Small Craft Advisory 388, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 PM AKDT Mon May 20 2019

.ANALYSIS AND UPPER LEVELS...
Unsettled weather continues today as a surface low pressure system
moving over the Kenai Peninsula spreads rain into the Susitna
Valley and showers over the Peninsula itself.

Another surface low over the Kuskokwim Bay area is working with
modest instability to keep showers moving off the Alaska Peninsula
toward the Kuskokwim Delta...while the sun was out over the area
this morning.

Aloft, the Pacific jet is well south of Alaska south of 50N, with
a sharp trough into the Pacific Northwest. The downwind side of
the trough is providing ample southeast flow over Southeast Alaska
and allowing for spin-ups of new waves working toward Western
Alaska.

Water vapor imagery shows a digging trough moving toward the
Aleutians which will help change the pattern heading into the
midweek.

&&

.MODEL DISCUSSION...
Models initialized well this morning and move together with track
and intensity through midday Tuesday. After that, we begin to see
deviation in strength and track, especially with regard to the
strengthening low moving north between 150-160 West. Overnight
model runs had the ECMWF moving the system into Southwest, but the
latest versions are more in alignment in both intensity and
track- keeping the low moving toward Kodiak Island. This, and the
placement of a trough over the Boundary Range later in the week,
will be quite enough for forecasters to focus on in the coming
days.

&&

.AVIATION...
PANC... Gusty conditions should taper off this evening with
somewhat variable breezes becoming southwesterly late tonight. VFR
conditions will continue.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Mon
evening through Wed afternoon)...

Residual showers associated with a departing upper level
shortwave will gradually continue to diminish this evening. High
pressure at the surface has begun to build over the Gulf
resulting in gusty southeasterly winds through the usual gaps and
passes along the Kenai Peninsula, Turnagain Arm, and Knik Valley
this afternoon. These winds will persist into the early evening
before tapering off overnight tonight.

Generally fair, quiet weather is expected across Southcentral for
Tuesday before a North Pacific low approaches moves into the Gulf
for Wednesday. The leading edge of the front associated with this
low will bring precipitation to Kodiak Island beginning Wednesday
morning before lifting to the northern Gulf coast by Wednesday
afternoon.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Mon evening
through Wed)... An upper trough crossing the Alaska Range this
afternoon will continue to rotate to the west across Southwest
Alaska through this evening, supporting numerous showers as well
as the bringing potential for an isolated thunderstorm or two.
Showers will diminish late tonight through Tuesday morning, but
with an upper level low remaining offshore over the eastern Bering
and weak troughing extending inland, scattered showers will
develop again Tuesday afternoon and evening. As the upper low
offshore weakens to a trough and rotates further to the west in
response to a strengthening North Pacific frontal system tracking
north towards the Alaska Peninsula, weak ridging building into
Southwest Alaska Tuesday night will end the showers over the area.
Easterly winds ahead of the front will increase through the gaps
and passes in the Alaska and Aleutian Ranges through Wednesday
morning with the frontal system reaching the Bristol Bay area
Wednesday afternoon. The front will shear apart Wednesday night
but continue to bring precipitation to the Southwest Alaska as the
vertically stacked low tracks slowly to the north.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Monday
evening through Wed)... A frontal system extends east from a
vertically stacked low along the Aleutian Chain near 180 to a
second low just to the north of Nikolski. The more western low
will track to the southeast tonight swinging the eastern low up
towards the Pribilof Islands. The upper low near the Pribilofs
will remain in that general location and weaken through Tuesday
night while upper level ridging builds slowly east towards the
western Aleutians. A strengthening North Pacific low and
associated frontal system will track towards the Alaska Peninsula
late Tuesday night through Wednesday morning bringing gale-force
winds with it.

&&

.MARINE (Days 3 through 5...Thu through Sat)...

A weakening low will drift north towards Bristol Bay, leaving
mostly benign conditions and broad cyclonic flow over the Bering
and Aleutians Thursday through Saturday. Winds will also diminish
in the Gulf during the day Thursday with a high building in the
North Pacific. That said, expect combined seas around 15 feet in
the central Gulf in swell originating from the weakening low. Seas
will subside late Thursday into Friday. South to southeast flow
will prevail in the Gulf Saturday.

&&

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

The long term commences with a compact, but weakening low in the
western Gulf shifting northward towards Bristol Bay. The leading
front will bring precipitation to Southwest early Thursday. As it
heads northeastward, precipitation will spread to the northern Gulf
Coast, but likely not make it far inland. The front will be
relatively fast moving, and expect downsloping to limit
precipitation chances in the Anchorage Bowl.

The next low will approach the Central Aleutians late Thursday
into Friday. There is some uncertainty with its strength and
track, but the general consensus is that there will be a wetter
trend for Southwest Friday into Saturday as the associated front
passes over. Deep southerly fetch ahead of the low will support
precipitation in southern mainland Alaska over the weekend. It's a
bit soon to say with confidence where the heaviest precipitation
will be, but we are leaning towards the focus being in
Southcentral. While the operational GFS projects a much deeper low
heading into Bristol Bay, thus keeping more of the energy farther
west, the ECMWF and FV3 continue to maintain a weaker low. This
is diminishing confidence in the GFS solution. So, the current
expectation is for persistent wet weather over Southcentral
Saturday and Sunday. Precipitation will taper off Sunday night
into Monday and another low will develop over the western
Aleutians, potentially bringing unsettled weather to the southern
mainland by mid next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warnings 150 155 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JER
MARINE/LONG TERM...MM