Gale Warning 549, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
534 AM AKDT Tue Aug 20 2024

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today
through Thursday)...

An upper level trough is shifting away from Southcentral this
morning towards the AlCan Border and AK Panhandle while a stout
upper ridge nudges into place behind it farther west. This is
helping to set up a fairly strong northwest flow for late summer
standards between the two features across Southcentral. Gusty
offshore gap winds are beginning to pick up along coastal sections
of the Kenai Peninsula and Prince William Sound, namely in
Whittier and Seward, where winds are already gusting between 20
and 40 mph at times. These winds will continue for much of the day as
the strong offshore gradient between the high pressure building
in from the west and the low/trough in the northern Gulf. Strong
offshore winds developing over higher terrain could also mix down
to sea level in Valdez later this morning, although this aspect of
the wind forecast is more uncertain. Scattered showers still
lingering over the Copper Basin should clear to the east as the
trough advances into Southeast AK by this afternoon. The remainder
of more interior parts of Southcentral should see a fairly mild
and mostly sunny day today, with much of the Mat-Su Valleys,
Anchorage and Kenai Peninsula set to see highs in the mid to upper
60s at lower elevations.

By this evening, offshore winds should finally begin to shut off
quickly as the upper level ridge moves directly overhead and as
the pressure gradient along the coast diminishes. Clouds will
quickly be on the increase by tonight as well ahead of a frontal
system expected to cross over the Alaska Range into Southcentral
early on Wednesday. This front will move in fairly quickly as the
ridge begins to flatten out and push a bit farther southeast into
the Gulf, and models have struggled still to get a handle on the
precipitation potential with this system. The most likely scenario
still appears to be for a quick round of rain progressing across
much of Southcentral from west to east between Wednesday morning
and Wednesday evening, with the highest rainfall amounts of around
a quarter to half inch focused across the Mat Valley down to the
Kenai Peninsula and Prince William Sound region. Southerly gap
winds will also pick back up as the front pushes through on
Wednesday into Wednesday night, but will not be as strong compared
to the southerly wind event a day or so ago.

By Thursday, the pattern effectively resets back to where we are
now, with another upper trough moving into place behind the
frontal passage and another area of weak surface low pressure
forming over the Gulf. Winds should again turn offshore and
northwesterly, becoming stronger and gusty through the typical
gaps near the AKPen and Kenai Peninsula throughout the day. Any
lingering rain will also give way to widely scattered showers
during the day, similar to what we saw yesterday across much of
the region.

-AS

&&
.SHORT TERM FORECAST SOUTHWEST ALASKA, THE BERING SEA/ALEUTIANS
(Days 1 through 3: Today through Thursday night)...

Relatively calm conditions prevail across Southwest Alaska, with
light winds being driven by surface high pressure directly
overhead this morning. Partly cloudy skies, punctuated by some
cirrus/cirrostratus in places has allowed for temperatures to
cool into the 30s in many places. Patchy fog has also developed
in local cold spots. Out west, the extratropical remnants of
Typhoon Ampil churn across the western Bering Sea. Warm advection
out ahead of the low is leading to stable conditions and a wide
area of stratus across the central and eastern Bering. A warm air
mass will move across the eastern Aleutians and Alaska Peninsula
today with southeasterly flow which should allow temperatures to
reach into the 60s today, potentially even approaching 70 for
Unalaska.

Ex-Ampil tracks northeast along the coast of Far East Russia with
storm force winds persisting near its center. By tonight, the low
center approaches Saint Lawrence Island and enters the Bering
Strait on Wednesday. Its front pushes into Southwest Alaska
Tuesday afternoon, bringing a brief shot of rain that sweeps west
to east. Generally three tenths of an inch or less of rain can be
expected through Wednesday morning, with the highest totals from
the Kuskokwim Delta coast into portions of the Lower Kuskokwim
Valley. Southerly winds along the Kuskokwim Delta coast remain
below gale force, developing and diminishing rapidly with the
frontal passage, so are not likely to produce significant coastal
flood impacts.

However, as the low tracks into the Bering Strait for Wednesday,
winds shift out of the southwest and approach gale force north of
Nunivak Island. Therefore, there is some potential for minor
coastal flooding, particularly northward along the coast from
Kipnuk, and likely focused in and around the community of Newtok.
A High Surf Advisory has been issued for this hazard, with water
levels expected to crest around 1 to 3 feet above the normal
highest tide line on Wednesday afternoon. Winds will gradually
decrease Wednesday evening, with a brief secondary push of
stronger winds Thursday morning as a weak low traverses the far
northern Bering Sea, before decreasing below 20 mph Thursday night
into Friday morning. Wind direction will remain from the west
southwest direction through Friday morning. Along the Aleutians,
moisture rides up over the North Pacific High, bringing periods of
showers on Thursday. Relatively light westerly flow continues
into the weekend across much of the Bering Sea and Southwest
Alaska.

Quesada

&&

.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...

Zonal flow on Thursday shifts to a more cyclonic flow by Friday.
Troughing develops across the mainland as a result of this upper
level pattern. The GFS and ECMWF agree that a shortwave starts
across the western interior Friday morning. This shortwave drives
into the Canadian Yukon by Saturday morning. The models show
another shortwave trough forming early Sunday in western Alaska,
but disagree on how far it will dip. The ECMWF has the trough
extending into the Aleutian Peninsula, while the GFS has it only
making it down to the western interior. This shortwave allows for
a surface low over the northern Gulf of Alaska to form on
Saturday, which exits by Monday. Also on Monday, an upper level
low moves into the Aleutian region from the Kamchatka Peninsula,
which will be our next weather maker. The ECMWF has this low
further to the north than the GFS. The GFS predicts this system
will arrive later and has the system further southward than the
ECMWF. Uncertainty of this track means that potential impacts are
currently inconclusive.

The strong gusty winds that accompanied the remnants of TS Ampil
across the Bering continues to dissipate on Friday. The shortwave
trough along the overall cyclonic flow gives rise to rainy
conditions for much of the state for the weekend. Additionally,
the surface low that this shortwave triggers in the northern Gulf
of Alaska allows for localized heavy rainfall across Prince
William Sound and the Copper River Delta until late Sunday. A
front ahead of the upper level low late Sunday will produce
localized heavy rainfall over the Aleutian islands. This front
drives onshore sometime Monday, but due to model disagreement, it
is uncertain exactly when and where the rain is going to fall.
Wind speeds increase in response to the upper level low, with
winds reaching gale force criteria locally over the Bering Sea on
Monday.

-JR

&&

.AVIATION...

PANC...VFR conditions will persist. North to northwest winds up
to about 10 kt most of today will become light southeasterly
tonight, increasing in speed to about 10-15 kt by tomorrow
morning.

&&