Small Craft Advisory 334, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 AM AKDT Sun Apr 28 2019

.ANALYSIS AND UPPER LEVELS...
The upper level pattern continues to be composed of a longwave
ridge over mainland Alaska, a North Pacific low centered roughly
500 miles south of the AKPEN, and a double barrel low impacting
the western Aleutians and Bering.

Warm south flow ahead of the North Pacific low continues to bring
precipitation to the Pacific side of the AKPEN and Kodiak Island,
with high clouds streaming into Southwest while mostly clear
skies under high pressure persist over Southcentral. The northern
low over the Bering Sea is bringing gales along its front per
recent ASCAT data. The southern low is gale force as well and is
centered around 500 miles southwest of Shemya, slowly shifting
northward.

&&

.MODEL DISCUSSION...
Models remain in general good agreement for the short term. The
main challenge is the position and track of the upper low
developing over the southern mainland Monday. The GFS continues to
have the farthest southeast track, thus is the driest solution
for Southcentral. Based on the consistency between the EC and
Canadian, we leaned toward the wetter solution for Southcentral
Monday and Tuesday.

&&

.AVIATION...
PANC...VFR conditions will persist, with winds increasing and
shifting to southeasterly in the afternoon. An upper low
developing over southwest Alaska this afternoon may support
stronger southeast winds than observed Saturday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Tuesday night)...
Expect another sunny and warm day across most of Southcentral
Alaska today. Clouds will then increase tonight ahead of a warm
front lifting northward across the Gulf of Alaska. Rain will reach
western Prince William Sound/eastern Kenai Peninsula on Monday.
Confidence has increased in the track of a closed upper level low.
It is expected to form overhead tonight then lift northwestward
across Southwest Alaska Monday night. This track will pull the
warm front and associated moisture across the Kenai Peninsula and
possibly into Anchorage and the Mat-Su. Have trended upward with
chance of rain in the latest forecast. As for timing, it looks
like Monday night into Tuesday morning would be the most likely
time period to see rain in these areas. The eastern half of
Southcentral, from eastern Prince William Sound to the Copper
River Basin should remain dry. A ridge of high pressure will then
build in behind the upper low on Tuesday, leading to a warming and
drying trend for all of Southcentral.

As for Kodiak Island persistent low level northeast flow out ahead
of the warm front will maintain steady light rain for much of the
island. The warm front will finally move through Monday night, but
another front moving up from the Pacific will lead to another
round of rain on Tuesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Tuesday)...

High pressure over Southwest Alaska will continue slowly
retreating into the interior today, as a slow-moving front
approaches the coast. This will result in more cloud cover across
portions of Bristol Bay, and otherwise a continuation of full
sunshine across the Lower Kuskokwim Valley. Some high cloudiness
is moving into the Kuskokwim Delta. Expect another afternoon with
low relative humidity values in the 15 to 25 percent range, mainly
across the Lower Kuskokwim Valley. In addition, the gap winds will
slowly increase through the day today, and as such, raise fire
weather concerns higher today than previous days. Clouds will
continue building across both Bristol Bay and the Kuskokwim Delta
on Monday, as an upper level disturbance teams up with the fronts
moving in from the Bering. Low relative humidities and gusty winds
may still be a concern across the Lower Kuskokwim Valley.

On Monday night, an upper level low and approaching front from the
Bering will cause the area's first widespread rain in quite a
while over much of Southwest Alaska. The rain will move from
southeast to northwest, so Bristol Bay will get the rain first,
then it will progress into the Kuskokwim Delta Tuesday morning,
exiting the area Tuesday afternoon. The Bering front will stall
along the coast, keeping conditions cloudy and unsettled into
Tuesday night all along the Bering coast. The disturbance will be
accompanied by a somewhat warmer air mass, which will combat the
expected widespread cloud cover. Thus, the warming trend will slow
by Tuesday.

&&

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

The first of a pair of lows over the western Bering is rapidly
weakening this morning as the second stronger low approaches. The
first low will be absorbed into the second one by this afternoon
as the leading front moves into the western Aleutians late this
morning. The second low will bottom out in the mid 970 mb range
tonight as it passes over or near Shemya. The low will then drift
and slowly weaken as it tracks off the Kamchatka Peninsula through
Tuesday. The leading front will be the main weather maker for the
eastern half of the Bering, as it moves through the Pribilofs
Monday evening, then slows to a drift itself just off the
Southwest Alaska coast through Tuesday. The front and both lows
will produce gale force winds across the western Bering through
Monday before both the low and the front weaken back to small-
craft advisory criteria levels. The primary change with this will
be basin-wide southerly winds, which will end the cold period
across the area and allow for a seasonably warm air mass to move
in. Recycled cool air will follow behind the front for the
western Bering late Monday into Tuesday.

&&

.MARINE (Days 3 through 5)...
(Tuesday through Thursday)

...Bering Sea and Aleutian Coastal Waters...

A far western Bering low on Tuesday will lift towards the
Kamchatka Peninsula, with it's associated surface front crossing
the central and eastern Bering south trough the Alaska Peninsula.
Both features will bring sustained winds of 25 to 30 knots across
most of the region. Then for your Wednesday, high pressure will
quickly build across the eastern half of the region, before
another weak surface low approaches the central Aleutians on
Thursday. Seas of 10 to 20 feet on Wednesday will decrease for
Thursday into Friday.

Forecast Confidence: High

...Gulf of Alaska...

Sustained winds of 20 to 25 knots for coastal waters on Tuesday
will weaken for your Wednesday and Thursday, with the offshore
waters generally seeing winds less than those speeds. A weakening
front looks to move into the Gulf by Thursday.

Forecast Confidence: High

&&

.LONG TERM FORECAST (Days 3 through 7)...

A challenging forecast is in store for the region for the, as the
models are in fair agreement when looking at the spaghetti plots
for the start of the period, and progress into a mess by the
weekend. If there's any type of continuity, it's that a series of
lows will continue to lift north across the western
Bering/Aleutians, a narrow high amplitude ridge is expected to
stretch from the Arctic through the center of the state, with a
low retrograding from interior Canada and another low well south
of the Alaska Peninsula.

However...timing, location, and amplitude of all these features
are in poor agreement, with some models having a mid-level low
right in the middle of a ridge axis on other numerical forecasts.
About the only consistent signal is for afternoon highs and lows
to be on the increase through the forecast period.

Forecast Confidence: Moderate at best

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales: 120 131 138 173 175 176 177 185 412 413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MM
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
MARINE/LONG TERM...PD