainland are likely to
see below normal readings through the period.
Fracasso
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
coastal Southcentral Alaska, Sun-Mon, Oct 25-Oct 26.
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