Small Craft Advisory 298, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
507 AM AKDT Tue Apr 10 2018

.ANALYSIS AND UPPER LEVELS...
The latest satellite image has the low located south of Kodiak
with moisture being entrained from multiple directions. The
precipitation shield from Bristol Bay extends across the AKpen
and towards the low. Additional moisture gets wrapped in from the
Gulf of Alaska. The low continues to deepen and move towards the
northwest. The latest scatterometer pass has detected gale force
winds in the Gulf of Alaska and Shelikof Straight in association
with this intensifying low. The Middleton Island radar (PAIH) has
an expansive rain band from Seward to Cordova and the Kenai radar
(PAHG) has also detected a band near Seldovia along with activity
over the Cook Inlet. There is a second surface low moving
through the western Bering. This system has already reached the
pinnacle of its life cycle and is beginning to fill. There are
still strong winds associated with this system. A peak wind of 52
knots was recorded at Shemya overnight.

&&

.MODEL DISCUSSION...
There is good consensus amongst the models with the synoptic
features over the Bering and the North Pacific through Friday. The
Gulf of Alaska is a little more tricky. The models do
exceptionally well with the low currently south of Kodiak and its
merger with the remnants of the frontal boundary over the AKpen.
And then the models all have the low near Kodiak pushing to the
southeast and decaying. But there is a second disturbance that is
near Southeast Alaska and that low is messy. The Canadian and the
ECMWF are in one camp. While the NAM and the GFS have diverged
and are taking that disturbance elsewhere which is reflected at
both the surface and the 500 mb.

&&

.AVIATION...
PANC...VFR conditions are expected through Wednesday. There are
some showers over Cook Inlet on the Kenai Radar (PAHG), however
they should remain southwest of PANC.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A deep vertically stacked low will pass just south of Kodiak
Island this morning. Buoy 46078, which is close to the low
center, indicates a central surface pressure of 968 mb as of 12Z.
The pressure may drop a few more millibars this morning before
beginning to fill and track back southward away from Kodiak
Island. The worst conditions will be this morning across the
western Gulf (including Kodiak Island and on up to southern Cook
Inlet) as the occluded front pushes up against the mountainous
terrain of the Alaska Peninsula, with pressure gradients at there
tightest. The occluded front wraps around along the north Gulf
coast where rain will continue, especially in western Prince
William Sound. The front will not make any further northward
progress, so the rest of Southcentral will remain dry today with
some mid to high level clouds.

Focus will quickly shift to the eastern Gulf this afternoon as a
short-wave trough lifts northward along the British Columbia and
Southeast Alaska coastline. This will lead to strengthening of a
surface low, which will then rotate westward across the Gulf
tonight, over top of the exiting low. Numerical guidance has been
all over with handling of this low, though there is increasing
confidence in a deeper low tracking to Kodiak Island. This will
bring another round of rain to Kodiak Island and western Prince
William Sound/eastern Kenai Peninsula late tonight through
Wednesday, along with small craft to gale force winds across much
of the Gulf and Kodiak Island waters. With a trailing upper level
trough moving into Southcentral, some showers or light precip
could make it inland, but expect most areas will remain dry and
warm.

&&

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

Two different easterly waves will be impacting portions of
Southwest Alaska through the short-term forecast. The first is
moving through this morning. It is producing isolated to scattered
rain and snow showers, but with temperatures above-freezing nearly
area-wide, any snow showers will not be producing any
accumulation. These very light showers will move off the coast
into the Bering today, as light offshore winds develop over the
rest of Southwest Alaska. The parent low of this wave is passing
south of Sitkinak Island, where it is forecast to stall later
today. Thus, much of Southwest Alaska will see overcast skies all
day, with less cloud cover the further north you go.

The next wave will take a similar track, but will be a bit further
north, likely tracking near the southern tip of Kodiak Island
Wednesday evening. However, this second wave is the weaker of the
two, and as such a similar situation to this morning is expected
again Wednesday night as the wave tracks westward into Bristol
Bay...namely precipitation that will be isolated, light, but most
likely near Bristol Bay with very little expected further north.

This otherwise cloudy, quiet pattern should make for a
continuation of the steady state temperatures we are seeing
currently across Southwest Alaska. Look for highs in the 40s to
near 50, with lows within 5 degrees of the freezing mark for the
next few days.

&&

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

Two separate lows and a front will be impacting the Bering over
the next couple days. First, a nearly stalled out low over the
western Bering is steadily weakening, but is still causing strong
winds over much of the western and central Aleutians, along with
numerous rain and snow showers that are quickly tracking eastward
across the chain. The low will continue weakening and shrinking as
it turns northwestward towards Kamchatka, no longer an influence
in the Bering by Wednesday. An associated front extends between
this low and the one currently south of Kodiak. This front is
acting as a conveyor belt for moisture to move across the Bering,
resulting in steady rains across the far southern Alaska Peninsula
and nearby islands. The front too is weakening and will largely
dissipate in place on Wednesday.

The second low is rapidly moving towards the western Aleutians
from the northwest Pacific this morning. The forecast track of
this low takes the center well south of the Aleutians tonight
through Wednesday night. Thus, the western and central Aleutians
will see only a quick glancing blow with some associated rain and
snow. The low is also a fast mover, so any one location should get
only about 12 hours of precipitation before the entire system has
passed them by. The northern fringe of precipitation will pass
entirely south of the Aleutians by Wednesday night, leaving the
eastern Aleutians and Alaska Peninsula precipitation-free from
this particular system. However, another unassociated weak
disturbance moving westward from the Gulf will bring its own
precipitation to the Alaska Peninsula Wednesday night.

&&

.LONG TERM FORECAST (Days 3 through 7)...
On Wednesday night the upper low south of the Aleutians and
tracking east will merge the broad upper low complex stretched
across and to south of the Gulf to consolidate into a single but
rather large upper level low centered over the far southwestern
Gulf Thursday. This upper low will shift slowly east Thursday
night through Friday as an upper level ridge builds north into the
central Bering. In the southwesterly flow to the west of the
ridge, a strong north Pacific low spins up and tracks north across
the western Aleutian Friday night or Saturday. The progression of
the upper level ridge through the weekend and the response of an
arctic low/trough over the Arctic Ocean and far northern Canada
remains uncertain with various model solutions producing
significant different outcomes for Southern Alaska.

The GFS has continued to trend more westerly, with a rather high
amplitude version of the upper level ridge centered over the
eastern Bering. This positioning and a strong arctic trough
digging south through resulting northerly flow over mainland
Alaska will bring a significantly colder air mass to southern
Alaska for the weekend and strong, dry north to northeasterly
outflow winds Saturday and Sunday. In contrast the 00Z EC and
Canadian solutions favor an eastward progression of the upper
level ridge with its axis building over the Alaska Peninsula and
Southwest Alaska Saturday and shifting east over Southcentral
Alaska on Sunday. The progressive ridge solution would bring
warmer temperatures for the weekend, accelerating this weeks
gradual warming trend rather than sharply reversing it as the cold
trough solution would.

While both of these scenarios would have significantly different
outcomes for the weekend weather, models do return to better
synoptic agreement for next week.  Westerly zonal flow and an
active storm track returns to the Aleutians and Bering Monday and
Tuesday and then progressively pushes east, breaking down/pushing
east whatever version of the ridge is left.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm 131 138 139.
         Gales 119 120 130 132 136 137 150 155 165 172 174-78
               351 352 411 413.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JPW
LONG TERM...JR