Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
757 PM EDT Wed Aug 30 2023
Valid 12Z Sun Sep 03 2023 - 12Z Thu Sep 07 2023
...Guidance Evaluation/Preferences...
Initial guidance preferences are tied to the handling of an
upper-level wave over the souther Gulf of Alaska/northern Pacific.
This wave has trended stronger over the past few days, and the
CMC/GFS are in better agreement with a slower eastward movement,
though the wave should remain to the south of the state and have
little sensible or hazardous weather impacts. The guidance has
then come into better agreement overall regarding the approach of
a couple systems upstream over the Arctic and Bering Sea. The GFS
has fallen more inline with the ECMWF and CMC in depicting the
Arctic system taking a more northern path while the wave over the
Bering Sea pushes east/southeastward over the
southwest/Southcentral and into the Gulf of Alaska. The GFS is
still a bit more aggressive in the strength of the Arctic system
and takes it on a more southerly path across the North Slope and
into the Yukon, with the spread in the guidance overall increasing
through this time frame. Predictability rapidly decreases in the
mid- to late period with significant spread in the guidance
regarding the next upper wave approaching upstream from eastern
Russia and tracking across the Bering Sea. The GFS develops a
deep, closed low that dives southeastward across the state and
into the Gulf of Alaska, while the upper wave stalls and weakens
over the Bering Sea as ridging amplifies a bit over western
portions of the state in the ECMWF. The upper wave is depicted to
a lesser extent in the ensemble means, with the GEFS faster and
the ECens similar in placement/timing to the ECMWF over the Bering
Sea, making any specific guidance preferences more difficult.
Regardless of the evolution of this wave southeastward in the GFS,
mean troughing looks to remain in place along the eastern Gulf of
Alaska and the Southeast.
The updated WPC forecast early in the period relies on a blend of
the GFS/ECMWF/CMC, with a preferential weighting to the CMC/GFS
initially given better agreement over the northern Pacific/Gulf of
Alaska before weighting more towards the ECMWF/GFS for the
forecast of systems upstream over the Arctic/Bering Sea. The blend
late period is a combination of the ECens and GEFS means given the
significant increase in spread in the deterministic guidance,
especially with regards to the closed low diving southeastward
through the state depicted in the GFS. A contribution from the
ECMWF is included to help weight the forecast a bit more towards
the discussed location/timing of troughing over the Bering Sea
given the disagreement between the ECens and GEFS means in this
vicinity.
...Weather/Hazards Highlights...
Following the departure of a strong system late this week
connected to the remnants of Damrey, the pattern then looks to
transition away from high amplitude as systems are excepted to be
embedded within much more progressive upstream flows both across
the Arctic and underneath out across the Bering Sea, with a trend
towards more uncertainty regarding their eventual evolution.
However, the signal remains consistent that energies from both of
these flows will continue the unsettled pattern late this weekend
into early next week. Rainfall will overspread the western
coast/Interior as well as into Southcentral, where a split of
upper level energies is expected to lead to a southern
wave/surface system moving southeastward into the Gulf of Alaska.
While conditions will be wet the trend in amounts in the guidance
has been downward over the last couple of days, with the
expectation that the progressive nature of the pattern will keep
rainfall duration more brief. To the north, concerns over high
stream flows sourced from the Brooks Range may be somewhat abated
by lowering snow levels and reduced runoff as well. The transition
between systems may also lead to some locally gusty gap winds
along the Alaska Peninsula. Precipitation chances will also likely
increase over the Southeast early next week as this southern
system continues to the southeast over the Gulf of Alaska. The
Arctic system will also bring precipitation chances to the North
Slope and eastern Brooks Range, but a preference towards a
weaker/more northerly track looks to keep amounts down overall
here as well. There looks to be a brief break for much of the
Interior in the Tuesday-Wednesday time frame before another
potential system approaches from the Bering Sea late period. The
strong disagreement in the handling of this system in the guidance
and overall weaker flow pattern make day-to-day specifics harder
to pin down in the late period, especially in terms of timing and
amounts of potential precipitation, although the overall trend
would again suggest continued off and on precipitation chances
with no particular threat of more significantly hazardous weather
for now.
Putnam
Hazards:
No significant hazards are expected over Alaska during this
forecast period.
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