Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
650 PM EST Fri Jan 15 2021
Valid 12Z Tue Jan 19 2021 - 12Z Sat Jan 23 2021
...Overview and Guidance/Uncertainty Assessment...
Today's guidance continues to show a pattern change toward lowest
heights aloft/deepest low pressure tending to be over the
northwestern Pacific/western Bering Sea while mean ridging aloft
becomes more prominent over the northeastern Pacific and Mainland
Alaska. As this new pattern becomes established there are still
likely to be progressive shortwaves that will bring waves/frontal
systems into portions of the mainland and Gulf of Alaska. However
these features should be weaker than some of the northeastern
Pacific systems in the recent past and into the short-range part
of the current forecast. Most models/means share similar ideas
for the general pattern but there are numerous detail
uncertainties to be resolved.
Already early in the period there has been considerable spread and
run-to-run variability for a compact but possibly vigorous wave
(with a narrow band of fairly strong winds ahead of it) expected
to be southwest of the Alaska Peninsula as of early Tue and track
northeastward thereafter. Solutions for track by early Wed have
varied from the southern coast to the Bering Strait, with some
possibility of merging with a wave along an initial Bering Sea
front. Most possible solutions are plausible relative to the
overall ensemble mean pattern, and the small scale of this feature
lends itself to low predictability until it reaches closer to the
valid time. Thus prefer a conservative blended approach that
allows for a wave at either extreme.
Strong low pressure expected to track into the western
Aleutians/Bering Sea mid-late week is a consistent feature but
guidance spread for track and evolution (consolidated versus
elongated) has not improved over the past day. If anything, the
modest weakening trend in some ensemble means over the past day or
so could suggest a slight widening of the envelope for the storm's
character. At the very least would favor leaning away from the
12Z GFS for this feature as it is on the fast/eastern side of the
full spread. The 06Z GFS was somewhat closer to remaining
guidance. Shortwave energy continuing eastward from this system
should push a leading front into the southwestern mainland by day
7 Fri. There remains potential for a wave along this front but
with only a modest signal in the guidance overall--in contrast to
the stronger 12Z GFS. Then the majority cluster (recent GFS/ECMWF
runs and GEFS/ECMWF/CMC means) has the shortwave energy cutting
through the mean ridge with a corresponding surface low in the
Gulf by early Sat. The full guidance average lies between the 12Z
ECMWF and faster 00Z ECMWF run, with the new 12Z ECMWF mean
providing confirmation of this intermediate timing. Meanwhile
there is decent agreement on another system coming into the
picture over the western Aleutians/Bering by the start of next
weekend. In contrast to the first Bering system, the 12Z GFS
compared better to consensus versus the fast 06Z GFS.
Based on considerations for each system, today's blend
incorporated the 12Z operational models early and then trended
toward somewhat more ensemble mean focus (plus splitting ECMWF
input between both recent runs) late in the period. Although not
a large weight in the blend to begin with, varying GFS ideas in
different runs led to 12Z run inclusion early, transitioning to
the 06Z run during the middle of the period, followed by a return
to the 12Z run late.
...Weather/Hazards Highlights...
From Tue into Wed expect a wave/frontal system to bring moderate
precipitation into parts of the Alaska Peninsula as well as the
southwestern mainland/along the southern coast. There may be a
band of brisk to strong winds just ahead of the surface wave. Low
pressure tracking into the western Bering Sea mid-late week and
its leading front will bring enhanced wind/precipitation into the
western and central Aleutians by Wed. The moisture, and possibly
still brisk winds over some areas, should extend into the
western/southern mainland by late in the week with some localized
enhancement of precip possible along and near the Alaska
Peninsula. Some of the moisture may reach as far eastward as the
Panhandle by the start of next weekend. Next system may bring
another episode of wind/precipitation to the western Aleutians by
next Fri-Sat. Above to much above normal temperatures should
prevail over most areas through the period. Highest anomalies
will likely be over northern/eastern areas to start the period on
Tue followed by a shift into the eastern half of the mainland
later in the week. Meanwhile the Panhandle may see enough of a
cooling trend to bring readings into the moderately below normal
range over southern locations by late week.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Mon, Jan 18.
- High winds across portions of the Alaska Panhandle and mainland
Alaska, Mon, Jan 18.
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