Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
559 PM EST Fri Dec 20 2019
Valid 12Z Tue Dec 24 2019 - 12Z Sat Dec 28 2019
...Below average temperatures expected across much of mainland
Alaska through next week...
...A couple of potentially strong storms may track near the
Aleutians, bringing precipitation and gusty winds...
...Wet pattern along the southeastern coast/Panhandle...
The medium range period (Tues/Dec 24 - Sat/Dec 28) begins with
persistent mean troughing over mainland Alaska with a surface low
centered over the southern coast region and shortwave energy
moving east across the Gulf/northeast Pacific. Guidance shows good
agreement that the upper trough over the mainland should persist
through the period, promoting an extended period of below normal
temperatures the core of which should shift slowly westward with
time next week. Meanwhile, energy rotating around a deep closed
low over the Arctic should send a couple of cold front into the
northern slope region through the period.
To the south, the guidance continues to show a very active North
Pacific/Aleutians stream that should include a series of
shortwaves/surface lows traversing near or south of the Aleutians.
Differences begin as early as day 4 with the first low moving near
the Aleutians Tues/Wed. The 12z GFS and CMC are quite a bit south
with this low compared to the ECMWF, with the means supporting a
solution closer to the ECMWF. As the first low exits into the Gulf
on Thursday, another low should again pass south of the Aleutians,
with yet another low approaching the western Aleutians by day
8/Sat. It's with this second low that differences are quite
significant with the 12z ECMWF bringing a rather deep low close to
the islands next weekend, while the GFS is displaced several
hundred miles to the southwest. The ensemble means would generally
support their operational counterparts, though spread is not quite
as great. This pattern supports renewed low pressure across the
Gulf of Alaska and persistent focus for precipitation along the
southeastern coast/Panhandle, some of which could be locally heavy
at times. Depending on proximity to the Aleutians and depth of the
various passing surface lows, a period of precipitation and gusty
winds is possible across much of the island chain.
Given the uncertainties described above, preferred a general blend
of the ECMWF with the ensemble means, with significant weighting
towards the means by days 7 and 8. This fits well with the
preference downstream over the CONUS as well.
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, Mon, Dec 23.
- High winds across portions of the Aleutians, Mon, Dec 23.
- High significant wave heights for coastal portions of mainland
Alaska and the Aleutians, Mon, Dec 23.
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