Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
752 PM EDT Thu Sep 9 2021
Valid 12Z Mon Sep 13 2021 - 12Z Fri Sep 17 2021
...Heavy rainfall likely early next week from the Peninsula region
to the Panhandle with accumulating snow possible in the higher
elevations...
...Pattern Overview and Model Guidance...
A strong upper level low is expected to be in place across
southwestern Alaska on Monday along with a sub 990mb surface low
and trailing front across the Gulf of Alaska. This low should
weaken and then a second upper trough/closed low moves in from the
Bering Sea by the middle to end of the week. Quasi-zonal flow
aloft should be in place across the North Pacific and this leads
to some model differences in terms of timing and magnitude. Of
note is the 12Z ECMWF that becomes considerably more amplified
over the Gulf by the end of next week compared to the CMC/GFS and
ensemble means. By Friday, the CMC is indicating more of a broad
upper ridge across the Gulf whereas the other guidance supports
lower heights aloft. Taking these factors into account, the WPC
fronts/pressures forecast was primarily derived from a
multi-deterministic blend through day 5, and then mainly
GFS/GEFS/ECENS/WPC continuity for days 6 and 7 and then mainly
ensemble means for day 8.
...Sensible Weather...
Much of southern Alaska is expected to be unsettled with multiple
rounds of moderate to locally heavy precipitation. The highest
totals are likely to extend from the eastern Alaska
Peninsula/southwestern mainland through the southern coast and
Panhandle, and this includes favored terrain over the southern
mainland. In addition, lowering snow levels may bring
accumulating snowfall to some of the higher mountains across
southern Alaska. Flow ahead of the upper low tracking from the
Bering Sea into the mainland should spread moisture northward
across the remainder of the mainland but most of this
precipitation should be lighter. Confidence remains lower than
desired for specifics of another system that could bring an
episode of rain and brisk winds across the Aleutians, Alaska
Peninsula, and nearby areas by the middle of the week. The most
likely scenario is for this system to track over the
Aleutians/southern Bering Sea and Alaska Peninsula.
Much of the southern mainland and Panhandle will see below normal
high temperatures through most of next week. Clouds and moisture
will maintain warmer anomalies for low temperatures over most
southern locations, with greater coverage of above normal
readings. Some areas in the southwest may still see modestly
below normal lows and coverage of negative anomalies may expand
somewhat northeastward.
Hamrick
Hazards:
- Heavy rain across portions of mainland Alaska, Sun, Sep 12.
- Heavy rain across portions of the Alaska Panhandle and mainland
Alaska, Mon, Sep 13.
- High winds across portions of mainland Alaska and the Aleutians,
Sun, Sep 12.
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