Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
749 PM EDT Mon Oct 10 2022
Valid 12Z Fri Oct 14 2022 - 12Z Tue Oct 18 2022
...Series of lows will produce rounds of heavy precipitation for
South-Central to Southeast Alaska...
...Overview...
An upper low is forecast to slowly drift from near the Aleutians
Friday into the Alaska Peninsula during the weekend. Energy coming
in from upstream should reinforce troughing across the
Aleutians/AK Peninsula and farther north into the western mainland
for early next week, while ridging generally persists farther east
(after a bout of troughing aloft at the end of this week). This
pattern and a couple of surface low pressure systems will direct a
long fetch of Pacific moisture into southern parts of Alaska and
lead to potential for heavy precipitation.
...Guidance/Predictability Assessment...
12Z model guidance is in rather good agreement with the initial
position of the upper low for late week near the Aleutians to AK
Peninsula, with a bit more spread in the positioning of the
associated surface lows. Models also show some low amplitude
Arctic troughing Friday retreating for the weekend while ridging
builds into eastern Alaska. The 12Z CMC appeared a bit out of
phase with the latter, but overall, deterministic guidance showed
good consensus and thus a multi-model deterministic blend of the
GFS/ECMWF/CMC/UKMET was able to be used for the early part of the
medium range period.
By Monday uncertainty begins to grow considerably with the details
of additional upstream energy that should reinforce a low/trough
across western parts of the state, with lower confidence in terms
of the exact evolution. Also in play for the forecast is a surface
low tracking east through the Pacific over the weekend that could
tap into this energy and strengthen in the northeast Pacific into
the Gulf of Alaska Monday-Tuesday. Deterministic models as well as
many individual ensemble members indicate this low could be quite
strong (some ensemble members and the 12Z GFS showing in the 940s
mb south of Alaska), but there is notable spread and the timing
and position of the low, so confidence in the details is low. The
CMC in particular seemed to be a northern outlier for the surface
low position and strongest with the mid-upper energy. The WPC
forecast relatively quickly transitioned to a blend including the
GEFS and EC ensemble means by half day 7 and more day 8 given the
spread in solutions of the deterministic guidance. The isobars
around the surface low were enhanced some manually for the WPC
fronts/pressures forecast, but if confidence increases for the
track of the surface low, it will likely need a lower central
pressure in future forecasts.
...Weather/Hazards Highlights...
Precipitation may continue across the AK Panhandle into Friday,
lingering from a strong short range system. Meanwhile an
upper/surface low is forecast to bring some precipitation to the
Aleutians and AK Peninsula on Friday, and precipitation amounts
should increase as the low moves slowly east over the weekend as
it taps into deeper Pacific moisture as the trough/ridge couplet
strengthens. Heavy precipitation is likely for Southcentral Alaska
eastward into especially northern parts of the Panhandle for the
weekend, enhanced by topography. Precipitation types are rather
uncertain at many locations and could be changing throughout the
event, but with a tendency toward more wintry precipitation
inland. Gusty winds may be a concern, but the surface low is not
forecast to be significantly strong enough to cause widespread
issues. Lighter snows also may be possible farther north into the
mainland.
Then as the next low may approach the Gulf of Alaska by around
Tuesday, southern Alaska could again see chances for heavy
precipitation, as well as potential for high winds if the low is
as strong as some guidance shows. Again the details are uncertain
at this point and will hopefully come into better agreement with
time.
Temperatures across the Brooks Range farther south into southern
parts of the mainland are forecast to be colder than normal under
the influence of the initial trough there. But as that retreats
and ridging builds for eastern Alaska, and Pacific moisture
increases across the state, temperatures are forecast to warm to
generally above average into early next week. Lows of 10-20F above
average should be common, with lesser anomalies for highs.
Tate
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
Hazards:
- Heavy precipitation across portions of coastal and nearby high
terrain of southern mainland Alaska into the Alaska Panhandle,
Sat-Sun, Oct 15-Oct 16.
- Heavy rain across portions of the Alaska Panhandle, Thu, Oct 13.
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