Small Craft Advisory 833, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
355 PM AKDT Wed Nov 1 2017

.ANALYSIS AND UPPER LEVELS...
A high amplitude pattern is over the area. There is a strong
upper level ridge oriented north to south over Alaska. Inversions
under this ridge are fairly strong for inland areas, and some fog
and stratus are the result. Offshore flow at the surface is
weakening, with breezy conditions still occurring through favored
gaps. There is a deep trough over western Canada and another over
eastern Russia. Some moisture is pushing northward across the
eastern and central Aleutians into the southern Bering.

&&

.MODEL DISCUSSION...
The numerical models are in excellent agreement through the short
term portion of the forecast. The main forecast issues deal with
the fog and stratus in the lower levels, which the models in
general have a hard time handling. Forecast confidence is higher
than normal.

&&

.AVIATION...
PANC...IFR to LIFR conditions in fog and low clouds are expected
through Thursday morning. There is a chance conditions could lift
to MVFR Thursday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
With high pressure moving over the area, the main forecast
challenges will continue to be the timing of low stratus/fog
around Cook Inlet along with offshore flow along the North Gulf
Coast. Beginning with the North Gulf Coast, winds throughout the
area should diminish over the course of this evening as the
pressure gradient along the coast weakens as high pressure moves
overhead. These winds will then redevelop late Thursday-Friday
from west-east as a weak surface trough moves through the gulf,
however winds are generally expected to remain below gale force.

Widespread dense fog is expected to redevelop over the course of
this evening throughout the Anchorage Bowl and Matanuska Valley as
high pressure provides increasing subsidence and light winds,
which looks to persist into Thursday morning. The fog/stratus
forecast is much less certain for Thursday night into Friday as
the offshore flow begins to strengthen along the Gulf Coast, but
only marginally near Cook Inlet. As a result of this the forecast
just includes widespread patchy fog, however there is a chance
that the down inlet flow will not be strong enough for dense fog
to redevelop again for the third night in the row.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

A weakening front will approach western Nunivak Island and
Kuskokwim Delta by Thursday afternoon. Although the front will be
in the weakening process, enough convergence is expected along it
for light precipitation to develop Thursday afternoon through
Friday morning, with area profiles showing all precipitation will
remain in the liquid form. Some cooling aloft associated with
this system should allow fog to become somewhat lighter, with
improved visibilities a good bet.

Farther south towards the Dillingham and King Salmon region,
temperatures aloft will continue to slowly warm compared to
further north, strengthening the nocturnal inversion. Overall,
look for a repeat scenario with regards to the fog development as
observed last night/this morning, with perhaps a slightly quicker
onset a distinct possibility. With that in mind, we bumped up the
timing a few hours in the forecast to account for this.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

Good model continuity is seen in the data for this region, with
only a few minor changes noted. The first such change was a trend
towards a stronger ridge aloft building in, with the numerical
guidance now indicating 850mb temperatures topping 10 degrees
Celsius. This will enhance the inversion strength, allowing for a
quicker onset/longer duration to the fog development tonight.
Given this trend and observed conditions this morning at Cold Bay
and further up the peninsula, most of this region (forecast zone
181) was expanded in coverage.

Additionally, locally dense fog will likely occur and we added
reduced visibilities under 1 mile to much of this region and
adjacent waters. This was included in the forecast for these
locations. There's some concern the fog may persist through the
day, and this may need to be extended in time with later forecast
updates. Otherwise, look for a steady light rain to persist along
a weakening front near Amchitka.

Farther east, light rain will develop from near Adak eastward
towards Nikolski as a surface trough extended north from a
developing surface low. This low will lift towards the region over
the next 48 to 72 hours, with a warm front approaching the area
just beyond the short term period of the forecast. The models have
trended stronger with these features and this will continue to be
monitored for earlier precipitation/fog development.

&&

.LONG TERM FORECAST (Days 3 through 7: Saturday through
Wednesday)...

Saturday will mark the transition from fairly zonal flow to a
highly amplified, progressive pattern that will last through early
next week. A trough over the west coast of Alaska will dig
Saturday, sending a strong upper-wave southward to the southwest
Coast Saturday evening. As the wave moves across the Alaska
Peninsula Sunday, a closed upper-level low will break off and
barrel into the northern Gulf Sunday evening. Models are not in
good agreement in handling the placement of the upper-level energy
or the associated surface low pressure system and it's
precipitation.  However, the setup for fairly widespread
precipitation over Southcentral Alaska Saturday night into Sunday
morning looks increasingly favorable with the more recent model
runs. The timing, precipitation type, and QPF amounts will depend
on the placement of the surface low and the timing of the upper
wave, but for now forecast confidence is fairly low for the
weekend.

A sharp surface ridge over the Bering will amplify Sunday night
through Monday. The ridge will be squeezed between the digging trough
over the Mainland and and a trough with a strong wave pushing in
from the west Monday. By Tuesday night, a closed low will develop
over the Bering Sea and fully cutoff the base of ridge, pinching a
closed surface high off over the northern part of the state. In
terms of sensible weather, this will result in a drying trend
Sunday over the Mainland as the ridge pushes in with a return to
onshore flow and precipitation moving in over the west coast by
Tuesday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...KVP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...RD