Small Craft Advisory 693, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
437 PM AKDT Sun Sep 22 2019

.ANALYSIS AND UPPER LEVELS...
A resident low is centered over eastern Kamchatka while a closed
system is tracking east just south of the eastern Aleutians and
Alaska Peninsula this afternoon. Abundant moisture is riding up to
the Gulf of Alaska between the eastward moving low and a ridge
centered over the eastern Gulf. At the surface...a low is just
south of Dutch Harbor bringing rain and gusty winds around its
core. Clouds are spreading across Southcentral after clearing
early this morning. Temperatures dropped to the freezing mark, or
just below, for the first time this season across the eastern
Anchorage area and at several stations across the Kenai.

&&

.MODEL DISCUSSION...
Models are in relatively good agreement with the overall synoptic
pattern through Tuesday and then differ on the strength of a
significant shortwave that dives across the west coast to
Southcentral into Wednesday. There is still some uncertainty on
the timing on max winds around the core of the current system
tracking toward the Gulf...however, the storm system should stay
on a quick pace and clear the central Gulf by Tuesday night. The
next system that moves across the mainland on Tuesday into
Wednesday is kicked inland by the cold core low currently over
Kamchatka...models differ in precipitation coverage and amounts.
The main changes in the forecast were to increase precip
chances...with snow added along mountain ranges.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through Monday
afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tonight
through Tuesday)...

A strengthening low centered near Sand Point is moving towards the
southern Gulf this evening. The low is near its peak intensity. As
it moves south of Kodiak Island tonight, strong winds are likely
to impact the island and especially the surrounding waters. High
end gale force winds (50 mph) will impact the surrounding waters
tonight, both on the Gulf side and funneling down Shelikof Strait.
The low is expected to move nearly due east across the southern
Gulf through the day Monday. While 30 to 40 mph easterly winds
will develop off the Gulf Coast as the low goes by, any
associated rainfall is expected to impact the Gulf Coast and
coastal mountains. That will be the extent of any impactful
weather associated with the low. For inland communities, variably
cloudy conditions and dry weather are expected from tonight
through Tuesday morning, along with seasonable temperatures.

On Tuesday afternoon, the first of two distinct upper level waves
and shots of cooler air will move into Southcentral. With the
timing of this first disturbance expected in the afternoon during
peak heating, all of the models show widespread rain shower
activity breaking out as the wave tracks eastward. It's likely
that the mountains will provide additional lift, and the chances
of rain and high elevation snow were raised significantly from
previous forecasts. Showers should also impact many lower
elevations areas as well, but since cold air usually struggles to
cross the Alaska Range and often lags behind, along with the mid-
afternoon timing of the wave, rain will be the dominant
precipitation type. More on the second round of precipitation
below in the long-term section.

&&

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

An area of low pressure centered just south of the Alaska
Peninsula continues to move eastward into the Gulf of Alaska.
Rain and gusty winds will continue through the overnight hours
and into Monday with the strongest gusts expected through the
usual gaps and passes along the Alaska Peninsula. Meanwhile, and
upper level low currently over Siberia will drift southward for
Tuesday, bringing colder air to the Kuskokwim Delta region. While
rain is expected with this shortwave, trace amounts of snow in
higher elevations are possible Tuesday evening. Behind the
shortwave, a weak ridge arrives over Southwest Wednesday before
the next front approaches Wednesday evening.

&&

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

As the low centered south of the Alaska Peninsula moves eastward
this evening, gusty winds will continue across the central and
eastern Aleutians before diminishing Monday. With north to
northwesterly flow, some gusts may reach storm force levels
especially out of bays and passes south of the Chain. Rain will
continue overnight before eventually tapering to showers by
Monday. Behind the low, a weak ridge builds briefly over the
central Bering for Tuesday. Farther west, the next system
approaches the western Aleutians Tuesday afternoon with the
potential for storm force winds over the Near Islands late Tuesday
evening. As the low tracks eastward across the Bering, models
currently indicate a weakening trend, though strong gales are
still likely.

&&

.MARINE (Days 3 through 5)...

Beginning the extended period marine forecast on Wednesday, there
will be a broad are of westerly cold air advection across the Gulf
of Alaska with gale force winds coming our of the Barren Islands.
Given the generally longer fetch, seas will likely be in the 10-15
foot range across the Gulf into Thursday before high pressure
moves in Thu night.

Over the Bering Sea, there will be a large cyclone slowly moving
across the Bering Sea, with broad southerly flow encompassing the
entire area as the front slowly shifts eastward, reaching the
west coast by Wed night and Thu. This storm will bring quite a bit
of swell into the Bering, so expect widespread seas of 15-20 feet
with some potential for higher seas over the far western Bering
Sea.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Beginning the extended period forecast on Tuesday evening, there
will be an active pattern in place over the Southern Mainland as a
series of mobile shortwave troughs will be moving east. This will
keep an active threat for rain, although light in nature across
most regions, as well as higher elevation snow (3000+ feet)
favoring the Talkeetnas/Chugach/Alaska Range.
One shortwave trough looks to move east Tue night across
Southcentral with another secondary wave tracking across Southwest
Alaska. The shortwave trough shifts east on Thu, and there is much
less certainty on the second, especially as it tracks into
Southcentral. Given the west flow regime, precipitation will
likely more favor the mountains and north Susitna Valley and
Copper Basin with a quick hit of rain and more higher elevations
snow. Westerly cold air advection will push stronger winds through
the Barrens and some local gaps, and will also dry the region out
as offshore winds increase Wed night.

Expect high pressure to dominate with dry weather through early
Saturday before the next Pacific low spreads more clouds and
moisture into Southern Alaska. Timing is uncertain, but most
global guidance favors increased rain chances for Southwest by
Saturday morning with chances increasing for Southcentral by
Sunday. This low will be tapping deep subtropical moisture and
will be a longer, slower moving trough, so this could be a high
rain maker Sunday through Tuesday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning...119 131 132 136 138 150 155 160 165 170
                        171 172

FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...JA