Alaska Extended Forecast Discussion
National Weather Service College Park MD
725 PM EDT Tue Aug 5 2025
Valid 12Z Sat 09 Aug 2025 - 12Z Wed 13 Aug 2025
..Heavy rain could cause flooding concerns in Southeast Alaska
this weekend...
...Overview...
An upper trough is forecast to extend from the Arctic Ocean
southward across the Bering Strait and near the western Mainland
as the forecast period begins Saturday. At least one embedded
upper low is likely to form within this trough but with
uncertainty in the details. Energies along the base of the trough
and a Gulf surface low will promote moist inflow into the southern
coast, especially Southeast Alaska, this weekend into Monday.
Moderate precipitation is also forecast across much of the
Mainland, along with cool temperatures. Into next Tuesday-
Wednesday, the upper trough should eventually track east and
flatten, while another round of energy upstream could bring
precipitation to the Aleutians and toward the Alaska Peninsula and
western Mainland.
...Guidance Evaluation/Preferences...
Even early in the forecast period this weekend, models disagree
with the details of shortwave energy within the relatively more
predictable trough that extends from an Arctic Ocean upper low.
12Z model guidance generally showed an embedded upper low forming
atop the western part of the Mainland late Saturday into Sunday,
with variations in the position of the trough. GFS runs,
especially the older 06Z run, were the farthest south, so leaned a
little north of those closer to the non-NCEP guidance. A
shortwave farther south in the northeast Pacific into the Gulf
should support a surface low there this weekend, but there is
spread in its position as well.
The variety in the main upper low position also affects the
trough moving eastward early next week. The GFS became the slowest
to track the trough eastward, maintaining it across the Mainland
into Tuesday, unlike other guidance that has the trough lifting
and weakening. Ensemble members show spread with the timing as
well, yielding fairly flat ensemble means. Meanwhile upstream, at
least one round of upper troughing should support surface low
pressure tracking near or over the Aleutians. Once again the
details will need to be refined in future forecasts, as models do
not show consensus on timing/position/depth of these features.
The WPC forecast used a multi-model blend early in the period. As
the forecast progressed, quickly ramped up the proportion of GEFS
and EC ensemble means to the full blend by Day 8 as spread with
the individual models increased.
...Weather/Hazards Highlights...
The primary hazard to watch out for through the period is a
worsening of potential glacial outburst flooding from the
Mendenhall Glacier this weekend as a period of wet weather is
expected to continue into the weekend and Monday. Rounds of
moderate rain are likely for multiple days in Southeast Alaska,
with total rain accumulations of multiple inches, due to a
potentially reloading moderate to strong atmospheric river at the
base of the Mainland upper trough. A Heavy Rain area is delineated
on the Day 3-7 Hazards map across Southeast Alaska for the peak
of the rainfall this weekend. Lighter showers are forecast late
Monday into next Tuesday-Wednesday.
Farther north, just ahead of the main upper trough axis and a
cold front sweeping through, generally moderate precipitation is
likely to track slowly west to east across essentially the entire
Mainland this weekend into Monday. Higher totals are possible in
upslope flow areas over mountains, while the highest peaks of the
Brooks and Alaska Ranges seeing some snow especially after the
cold front passes. This precipitation will also help extinguish
remaining fires in the Interior. Precipitation should also become
lighter across much of the Mainland into next week as well.
Meanwhile, upper level energy and a surface frontal system is
forecast to move across the Aleutians early next week and toward
the Alaska Peninsula and Y-K Delta by Tuesday-Wednesday, spreading
light to moderate precipitation across those areas. The surface
low generally in the 1000s or possibly 990s at times should not
yield terribly strong winds.
The upper trough will promote cooling conditions across much of
the state by the weekend into early next week. Temperatures are
expected to be around 5-15F below average for highs and a few
degrees below average for lows. Most areas can expect highs in the
50s and 60s. Temperatures may gradually warm into mid-next week
with the initial trough's exit.
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