Small Craft Advisory 224, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
628 AM AKDT Mon Mar 18 2019

.ANALYSIS AND UPPER LEVELS...
The overall pattern continues to be very slow to change. The
highlights remain cold air over the Bering, accumulating snow over
SW AK, and the continued intrusion of moderately strong storms
into the Gulf of AK coast. The upper level (30,000') jet is
extremely remarkable as it moves from 35N all the way through
Southcentral at 135kt. It is flowing over a closed ridge of high
pressure centered near British Columbia. The jet is helping to
ferry an abundance of moisture and warm air from the North Pacific
towards the state.

Along the jet, successive areas of low pressure continue to form.
While the storms themselves do not have very deep central
pressure, they are forming in a favored spot to create some
problems along the Gulf Coast. The first of these is heavy QPF.
Most of this is falling as rain, with snow levels climbing up and
above 2000' in this pattern. The second is strong winds and high
seas in the barrier jet region along the coast. These have ebbed
between small craft and storm-force winds.

SW Alaska is caught between cold air from the Bering and this
warm, moist air from Gulf of AK. The result is a deformation band
that put down several inches of snow across parts of Bristol Bay
and the Lower Kuskokwim Valley on Sunday. This band is still
holding on over King Salmon and Aniak this morning. The Bering
remains very quiet with just some cold air snow showers. A low
skirting south of the chain is increasing the easterly flow across
the Central Aleutians.

&&

.MODEL DISCUSSION...
Models remain in very good synoptic agreement. Thus, forecast
confidence is high. There are still some variances with the exact
location and intensity of the deformation band across Interior
Bristol Bay. We used the NAMNest and hand edits to update the
forecast overnight.

&&

.AVIATION...
PANC...VFR conditions will persist. Winds will be problematic
today and uncertainty is fairly high in the exact timing.
Southeast winds will remain gusty this morning. But there could be
a brief period of down-inlet flow this afternoon before
southeasterlies return tonight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tonight
through Tuesday)...

Precipitation (mostly rain below 2500 ft) will generally be confined
to the coastal areas today as strong southeast cross-barrier flow
keep most areas to the lee of the Chugach mostly dry. The
precipitation pattern will change some across the Cook Inlet to MAT-
SU Valley corridor tonight as a surface low moves north and an upper
trough sweeps across the area. The trailing cold air looks like it
will be slow to arrive and wet-bulb zero heights through midnight
will likely remain high enough to keep any precipitation mainly as
rain (with a mix at higher elevations). The cold air finally
arrives after midnight tonight along with the upper trough so
precipitation should become a mix or all snow. The current thought
is that any snow accumulations should be pretty minimal at lower
elevations with a couple of inches at higher elevations possible.
Elsewhere, showers will continue along the coast.

A weather front moving west across the Gulf of Tuesday/Tuesday night
ushers warmer air back in and renews windy and wet conditions
across the Gulf. This system will also bring warming temperatures,
increased chances of rain and snow, and locally windy conditions
to Southcentral.

&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

A weak surface trough and upper-level low will track north across
interior southwest Alaska today. These features will continue to
bring light snow across the area, with areas from the Nushagak
Hills north seeing another couple of inches of new snow
accumulation. The precipitation will diminish by early Tuesday
morning as as a surface ridge builds over the southwest mainland,
ushering in colder and drier conditions. The combination of mostly
clear skies, light winds, and radiational cooling will increase
the chance for fog, especially over the Kuskokwim Delta and
Bristol Bay region Monday night into early Tuesday morning.

The next surface low then moves northwest from the Gulf of Alaska
to Kodiak Island late Tuesday, pushing a front over the AKPEN and
Bristol Bay. Snow will develop over Bristol Bay and the Nushagak
Hills by Wednesday morning and spread northwest to the Y-K Delta
by late Wednesday afternoon. A strong southeasterly downslope
wind in association with the front will limit precipitation along
the lee of the Alaska and Aleutian Ranges while warmer air
advecting in aloft will help change the snow to rain/snow mix. For
areas from Dillingham and Sleetmute on west, enough cold air will
remain in place for the precipitation to remain all snow. At this
time, accumulations are expected to be light, on the order of 3
to 6 inches; however, a strong pressure gradient between the
northward advancing low and the retreating ridge will bring gusty
northeasterly winds along the front, producing areas of blowing
snow with this event. The front then stalls over the Y-K Delta
Wednesday night, with light snow continuing along the boundary
into Thursday as the parent low weakens and drifts toward the
eastern Aleutians.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

A surface ridge over the central Bering will keep conditions
rather tranquil through midweek. The exception will be the chance
for scattered snow showers today as a weak upper-level shortwave
slides over the ridge. Elsewhere, a low well south of Atka will
continue to move east today, stalling south of Unalaska Tuesday
with minimal impacts for the central Aleutians. A ridge will
build back in over the western Aleutians in the wake of this low.
This ridge will then slide to the northeast as a Kamchatka low
moves west of Shemya, lifting a front across the western Aleutians
on Wednesday.

&&

.MARINE (Days 3 through 5, Wednesday through Friday)...

...Bering Sea/Aleutians...

There is increasing confidence of northeasterly gales developing
over the eastern half of the Bering on Wednesday through Friday as
a series of lows track south to north over SW Alaska and the
AKPEN. Right now, there is more uncertainty regarding the exact
location and strength of the winds on Wednesday/early Thursday
due to model disagreement of the surface low moving over the
AKPEN. Model guidance shows better agreement with a stronger low
moving towards the AKPEN on Friday, thus more confidence that
there will be northeasterly gales developing from the Pribilofs
east. Looking towards the end of Friday, there is decent
agreement that a storm force low will push across the western
Bering/Aleutians.

...Gulf of Alaska...

Wind and wave heights are expected to subside through the day on
Wednesday as the region will be in between systems. There is
increasing confidence that a gale force low will enter the Gulf
by Thursday with easterly gale force winds spreading across most
of the region on Thursday and Friday. Wave heights in excess of
20-25 ft are also likely by Friday with increasing long fetch
winds from the east and eventually south/southeast. Storm force
winds could also develop near the North Gulf coast Thursday and
Friday during this time associated with an easterly barrier jet.

&&

.LONG TERM FORECAST (Days 3 through 7, Wednesday through Sunday)...

Not much change is expected in the longwave pattern this week
with persistent and strong southerly flow over the Gulf supporting
a barrage of lows over the southern mainland. These lows will
continue to tap into subtropical moisture leading to very heavy
precipitation over the coast. Models show total liquid
precipitation amounts over the coastal mountains along Prince
William Sound approaching 10 inches through the end of the week.
The southerly flow will also pump in warm air with snow levels
hoovering between 1500 and 2500 ft. Strong cross barrier flow
(from the east) will continue to promote downsloping for locations
on the west side of the mountains, including the western Kenai,
Anchorage, and the Mat-Su. Weather in these locations will be
cloudy but mostly dry with temperatures well above normal, climbing
into the 40s each day.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales 132 138 174.

&&

SYNOPSIS AND MODEL DISCUSSION...MO
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...KVP