Small Craft Advisory 361, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
547 AM AKDT Thu May 3 2018

.ANALYSIS AND UPPER LEVELS...

A shift in the pattern is well underway this morning as a broad
upper level trough centered over the western Bering Sea shifts
east. Leading this progression east is a leading shortwave trough
which is amplifying along the Alaska Peninsula and eastern Bering
Sea. Broad isentropic lift over the Bristol Bay region is already
bringing widespread rain (and mixed snowfall) to the area, with
some of that activity spreading over the Kilbuck Mountains and
Kuskokwim Mountains. Lingering showers continue over the Susitna
Valley and along the North Gulf Coast which is likely being aided
by cold air driven atmospheric instability. Satellite imagery
depicts a secondary basal shortwave trough, trailing the leading
one moving into Bristol Bay, moving northeast along the Western
Aleutian Chain. This surface low is expected to intersect a moist
warm front then undergo rapid cyclogenesis as it lifts towards the
AKPEN Thursday night.

Closer to home, with the approaching warm front lifting through
the southern Gulf of Alaska, the pressure gradient has
strengthened sufficiently along the Chugach Mountains to aid in
local Turnagain winds moving into SW Anchorage and along the
Hillside. Rain is also moving toward the southern Kenai Peninsula
in association with the aforementioned warm front. This front will
spread more precipitation toward the North Gulf Coast today.

&&

.MODEL DISCUSSION...

The models are in lock-step with the general synoptic pattern
heading into early next week. They are also in much better
agreement with the deep low expected to move into Southwest Alaska
Friday. There is little to no model discrepancy this morning, and
forecast challenges will be related to the impacts of the upcoming
storm, but not the models.

&&

.AVIATION...
PANC...Gusty Turnagain winds are anticipated to rapidly diminish
early this morning below 10 knots. Sprinkles/showers will be
possible at the terminal the whole TAF period, but CIGS/VSBY
expected to remain VFR.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper level trough near Kodiak Island is sliding north into
Southcentral this morning. The surface front will reach the north
Gulf coast early this afternoon, then work its way inland later
today. Most of the precipitation with this system will fall along
the Gulf Coast and the coastal mountains. Strong easterly
low level flow will limit precipitation for inland areas. Winds
and rain will diminish tonight as the system moves inland and
dissipates. A much stronger and wetter system will move into the
area on Friday. A rapidly deepening surface low will cross the
Alaska Peninsula and be in Bristol Bay at about 972 mb Friday
morning. Winds will increase across the area, especially through
gaps in the terrain such as Turnagain Arm. This system has a
strong tap of subtropical moisture, which will push into Kodiak
Island late tonight, and then to the Kenai Peninsula Friday
morning. The bulk of the moisture will then slide slowly east,
with most of rain falling from Prince William Sound eastward on
Friday night. A fairly intact front will push through inland
portions of Southcentral Friday afternoon and evening, and most
inland locations will see some precipitation.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A 992mb low moving toward Cape Newenham this morning is swinging
a front across the AKPen and into Bristol Bay. Widespread
precipitation has moved in along the coast and will spread
northward through the YK Delta today. Colder 850mb temperatures
and lower freezing levels along the north coast of Bristol Bay has
resulted in some locations reporting snow overnight (Dillingham).
Any pockets of snow along the west coast are expected to mix with
and change to rain by mid-morning as warmer air advects in behind
the front. Easterly wind gusts in favored gap locations across
the AKPen will also diminish this morning as the front tracks
northward.

This system will retrograde back to the south tonight as a
vigorous upper-level trough digs southward across the central
Aleutians. Strong vorticity advection, coupled with a robust
north-south oriented jet streak will spin up another, more
vigorous surface low that will track across the AKPen and into SW
AK Friday. Warmer air advecting in with this system will keep
freezing levels higher than previous systems, meaning most low-
elevation locations will see mainly rain. The heaviest of the
rainfall is expected to taper off from south to north beginning
Friday night as the low tracks into the Kuskokwim River valley.
Southeasterly winds will again become quite gusty (35mph to 45mph)
in favored mountain gap locations on the lee side of the Alaska and
Aleutian ranges as the low approaches and moves across the AKPen
overnight Thursday into early Friday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A complex series of low pressure systems will impact the Bering
and Aleutians bringing strong northwesterly winds and
precipitation to the region through the beginning of the weekend.
The strongest of these systems will rapidly strengthen south of
Sand Point overnight Thursday and move quickly northeast toward
Bristol Bay. Cold air wrapping around this system coupled with a
tightening pressure gradient across the central Bering will lead
to development of a band of northerly gale force winds from the
Pribilofs to Dutch Harbor through Friday evening. A ridge building
in the western Aleutians will keep things quieter from Adak west.
However, a cold, unstable northerly flow will keep scattered snow
showers in the picture through the first half of the weekend.

