Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
658 PM EST Mon Feb 06 2023
Valid 12Z Fri Feb 10 2023 - 12Z Tue Feb 14 2023
...Overview...
While the shift in guidance is not unanimous yet, there is some
degree of follow-through from yesterday's emerging trend toward a
faster Aleutians storm with less extreme depth as of the start of
the forecast period early Friday. Behind this system the guidance
is more in agreement about trending toward a deeper upper trough
over the western/southern mainland by next Sunday-Tuesday. There
has been somewhat of a signal for trailing low pressure to reach
over or near the Gulf of Alaska by Sunday-Monday and the trend
with the upper trough leads to improved larger-scale support for
such a low, while also bringing colder temperatures to the western
mainland. Farther west, guidance still shows potentially strong
low pressure emerging from the western Pacific and possibly
affecting the Aleutians early next week, but with a lot of
uncertainty in the latitude of the storm track..
...Model Guidance/Predictability Assessment...
For the late short-range northeastern Pacific system forecast to
reach the Panhandle by the start of the extended period early
Friday, the majority cluster has maintained good continuity with
minor adjustments from individual models. In today's case the 12Z
UKMET has gravitated more to consensus versus earlier runs that
were strong/northwest while the 12Z CMC has dropped a bit to the
south. A model average looks good overall.
The GFS/UKMET/CMC and partial support from the GEFS/CMC means
(some eastward elongation) lead to a majority cluster now showing
a more progressive and less deep initial Aleutians storm late this
week. The 00Z ECMWF was also a member of this cluster. The
00Z/12Z ECMWF means and the 12Z operational ECMWF still hold the
system farther back for a time, at least in the 12Z ECMWF due in
part to incorporation of upper dynamics immediately to the west.
There is a general signal for another wave to pass to the south of
the Aleutians around Saturday and it is possible that the ECMWF
means reflect some merging of members that hold the initial system
back and the second wave. The Friday-Saturday part of the
forecast still looks quite uncertain, but in one way or another
there appears to be some convergence among models/means favoring
low pressure gravitating toward the Gulf of Alaska region by
Sunday-Monday with the deeper western mainland upper trough
helping to enhance this signal versus a somewhat less prominent
variation of the theme yesterday. Preference today was to
emphasize the 12Z GFS/CMC/UKMET and 00Z ECMWF early in the period
to reflect a coherent evolution, with the understanding that some
ideas from the ECMWF means and 12Z ECMWF are still possible. The
forecast excluded the CMC after day 5 Saturday since that model
depicted a farther east northeastern Pacific wave than consensus
by Sunday, while a compromise among the models and ensemble means
seemed reasonable mid-late period.
The system emerging from the western Pacific and possibly
affecting the Aleutians by early next week has displayed much
better agreement for eastward timing than for latitude of the
storm track, and the ensemble means have differed among each other
for strength as well. Latest GFS runs have been on the southern
extreme, only reaching somewhat closer by around the end of the
forecast. On the other hand latest CMC/CMCens runs have
consistently been on the northern side of the spread. This leaves
recent ECMWF runs in the middle of the spread. Even with this
spread, operational models as a whole advertise a fairly strong
storm. Among the means, GEFS runs have been remarkably weak given
the signals from other guidance, while CMCens means have tended to
be strongest. A composite of recent ECMWF runs and ECMWF means
provides the best intermediate solution at this time for the
overall track and strength of this system.
As mentioned above, early preferences led to starting the forecast
with the 12Z GFS/UKMET/CMC and 00Z ECMWF for days 4-5
Friday-Saturday. The blend started to incorporate some 12Z
GEFS/00Z ECens means while phasing out the CMC/reducing UKMET
input by early day 6 Sunday. Improving consensus for low pressure
over the Gulf and western mainland upper troughing, along with
ECMWF/ECMWF mean preferences for the late-period storm affecting
the Aleutians, led to a days 7-8 Monday-Tuesday transition to 60
percent total 00Z/12Z ECMWF weight and the rest a 25/15 proportion
of the 00Z ECens/12Z GEFS means respectively.
...Weather/Hazard Highlights...
The most likely storm track during the period will tend to focus
the most active weather from the Aleutians through the southern
coast and Panhandle. The late week Aleutians system should spread
an initial area of precipitation and brisk winds along the Alaska
Peninsula and surrounding areas, with a combination of moisture
from this system and another North Pacific wave right behind it
most likely producing additional precipitation across the southern
coast and Panhandle during the weekend. This activity should
become noticeably lighter early next week. There may be some
areas of locally moderate to heavy precipitation, but currently
expect amounts at most locations to stay below hazardous
thresholds on a daily basis. Multi-day totals could be meaningful
though, especially over the Panhandle. While the track of the
next storm that could affect the Aleutians next Monday-Tuesday
remains uncertain, the majority of guidance suggests at least some
enhancement of winds and precipitation over the islands in that
time frame. The upper trough developing over the western/southern
mainland by early next week may produce some areas of light snow
in its vicinity.
Below normal temperatures over the north and west may moderate
briefly into the weekend (more so over the west than the North
Slope). Then the upper trough developing over the
western/southern mainland will likely lead to a return of below
normal readings early next week, with colder trends corresponding
to the deeper trough that is currently forecast. Eastern areas
may still hold onto some above normal readings early next week but
with a cooling trend and at least some pockets of below normal
temperatures developing.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle,
Thu, Feb 9.
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