Small Craft Advisory 497, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
1003 PM AKDT Wed Jun 24 2020

.ANALYSIS AND UPPER LEVELS...
A cold upper trough (-28C at ~18,000') and associated weakening
frontal system are moving north across Southcentral today. The
combined cooling aloft and instability with the surface front is
producing scattered to widespread showers across Southcentral.
Inland areas are showing significant cumulus development and
increasing potential for thunderstorm development. Gusty gap flows
were observed along Turnagain Arm, out of the Knik River Valley,
and along the Copper River.

Further west a cold front is slowly moving east across the
Kuskokwim/Yukon Deltas. Rain was observed along the frontal band
over the Delta with some patchy fog on the backside of the front.
As the front progresses east it should get into more unstable air
and may kick off some thunderstorms. Low stratus and fog continue
over the Bering Sea and Aleutians as high pressure remains in
place.

&&

.MODEL DISCUSSION...
Models overall are painting a similar synoptic pattern. The main
challenges today/tonight will be local wind regimes and thunderstorm
potential for Southcentral.

&&

.AVIATION...
PANC...VFR conditions in showers. Gusty southeast winds are expected
across the airport complex through late this evening before diminishing.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Thu and
Fri)...

An upper level trough lifting inland over Southcentral will keep
unsettled weather conditions in place this afternoon and evening.
Showers and isolated thunderstorms are expected over the Kenai
Peninsula, Talkeetna Mountains, and northern Copper River Basin
along the foothills of the Alaska Range. A strengthening coastal
ridge along the northern Gulf coast will keep gusty southeasterly
winds in place through Turnagain Arm, the Knik River Valley, and
the Copper River Basin through Thursday. A second low entering the
western Gulf will lift a gale force front over the northern Gulf
Thursday afternoon, which could bring another round of gusty
southeasterly winds to Southcentral late Thursday. Friday will
bring generally dry conditions to start with another chance for
showers and isolated thunderstorms to develop in the afternoon
hours.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

The weak low over the Kuskokwim Delta will drift northeast and
dissipate over the western interior by Thursday afternoon. We still
see enough instability to allow for some isolated thunderstorms
along the Alaska Range and Aleutian Mountains this evening. A strong
cold upper level trough is dropping south out of Siberia across the
Bering Sea through Friday morning with an upper low forming in the
base of the trough. As this upper low moves over southwest Alaska,
surface low pressure forms in the Dillingham region. Along with
this, lower level cold air advection is on tap into the southwest
mainland that will inhibit the normal instability expected with a
cold upper level low. While thunderstorms can not be ruled out, if
any do occur they will be isolated Friday. This colder airmass
associated with the upper trough will hold temperatures down of the
southwest mainland, and along with the lower temperatures we will
see humidities remain higher, thus mitigating any threat for
wildfire activity over the next several days.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

A low is moving up toward the Alaska Peninsula tonight. The
associated front will push to just south of the Alaska Peninsula
and generate near gale force winds south of the Peninsula tonight.
The low will continue east as high pressure moves in behind it.
This high pressure will be the main weather feature across the
Aleutians and Bering Sea through Friday. The trough/front that is
dropping south out of Siberia will generate near gale force winds in
the northern portions of the Bering Sea offshore marine areas late
Friday into Saturday. A storm will push into the Aleutians late
Friday with the associated front swinging northeast to the Alaska
Peninsula by Saturday morning. Near gale force winds are expected in
advance of this front along the central and eastern Aleutians to the
Alaska Peninsula.

&&

.MARINE (Days 3 through 5: Saturday through Monday)...

Aleutians and Bering: A front swings across the Bering through
Mon. Confidence is good for southwest small craft winds and waves
across the Bering to increase to gales in the Eastern Bering on
Sat. A North Pacific low and front slides over the Western and
Central Aleutians for Fri and Sat. Confidence is good for easterly
gales over the Aleutians with small craft winds and waves
spreading towards the Alaska Peninsula.

Gulf of Alaska: High pressure moves across the Gulf through Sun.
Confidence is good for Southerly small craft winds and waves to
spread across the Gulf ahead of a low and front diminishing Sun
along with westerly small craft winds and waves in the Western
Gulf around Kodiak Island on Sun and Mon.

&&

.LONG TERM FORECAST (Days 4 through 7: Sunday through Wednesday)...

Models are coming into better agreement through the weekend, but
begin to diverge with solutions by Tuesday and Wednesday. Biggest
difference from previous model runs is the parent low near the
northern Bering Sea will not absorb the low over the Aleutians.
This will keep this low from strengthening and being a bit
colder. This will allow for the colder temperatures to remain over
northern and western Alaska but will allow the south mainlands to
remain a bit warmer than previously expected.

Sunday features a pair of low pressure systems. The parent low
over the northern Bering Sea and a weaker low centered over the
Aleutians with the upper trough extending near western Alaska.
This sets up broad southwesterly flow aloft across the Southern
Mainland. This flow taps into moisture from weakening surface
fronts moving across the Southern Mainland which will bring cloudy
conditions with rain and showers Sunday and Monday. A transitory
ridge will move across the Copper River Valley bringing brief
drier and warmer weather Sunday. By Monday, the upper low and
short wave over western Alaska Sunday will mover east of the area.
However, an upper low near the Seward Peninsula keeps generally
westerly flow across the mainland. So expect generally showery
conditions Monday. Upper ridging builds in across the southern
Mainland late Tuesday into Wednesday which will bring decreasing
shower activity. However, there may be potential for afternoon
and evening showers inland with a strengthening thermal trough.
Model differences arise Tuesday and Wednesday which lower forecast
confidence.

The Bering Sea will be generally under high pressure through
early Monday, then transition toward more active weather as a low
and front approach from the south and west Tuesday and Wednesday.
Again, confidence is low as significant model differences arise
in positioning and intensity of these systems.

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 119.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RMC
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SA
MARINE/LONG TERM...MK/RC