&&

.LONG TERM FORECAST (Days 3 through 7...Sat through Wed)...
The weekend will start out with yet another strong low pressure
system moving inland. This system will bring strong winds, heavy
rain, and even the chance for heavy snow for higher elevations.
The low pressure center at the surface will drive from south to
north across SW AK early Sat morning. This will bring strong winds
to parts of Kodiak Island, into Bristol Bay, and along the Gulf of
AK coastline. The difference with this system from yesterday is
that it does not hold together and move far enough north to spread
much of these winds inland across Southcentral and the Cook Inlet.

This system will be unique in how far south it taps into moisture.
The fetch will extend all the way down to nearly 30N. This is
known as an atmospheric river. And at least to start the weekend,
it will be pointed at the Gulf Coast. The heaviest precipitation
does slide slightly east late Sat, but is quickly reinforced along
the coast by another shortwave moving into the area early Sun.
This heavy precipitation will work in tandem with some cold air
that filters into this system to provide some snow for the higher
elevations such as Thompson Pass. Being that it is early May, it
becomes harder and harder to get significant snowfall, but
certainly not impossible. There are many variables that need to
come together to keep the precipitation from turning over to rain,
but we have introduced a healthy (5-10") of snow to the coastal
mountains with this package.

By Sun and Mon, the pattern starts to shows two main features: a
building North American Ridge and a very broad, unseasonably deep
low over the Central Aleutians. The ridge will try to build up
from SE AK, but as it does so, it ushers in another shot of
moderate rain and wind for the Prince William Sound and areas
east. By the middle of next week, all models are in fairly good
agreement that the Aleutian low will slide very near the Cold Bay
area. The trend has been to take this low slightly further south
and try to build some weak ridging over the mainland from the
Yukon Territory. However, for now, it looks to continue to be
mostly cloudy and slightly warmer for Southcentral with continued
wet along the Gulf Coast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Watch 155.
MARINE...Gale 119 130 131 132 136 137 138 139 150 155 170 173 174
179 180 181 351 352 414.

&&

SYNOPSIS AND MODEL DISCUSSION...Ahsenmacher
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
LONG TERM...MSO

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
109 PM AKDT Thu May 3 2018

AKZ101-111-040915-
Anchorage-Matanuska Valley-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Palmer, Wasilla, Sutton, and Chickaloon
109 PM AKDT Thu May 3 2018

A fairly fast moving weather system is moving over the Western
Gulf bringing strong southeasterly winds over the Anchorage Upper
Hillside, Turnagain Arm, and Matanuska Valley through the
weekend. These winds will not be as strong or as widespread as
those experienced last week, but there will be occasional strong
gusts in the normally windy locations.

Winds are expected to begin increasing tonight reaching their
peak speeds by Friday afternoon. Gusts up to 70 mph are possible
along the upper Anchorage Hillside and Turnagain Arm, while gusts
up to 45 mph are possible along the Matanuska Valley. The
Anchorage Bowl and Susitna Valley will experience gusts up to 35
mph at times. Winds will start to diminish Saturday morning,
though breezy conditions will persist through Monday.

To get your most current forecast near you, please visit
www.weather.gov/anchorage

AKZ125-131-135-040915-
Western Prince William Sound-Northeast Prince William Sound-
Southeast Prince William Sound-
Including the cities of Whittier, Seward, Girdwood, Moose Pass,
Valdez, Thompson Pass, and Cordova
109 PM AKDT Thu May 3 2018

An atmospheric river moving into the Northern Gulf Coast from the
Pacific will bring heavy rainfall to Prince William Sound. The
heaviest rain will fall Friday afternoon, then taper off as the
front moves eastward. Rainfall up to 3 inches is possible in and
around Valdez. Up to 3 inches of rain is possible in Cordova. Higher
amounts near 4 inches will be possible at higher elevations. While
rain will continue through the weekend, rainfall intensity will
diminish Saturday afternoon.

To find your most up to date forecast, please visit
www.weather.gov/anchorage