Small Craft Advisory 727, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
517 PM AKDT Mon Sep 24 2018

.ANALYSIS AND UPPER LEVELS...
The low pressure system that entered the area from the North
Pacific on Sunday has now passed over Bristol Bay as is
dissipating over the Southwest Mainland. This low not only is
bringing widespread rain showers to the Southwest Mainland, but
also into the Gulf from the remnants of its associated front. In
addition to rainfall, this front also brought gale force winds to
the Gulf and areas of Prince William Sound this morning, as well
as some isolated wind gusts in excess of 70 mph along Turnagain
Arm and the Anchorage upper hillside. Strong winds also exist
behind this front, which has resulted in abrupt wind shifts as the
wind turned southwesterly along the Kenai Peninsula this morning.
This front is spreading northward where it is expected to have a
similar impact along the Anchorage Bowl and MatSu Valley's this
afternoon. The weather behind this low over much of the Bering Sea
is much quieter with broad northerly flow ahead of a ridge of
high pressure over the Western Aleutians. This ridge will continue
expanding to the east this week, and eventually bring a period of
quiet weather to much of the AOR by mid week.

&&

.MODEL DISCUSSION...
With a fairly quiet and slow moving pattern into the middle of
this week, the models are in very good agreement through
Wednesday. As a result, forecast confidence is above normal for
most of the AOR through Wednesday, with the primary challenges
being the possibility of stratus and fog around Southcentral.
Starting Thursday, agreement begins to break down as a secondary
low from a North Pacific low well south of the area develops near
the AK Pen. This disagreement primarily impacts the confidence in
the rainfall forecast, especially from Anchorage northward where
there is uncertainty in how far northward the rainfall will
spread.

&&

.AVIATION...
PANC...Gusty southerly winds will continue this evening as rain
showers pass through the area behind the front. This may produce
short lived periods of MVFR as heavier showers pass through,
however these are not expected to be persistent. Winds will then
diminish early Tuesday morning, leaving VFR conditions and light
winds for the rest of the day Tuesday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Monday night through
Thursday)... A stout front that rolled through the Anchorage area
this afternoon is rapidly progressing northeast this evening
bringing a line of heavy rain and thunderstorms across the Chugach
and Talkeetna Mountains and moving into the Western Copper River
Basin and the Northern Susitna Valley. Cold air advection behind
the front combined with rapid pressure rises are leading to very
strong southerly winds up the Cook Inlet. These will slowly
diminish through the night as conditions throughout the area will
improve from southwest to northeast. Strong southwesterly flow
will will promote good orographic lifting along southwest facing
mountains overnight so scattered showers can be expected to
persist over the higher terrain especially the Talkeetna
Mountains. Cold air aloft will drop snow levels down to below 4000
ft overnight, bringing accumulating snow to the local peaks.

Conditions will continue to improve tomorrow as high pressure
builds in from the west. Clearing skies and light winds will likely
lead to the coldest temperatures of the season so far, with many
locations approaching or dropping below the freezing mark
Wednesday morning. The next low loves into the Northern
Pacific/Southern Gulf Wednesday afternoon increasing cloud cover
and spreading precipitation into Kodiak and the Southern Kenai
Peninsula by early Wednesday evening. Models have disagreed as to
how far north the precipitation gets, but recent runs have
trended north into the Anchorage and Mat-Su Valley by Thursday,
thus increased cloud cover and POPs over these areas.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Tonight through
Wednesday)...

The current pair of lows from Norton Sound south to near Iliamna
slowly rotate around each other as they drift off to the northeast
through Wednesday as a surface high pressure ridge moves east
across the Aleutians and Bering Sea. The wrap-around southwest
flow is resulting in a combination of showery conditions across
inland Bristol Bay and low level moist advection along the Bering
Sea coast resulting in stratus and fog. Gusty southwest wind
across the Bristol Bay zone will taper off this evening. The ridge
of high pressure builds over the southwest mainland late Tuesday
night bringing stable conditions coupled with low level moisture
that should result in more widespread fog over the Bering Sea
coastal regions Tuesday night and Wednesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Tonight through
Wednesday)...

High pressure will build and move across the Bering Sea and
Aleutians to the mainland through Wednesday. A low pressure system
and the associated front move into the western bering Sea Tuesday
morning. The front will move to the Central Aleutians through
Wednesday. Expect minimum gale force wind in advance of this front
as it swings across the Aleutians.

Another low is tracking north toward the Alaska Peninsula and the
warm front is expected just south of the Peninsula. by Wednesday
afternoon. This will bring up gale force wind along the south side
of the Peninsula Wednesday.

&&

.LONG TERM FORECAST (Days 4 through 7: Thursday Night through
Sunday)...
What looked like a sunny pattern starting on Thursday across
southern Alaska is not so certain now as a significant disturbance
will move through Southwest and Southcentral Alaska Thursday into
Friday before the ridge moves over the region.
While it looks like the upper level ridge will establish itself
over the mainland part of the state this weekend and into early
next week, the GFS model is trying to elongate it quickly which is
signaling that the GFS thinks the ridge may not be as strong as
previously expected. However, with the ECMWF, Canadian and WPC
solutions all indicating the ridge building over the state, the
odds are that the ridge will indeed build over the state and usher
in some mild and sunny weather over Southwest and Southcentral
Alaska this weekend into next week. This pattern will keep the
storm track over the Bering Sea as the ridge blocks the storms
from the mainland part of the state.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 140 178.

&&

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...KVP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SA