Small Craft Advisory 370, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
535 AM AKDT Mon May 2 2022

.ANALYSIS AND UPPER LEVELS...

A low pressure system is currently over the western Alaska
Peninsula is moving eastward. The jetstream remains south of 50N
west of 150W and east of 135W.

Closer to the surface, radar imagery shows precipitation moving
northwestward across the northern Gulf of Alaska into Prince
William Sound and along the windward side of the Chugach, Kenai
and Alaska Ranges.

&&

.MODEL DISCUSSION...

Guidance remains in very good agreement with the overall synoptic
pattern. However, it is important to note that this pattern with
multiple shortwaves and widespread weak instability is very
challenging for the models to resolve the timing, strength,
amounts of any winds and precipitation. Forecast confidence
remains moderate overall.

&&

.AVIATION...
PANC...VFR conditions are expected during this TAF period.
Southeasterly Turnagain Arm winds are expected to begin near
02nd/14z and continue through 02nd/22z. Low level windshear is
likely between 02nd/22z and 03rd/05z.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3;
Sunday evening through Wednesday afternoon)...

A surface trough and associated upper-level wave are moving north
across the Kenai Peninsula this morning. Gap winds have increased
in advance of these features with a gusty southerly wind
developing along the southern coast of the Kenai Peninsula in the
wake of the trough. A widespread area of moderate precipitation
also extends from the northern Gulf to Prince William Sound and
the eastern Kenai Peninsula this morning. While much of the
precipitation is falling as rain along the immediate coast, it is
cold enough ahead of the trough for snow from Seward north to
Turnagain Pass. Snow levels will rise today, though, as the trough
lifts north, with snow changing to rain for locations less than
1,200 ft in elevation.

Gap winds will also increase today, not only in association with
the trough this morning but with the next frontal system moving
into the northern Gulf on the heals of the current system. Expect
an increase in winds along Turnagain Arm, the Upper Hillside of
Anchorage, Knik River Valley, and the Copper River Valley
beginning this morning and likely peaking early to mid afternoon.
The biggest challenge to the wind forecast will be whether the
Turnagain Arm wind bends over the western half of Anchorage later
this morning, and whether the gusty winds on the Upper Hillside
move down into Anchorage proper. This uncertainty is due to
competing pressure gradients as a low south of the AKPen tries to
bend the winds down Cook Inlet while the aforementioned upper-
level trough induces a lee-side trough along the Alaska Range from
Southwest Alaska across the interior of the state. Nonetheless,
the Upper Hillside of Anchorage and Turnagain Arm could see gusts
to 60 mph by early afternoon with gusts approaching 35 to 40 mph
for Palmer. Winds are then expected to wind down through the
evening and overnight hours.

A southerly flow in the wake of the first trough this morning will
bring a round of heavier precipitation into places like Seward.
However, as the flow aloft shifts more easterly in advance of the
next front, the bulk of the moisture will move over the coastal
mountains and across locations such as Whittier and Portage. A
strong cross-barrier flow will keep all but a few sprinkles or
isolated showers from making it over the mountains to the
population centers.

The front will weaken and hang up along the coast Monday night
into Tuesday as the associated surface low moves to the western
Gulf. The low will drift east through Wednesday with rain
persisting along the coast and isolated showers over interior
Southcentral.

&&

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

The easterly wave that set off ample shower activity across the
Middle Kuskokwim Valley and Bristol Bay yesterday will advance
into the Kuskokwim Delta later today. The afternoon should play
out in a similar fashion to yesterday (but shifted farther west)
as the upper wave interacts with increasing low level instability
developing with daytime heating, resulting in a new round of
pop-up shower activity across namely the Kuskokwim Delta. While
this may bring a small amount of needed rainfall to the region,
wetting rainfall will be light and spotty, with areas across the
delta that do see rain picking up only a few hundredths of an
inch. Farther west, cloud cover and areas of light rain will
continue around a low drifting from the Alaska Peninsula into the
Gulf along with gusty northeast winds through much of Bristol Bay
through this evening.

Little change is expected for eastern parts of the outlook area on
Tuesday, with the broad low over the western Gulf continuing to
influence most of the region, with cloud cover staying put along
with additional diurnally driven shower activity. Bigger changes
are expected across the Kuskokwim Delta, where a polar low
dropping south into the Bering will send a glancing blow of cool,
dry air across between Tuesday and Wednesday. Early morning lows
could dip down well into the 20s on Wednesday, particularly close
to the coast between Toksook Bay and Kipnuk. The rest of Southwest
will be spared by this cold snap, with the bulk of the polar air
mass expected to continue diving south into the Bering. This will
keep fairly benign conditions in place for the remainder of the
outlook area, with light northerly winds and chances for afternoon
showers continuing through midweek.

&&

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

Northerly winds will expand from west to east across the Aleutian
Chain through tonight as a broad, stacked low shifts from the
southern Alaska Peninsula into the western Gulf. The strongest
winds will ramp up through the typical bays and passes, with gusts
to around 35 kts possible in places like False Pass and Cold Bay
through Tuesday. Rain showers will continue along mainly the
eastern Aleutians through Tuesday, potentially changing to snow
showers by Tuesday afternoon before ending as cooler air filters
in from the north. Meanwhile, a polar low dropping south out of
the Bering Strait will enter the northern Bering with an attendant
pocket of seasonably cool air, sending temperatures down into the
mid to upper 20s across the northern Bering and Pribilofs by
Wednesday morning. An effective cold front will sweep across the
Bering ahead of the approaching polar low from late Tuesday into
Wednesday, with low level temperates cooling enough for all snow
to fall with shower activity along and behind the front across the
Pribilofs. Expect snow and rain showers to expand south into the
Aleutians as the polar air mass continues to shove its way south
during the day on Wednesday.

&&

.Marine (Days 3 through 5: Wednesday through Friday)...

Brief gales are possible Thursday behind a low in the western
Bering. Sub-gales and low seas will otherwise persist across the
rest of the Bering/Gulf.

&&

Long Term Discussion (Days 4 through 7: Thursday through Sunday)...

Unsettled weather is expected in the long-term as a broad, upper
level trough remains over Alaska. The main features to watch are
an occluding low in the Gulf and a compact, Arctic low curving
across the Central Aleutians and into the Gulf. The occluding Gulf
low will bring precipitation to the north Gulf coast and Kodiak
Island until it dissipates on Friday. Upper level shortwaves
around this low could also bring showers to Southwest Alaska.
Meanwhile, as the Arctic low moves into the North Pacific, the
Central and Eastern Aleutians should expect gusty winds and
rain/snow showers on Thursday and Friday. The low weakens as it
moves into the Gulf, so conditions should remain fairly settled
across the region through Saturday. Active weather resumes Sunday,
but confidence is low with the details. Models are consistent in
showing an increasingly amplified pattern as ridges build in the
western Bering and eastern Pacific, while lows develop in Siberia
and south of the Central Aleutians. How amplified the ridges
become will determine whether the Siberian low pushes into
Southwest Alaska and when the Aleutian low curves into the Gulf.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 119.
Gale Warning 120 125 131 351 352.
Heavy Freezing Spray Warning 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...ED
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AS
MARINE/LONG TERM...AP/KC