Gale Warning 353, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
550 PM AKDT Tue May 5 2020

Updated Aviation Discussion

.ANALYSIS AND UPPER LEVELS...

Satellite imagery shows an area of low pressure across the north
Pacific making headway as it heads towards the southwestern Gulf.
Further west, low stratus continues to blanket most of the Bering,
with clouds becoming more scattered south of the Aleutian chain.

&&

.MODEL DISCUSSION...

The NAM and Canadian solutions have trended towards the more
consistent GFS/ECMWF (European) guidance, with all models now in
very good agreement in the synoptic pattern through the weekend.
This leads to an above average confidence level in the forecast.

In general, they show a mid-level area of low pressure just south
of the Gulf, with several disturbances disturbances diving south
southeast from the Chukchi Sea. These disturbances then get
ingested into the base of a trough, rotating around the low in a
Fujiwhara-like fashion. Further west, a transient shortwave ridge
can be seen in the model data, as it stretches across the west
central Bering Sea and Aleutians. Finally, the models show a
shortwave trough poised to the southwest of Shemya.

&&

.AVIATION...

PANC...VFR conditions will persist through the period. A brief
period of southeast winds off the Turnagain Arm are possible this
afternoon-evening, but they will not be very strong or last long
due to weak pressure gradients.

&&

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

Mostly sunny skies and above average temperatures are expected
across much of Southcentral today as weak high pressure and drier
air builds over the region. Marginal instability may still
develop over the northeast Copper River Basin this afternoon and
thus isolated weak thunderstorms remain possible. Similar
conditions are expected for Wednesday. Clouds will increase
Thursday as a front moves into the Gulf. Though model
discrepancies exist regarding its position and timing, it appears
that any precipitation should remain confined to the coast before
possibly moving farther inland by Friday.

&&

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

There are a couple of weak shortwaves pushing across SW AK this
afternoon and into the evening. Isolated light showers were
detected this afternoon on both the Bethel and King Salmon
radars. This energy doesn't really clear out until Wednesday
morning. A warming trend is expected across Southwest. Above
normal temperatures will continue. Multiple interior valley
locations will reach 60 degrees Fahrenheit.

&&

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

The latest CIMSS Natural Color RGB satellite image has a swath of
fog and low stratus from the Chain to NE Russia. There is a break
in coverage near Saint Matthew Island. There is a deep low near
Kamchatka and the leading edge of the warm front has moved over
the Western Aleutians. As time elapses, the surface low will hug
the coast of Kamchatka. The primary impacts will gale force winds
in the Western Bering and enhanced seas. A second low will follow
in its wake although the actual surface low will come closer to
Attu Island on Friday.

&&

.MARINE (Days 3 through 5)..
(Friday through Sunday)

...Bering Sea and the Aleutian Coastal Waters...

Sustained gales are expected for the southern coastal waters of
Alaska Peninsula Thursday afternoon and evening, mainly east of
the Shumagin Islands. Here, waves building into the 15 to 20 feet
range are anticipated.

Otherwise, look for a front to enter the western Bering and
Aleutians on Friday, then it will strengthen with winds
approaching low-end gales on Saturday as it reaches the central
Bering/Aleutians, before weakening on Sunday as it approaches the
mainland. Given a decent fetch length, seas will gradually build
into the 15 to 20 feet range for locations along the front and
across the western portions of the chain.

...Gulf of Alaska...

An area of low pressure just south of the region will bring a
large swath of sustain gales to the Gulf, Barren Islands, and
Kodiak coastal waters on Friday. A few gusts near storm force are
possible. Winds will decrease some as you get near the northern
Gulf Coast. However, due to a long fetch length, seas of 15 to 20
feet will be fairly widespread, with the exception being Shelikof
Straight, where waves of 10 to 15 feet are more likely. These
winds and waves will decrease in intensity for the weekend as the
system moves away from the area.

&&

.LONG TERM FORECAST (Days 4 through 7)...
(Saturday through next Tuesday)

Our long-time resident low just south of the Gulf will continue to
wobble in place, sending multiple shortwaves northwestward into
the mainland. This will bring periods of rainfall to portions of
Southwest and southcentral Alaska. The exception to this is the
Copper Basin. This area may see less upper-level support, as a
ridge of high pressure tries to build westward from the Yukon.
As it tries to do so, 850mb temperatures climb towards 14 deg. C,
with soundings showing a dry airmass in place by Monday. This
could potentially elevate some fire weather concerns.

Further west, a front located over the central Bering and
Aleutians on Saturday will bring widespread precipitation ahead of
it. This feature will then weaken on Sunday as frontolysis (the
weakening of a front) commences, as it's associated surface low
lifts north towards Siberia. Another low and associated front then
approaches the western Bering and Aleutians by Monday, with
precipitation chances rapidly increasing once again, while also
bringing another surge of warmer air.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE... Gale Warning 178, 411
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...AP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...PD