Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
650 PM EST Wed Dec 18 2019
Valid 12Z Sun Dec 22 2019 - 12Z Thu Dec 26 2019
...Below average temperatures expected across parts of mainland
Alaska into next week...
...Two potentially strong storms may track near the Aleutians,
bringing precipitation and strong winds...
...Wet pattern along the southeastern coast/Panhandle...
Today's models and ensemble means display a number of similar
themes but there are ongoing uncertainties for important
specifics. One aspect of relative convergence compared to
yesterday is with the development and persistence of mean
troughing aloft especially over the northern and western
mainland--initiated by an upper ridge that migrates from the
northern Bering across eastern Siberia from late weekend into
early next week. 24 hours ago operational guidance had a better
signal for this evolution than the ensemble means but since then
the means have trended toward more mainland troughing. Farther
south the guidance continues to show a very active North
Pacific/Aleutians stream that should include two vigorous
wind/precipitation-producing systems. Also this pattern will
likely support mean low pressure over the Gulf of Alaska and a
persistent focus for precipitation along the southeastern
coast/Panhandle.
For the system likely to be over the southern Bering at the start
of the forecast early Sun, guidance remains divided over the
ultimate track of the parent low. Current guidance majority
including the 12Z GFS/CMC/UKMET/NAVGEM and latest GEFS/ECMWF means
would have the initial surface low linger over the Bering at least
into day 5 Mon versus the past couple ECMWF runs that whisk it
into the northeastern Pacific. Some GFS runs keep the parent low
over the Bering longer than consensus though. Based on history
this area is notorious for such sensitivity, favoring a
consensus/continuity approach until guidance trends emphatically
suggest otherwise. Strength has been a less contentious issue,
with the majority of guidance still leaning a little weaker than
GFS runs.
The trailing system that may affect the Aleutians around
Tue-Wed/Christmas Eve-Christmas Day still has a decent signal in
the guidance but exhibits considerable spread for
strength/track/timing. The most basic question mark comes down to
whether and for what duration the upper dynamics will be a mere
open wave or close off an embedded upper low. Considering this
energy originates from fast/flat flow emerging over the western
Pacific just a day or so before development, this seems to be a
detail too small in scale to have much predictability several days
out in time. Therefore the ultimate preference for the
deterministic forecast is a model/ensemble compromise--more GEFS
mean due to a deeper low than the ECMWF mean into day 7 and
inclusion of enough operational input (but not the very slow 12Z
CMC) to enhance the means though not to the extent of relying too
much on any one model scenario. By day 8 Thu the GEFS mean may be
too slow to transfer energy into the Gulf of Alaska though.
Behind this system the models/means depict another general area of
low pressure, perhaps consisting of multiple centers, that will
more likely track to the south of the Aleutians after midweek. A
general guidance blend provides a reasonable starting point given
low confidence in specifics that many days out in time.
Across the northeastern Pacific/Gulf of Alaska, solutions agree on
one system just off the southeastern coast/Panhandle early Sun.
Then a wave along the first Bering system's front should reach the
Gulf early next week. Model/ensemble consensus recommends a low
track somewhat north of the past couple ECMWF runs, though at
least the 12Z ECMWF did trend favorably from the 00Z version.
Whether from persistence or re-development (including some support
from energy within the mainland upper trough) the guidance
suggests northern Gulf low pressure extending into Wed/Christmas
Day. Then energy associated with the second Aleutians storm
should arrive by day 8 Thu with another well-defined surface low.
Perhaps reflecting slightly greater predictability around this
time, the past two ECMWF runs trend closer together over the Gulf
after showing very different low tracks/depth upstream.
As noted earlier the guidance shows much better overall agreement
today for the upper trough over the mainland. It remains to be
seen whether some of the embedded energy will extend as far
westward into the Bering as some GFS runs and to some extent the
12Z GEFS mean suggest--or whether energy in the southern mainland
will be as deep as in ECMWF runs. Given modest spread in
location, a consensus approach still looks good for positioning
the upper low over/near the Canadian Archipelago. Flow around
this circulation should support a couple cold fronts that will
brush the extreme northern mainland. Expect the pattern evolution
to yield a westward shift in the axis of below normal temperatures
compared to the start of the period, while anomalies may become a
little less extreme with time.
Based on the composite of forecast preferences, today's blend
started with components of the 12Z GFS/ECMWF/CMC early in the
period. This yielded an evolution close to the ensemble
means/continuity in principle while maintaining better
detail/strength. The forecast then adjusted to a 12Z GFS/GEFS
mean and 12Z-00Z ECMWF compromise for days 6-7 followed by
introduction of the 00Z ECMWF mean from late day 7 onward. Total
ensemble mean weight reached 50 percent by day 8.
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
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Mon, Dec 23.
- High winds across portions of the Aleutians, Sun-Mon, Dec 22-Dec
23.
- Much below normal temperatures across portions of mainland
Alaska, Sat-Mon, Dec 21-Dec 23.
- High significant wave heights for coastal portions of mainland
Alaska and the Aleutians, Mon-Tue, Dec 23-Dec 24.
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