Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
656 PM EST Sat Jan 15 2022
Valid 12Z Wed Jan 19 2022 - 12Z Sun Jan 23 2022
...Heavy precipitation threat along the southern coast and
Panhandle late week into the weekend...
...Overview...
Latest guidance still shows a mean ridge aloft over the far
eastern Pacific extending its influence farther north/northwest
into Mainland Alaska and northwestern Canada by late week into the
weekend, leading to period of warmer and wetter weather.
Consensus shows the best potential for heavy precipitation will be
along parts of the southern coast and Panhandle from Friday into
the weekend. Meanwhile the North Pacific will be active during
the period. One wave is likely to track into the Gulf of Alaska
by Thursday followed by another system that may affect portions of
the Aleutians and Alaska Peninsula/far southwest mainland and then
yet another storm that comes into the picture south of the
Aleutians next weekend. There is still some uncertainty over what
becomes of an elongated upper trough over the northwestern
mainland as of early Wednesday but there are some hints at gradual
convergence among latest solutions.
...Guidance/Uncertainty Assessment...
For the evolution over the mainland early in the period, it is
still a story of the operational models versus the ensemble means.
Models continue to form a compact closed low from the energy
within the early Wednesday elongated upper trough with slightly
delayed influence from the upper ridge to the southeast while the
means still depict a mere weak trough with stronger ridging to the
southeast. Today's 12Z cycle provided a notable westward
adjustment in the ECMWF that had been on the southeast side of the
spread with its upper low (keeping it over the mainland for a
longer time). By Thursday-Friday the latest consensus has yielded
a moderate southwestward adjustment from yesterday's preference
for the upper low position but maintains the theme of having the
upper low retrograde into the Bering Sea by late in the
week/weekend. During the latter half of the period some of the
model/mean differences persist with the 12Z GFS/ECMWF keeping the
upper low east of the means. The new 18Z GFS has trended westward
though. Forecast preferences still reflect the better detail of
the models early in the period followed by a transition to a
model/mean compromise.
Various detail differences persist for North Pacific systems. The
model blend for the leading wave expected to reach the vicinity of
the Gulf of Alaska by Thursday provides good continuity in
principle (with typically improving definition getting closer to
the valid time) and fits within the pattern of the means. There
are small-scale details that lead to relatively lower
predictability so some changes in specifics may be possible for a
while. Differences with the stronger system farther west derive
in part from the issues noted between the models and means farther
north. In general the models suggest a triple point wave may
track toward the Alaska Peninsula by Friday-Saturday while the
parent low remains farther southwest, in contrast to the means
that show the parent low reaching farther north near the eastern
Aleutians. The time frame of interest is during the transition
between model emphasis and model-mean compromise, with the end
result somewhat closer to the model cluster. The next system
comes into the picture next weekend. Thus far some GFS runs have
been on the deep and northwestern side of the spread while it
remains to be seen if it merges into the overall mean low pressure
the way CMCens/ECens means depict. A model/mean blend provides a
reasonable starting point given the various uncertainties.
Guidance preferences led to starting the forecast with the 12Z
GFS/ECWMF/UKMET/CMC from Wednesday into early Thursday, followed
by gradually increasing 12Z GEFS/00Z ECens inclusion so that the
means had 50-60 percent weight by days 7-8 Saturday-Sunday.
...Weather/Hazard Highlights...
The leading Pacific system tracking into the Gulf of Alaska by
Thursday should produce an initial episode of precipitation,
mainly from the eastern Kenai Peninsula through the Panhandle.
Some activity could be locally moderate to heavy. Then guidance
shows a persistent and perhaps increasing signal for heavier
precipitation from the southern coast into Panhandle from Friday
into the weekend as strengthening southerly winds over the
northeastern Pacific ahead of trailing North Pacific low pressure
bring in more moisture. Most solutions suggest the highest
precipitable water values will be around Friday with latest
GFS/ECMWF runs reaching at least three standard deviations above
normal over some locations and the GEFS/ECens somewhat lower as is
typically the case. To a lesser extreme the moisture anomalies
generally remain elevated through the weekend. Mean flow between
the upper low expected to be over the Bering Sea and the ridge to
the east/southeast may also bring some moisture farther north into
the mainland. Early in the period the combination of the upper
trough/low over the northwestern mainland and a front to the
southeast may produce some light snow over the Interior. The
Aleutians may see periods of unsettled weather though with
uncertainty in effects from individual systems.
During mid-late week the North Slope and parts of the far western
mainland will tend to see below normal temperatures while above
normal readings should be more prevalent elsewhere. Temperatures
may rise slightly during this time frame. As the upper ridge
becomes more established over the mainland and western Canada,
expect a more pronounced warming trend next weekend with
increasing coverage and magnitude of positive anomalies. Warmest
readings versus normal should be over Southcentral. In most cases
the anomalies for min temperatures will be warmer than those for
highs.
Rausch
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
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