Small Craft Advisory 33, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
853 AM AKST Thu Jan 10 2019

.ANALYSIS AND UPPER LEVELS...
An impressive 933 mb vertically stacked low situated south of the
western Aleutian Chain is the dominating feature across Alaska
this morning. Satellite imagery shows this quick moving system,
which has reported lightning strikes, moving across to the central
Aleutians early today and will continue to progress eastward. The
ASCAT scatterometer pass has already shown gale force winds
associated with this low that will continue to impact the
Aleutians and Bering. A weak front stretching down from the
northern Bering, associated with an upper level trough, continues
to lift northward above the Pribilof Islands.

Southwest and Southcentral are seeing a different weather story
unfold this morning as temperatures continue to drop down from
Interior Alaska. A Wind Chill Advisory is in effect for Thompson
Pass until this afternoon, which has already seen wind chills
near 50 below zero through the overnight hours. The Kuskokwim
Delta also had a Wind Chill Advisory through Saturday. These
areas, including Bethel and Hooper Bay, have experienced wind
chill values between 30 and 40 below zero starting late last
evening. Cold temperatures continue to spread across Southcentral,
as well as dry conditions associated with a weak upper level
trough situated over the area.

&&

.MODEL DISCUSSION...
Models tend to stay in the same general agreement with the low
over the western and central Aleutians through Friday afternoon.
However, models are struggling with the position of a second low
south of the Aleutians that moves to the central Aleutians by
Saturday afternoon. The NAM tends to by the outlier as it sits
over the Chain, while the GFS and Canadian are stronger and
sitting further south.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Thursday through Saturday)...

The broad upper low will drift south to western Alaska later
today then lift north on Friday. Lingering cloud cover over inland
areas of Southcentral will dissipate tonight as cold and dry air
pushes in from the west. So expect fairly widespread below zero
temperatures overnight with the coldest temperatures over the
Susitna Valley and Copper River Basin. Gap and outflow winds
across the North Gulf and the Eastern Kenai Peninsula should peak
this morning then slowly diminishing through tonight.

A strong weather front heads north Friday night and should be over
the Kodiak Island and northern offshore waters by Saturday
afternoon. This system will bring snow initially to Kodiak Island
Friday night, which should then quickly transition to rain or mix
Saturday morning. Snow will then spread north to the Southcentral
Mainland/Gulf coast Saturday, though there still some uncertainty
as to how quickly as models are still struggling with the
handling of this system. The one common factor is that warmer,
windier, and cloudier conditions are on the way this weekend.

&&

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

There is a deep arctic vortex over most of mainland Alaska with a
low of 500 DAM at 500MB. The H5 low slowly pushes south of the
Brooks Range bringing that brutally cold air into Southwest
Alaska. As this H5 lows descends, the models actually have the low
deepening to 495 DAM. The coldest temperatures will arrive
tonight, with values as low as 38 to 40 degrees below zero. In
addition to dropping temperatures, offshore flow will add a
windchill factor over the Kuskokwim Delta, thus a Wind Chill
Advisory will be in effect Thursday evening through midday
Saturday. There is a pattern shift this weekend as the ridge axis
builds into Southcentral and Southwest Alaska. As that ridge moves
into southwest, temperatures will begin to moderate.

&&

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

Meanwhile, there is a fierce and beastly baroclinic low in the
North Pacific that is southwest of Adak and has a surface pressure
of 935 mb. This storm is a classic bomb cyclone (one which
deepens 24 mb or more in 24 hrs). The pressure gradient forces on
this storm will hold through Friday bringing rough seas and strong
winds on both the Pacific and the Bering side of the chain. The
surface low is barreling towards Adak and the orientation of the
upper level trough will become more negatively tilted. The surface
boundary then stalls. A secondary impulse will move towards Adak
and eventually the surface low will reach the Pribilof Islands
with a surface pressure of 960 mb. Expect a prolonged period of
unsettled weather.

&&

.MARINE (Days 3 through 5)...

An active storm track will continue to bring a series of lows and
frontal systems north over the marine waters of the Aleutians,
Alaska Peninsula and western Gulf through this weekend. While the
precise details of timing and track of each system remains
uncertain it is likely that gales will impact much of these
waters at times. For the Bering, the persistent, cold east to
northeasterly gales and possible storm force winds in the
southwest Bering on Friday and Friday night will ease somewhat
through the weekend. The stronger winds will turn more northerly
in direction and shifting further west and north as vertically
stacked surface lows begin pushing further north and stalling out
over the east and central Bering.

&&

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

On Friday and Friday night, upper level ridging will build into
the Gulf and southern Mainland Alaska pushing the cold upper level
trough further north and into interior Alaska. Warming air aloft
will spread across the southern third of the state on Saturday,
however northeasterly offshore flow will keep low level
temperatures cooler with moderate to strong low level inversions
remaining in place. A surface low will lift north to Kodiak Island
on Saturday with a second deeper low lifting north to the eastern
Aleutians. A third larger vertically stack low will be located
south of the western Aleutians on Saturday with a cold upper low
over the northwestern Bering.

On Saturday night and Sunday, upper level ridging will continue
to build over western Canada and mainland Alaska with the surface
low over the eastern Aleutians tracking north across the Pribilof
Islands and into the central Bering. Southeasterly flow will
strengthen and build north into the Gulf with east to
northeasterly flow and overrunning precipitation a potential
concern for Southwest Alaska. In addition to windy conditions,
moderate to heavy rain will spread north across Kodiak Island late
Saturday night through Sunday morning. Rain at the lowest
elevations along the coast and snow inland over the Kenai
Mountains will continue north along the eastern Kenai Peninsula,
over Prince William Sound and to the northern Gulf Coast Sunday
through Sunday evening.

A series of additional North Pacific low pressure systems will
track north across the Alaska Peninsula and into Southwest Alaska,
Bristol Bay and eastern Bering through the first half of next
week. The associated frontal system will lift north across the
Gulf and into Southwest Alaska. While each of these systems will
continue to bring an additional round of rainy and windy
conditions to the Gulf coast and gradually warming temperatures
further inland, persistent offshore flow will keep much of
southcentral Alaska somewhat drier with temperatures persisting
at or below freezing. For Southwest Alaska precipitation type is
going to be an ongoing challenge with the strength of the
northeasterly flow for each system determining whether rain, snow,
blowing snow or freezing rain will be the primary impact.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 155 131.
MARINE...Gale 414 185 179 174 173 172 170 127 130.
 Storm 412 413 411 178 177 176 175.
 Heavy Freezing Spray 412 413 411 178 177 176 175.

&&

SYNOPSIS AND MODEL DISCUSSION...PAMBER
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...JR