Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
745 PM EDT Mon Aug 05 2024
Valid 12Z Fri Aug 09 2024 - 12Z Tue Aug 13 2024
...Atmospheric river will wind down across Prince William Sound
and vicinity late week...
...Rain and gusty winds spread to western Alaska late week,
shifting into Southcentral over the weekend/early next week...
...Overview...
A rather amplified upper-level pattern in the short range will be
relaxing a bit by the beginning of the medium range period, but
lingering troughing across the western half of Alaska will support
deep layer moist southerly flow causing some heavy rain remaining
near Prince William Sound. As the upper trough continues to weaken
and lift as it moves east, another trough is forecast to move into
the Bering Sea late week and across the state early next week.
This trough and an associated surface low pressure system will
promote rain and gusty winds spreading west to east across the
state through the period.
...Guidance Evaluation/Preferences...
Model guidance continues to show good agreement at the start of
the period, indicating the first trough tracking eastward and
lifting/weakening late week while the next trough pushes from
eastern Siberia into the Bering Sea and becomes the dominant
feature of the pattern. With the first trough, models have
converged better today with the timing of the trough lifting out,
though the 12Z CMC remained on the slower side and this was not
favored. The second trough will likely have an embedded upper low
around Saturday and will support a surface low that could be
sub-990 mb for a time in the Bering. Placement and strength
differences start out relatively minor with these features, but
model solutions diverge more by Sunday-Monday. Most guidance (like
the 06Z GFS and 12Z ECMWF and CMC) continues to track these
features east early next week, but the 12Z GFS becomes an outlier
as it spins the upper/surface lows in place, as differences in
surrounding energies affect the track. The 12Z ECMWF and CMC
maintain reasonably good agreement and are most consistent with
the ensemble means at this time.
Thus the WPC forecast blend used a multi-model deterministic blend
early on, including the 06Z and 12Z GFS and 12Z CMC and UKMET, but
heavily favored the 12Z ECMWF. Reduced the proportion primarily of
the GFS runs as the period progressed while gradually increasing
the proportion of the GEFS and EC ensemble means to over half by
Day 8 amid the increasing uncertainty.
...Weather/Hazards Highlights...
Though the heaviest rainfall amounts have phased into the short
range, heavy rain may remain across Prince William Sound and
vicinity Thursday, lessening Friday as the atmospheric river event
winds down with upper-level and surface support dwindling. Some
moisture should spread farther north across the eastern Mainland
for more modest rainfall amounts, but locally enhanced in the
eastern Alaska and Brooks Ranges. Then the next upper trough
coming into the Bering Sea and western Alaska, along with a
surface low that should be relatively strong for the warm season,
are forecast to spread precipitation chances into the state from
west to east. The Aleutians can expect moderate to heavy rain
Friday-Saturday, shifting into the southwestern Mainland and
Alaska Peninsula over the weekend, and Southcentral Alaska early
next week. Since the duration of the heavy rain does not look to
affect any individual area too long, held off on Heavy Rain areas
on the Day 3-7 Hazards for the Aleutians to Southwest
Mainland/Alaska Peninsula areas, but WPC did delineate a Heavy
Rain area for the typically favored area of Prince William Sound
again for Sunday. Once again more modest rainfall totals are
forecast farther inland across the Mainland. The southerly flow
may cause some gusty winds to 30-40 knots across the Aleutians
toward the Alaska Peninsula and Bristol Bay, which may cause minor
coastal flooding on south-facing shores. This is all below
criteria for what would typically be considered hazardous in these
areas though.
The rounds of upper troughing coming atop the state will generally
lead to modestly below normal high temperatures and around average
low temperatures for most areas late week into early next week.
However, parts of Southeast Alaska will be the exception
especially late this week, staying warm with highs in the 60s to
even over 70 in some locations. These may cool/moderate closer to
normal early next week.
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