Small Craft Advisory 731, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
434 PM AKDT Tue Sep 25 2018

.ANALYSIS AND UPPER LEVELS...
The remnants of the low that brought wind and rain to much of the
South Mainland yesterday has largely moved north of the area,
leaving generally clear skies and light winds for the area today.
Over the Gulf, a surface trough stretches from the Southern Gulf
into the Panhandle, but is only impacting the marine areas with
rainfall. Impacts of this trough can be seen stretching well to
the south with a significant plume of moisture riding along the
western edge of a ridge over the Northeast Pacific. This moisture
will begin to impact our area as the ridge slides northward later
in the workweek. A broad ridge of high pressure is also present
from the Western Aleutians into the Eastern Bering, providing very
benign weather to most of that region without even significant
fog or stratus to speak of. Further to the west, a new gale force
front associated with the low pressure system is off the coast of
Kamchatka. This front will continue tracking eastward today,
forcing the ridging over the Bering Sea to spread into the south
Mainland.

&&

.MODEL DISCUSSION...
Model agreement remains very good through Wednesday with the major
synoptic features, and is beginning to improve on Thursday with
regards to the weak secondary low that will develop near the AK
pen. The NAM especially has come into much better agreement with
this low, and so now all of the guidance brings rainfall into
Southcentral for Thursday. As a result POP's were increased with
increasing confidence for Thursday, especially for interior
Southcentral and along the Gulf Coast. Afterwards agreement
remains good into Friday, with both the ridging that will build
into the Mainland from the south, as well as a persistent trough
over the Bering. The result is forecast confidence that is at or
above average for the remainder of the work week.

&&

.AVIATION...
PANC...VFR conditions with light and variable winds will continue
into Wednesday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 3/Tonight
through Friday)...
Skies will continue to clear and winds will diminish this evening
as high pressure nudges in over Southcentral from the west. The
combination of clear skies and light winds will likely lead to the
coldest temperatures of the season so far, with most inland
locations approaching, or dropping below, the freezing mark. The
upper ridge over Southcentral will be short lived as a frontal
system to the south moves up the Gulf through the day tomorrow.
Clouds and rain will spread north through the day, reaching Kodiak
by the afternoon, the Kenai Peninsula Wednesday night, and the
Northern Gulf Coast, Anchorage Bowl, and Mat-Su Valley by Thursday
morning/early afternoon. This system will tap into tropical
moisture, so heavy rain will fall at times across Kodiak and the
Northern Gulf Coast.

By Friday, a large upper ridge over the Northern Pacific will
build north into the Gulf, acting to pinch off the front, pushing
most of the moisture rapidly to the east. Recent model runs have
suggested a weak disturbance will get wrapped over the top of the
ridge and pushed north across the southern Gulf and Kenai
Peninsula, which could bring more cloud cover and some rain to
those areas Friday, though there is still a fair amount
uncertainty this far out.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Friday)
Isolated showers continue to affect the western half of the
region, as cooler air aloft in the cyclonic flow continues to feed
moisture into the area, with onshore flow and diurnal heating
giving these showers a boost in coverage and intensity. With the
loss of daytime heating any precipitation will become isolated at
best and be confined to along/north of the Kuskokwim river. With
the added moisture, some patchy fog will probably develop along
the coast and across interior valleys.

Our eyes then turn to the next storm approaching from the south.
Today, the numerical models have come into much better agreement
with the overall pattern compared to this time yesterday. Todays
runs show a low crossing near Dutch Harbor, dragging a warm front
to the chain and peninsula before briefly stalling, as the initial
low weakens on Thursday. widespread precipitation will develop
along this front, and even to its north as an over-running setup
develops. Then, a second low quickly moves in right on its heels
with a similar track for Friday, lifting the front and rain shield
further north.

Given the increased agreements amongst the numerical forecasts,
some significant changes were made to the forecast. These include
speeding up frontal timing, changes in positioning, increasing
wind speeds, cloud cover, rainfall chances, and amounts.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Friday)
A ridge axis stretching from the west central Aleutians to Saint
Matthews Island will decrease in size initially before re-
centering to the northeast across the northern part of the state.
This occurs due to a strong low with an associated cold front
trying to push east. This front is currently located across the
western Bering Sea to near Shemya, and it should push eastward
another 100 miles or so late this evening before stalling briefly,
as the parent low southeast of Kamchatka weakens as it fills.
However, an incoming vort max in the cyclonic flow aloft will give
the system new life, with another area of surface low pressure
developing near Shemya overnight. As the low moves to northeast of
Shemya by morning, the front will once again accelerate to near
Atka, before stalling a second time. Widespread sustain gales with
rough seas and widespread rainfall will precede the frontal
arrival.

The low will then continue to dumbbell around the region with
frontolysis (a decaying front) taking hold, as a series of lows
trek north to northwest across central portions of the
chain/Bering, each once bring a push of warm moist air to the
eastern half of the sea and eastern portions of the islands, as a
series of warm fronts cross the area. Widespread showers will
accompany each low and front, with showers developing elsewhere.
Winds will also increase with each frontal passage here.

&&

.LONG TERM FORECAST (Days 4 through 7)...
Beginning the extended period forecast Friday night, there will
be a building high amplitude ridge over Southern Alaska with a
deep trough over the Bering Sea. The main feature of this pattern
will be the 3+ sigma (standard deviations) closed blocking high
over the state of Alaska, with some model guidance hinting at an
amazing 4 sigma standard deviation in upper level heights by
Saturday night. What this means is the main storm track will
remain over the Bering Sea with beautiful fall weather for all of
Southern Alaska. This will last through the entire weekend as
transient shortwave troughs traverse the ridge and move down the
ALCAN border, thus reinforcing the dry weather. By Monday
afternoon or so, the continuity in the model suite begins to
degrade as both the ECMWF and GFS have shown inconsistency in how
quickly the upper ridge breaks down before the Bering low begins
to exert a greater influence on Southern Alaska. The current 12z
deterministic ECMWF now brings the next front into southern
Alaska Monday night and Tuesday whereas the GFS holds off on
stronger ridging through the latter part of the week. This is a
complete switch in the deterministic models since yesterday, when
the ECMWF held onto a much stronger ridge than the GFS. However,
the ensemble means both depict a stronger and longer lasting ridge
at least into mid week, so this forecast will follow suit with
that general thinking...a slower eastward progression of the front
which will likely result in mostly dry weather lasting into
midweek next week.

Over the Bering Sea, there is low confidence on timing of each
individual low expected to pass through the Bering, but high
confidence that there will be a series of low, thus keeping the
Bering Sea pattern rather busy in the extended time range.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 131 132 138 150 155 175>178 185.

&&

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