Small Craft Advisory 806, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 PM AKDT Thu Oct 18 2018

.ANALYSIS AND UPPER LEVELS...
An upper level long wave trough fills most of the Bering Sea and
southern Alaska today. The closed low still sits near the center
of the trough in the eastern Bering while the trough digs into the
North Pacific. There are several shortwaves embedded in the flow
of the trough, most notably along the Gulf of Alaska coastline.
This shortwave is associated with a band of higher vorticity and
instability which encompasses much of the western Gulf. Behind the
trough, the weak ridge that once sat over the central Aleutians
is being whittled away as the next upper level low is moving in
from the west. This low is starting to deepen as it sits in a
favorable position of the jet stream which is arching just south
of the western Aleutians.

At the surface, ridging in the eastern Gulf is advecting warm air
in from the south. This, combined with cooler air aloft, the
short wave trough, and relatively warm sea surface temperatures,
has caused the atmosphere to become unstable. The instability is
leading to convective storms to form along the north Gulf coast
and showers to push inland over Southcentral. Satellite imagery is
picking up on the boundary between the cool and warmer air masses
with more scattered cloud cover on the leading edge of the warm
front and a stratus deck behind it. With the east being fairly
active, western and southwestern Alaska is relatively calm. The
occluded front associated with the eastern Bering low continues to
decay while a front connected to the Kamchatka low slowly moves
eastward over the western Aleutians.

&&

.MODEL DISCUSSION...
Models have come into good synoptic agreement and have locked
onto a track for a low pressure system stemming from the Pacific.
The one notable change from yesterday is that all models are now
bringing the low inland after moving through Prince William Sound.
Previous runs were bringing the low over the Kenai Peninsula.
This is a small difference, but it will have an impact on the
eastern Kenai Peninsula and Prince William Sound forecasts.

The next low that will move into the western Gulf to eastern
Bering Sea this weekend has a tighter solution between the models
which is a big improvement from yesterday. The most likely track
now looks to be right over Kodiak Island Saturday night and then
into the Bristol Bay region.

Models are also handling the Kamchatka low well early in the
period. Models begin to diverge on Saturday as there becomes a
spread in the low center. The EC leads the pack with the NAM being
the slowest of the solutions. This pattern continues through
Sunday.

&&

.AVIATION...
PANC...Winds will remain northerly and increase as the night goes
on, diminishing late Friday morning. Rain will begin Friday
morning, but VFR conditions are expected.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Today through Saturday)...
A strong storm will bring gusty winds and heavy rain across the
gulf and surrounding coasts tonight and Friday then pushes inland
Friday night. The surface low is expected to deepen to 965 mb as
is moves across the central gulf Friday morning, so expect
directional wind shifts through Friday night. The heaviest
precipitation with this system will be along Prince William Sound
and the northeast gulf with rainfall amounts amounts between 3 to
5 inches. Another precipitation challenge with this system will be
snow as freezing levels decrease Friday into Saturday. The most
likely areas to see snowfall will be along the western Copper
River Valley and along the ranges across all of Southcentral.

Then next system quickly follows with a warm front pushing into
the southern Gulf Saturday afternoon. The associated low deepens
to 970 mb as it tracks to Kodiak Island Sunday morning, then fills
as it moves inland over Bristol Bay during the afternoon. The
highest precipitation accumulations with this storm will be along
the eastern Kenai and Prince William Sound locations with amounts
up to 4 inches, although locally higher amounts are possible.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Thursday
night through Sunday)...

Showers associated with the weakening low over Bristol Bay will
taper off this evening. As the low in Bristol Bay weakens, a
strong low moving into the Gulf of Alaska will attempt to push
moisture over the Aleutian/Alaska Ranges. Based on a further east
storm track, we do not expect moisture to move as far west as we
did yesterday. The low in the Gulf of Alaska will also induce
northeast flow for the Kuskokwim valley tonight into tomorrow,
which will actually scatter skies out and create drying
conditions for the area. This will lead to some patchy valley fog
tonight, especially along waterways. This flow will also cool
temperatures to around freezing overnight Friday into Saturday
making it possible for a rain/snow mix when moisture works its way
back into the area Friday night into Saturday morning.
Precipitation is then expected to transition back to rain as
warming temperatures and cloud cover move back into the area.

