Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
626 PM EST Sat Nov 23 2019
Valid 12Z Wed Nov 27 2019 - 12Z Sun Dec 01 2019
The majority of today's guidance agrees fairly well for the large
scale pattern evolution but with some important detail
uncertainties yet to be resolved. The forecast also reflects a
combination of continuity and some adjustments from yesterday. A
strong and impactful short range storm crossing the
western/northern Bering Sea will track across the Arctic from day
4 Wed onward, with latest model runs noticeably farther northward.
The next storm in the series is most likely to track over or near
the western Bering Sea late week into the weekend--with a leading
front that may contain one or more waves whose development and
track are very uncertain. Farther east expect a wave to travel
near the Alaska Peninsula/Kodiak Island into the northern Gulf of
Alaska during the mid-late week time frame. Along with the
changes in track for the Arctic system, the other most notable
continuity change involves the mid-latitude Pacific system well
south of the Peninsula. Best consensus now shows this system
remaining more separated from flow to the north, keeping it well
south of the northeastern Pacific/Gulf.
As for some of the forecast specifics, there is decent agreement
on existence of the mid-late week wave that may track into the
northern Gulf. However most guidance suggests that the wave will
be somewhat better defined and make more eastward progress than
latest GFS runs. Leaning more in the direction of the majority,
the potentially significant precipitation with this feature should
taper off over the Peninsula/Kodiak Island earlier than the GFS
and extend farther eastward. The 12Z CMC is suspect in its
amplitude of shortwave energy aloft crossing the Gulf though.
Meanwhile an average of ECMWF/GFS runs and their means will
reflect the most likely scenario for the system tracking near the
western coast of the Bering Sea. The 12Z CMC is farther south
than this cluster. Individual model runs and ensemble members
offer a variety of possibilities for one or more waves that may
develop along the front anchored by the primary low. The best
signal for any particular wave exists from the mid-latitude
Pacific just west of 180 degrees longitude early day 7 Sat and
then northeastward from there, as upper troughing moves
into/across the western Pacific. Model runs still vary
considerably on this possible late-period wave (but within a
fairly broad ensemble envelope), favoring a model/ensemble mean
blend as the best way to reflect some wave potential but without
going as deep as the 12Z GFS and much deeper 00Z ECMWF. On the
other hand the 12Z ECMWF switched to a weak/suppressed wave.
Based on today's forecast considerations the initial blend used
input from the 00Z-12Z ECMWF and 12Z GFS/UKMET early in the period
followed by a gradual increase of the 12Z GEFS/00Z ECMWF means so
that total ensemble weight reached 60 percent by day 8 Sun. Cold
frontal progression across the mainland Wed-Thu will bring a drier
trend from west to east while the wave near the southern coast
should produce a period of focused precipitation in its vicinity.
The wavy frontal system approaching from the west will likely
bring precipitation first across the Aleutians and then into
portions of the mainland--but with considerable uncertainty over
coverage as well as intensity at any particular location.
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