Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
656 PM EST Wed Dec 15 2021
Valid 12Z Sun Dec 19 2021 - 12Z Thu Dec 23 2021
...Likely precipitation with significant amounts and strong
southwesterly winds over the western mainland on Sunday and
Monday...
...Overview...
A significant pattern change is still expected over Alaska early
next week. After periods of shortwave troughing moving across the
state during the short range period, deep layer ridging from the
surface to upper levels will set up across the Pacific, with its
northern periphery spreading northeastward and pressing across the
mainland Sunday-Monday. This pattern will lead to strong, moist,
and unseasonably warm southwesterly flow over much of the state
for the early part of next week, causing the potential for gusty
winds and heavy precipitation along with much milder temperatures.
A quick shortwave trough could disrupt the pattern on Tuesday
before another bout of ridging sets up for midweek.
...Guidance Evaluation and Preferences...
Guidance continues to have a good signal for the aforementioned
Pacific ridging coming into the state and directing southwesterly
flow initially to the Aleutians/western mainland and spreading
eastward. However, there are some model differences especially
west of the state with the details of low pressure systems. Latest
12Z guidance generally advertises two lows tracking northward
across eastern Siberia and into the Chukchi Sea early next week,
one behind the other taking generally the same track, and
consolidating later on Monday. There remain some slight timing and
position differences, but all guidance has the same general idea,
so went with a multi-model deterministic blend (favoring the
06Z/12Z GFS and 12Z ECMWF) for the early part of the medium range
period. NAEFS and ECMWF Ensemble Situational Awareness Tables
agree that water vapor transport values will be nearing their
climatological maxima for parts of the western/central/northern
mainland on Sunday and Monday.
By Tuesday and Wednesday, an upper low tracking to the north of
Alaska is forecast to send energy southward and create a shortwave
trough passing through the state cutting through the ridge axis
(with additional ridging behind it moving into western Alaska
early Wednesday). Model guidance remains somewhat inconsistent
with the position and especially with the strength of this
shortwave. The 12Z CMC did not line up as well with its shortwave
axis, with the GFS runs and ECMWF showing somewhat better
agreement--though GFS runs are notably stronger with closing off a
mid-upper low offshore of the Panhandle by early Wednesday
compared to the open ECMWF, along with some differences in timing
of strengthening of a surface low and with its position. Ensemble
mean guidance is of course open with its troughing but likely due
to model differences averaging out rather than a true signal. The
WPC model blend took a middle ground approach, transitioning the
CMC out of the blend but keeping both the GFS and ECMWF as
components, and ending up with percentages of 30 GFS/30 EC/20 GEFS
mean/20 EC mean in the blend by day 8. Overall model guidance is
reasonably well clustered for a high latitude medium range
forecast.
...Weather/Hazard Highlights...
Precipitation is likely for western and central parts of the
mainland on Sunday and Monday given the increasing deep layer
moist southwesterly flow into the state to the west of the Pacific
ridge. From the Brooks Range southward into parts of western and
central Alaska, snow may be heavy over terrain favored by strong
southwesterly winds on Sunday and Monday. Brisk low level winds
are expected, which along the coast may even lead to areas of
coastal flooding. Areas with a combination of snow and strong
winds could see blizzard conditions. Precipitation and winds
should trend lighter on Tuesday after the cold front passes,
though some could linger for the eastern part of the state
including the Panhandle. Another round of precipitation may affect
the Aleutians and southwestern Alaska around next
Wednesday-Thursday, but those details will have to emerge in
future forecast cycles.
With the relatively warm inflow for Alaska, the western half of
the mainland will see above normal temperatures Sunday, increasing
and spreading eastward on Monday and Tuesday. Temperatures could
be 20 to as much as 40 degrees above average, with some
temperatures rising above the freezing mark especially in the
southwest part of the state. The Panhandle will be the main area
with below normal temperatures through the first half of next
week, while parts of the southeastern mainland could be around
normal. By Wednesday temperatures should moderate closer to normal
for much of the state after a cold frontal passage.
Tate
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
Hazards:
- Heavy snow across portions of mainland Alaska, Sun, Dec 19.
- Heavy snow across portions of mainland Alaska, Sun-Mon, Dec
19-Dec 20.
- Heavy snow across portions of mainland Alaska, Mon, Dec 20.
- High winds across portions of mainland Alaska and the Aleutians,
Sat-Sun, Dec 18-Dec 19.
- High winds across portions of mainland Alaska, Sat-Mon, Dec
18-Dec 20.
- High winds across portions of mainland Alaska, Tue-Wed, Dec
21-Dec 22.
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