Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
616 PM EDT Wed Aug 23 2023
Valid 12Z Sun Aug 27 2023 - 12Z Thu Aug 31 2023
...Heavy rain likely this weekend along the southern coastline as
wet pattern lingers into next week for southern Alaska...
...Overview...
Wet weather associated with a lingering front across northern
Alaska this weekend is forecast to give way to drier conditions by
early next week as high pressure moves over the region. Meanwhile,
a heavy rain event appears in store for the southern coastal areas
of Alaska through this weekend. The heavy rain threat should
lessen by early next week but the associated wet pattern is
forecast to linger, keeping the weather unsettled across southern
Alaska well into next week as a couple of surface lows lift
through the Gulf. Uncertain system energies from a likely cyclone
currently in the West Pacific may approach the Alaska domain next
week, helping to deepen a surface low that moves in towards the
central-eastern Aleutians next Wednesday-Thursday.
...Guidance Evaluation/Preferences...
Overall, there's fairly good agreement through the period
regarding the upper pattern over Alaska but with plenty of
uncertainties in the details and timing of individual systems.
Amplified troughing looks to move through western/northern Alaska
Sunday-Monday atop a strengthening upper ridge over southeast
Alaska and western Canada. Models show good agreement this ridge
should build over much of Alaska through at least the middle of
next week. A large scale upper low moves in to the Bering Sea
around Monday-Tuesday with lots of uncertainty in the details of
multiple waves of energy rounding the base, with weak surface lows
moving towards the Peninsula/Southern Coast. One wave next week,
possibly further amplified by extratropical energy should work to
deepen a surface low near the Aleutians late in the period. The
GFS, ECMWF, and CMC all so good agreement on the presence of this
low, but some differences in placement and strength. The ensemble
means support this evolution as well, though much weaker. Today's
WPC forecasts for Alaska were based on a general deterministic
model blend days 4-5. The UKMET was an outlier by day 6 with an
initial weak low near the Aleutians and was excluded from the
blend. Introduced modest amounts of the ensemble means day 6 and
beyond, eventually reaching a 40/60 blend ensemble guidance with
deterministic guidance. This ensured some needed definition to the
typical washed out depiction from the means, as well as sufficient
depth to the late period low near the Aleutians.
...Weather/Hazards Highlights...
Heavy rain looks likely as the period begins this weekend along
the Southern Coast and coastal ranges as southerly flow directs
ample moisture into the region. Models continue to advertise the
potential for several inches of rainfall. Heavy rainfall potential
should wane early next week, but remain unsettled and showery (as
well as into parts of the interior). Another weaker wave may move
into the region next week bringing a more uncertain threat for
heavy rains along parts of the immediate Southern Coast. Rain,
locally heavy in spots, also looks likely into Monday across
interior and northern parts of the state associated with a frontal
passage but should dry out across northern portions thereafter as
surface high pressure moves in. Increasing precipitation chances
should move into the Aleutians around mid next week, with some
potential for gusty winds as well (depending on strength of the
deep low).
In terms of temperatures, the coldest anomalies (10-20F below
normal for daytime highs) will be Sunday and Monday across western
Alaska and the Brooks range as upper troughing moves through these
regions. After Monday, temperatures across much of the state
should moderate closer to normal (or above) as upper level ridging
builds back in. The northern coast, eastern interior, and southern
Panhandle should stay near or above normal the entire period.
Santorelli
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
Hazards:
- Heavy rain across portions of the Alaska Panhandle and mainland
Alaska, Sat-Sun, Aug 26-Aug 27.
- Heavy rain across portions of mainland Alaska, Tue-Wed, Aug
29-Aug 30.
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