Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
625 PM EST Tue Nov 21 2023
Valid 12Z Sat Nov 25 2023 - 12Z Wed Nov 29 2023
...Heavy precipitation continues late week for Southcentral and
pivots into Southeast Alaska early next week...
...High winds possible for the Aleutians to St. Lawrence Island
and the Seward Peninsula this weekend...
...Overview...
As the medium range period begins late week, surface and upper
lows will be stacked near the Alaska Peninsula, though weaker than
their depth in the short range. This will still direct enhanced
Pacific moisture with deep southerly flow into Southcentral Alaska
with heavy precipitation lasting particularly into Friday. Rounds
of additional precipitation are likely over the weekend with a
general eastward trend toward Southeast Alaska into early next
week as the energy shifts east. Farther west, a large upper
trough/low looks to track over the Kamchatka Peninsula with some
troughing spreading east into the Bering Sea early next week. A
surface low reflection that stays well west of Alaska will
nevertheless push some windy conditions and precipitation with a
frontal passage across western parts of Alaska including the
Aleutians, St. Lawrence Island, and the Seward Peninsula over the
weekend. Meanwhile, mean upper ridging looks to persist across
eastern parts of the state through much of the period, though
periodically could be suppressed by shortwaves moving through.
...Guidance Evaluation and Preferences...
The 12Z model guidance suite is reasonably agreeable with the
pattern described above on the large scale, with typical
differences in the details. Just minor differences were seen
regarding the narrow trough and a couple surface lows tracking
across the Alaska Peninsula into the Gulf over the weekend,
including a slightly faster CMC/CMC mean trough axis, but a
multi-model deterministic blend handled it well. Regarding the
large upper trough/low over Kamchatka and stretching to the Bering
Sea, there is high confidence in this feature emerging initially
but questions arise with time and as it elongates east. The first
model that seemed to become an outlier was the 12Z UKMET, which
dove strong upper/surface lows into the southern Bering Sea
nearing the Aleutians by Sunday, much farther southeast than other
models, so this was not favored. Other guidance also shows
increasing spread as well into early next week though. GFS runs,
particularly the 12Z run, drift the centroid of the upper low
within the trough back west of Kamchatka, where additional
possible energy upstream may come into play. The 12Z GEFS mean is
pretty elongated from west of Kamchatka across it and into the
Bering, indicating spread in its members, but the ECMWF and CMC
and EC mean seem to be better clustered in showing a low center
over the Bering. So leaned toward the latter cluster of the
non-NCEP guidance that was more agreeable. Then on the eastern
periphery of the upper trough, periodic shortwaves and surface
lows could track near and across Alaska. These seem to become a
dominant part of the Alaska pattern by next Tuesday-Wednesday and
since these smaller features have lower certainty, confidence for
the resulting forecast decreases. Given these considerations, the
WPC forecast started with a blend of the 06/12Z GFS and the 12Z
ECMWF and CMC, gradually incorporating and increasing the
proportion of the ensemble means to about half the blend by Day 7
and over half by Day 8 amid the increasing uncertainty, while
reducing the proportion of the deterministic models (especially
the GFS).
...Weather/Hazards Highlights...
Ample moisture streaming in with southerly flow ahead of a low
tracking from the Aleutians to the Alaska Peninsula to the Gulf
will bring heavy precipitation lasting into at least Friday across
Southcentral Alaska. Coastal rain and higher elevation and inland
snow are forecast with this precipitation activity, with some
areas like Anchorage perhaps flipping between precipitation types.
The precipitation may weaken to more moderate levels into the
weekend there, but numerous showers should continue within a
generally unsettled pattern and onshore flow. The axis of highest
moisture shifts into Southeast Alaska by Sunday and will likely
provide heavy rain and snow to that region.
A frontal system is forecast to track across the Aleutians this
weekend, bringing some precipitation. Strong southerly flow ahead
of the cold front and winds with a westerly component behind the
front could both be gusty to strong especially on Saturday for the
Aleutians and shifting into St. Lawrence Island and the western
Mainland by Sunday, as the frontal system and energy aloft push
into those regions. Precipitation chances should also increase for
the Alaska Peninsula and the southwestern Mainland by early next
week and then focusing across Southcentral. Once again, overall
coastal rain and inland snow are expected, but with perhaps some
lower snow levels compared to late week given the troughing aloft.
Temperatures across the state should be considerably milder than
in the short range given the (relatively) warm southerly flow into
the state and the ridging aloft in the east. Lows look to be
around 20-40F above normal, staying above 0F except in a few
pockets of the North Slope and Interior Saturday, and generally in
the teens and 20s across the Interior and in the 30s to low 40s in
coastal areas farther south. Highs should be closer to 10-30F
above normal, with actual temperatures mainly in the teens even
for the North Slope, with increasing values going south into the
20s/30s/40s.
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
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