Small Craft Advisory 919, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
543 AM AKST Thu Nov 22 2018

.ANALYSIS AND UPPER LEVELS...

An upper level low is positioned over the central Gulf of Alaska
at the base of the long wave trough that encompasses most of mainland
Alaska. Northerly flowing cold air advection is occurring along
the backside of the trough over the western Mainland, MAT-SU, and
Kodiak Island. Fog with dense low level stratus persists in the
Lower Kuskokwim Delta while off-shore flow is keeping much of
Southcentral fog free. Scattered precipitation continues over the
North Gulf Coast while it has dissipated in the west. As the low
travels southeastward, a ridge is building over the central
Bering. The Bering is mostly cloud free with a cloud cover
increasing over the land areas of the Aleutian chain. The next
system is emanate in the west as vertically stacked, gale force
low stemming from the North Pacific skirts the western Aleutians.
The associated front is pushing northeastward over the western
Aleutians into the Bering. Scatterometer wind detection shows gale
force winds just south of Adak.

&&

.MODEL DISCUSSION...
Models are in general agreement in the broad synoptic sense. There
are still significant differences in timing, position, and
strength of the next low heading into the western Gulf Saturday.
The GFS at this point looks like a good middle ground. Even though
there are significant differences the take away is that wet,
windy, and warming temperatures are in store for the Gulf and
Southcentral this weekend.

&&

.AVIATION...
Generally light winds will prevail. There is a possibility that
there will be a period of IFR/LIFR CIG and VSBY with fog this
morning. These conditions, if they develop, should dissipate by
afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

An inverted trough with embedded low centers continue over the
Gulf. One of these centers is drifting west across the northern
Gulf producing showers and gusty easterly winds. The pinch in
pressure gradient from the large scale Gulf trough and higher
pressure inland has given rise to gusty outflow winds across the
North Gulf Coast. These conditions will continue through today
then weaken this evening. Widespread low clouds and fog over the
Copper River Basin will lift some today, but should settle back
in tonight and linger into Friday afternoon. Areas of fog over
Western Kenai to the lower Matanuska Valley should break up
through today. The inversion should weaken with an upper trough
moving across the area tonight, so a repeat performance is not
likely.

A weather front approaching the western Gulf brings winds and rain
to Kodiak and surrounding marine and offshore waters Friday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
A ridge builds across the Southwest today which will improve fog
conditions and bring colder than normal temperatures through
Friday. The next front brings increased precipitation chances for
the start of the weekend. The surface low driving the front
positions just south of the Central Aleutians on Friday afternoon.
The warm boundary runs ahead, pushing to the Bristol Bay coast by
late afternoon. Light snow is expected with this front as the low
loses momentum through Friday night. The highest accumulations
between 2 to 4 inches are possible across Bristol Bay as warm
moist air advects over the cold pool inland.

A second developing North Pacific low races toward the Gulf on
Saturday. This system brings a re-enforcing push of moisture
along with forcing of a warmer air mass across the Southwest.
North to easterly flow limits precipitation with this system;
however, rain will be the main challenge with this second system
as the warm air mass pushes inland across the Aleutian Range.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
An active weather pattern is expected through the start of the
weekend as low pressure along the Aleutian Chain ushers several
fronts toward the southern mainland. The first front slides across
the Central Aleutians this morning bringing storm force easterly
winds on its track to the southern Bering through this afternoon.
The boundary elongates eastward as the low becomes stationary just
south of Adak. Gusty winds and rain associated with this system
increase across for Dutch Harbor this afternoon and then overnight
for the Alaska Peninsula and Pribilof Islands. Rain is expected
for locations along the southern Bering waters with snow and for
locations along the Western Capes to the Pribilof Islands. The
frontal boundary diminishes as it reaches the northern waters
early Friday morning.

There is another system that races toward the eastern Aleutians
early Saturday morning, however there is still uncertainty on the
track and timing with this second system. Colder air is expected
to advect back toward the western Aleutians on saturday as the
track of this low forces the winds to back to north from the
Pribilof Islands west.

&&

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

The latest model runs have come into better agreement regarding
the evolution of the weather pattern on Sunday. A strong low
centered over the North Pacific well south of Cold Bay will have
developed a secondary triple point low near the southern Gulf late
Saturday night. While the secondary low is expected to track near
the southern end of Kodiak Island by Sunday afternoon, its strong
warm/occluded front off to its east will ultimately be what causes
active weather across Southcentral throughout the day Sunday.
Along and behind the front, an atmospheric river will develop that
is likely to produce heavy rainfall along the Gulf Coast.
Currently it appears the target for the heaviest rain will be from
Prince William Sound eastward, though that is subject to change
with future guidance as they get a better handle on the synoptic
situation. Either way, the models are in good agreement that
additional triple point lows are likely to develop ever further
east with time, which in turn will move the atmospheric river east
as well. Thus, the relatively brief time the atmospheric river
will be over any once location should mitigate any flooding
concerns. In addition to the rainfall, the front will increase the
pressure gradient across the gaps out of the Copper River Basin,
including Thompson Pass, the Matanuska Valley, and the Copper
River Delta. This should result in a period of strong winds on
Sunday through those gaps, though exact wind speeds are still yet
to be determined.

After the first low moves past Kodiak on Sunday afternoon, it
looks to continue tracking northwestward into Southwest Alaska or
along the coast. This will bring warmer air into that region,
which in turn appears favorable for another round of mixed
precipitation and freezing rain for Southwest Alaska Sunday night
into Monday.

A weaker, slower moving low will follow behind these systems,
moving into the Gulf Monday afternoon and evening. While there is
increasing disagreement on what the low will do once it gets
there, the general consensus right now is for it to stall over the
Gulf, resulting in a continued wet pattern, especially along the
Gulf Coast right into midweek.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Ashfall Advisory: 181.
         Dense Fog Advisory: 111.
MARINE...Gale Warning: 170, 172, 177, 178, 411, 412, 414.
         Storm Warning: 173, 174, 175, 176, 413.

&&

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