Small Craft Advisory 793, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
518 PM AKDT Sun Oct 14 2018

.ANALYSIS AND UPPER LEVELS...
A large occluded low is situated south of Dutch Harbor this
afternoon with high pressure in the eastern Gulf of Alaska. The
southerly flow aloft between these features will increase tomorrow
as a front in the north Pacific moves into the central Gulf and
then to Prince William Sound. This front will tap into some
subtropical moisture and push an atmospheric river into Prince
William sound and surrounding coastal areas.

&&

.MODEL DISCUSSION...
Models remain in good synoptic agreement when all the 12Z and 18Z
model runs are compared. There are some important changes in the models
from yesterday afternoon and evening which are impacting the
forecasts along the North Gulf coast and Prince William Sound.
The first issue is the low and associated front this afternoon in
the central Gulf of Alaska. This low is moving eastward a bit
faster today and will be out of the forecast area this evening.
More importantly is the front in the Gulf Monday and Monday night
which will be much stronger than previously indicated as it brings
a barrier jet along the North Gulf coast and into Prince William
Sound. After this models indicate a similar solution for the
region through the middle of the week.

&&

.AVIATION...
PANC...VFR conditions and light northerly winds will persist
through the night. Rain will move into the area around noon Monday
and ceilings will lower at that time but should remain VFR
through Monday afternoon.

&&

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

A large area of low pressure centered near Dutch Harbor will tap
into some tropical moisture tonight, guiding the moisture stream,
otherwise known as an "atmospheric river" northward into Prince
William Sound during the day on Monday. This will have a number of
impacts. First all the sunshine we are experiencing now across
Southcentral will be long gone by Monday morning as the rainfall
moves in. Second, the atmospheric river will cause a prolonged
period of heavy rain throughout much of Prince William Sound
starting from about midday Monday and persisting into late Monday
night before moving off to the east. Fortunately, since this will
be a relatively brief period of time for the atmospheric river to
persist, rainfall totals are expected to be in the 3 to 5 inch
range for areas such as Cordova, Valdez, and Whittier through
Tuesday morning. Since Seward is further west, it will only be
caught up in the heavier rain for a much shorter time, so about 2
inches of rain is expected there. While the atmospheric river is
aimed into northwestern Prince William Sound, some of the moisture
is expected to succeed at crossing the mountains and cause rain
over Anchorage and the Mat-Su Valleys, especially Monday night.
Around 1 inch of rain is expected in these areas.

In addition to the widespread rainfall, wind is also expected to
become very strong along the north Gulf Coast and Prince William
Sound. This is a significant change from previous forecasts as the
atmospheric river previously was not expected to get as far west,
nor the front to get quite as strong. Thus, widespread gales and
localized storm force winds are now expected on Monday night from
a "barrier jet". The barrier jet is a corridor of strong winds
that forms along mountainous coasts when a front oriented parallel
to the mountains but moving towards them temporarily creates an
invisible tube through which the wind accelerates to much higher
speeds. Winds of 40-50 mph with gusts of 55-65 mph are expected
along the coast and into Prince William Sound Monday night. Waves
are expected to get as high as 14 feet in Prince William Sound and
over 20 feet out in the Gulf. Those who will be in the area
Monday afternoon through Tuesday morning should prepare for
hazardous winds and seas.

After the front moves into the mountains, the winds will taper off
very quickly Tuesday morning back to the 10-20 knot range.
However, because the large-scale winds will continue to be onshore
through Wednesday, upslope flow will continue to produce rain
throughout Prince William Sound, though with slowly diminishing
rainfall rates. Scattered to numerous showers will continue
over the Kenai Peninsula, Anchorage, the Mat-Su Valleys, and the
Copper River Basin both Tuesday and Wednesday as well.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3...Mon
through Wed)...
Today is the relative "calm" before the storm. A broad, mid-960 mb
range is slowly churning east near Dutch Harbor. A broad occluded
front associated with it is pushing from SW to NE through Bristol
Bay. This front and low are the reasons for the gusty offshore
winds today. The system also produced some strong cells near the
Kuskokwim Bay and Nunivak Island making for a few lightning
strikes earlier today. The gusty E-SE winds will continue to
increase this evening before they peak right around daybreak Mon.
A slight chance of thunderstorms will remain over the marine zones
near Nunivak through the evening.

Shower activity will be on the increase tonight into Mon. The
southeast cross-barrier winds will help to keep some of the
population centers in the lee of the mountains (Bethel, King
Salmon, Iliamna, etc) a bit drier. The front will bring more
widespread steady rain across all of SW AK as we march through the
day Mon. With some decent instability and potent lift, we can't
rule out the possibility of a strike or 2 along the coast on Mon
afternoon as well.

Tue the low will track right along the coast. This will make for
more unsettled weather with abundant showers and gusty winds.
Overall flow will remain out of the south, so expect the mild
temperatures to continue.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3...Mon
through Wed)...
A very large low will dominate the weather across the region. The
low itself is currently near Dutch Harbor. It will continue a slow
east to northeast track through the start of the week. The
strongest winds (gale-force) associated with it are along its
occluded front over the Eastern Bering. To the west of the low,
across the Central and Western Bering, winds have shifted to a
N-NW direction. These winds are generally solid small craft
level with some gale-force gusts. Along the front and under the
low center there remains a slight chance of some thunderstorms (as
we saw this morning). This chance will remain through the next
several days with warm ocean waters and cold air aloft.

As the low drifts east of St Paul by Tue, another shortwave will
drop in through the western half of the trough. This shortwave
will usher in some of the coldest air of the season and it too
will bring a chance for thunderstorms. With some slightly better
mixing in colder air, expect another swath of gales to move from
south to north along with this wave.

&&

.LONG TERM FORECAST (Days 4 through 7)...

At the onset of the forecast period, the synoptic pattern
features a low near Kenai, a second low south of the Aleutian
chain and a low near Kamchatka. Anticipate a very progressive
pattern. The guidance is coming into better agreement. Expect back
to back lows to bring a healthy amount of precipitation to PWS
and the Anchorage metro area. Even though the third week of
October is considered to be part of the cold season for Alaska,
the temperatures with both storms will remain above freezing in
the low elevations. Higher elevations on the Kenai and other
portions of South Central can expect a fresh blanket of snow this
forecast period.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm 119. Gales 120 125 130 131 150 160 165 174 176 180
185 351 352 414.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
LONG TERM...PS