Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
755 PM EDT Wed Aug 21 2024
Valid 12Z Sun Aug 25 2024 - 12Z Thu Aug 29 2024
...Upper/surface low pressure system could spread heavy rain and
wind from the Aleutians across the Southern Coast next week...
...Overview...
Early next week, an upper trough that is trending deeper will be
atop most of Alaska, promoting cool temperatures and precipitation
chances, including moderate to heavy rain over Southeast Alaska. A
brief period of upper ridging is possible behind this feature,
while upstream a notable upper trough/low and surface low will
track across the Bering Sea/Aleutians early week eastward into the
southwestern Mainland by Tuesday-Wednesday. Precipitation chances
with modest to heavy rain amounts are possible near and ahead of
the low track, i.e. for the aforementioned areas east into parts
of Southcentral Alaska into the southeastern Mainland, and
enhanced winds are also possible.
...Guidance Evaluation/Preferences...
Model guidance is reasonably agreeable with the large scale
pattern and less agreeable on the medium and smaller scales into
next week. First, at least the trend is becoming clearer for
troughing Sunday-Monday to be fairly deep with stronger energy
than some guidance indicated yesterday, leading to higher
confidence in moderate to heavy rain amounts. The 12Z GFS moves
the energy slightly faster east/out of Alaska on Monday, with the
CMC a little slower given another shortwave coming around. The
ECMWF seemed like a good middle ground. Upper ridging should move
across the state briefly behind this trough.
Then upstream, model guidance has shown pretty good agreement on
the existence of the upper and surface lows tracking across the
southern half of the Bering Sea for several cycles now. But there
are some differences on the timing of the lows pivoting east. With
this feature as well, GFS runs were on the faster side with the
CMC on the slower side, with the ECMWF and the UKMET (during its
forecast time frame) were a good compromise. The 12Z EC was a bit
slower than its 00Z run, and this was also reflected in the EC
ensemble means. At forecast generation time, only the 00Z EC
ensemble mean was in, and was faster than the 12Z GEFS mean. So
favored the GEFS mean timing a little more, and now the newer 12Z
EC mean has come in slower than its previous run. By
Wednesday-Thursday there is more spread with the evolution of this
trough generally weakening, perhaps joining with an Arctic trough,
and made more complicated by another upstream shortwave.
The WPC forecast used a blend of the deterministic models early
on, but gradually transitioned to more GEFS and EC ensemble means
in the blend, reaching well over half means by the end of the
period as spread increased.
...Weather/Hazards Highlights...
The upper trough pattern by Sunday will provide support for
precipitation lingering across the southeastern quarter of the
Mainland, with some snow in higher elevations. As the energy
within the trough trends stronger/deeper today, this should allow
for deeper moisture in the form of a weak to moderate atmospheric
river to flow into Southeast Alaska Sunday and provide for some
locally heavy rainfall totals. Rain there should be tapering off
on Monday.
The upstream system tracking west to east across the western to
central part of the state will also increase precipitation chances
and winds. The Aleutians can expect modest rain amounts
Sunday-Monday, while maritime and Aleutians winds are expected to
be in the 30-40kt range lasting behind the low system into
Tuesday. Wave heights may be enhanced near the Aleutians into
Bristol Bay and near the Alaska Peninsula as well. Enhanced rain
amounts are forecast to move into favored areas of the
southwestern Mainland, Alaska Peninsula, and Kodiak Island late
Monday into Tuesday, and into the southern Kenai Peninsula/Prince
William Sound and the southeastern Mainland to the Yakutat area
Tuesday-Wednesday. Most precipitation will be along the southern
coast, but light to modest amounts could spread especially into
the western and central parts of the Mainland and the Alaska
Range, with some snow in high elevations once again.
The troughing aloft to start the period Sunday will promote cooler
than normal temperatures across the state, particularly for highs.
Temperatures are likely to gradually moderate closer to normal
during the workweek, and even reaching a bit above normal for lows
as warmer southerly flow comes in ahead of the second upper
trough--but precipitation chances could hold high temperatures
down below normal.
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