Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
603 PM EST Mon Dec 28 2020
Valid 12Z Fri Jan 01 2021 - 12Z Tue Jan 05 2021
...Deep system in the western Bering late this week with strong
winds and significant waves...
...Overview and Guidance/Uncertainty Assessment...
An extremely deep area of low pressure in the far western Bering
Sea Friday will split off its triple point low into the Gulf as
the parent low slowly decays. The upper pattern will favor
troughing in the Bering to remain into early next week with an
extension into the Gulf. Upper ridging over the Interior Friday
will slip northwestward into northeastern Russia to be replaced by
troughing from NW Canada, ultimately merging with troughing in the
eastern Bering. This keeps the storm track south of the Aleutians
after the lead deep Bering system, with an aim generally on the
Panhandle. Strong 200kt jet across the North Pacific later this
week will translate eastward and weaken, perhaps to be
reinvigorated next week off Japan.
The 12Z guidance was in relatively good agreement, though the
GFS/ECMWF clustered better with the 12Z GEFS and 00Z ECMWF
ensemble means for the Fri-Sun period. Utilized a deterministic
blend to start in order to maintain an appropriate depiction of
the Bering low (930s mb, which was in between the deterministic
models in the 920s to low 930s and the ensemble means in the 940s
to around 950mb). Progressive flow beneath the upper low will keep
that system and the next couple systems moving from west to east,
with less agreement through the period (though the GFS/ECMWF were,
perhaps by happenstance, closer than expected out to next Monday).
Incorporated more ensemble weighting by next week to tame down any
feature as cycle-to-cycle consistency has been lacking in the
progressive pattern.
To the north, guidance differs on how the bring in lower heights
across the Interior (or North Slope), but ensemble consensus seems
to take the bulk of height falls WSW toward the Seward Peninsula.
This should get caught up with the leftover Bering troughing to
move across Bristol Bay/AKPen but extending back along western
Alaska next week.
...Weather/Hazards Highlights...
Deep Bering system will bring at least a period of modest
rain/snowfall and strong winds (40-50kts+, especially over the
water) around the center. Significant waves are also possible
(over 40ft). Please consult the Ocean Prediction Center for more
information. Parent low will take several days to wind down,
maintaining scattered precipitation for the Aleutians/Bering
Sat-Tue. Triple point low detaching from the occlusion could
provide more focus for rain/snow as it moves into the Panhandle
late Fri into Sat. Trailing system may again bring more rain/snow
to the Panhandle late Sun into Mon. Locally heavier amounts are
possible.
Temperatures near to above normal over western areas of the
mainland Friday will turn colder this weekend into next week as
colder air filters in from the east. Readings may be 10-15 degrees
below normal north of the Alaska Range. For colder valleys this
could mean low near 30 below and highs only around 10-20 below.
For much of Southcentral into the Panhandle, near to above normal
temperatures are expected.
Fracasso
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,
Fri-Sat, Jan 1-Jan 2.
- High winds across western Aleutians, Thu-Fri, Dec 31-Jan 1.
- High significant wave heights for coastal portions of the
western Aleutians, Thu-Fri, Dec 31-Jan 1.
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