On Sunday, the next low pressure system will move into the Gulf
of Alaska, with the low center located south of Kodiak Island.
With increasing confidence in the storm track, we have increased
winds and precipitation for the Alaska Peninsula/Bristol Bay
area.

&&

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

In the Bering Sea, the low that has been generally in place will
continue to spin slowly southeast and weaken into an elongated
trough overnight tonight. Northwest winds at about 25kts on the
backside of this low will continue until the low dissipates. Some
funneling of winds is expected to occur on the south side of the
eastern Aleutians and Alaska Peninsula as well. A low moving into
the Gulf of Alaska tomorrow will shift winds to a general
northwest direction the next couple of days, with just some rain
showers present. A shortwave ridge moving through the central
Aleutians will keep that region relatively quiet.

Meanwhile, a large low pressure system will push into the western
Aleutians tonight. The strong southerly winds ahead of this system
will push east tonight, weakening as they do. Winds at Shemya were
stronger than initially forecast in this band of wind, so
strengthened them slightly and slowed their weakening until
tomorrow. The low will slide south of the Aleutians the next
couple of days, keeping the strongest winds out of our forecast
area. Enough instability is present in this system that we kept
slight chance of thunderstorms in the forecast for the western
Aleutians.

&&

.LONG TERM FORECAST (Days 4 through 7: Sunday Night through
Thursday)...

Sunday night starts off with the remnants of the low slowly
diminishing overnight. Onshore southeasterly flow is likely along
the entirety of the Gulf Coast as the low center moves into
Southwest Alaska and slows its northwestward movement
significantly. Unfortunately for areas such as Seward and the
eastern Kenai Peninsula, this will allow the associated rainfall
to continue through much of the overnight and into at least the
first part of the day on Monday. However, rainfall rates will be
diminishing throughout the night as the parent low quickly
weakens. The low's westward track will bring in a renewed slug of
warm air causing wintry precipitation to remain confined to
higher elevations. Rain will persist with diminishing rates
across portions of Bristol Bay and the Lower Kuskokwim Valley on
Monday. The low will also help to draw colder air southward
across the Bering on Tuesday, which may threaten the Pribilofs
with some snow showers.

Turning back to the east on Tuesday, the models are coming into
much better agreement compared with yesterday's runs concerning
the development of another strong low in the Gulf. As expected,
there is still plenty of disagreement on the details, but all are
showing at least a gale force low, with perhaps storm force winds
in localized areas. It looks to develop over the southeastern
Gulf and track northwestward, reaching the central Gulf Tuesday
afternoon, then moving into the eastern Kenai Peninsula Tuesday
evening. Along this track, moderate to heavy rain at times will
overspread much of the Gulf Coast starting Tuesday morning and
then diminish Tuesday night. At the time of this forecast, this
storm does not look to have the same moisture feed as the Sunday
storm. Thus, while excessive rainfall amounts are not expected
anywhere, being the latest in this train of storms will not help
any ongoing flooding concerns in places like Seward.

In similar fashion to Sunday night behind the storm Tuesday night,
onshore flow and continued upslope rain will persist through the
day along much of the Gulf Coast. The low will leave nearly all
of its moisture along the coast, with relatively little expected
to make it inland. What the low will do out west however, is help
to draw increasingly colder air southward out of Russia into the
Bering. As a result, snow showers become increasingly likely as
far south as the Aleutians Wednesday and Wednesday night. By
Thursday, the models hint at...guess what...ANOTHER strong low
tracking northward out of the Pacific into the Gulf, but by that
time uncertainties are very high. There is also growing consensus
that some of the cold air over the Bering getting into Southwest
Alaska, particularly the Delta on Thursday, may produce some
below average temperatures somewhere in the area for a change.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 119 120 130>132 139.
 Gale Warning 125 127 136>138 140 141 150.

&&

SYNOPSIS AND MODEL DISCUSSION...RA
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
LONG TERM...JW