Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
630 PM EST Sat Jan 02 2021
Valid 12Z Wed Jan 06 2021 - 12Z Sun Jan 10 2021
...Northeastern Pacific/Gulf of Alaska and Aleutians storms to
bring potential for significant precipitation/strong winds...
...Overview and Guidance/Uncertainty Assessment...
Guidance continues to advertise a stormy period, with one area of
focus over the northeastern Pacific and vicinity while a deep
system expected to track along the Aleutians from late Fri into
the weekend becomes an increasingly dominant feature late in the
forecast. Relative predictability/confidence of specifics for
these two areas are fairly similar to recent days with the
Northeast Pacific region significantly lower than for the
Aleutians system.
For the Northeast Pacific, forecast preferences incorporated
operational guidance that best matched the general pattern of the
ensemble means which among themselves offered good clustering in
light of operational model spread and run-to-run variability.
This led to using the 12Z/00Z ECMWF and 12Z/06Z GFS (slightly more
weight to the 12Z runs early-mid period) as the primary
input--leading to a somewhat stronger version of the means. In
light of the differences among even the GFS/ECMWF runs for the
initial storm near Kodiak Island, plus greater differences in the
CMC/UKMET, confidence is still not especially great. The forecast
appears to be quite sensitive to the exact details of flow aloft
between a mean trough extending from the northern/eastern Bering
into the northeastern Pacific/Gulf and a western Canada ridge that
could extend its influence into the mainland at times. Thus it is
yet to be determined whether the Kodiak Island system may linger
in that vicinity for a time (majority consensus) or perhaps lift
northward but still leave behind residual low pressure in that
area. During next weekend the ensemble means generally side with
an average of the past two ECMWF runs for the general evolution of
another northeastern Pacific system, with faster northwestward
motion than seen in the 12Z GFS. At least in terms of timing the
06Z GFS was closer in principle but was a bit northward.
Thus far the models and ensembles have been fairly consistent and
agreeable for the late Fri into weekend Aleutians system, though
some east-west spread does develop with time. Recent ensemble
mean trends add support for depth closer to the operational
models, possibly into the 950's mb by early Sat--quite strong
though not as extreme as its recent predecessor. Thus the
operational model focus in the blend extended into day 7 Sat with
only 20 percent total input from the GEFS/ECMWF means. Ensemble
mean input increased somewhat for day 8 Sun in light of increasing
model divergence for the system farther east.
...Weather/Hazards Highlights...
One or more northeastern Pacific/Gulf systems should bring a
multi-day period of occasionally moderate to heavy precipitation
to areas along the southern coast and Panhandle. Best confidence
in highest five-day totals extends from near the Kenai Peninsula
eastward. Locations farther west will be more sensitive to the
exact track of low pressure centers. These systems may produce
one or more episodes of brisk to strong winds as well. Meanwhile
strengthening winds and increasing precipitation should reach into
the western Aleutians around Fri and then extend across the rest
of the islands during the weekend as the associated storm
progresses eastward. The upper level pattern will favor
establishment of above to well above normal temperatures over most
areas during the period. Locations over the extreme northern and
western mainland will likely be the last to see above normal
readings. The above normal anomalies for min temps should
generally be greater than those for max temps.
Rausch
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