Small Craft Advisory 37, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
544 AM AKST Fri Jan 11 2019

.ANALYSIS AND UPPER LEVELS...
The weather setup this morning is a persistent pattern, with an
upper level low situated over the central interior and a second
vertically stacked low situated south of the western Aleutians.
Focusing on Southcentral first, an upper-level shortwave
associated with the mainland low brought seasonably cold
temperatures overnight across much of the area. The Anchorage
Bowl, which recorded temperatures below zero, also saw a few snow
flurries overnight. Moving the focus to the west, the warm front
associated with the low continues to bring strong winds and mostly
rain, with a few snow showers, to the Bering and central Aleutian
Chain.

&&

.MODEL DISCUSSION...
Models continue to be in decent agreement in the near term with
the most significant low over the western Aleutians. This system
is expected to have multiple strong short waves which develops
several new surface low centers heading into Friday afternoon. Models
start to diverge on the solutions of these lows for Saturday's
forecast timeframe, making the MSLP comparison look like a true
spaghetti plot with differences in placement and strength of the
low centers as they move eastward. Forecast confidence remains
below normal for this time period.

&&

.AVIATION...
PANC...MVFR conditions and light snow are expected during the
morning as a strong upper level wave moves across the southern
mainland. Condition are expected to improve by early afternoon
with light northerly flow persisting through late Saturday
morning.

&&

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

The lingering upper low hovering over Southcentral is wringing
moisture out of stratus in the form of flurries from the northern
Kenai to MAT-SU Valleys this morning. Should see clearing skies
as the day progresses, however, there may be some lingering fog
across the Anchorage Bowl (Knik Arm) into the early afternoon
hours. Patchy fog will likely redevelop across the Anchorage Bowl
Saturday morning. Outflow winds will diminish across much of the
North Gulf and Eastern Kenai Peninsula today.

A developing surface low, well south of the Eastern Aleutians and
Associated occluded front will push north and usher in cloudier,
warmer weather with increased chances of precipitation this
weekend. The front rolls up toward Kodiak overnight and will
bring windy conditions to much of the Western Gulf. Precipitation
will likely start as snow at Kodiak but will rapidly change over
to rain. The front continues north through Saturday and will
likely bring snow to coastal Kenai Peninsula and Cook Inlet later
Saturday and eventually to the north Gulf/Prince William Sound
Saturday night. The cold air should be deep enough to the lee of
the Chugach range to mitigate downslope effects typical with
southeast cross-barrier flow. So expect some light overrunning
snow to likely make it across the Chugach Saturday night as front
heads north. In addition to very windy conditions over the marine
areas, gusty gap winds will likely develop in favored locations
(Portage Valley, Turnagain Arm) on Saturday. The onset of snow and
winds could make for some blowing snow concerns for Portage
Valley and eastern Turnagain Arm Saturday evening/night.

&&

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

More brutally cold air is on tap this forecast period. There is a
deep arctic vortex over mainland Alaska with the upper level low
hovering between the Brooks and the Alaska Ranges. This deep
freeze is going to last another 24 hrs, then the models ping into a
pattern shift. As time elapses, a ridge axis will nose its way
into Southcentral and Southwest Alaska. As that ridge builds into
southwest, it will cause the arctic airmass to retreat and will
bump temperatures up a noticeable amount (20F+ degrees).
Improvement is expected on Sunday and especially Monday.

&&

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

There is a 950 mb surface low that is south of Amchitka with an
occluded front wrapped across the Central Aleutians. There is a
secondary surface low southwest of the Amchitka low that has a
pressure of 980 mb. There is another low south of Unalaska with an
approximate pressure of 975 mb. There is a lot of action along the
chain. There is a tight pressure gradient north and south of the
Aleutians resulting in strong winds (storm force and gale force)
and rough seas. A fourth surface low enters the picture on
Saturday south of Attu Island. Expected unsettled weather this
entire forecast period. With a surface low pushing across the
Pribilof Islands early Monday.

&&

.MARINE (Days 3 through 5)...

Beginning the marine extended period forecast on Sunday, there
will be a gale force low moving into the eastern Bering Sea. This
low will move inland Monday, leaving behind weaker and broad
cyclonic flow over the Bering Sea, which will be a reduction in
winds and seas compared to the preceding two days. There is
uncertainty with the low tracks by Tuesday, but the storm track
will shift east into the Gulf of Alaska with a high likelihood of
a series of gale force fronts/low tracking in succession toward
the North Gulf Coast by Sunday and potentially lasting into
midweek. Confidence on timing or exact track by that juncture is
very low, but the pattern will be more active than it recently has
been for the Gulf.

&&

.LONG TERM FORECAST (Days 4 through 7)...

The state of the atmosphere will be rapidly changing on Saturday
evening as the deep upper level arctic trough over the interior
shears apart and retreats to the north. This will allow for a
barrage of fronts/lows to assault the Southern Alaska mainland.
This pattern will be messy to say the least, with warming
temperatures and threats for the entire spectrum of wintery
precipitation. The signal in the noise of this pattern is that a
very high amplitude negative tilt ridge axis will build over the
mainland through Wednesday. Low tracks are uncertain, but this
pattern will be active through at least middle of next week
before the storm track relaxes. However, that is still uncertain
with some guidance (the GFS, most specifically) keeps the
southerly flow onslaught through Thursday before upper ridging
tries to build. Key takeaway: expect warming temps and threats for
rain/freezing rain/mixed snow for most of Southern Alaska
beginning Saturday night and Sunday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 155 161.
MARINE...Gale 120 130 131 138 139 165 170 172 173 175 177 179 185.
		 Storm 178.
		 Heavy Freezing Spray 127 139 160 180 185.
FIRE WEATHER...NONE.

&&

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