Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
634 PM EST Mon Dec 14 2020
Valid 12Z Fri Dec 18 2020 - 12Z Tue Dec 22 2020
...Guidance and Uncertainty Assessment...
Models and ensemble means are consistent in showing a transition
of the pattern from one of complexity and various uncertain
relatively small-scale embedded features toward increasing
dominance of a deep and large-scale storm tracking into the
Aleutians/southern Bering along with a downstream upper ridge that
eventually builds into the mainland.
From late week through the weekend, the details at the surface and
aloft continue to have only moderate confidence albeit with some
prevailing themes. Within an axis of relatively low heights aloft
from the Bering Sea through the mainland and northern Canada there
is a general consensus that energy from the Canadian Archipelago
should retrograde into/across the mainland around a gradually
retreating Arctic upper ridge/high. Guidance has varied among
each other and consecutive runs for specifics of this evolution.
Meanwhile the details of surface lows over the Northeast
Pacific/Gulf of Alaska as well as the Bering will depend on
specifics of low-predictability shortwaves carried in progressive
North Pacific flow as well as possible interactions with lingering
energy within the Bering-mainland weakness aloft. Over the Gulf
and vicinity the most common idea is for one system to reach the
area by the start of the forecast early Fri, followed by arrival
of another wave about 24 hours or so later (may interact with
initial Gulf low pressure). Then low pressure over the
Gulf/southern coast should persist but steadily weaken into early
next week. A 12Z operational model blend represented these
features well and with reasonable support from the ensemble means.
The models show more detail for a defined low that may be over
the southeastern Bering as of early Fri.
In principle recent guidance has been fairly consistent and
agreeable for strong low pressure tracking into the Aleutians
during Sun-Tue. As might be expected the operational runs are
showing more signals for potential wave development along the
front extending out from the parent low, with some runs including
the past couple cycles of the ECMWF suggesting that the leading
wave development reaching the southeastern Bering near the end of
the period could end up producing a deeper low than the initial
one. Although some 12Z CMC specifics on the leading side of the
system become a bit of a question mark, in general the past couple
days or so of ECMWF/CMC runs have seemed to offer somewhat more
coherent evolutions overall than a number of GFS runs. The
typically more consolidated GEFS/ECMWF means have been similar and
stable for track/timing over the past day after adjusting somewhat
faster beforehand, adding to confidence. Guidance
behavior/comparison thus far favored adjusting the late-period
forecast toward highest weight of the past two ECMWF runs,
followed by 12Z CMC input and then lesser weight of the 12Z
GFS/12Z GEFS mean/00Z ECMWF mean. This blend with minor manual
enhancement yielded a somewhat deeper system relative to the means
while also accounting for detail uncertainty. Farther east this
solution represented consensus for the upper ridge that builds
into the mainland and displaces the initial east-west weakness
northward.
...Weather/Hazards Highlights...
Specifics are still uncertain for individual surface lows over and
near the Gulf of Alaska but the general pattern continues to
support the potential for meaningful precipitation over the
Panhandle and southern coast of the mainland from late this week
into the weekend. Some moderate to heavy totals are possible.
Meanwhile most of the mainland will likely see dry conditions or
at most light/scattered precip during this time frame. The deep
storm forecast to track into/along the Aleutians Sun-Tue will
bring a broad area of enhanced precipitation and winds across the
Aleutians/Bering Sea, perhaps reaching into the southwestern half
of the mainland by next Tue.
The upper level energy expected to move into/across the mainland
from the east-northeast should result in a colder trend from late
this week into the weekend. By the weekend much of the mainland
will likely see below to much below normal temperatures,
especially for highs. The far southeast corner of the mainland
and parts of the Panhandle could remain a little above normal.
The below normal readings should persist into early next week but
then trend warmer by next Tue as an upper ridge builds into the
mainland ahead of the Aleutians storm.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Thu-Sat, Dec 17-Dec 19.
- High winds across portions of mainland Alaska, Thu, Dec 17.
- Much below normal temperatures across portions of mainland
Alaska, Thu-Mon, Dec 17-Dec 21.
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
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