Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
623 PM EDT Wed Oct 27 2021
Valid 12Z Sun Oct 31 2021 - 12Z Thu Nov 4 2021
...Overview and Model Guidance...
The medium range period will begin on Sunday with a rather
pronounced upper ridge over western Canada and an elongated trough
axis extending from the Bering to the northern Gulf. The ridge
axis is expected to hold firm through mid-week over eastern Alaska
and into Canada, whilst a large upper low with multiple shortwave
perturbations takes up residence over the Bering and the
Aleutians. The potential exists for another organized storm
system to develop over the Gulf by the end of the forecast period.
Model agreement is greatest with respect to the anomalous upper
ridge near the west coast of Canada and extending northward to the
Arctic through mid-week. However, differences in the guidance are
apparent as early as this weekend across the western Bering
regarding the broad upper low evolution. The 12Z ECMWF/CMC are
out of phase with a broad ridge over this region in contrast to
the UKMET/GFS/GEFS mean with more of a trough. These differences
continue into early next week with more in the way of shortwave
energy lifting north across western mainland Alaska. Elsewhere
across the forecast domain, model differences appear less
significant.
The WPC forecast was primarily derived from a GFS/ECMWF/UKMET
blend along with some previous WPC continuity, and this included
some use of the ensemble means early in the forecast period owing
to the differences noted above.
...Weather/Hazard Highlights...
Heavy precipitation is expected for southern portions of mainland
Alaska for Sunday and into Monday as a steady surge of moisture
advects northward from the northeast Pacific, although the
intensity of the atmospheric river event is expected to wane some
by late on Sunday with the heaviest rainfall likely on Saturday.
The Kenai Peninsula and the coastal mountain ranges as far east as
the Yukon border appear to have the greatest prospects for
excessive rainfall that could result in flooding and run-off
concerns, and very heavy snowfall for the higher elevations. Some
strong southerly winds are also expected in those areas where
gusts could exceed 40 mph. Precipitation should continue through
the middle of next week, albeit abating some in intensity.
Farther north, light snow is likely to spread across much of the
western and central parts of the state with the moist inflow from
the low pressure system.
In terms of temperatures, most locations across central and
eastern portions of the mainland should enjoy above normal
readings next week, with some areas up to 20 degrees above normal.
The upper trough over the Bering will tend to keep readings
closer to normal or perhaps slightly below for the western portion
of the mainland, and this also holds true for the Aleutians.
Hamrick
Hazards:
- Heavy precipitation across portions of southern mainland Alaska,
Sat-Mon, Oct 30-Nov 1.
- High winds across portions of southern mainland Alaska and the
Aleutians, Sat, Oct 30.
- High winds across portions of southern mainland Alaska, Sat-Sun,
Oct 30-Oct 31.
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