Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
601 PM EST Tue Dec 03 2019
Valid 12Z Sat Dec 07 2019 - 12Z Wed Dec 11 2019
...Deep Lows lend a Maritime Hazard and Southern AK Tier Heavy
Precipitation Threat...
...Guidance/Uncertainty Assessment...
The 12 UTC GFS/ECMWF and 12 UTC GEFS/NAEFS/ECMWF ensembles are
reasonably well clustered Saturday-Sunday and seem to offer above
normal predictability and continuity for a stormy Alaskan weather
period. A blended approach (with more weighting on the
deterministics) provides a good forecast starting point.
Deterministic model forecast spread and uncertainty increases
rapidly by next week, particularly regarding
timing/placement/intensity of various surface low developments
towards the Aleutians/Gulf of Alaska early next week. This lends
to a transition towards more weighting on the better clustered
ensemble means with smaller contributions from the deterministic
runs just for some definition.
...Weather/Hazard highlights...
Mid-upper ridging from northeast Russia to the Arctic will favor
deep mean closed Bering Sea troughing. This will support
development and reformation of several deep and stormy surface
systems over the southern Bering Sea through the weekend. Slow
movement favors locally heavy rain/snow downstream whose forcing
mechanisms should prove slow to progress eastward, but eventually
work across the Gulf of AK to the Southeast Panhandle early next
week with pattern transition. Meanwhile overtop in the interior,
high pressure will slowly shift into Canada this weekend.
Deep lows offshore will remain a significant maritime hazard to
include high winds and waves. Multiple periods of heavy
precipitation will also work onshore and focus over the
AKPEN/Southwest AK heading into the weekend and then expand across
Southcentral then Southeast AK where it could be locally heavy
(coastal rain, inland/terrain snow). This transition occurs into
next week as triple point developments focus near Kodiak Island.
Ensemble QPF guidance highlights much of the south side of the
AKPEN to coastal regions of the Prince William Sound (including
Kodiak Island) where 5-10" (or more) of liquid-equivalent QPF is
depicted. This activity will be fueled by long fetch and slow to
translate plumes of deep lower latitude originated moisture.
Santorelli
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
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Sat-Sun, Dec 7-Dec 8.
- High winds across portions of the Alaska Panhandle and mainland
Alaska, Sat-Sun, Dec 7-Dec 8.
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