Small Craft Advisory 118, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
417 PM AKST Tue Feb 7 2017

.ANALYSIS AND UPPER LEVELS...
There is an upper level ridge over most of southern Alaska
extending across the Alaska Peninsula into the central Aleutians.
There is a closed cut-off low centered just to northwest of
Nunivak island. This is a fairly dry low and only minimal
precipitation is associated with it at this time. The rather
strong Polar Jet is pushing into northern Alaska and beginning to
drop southward.

&&

.MODEL DISCUSSION...
The numerical models are in fairly good agreement through the
short term portion of the forecast (Thursday afternoon). The Nam
begins to diverge from the other models Thursday morning with the
low moving from Bristol Bay into the Gulf. The Nam opens it up and
has it further north than the other solutions.

&&

.AVIATION...
PANC...LIFR conditions are expected to increase again this
evening. Conditions should improve later tonight as a mid-level
deck of clouds (bases around 3500-4000 feet) moves in. This deck
of clouds will help to break up the surface based inversion a bit.
Confidence in the exact timing of this is low, as well as with how
quickly conditions will improve.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Another night of fog around the northern Inlet, followed by
increasing chances for snow late tomorrow. Fog is still hanging
out around Turnagain/Knik Arm, though it has eroded a bit more
than yesterday. There should be some fog pulsating around the area
tonight, however, conditions are not as conducive as the previous
two nights. While an upper ridge will be overhead, heights will be
falling, suggesting cooling in the low/mid levels. Hence, the
inversion in the low-levels should be weakening. The other aspect
going against fog is the mid-level cloud deck building over the
Kenai Peninsula. As the deck advects/builds northward cross-
barrier flow increases over the Alaska/Aleutian range which will
try and dry the low-levels out at the same time. The question for
late tonight/tomorrow will be how long does the mid-level deck
hang out.

Wednesday evening a weak disturbance in the flow will bring a
low-level trough into the Cook Inlet region. Weak isentropic lift
in the southerly flow ahead of the trough should bring some
flurries to the Kenai through the Anchorage Bowl and Matanuska
Valley. Things quickly dry out on Thursday in cold advection.
Overall expect below normal temperatures and generally light
winds.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The low over the northeastern Bering is bringing onshore flow to
southwest Alaska with low stratus and light snow. The low will
track southeast through the eastern Bering and then across the
Alaska Peninsula Wednesday night. As the low passes, flow across
southwest Alaska will shift to a dry offshore flow that will bring
colder air to the area.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The main feature to impact the Bering and Aleutians is a front
that is approaching the Western Aleutians from the North Pacific.
It will make it to the islands tonight and then begin to stall in
the southern Bering stretching across the Central Aleutians as
the parent low weakens. This system will bring stronger winds as
it initially pushes through that will weaken into Thursday. It
will also have a strong push of warm air that will keep most of
the precipitation from this system falling as rain.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A cold longwave trough over interior Alaska will begin to drop
down over Southern Alaska Friday morning, bringing colder
temperatures to much of the region through Sunday. As this colder
airmass settles in over the area Friday, a surface low over the
Gulf of Alaska begins to strengthen and move over Prince William
Sound. There is good agreement in the models and higher confidence
in precipitation developing along the Northern Gulf coast areas
and over parts of the Copper River Basin Friday and into Saturday.
However, models are still struggling with the positioning of the
upper level low and various surface features, which has lead to
lower confidence in snow potential over the Anchorage, Mat-Su
Valleys, and Western Kenai areas. One can say with high confidence
that several areas of low pressure will move into the Gulf of
Alaska this weekend through the middle of next week, but it's
nearly impossible to even imagine where these features may track
given the poor agreement amongst the models.

Out west, a pattern change will also be underway over the Bering
Sea and northern Pacific, as the receding high pressure over the
areas gets pushes westward by the above mentioned cold longwave
trough tracking through the mainland. This will make way for the
upper level trough to set up over the Bering Sea and Northern
Pacific by early next week, with multiple systems at the surface
mainly staying along or south of the Aleutian chain through the
middle of next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 177 178 Heavy Freezing Spray 180 .

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...ML
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...PEPE