Small Craft Advisory 132, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
526 PM AKST Sat Feb 11 2017

.ANALYSIS AND UPPER LEVELS...

An area of low pressure continues to slide to the Northeast just
to the West of Yakutat this afternoon bringing show showers to
Eastern Prince William Sound. Another low south of the Aleutian
chain is helping to keep a fairly persistent NE wind regime across
the Bering today. At the surface and closer to the Anchorage
Metro, a fairly light wind and moisture rich boundary coupled with
a stout inversion at 2500 ft are helping to lock in a persistent
layer of clouds out over the Inlet. Out West and across the
Kuskokwim Delta, temperatures remain well below zero and any hints
of wind continue to bring near advisory criteria for windchill.

&&

.MODEL DISCUSSION...

Models all appear to be in good agreement in the short term. All
solutions seem to have a similar handle on how an upper level
polar low will dip out of the Arctic and get pulled into the flow
of the strong pacific jet. Differences start to show near the
Monday afternoon timeframe with the placement of a surface low
that will be kicked out of the Pacific at that time. The overall
idea is that a low will track to the North and Northwest, slipping
across or just West of Kodiak Island. Flow for areas east will be
southerly and moisture filled. By Monday afternoon all models
highlight the tipping point crossing Anchorage where precipitation
type flips from snow to rain. However, the details from high
resolution guidance suggest that Anchorage will remain primarily
rain shadowed but above freezing.

&&

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

A big pattern change is on the way for Southcentral Alaska as the
upper level pattern is quickly changing. Previously, the jet
stream was located over the northern Pacific Ocean south of
Alaska. However, a series of upper level disturbances have helped
amplify the longwave pattern and in return has dug out a deep
trough over the western Pacific Ocean with the eastern part of the
upper level trough pointed straight at Southcentral Alaska. This
will allow a tap into very moist and very warm subtropical air to
develop and advect northward toward Alaska. In the mean time,
temperatures have dropped off significantly the past 24 to 36
hours in the wake of a very cold arctic trough that brought snow
showers to much of the region. Well below normal temperatures will
continue through tonight and Sunday morning with many interior
locations dropping well below zero tonight and coastal locations
generally in the single digits to lower teens.

The period of dry and colder weather will be short lived as a
series of lows and front from the north Pacific quickly move
northward into the Gulf of Alaska. As this happens, a very warm
air mass will begin to erode the cold air mass that just situated
itself over Alaska just yesterday. Initially, it looks like heavy
snow will upslope along the eastern Kenai Peninsula and Prince
William Sound Area but by Monday afternoon, many locations along
the coast will begin transitioning to rain. Models have trended
further west with the low placement toward the west side of Kodiak
Island which will likely keep Anchorage downsloped through much of
the event although a significant warm-up is still expected. Some
snow showers, then possibly a short duration of freeing rain and
then finally intermittent light rain will be possible for Anchorage
on Monday which could really cause travel and commuting issues.

The Susitna Valley will likely continue to pull in colder
temperatures from the interior of Alaska and see some decent
snowfall. A winter storm watch has been issued for the Susitna
Valley Sunday night and Monday as blowing snow concerns through
broad pass are expected with moderate snowfall and winds between
25 and 40 mph. In addition, snowfall between 8 to 12 inches with
much higher amounts near mountains look quite likely west of the
parks highway as moisture upslopes along the terrain.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Most of southwest Alaska will stay under clear skies that will
allow for cold temperatures overnight. However, winds should stay
light so wind chills should not drop much further than the
temperatures. Sunday into Sunday night a front will approach from
the south helping to increase winds across all of southwest Alaska
through the night.

For areas of the lower Kuskokwim Valley, temperatures and winds
will be at their lowest early on and increase overnight, which
should keep wind chills warmer than -40F. The Kuskokwim Delta
temperatures Sunday night look to stay more steady throughout the
night and with the winds wind chills will drop into the -30s.
Areas around Bristol Bay will see their winds increase as snow
moves in with the front Sunday night which will bring some blowing
snow to the area. For all of southwest Alaska there is some
uncertainty as to how fast and far the front will bring in warmer
air and how much it will increase the winds.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
With high pressure to the west and low pressure to the east, the
Bering and Aleutians will stay under cold northerly flow through
Monday. This flow will also bring light snow to the area.

&&

.LONG TERM FORECAST (Days 3 through 7)...
By Monday night the next deep low pressure center will be
rotating across the southern end of Kodiak Island while the
associated frontal system lifts north across the northern Gulf
Coast and into Southcentral Alaska. With this being the second
intrusion of warm, moist air inland and with more pronounced
downsloping southeasterly flow...expect rain to spread further
inland with the system. There is still significant uncertainty
regarding where in the Susitna Valley the rain/snow line will set
up but further south, above freezing temperatures look very likely
for the lower to mid elevations (below 1500 ft inland and 2500 to
3000 ft near the Gulf Coast).

Conditions also look favorable for a strong barrier jet to
develop Monday Night and Tuesday bringing at least strong Gales
to the northern Gulf coastal waters and extending into Prince
William Sound. These strong east to southeast winds will funnel
and accelerate through Portage Valley and Turnagain Arm as well as
along the ridges of the Kenai Peninsula and Chugach Mountains.
Models rapidly diverge Tuesday and Wednesday with regards to
timing and position of lows but agree on another strong frontal
wave or two lifting across the Gulf through midweek. Cooler
temperatures return with an upper level trough and southwesterly
flow Thursday and Friday, bringing the snow level back down to
sea level. Numerous lows and troughs rotating in off the western
Gulf will bring the potential for snow at times to Southcentral
from Thursday through the weekend.

For Southwest Alaska easterly waves rotating across the Alaska
and Aleutian ranges will be the primary concern through midweek as
an upper level low remains centered over the western Alaska
Peninsula with an accompanying deep pool of cold air. The initial frontal
wave crossing Southwest Alaska Monday night will bring a good
potential for snow, followed by several more Tuesday and
Wednesday. To the west of the upper level low, northerly flow will
continue with scattered to numerous snow showers continuing across
much of the Bering. An arctic upper level trough becomes
established over western Alaska Thursday and Friday bringing a
drying trend and a return to very cold temperatures.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Watch 145.
MARINE...Gale 119 120 125 130 131 138 139 150 155 172.
 Heavy Freezing Spray 121 126 127 129 130 139 141 150 165 179 185.

&&

SYNOPSIS AND MODEL DISCUSSION...SS
SOUTHCENTRAL ALASKA...MC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...DS