Small Craft Advisory 92, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 AM AKST Tue Jan 29 2019

.ANALYSIS AND UPPER LEVELS...

This morning's analysis showed what's essentially a positively
tilted Rex Block that has been in place from the eastern tip of
Siberia through the far western Aleutians, with one of the vort
max ejecting from the Aleutians low currently located south of
Nunivak Island. A resultant trough extended southeast across
Kodiak Island into the northern Gulf. Further east, an inverted
ridge extended from the eastern Pacific northwestward to the Yukon
border. This type of synoptic pattern has resulted in deep
southerly flow across southcentral, as the latest CIMSS MIMIC Layer
precipitable water image loop showing both lows (western
Aleutians and the low approaching the Kenai Peninsula) clearly
shows the moisture feed is of tropical origin.

A surface low continued to deepen somewhat as latest analysis
showing this system's strength to be near 995 mb as it approaches
the Barren Islands (just off of the Kenai Peninsula).

&&

.MODEL DISCUSSION...

The models remain in excellent agreement at the surface and aloft,
with todays numerical solutions offering a possible pattern change
towards the end of the work week. The amplitude of such change is
much greater and faster than what was seen yesterday, and this is
sometimes a normal bias. As such, given the amplitude of the
various upper-level features mentioned above, will treat todays
projections with a bit of temporal skepticism.

Model guidance is another story though, as various solutions have
been too warm for many locations when compared to surface/upper
air observations for southcentral, while better agreement exists
further west. This has been the trend over the last 24 hours and
we expect this to continue through the day today.

&&

.AVIATION...

PANC...Look for light snow to continue through at least mid
afternoon, as this mornings RAOB sounding showing a steep
inversion around 925 mb, which has also been confirmed in ACARS
soundings. The inversion's max temp is around +0.5 deg.C/31 deg.
F...and the model guidance has been struggling to play catch up,
as it's been over-aggressive with any warming in the lower levels
over the last 24 hours.

The latest guidance and our belief is that snow will be the
predominant precip type with MVFR visibilities and ceilings continuing
through most of the day. It remains to see if enough warm air will
be sufficient to change the precip to a mix variety, and this is
still possible, but confidence in this is decreasing.

&&

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

Precipitation type and snow amount have been the biggest questions
with this morning's forecasts. There has been a battle with the
warmer air moving in aloft with the colder air at the surface the
past few days. In spite of the warmer air precipitation type
outside of the immediate Gulf coast has been mostly snow with just
some pockets of mixed precipitation. The warmest air (though not
by much) has been expected to move over Anchorage and the
Matanuska Valley and eastward into Prince William Sound today.
That does keep in the possibility of the snow mixing-with or
turning- to rain. An interesting caveat is that the warmest level
aloft on the 12Z anchorage sounding is at 925 mb where it was less
than a degree above freezing. NAM model data shows that warm
layer at 925 mb at its warmest early this morning and then slowly
cooling. If that is the case, precipitation will remain all snow.
Another factor indicating the precipitation mixing is that there
is likely the wet-bulb effect cooling the air column due to the
precipitation rate. Therefore as the precipitation rate lightens,
the remaining precipitation could turn to rain as the air column
then becomes warmer. That is the battle going on and why there
remains mixed precipitation type over a good part of the area
today.

Snow will continue in the Copper River Basin as the warm moist air
overrides the colder air in the Basin. Not much data is available
early this morning to verify snow amounts so far, but a few more
inches should be in store before snow tapers off later today.

Wednesday will be a day where some lingering light snow may still
persist over the area before the cold air begins to move in
Wednesday night into Thursday. This cold air will also kick up
some gusty northerly winds through channeled terrain and along the
coast.

&&

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

Southwest Alaska is currently seeing light snow from a weak
deformation zone that is associated with a low in the Gulf of
Alaska. This broad area of precipitation moved in as expected.
However, looking at satellite and RADAR in the area, the light
snow will hang around a little longer than forecasted this
morning. Therefore, upped the PoPs until late this morning. It
still looks like the system will move out around midday, with a
chance of showers remaining. Patchy fog has also been something to
keep an eye on, with fog remaining a possibility tomorrow,
especially in areas that have received fresh snowfall.

Temperatures remain close to freezing in many areas this morning.
However, a cooling trend is still expected with low level
northeast flow filtering in colder air. This means highs should
be this morning, with temperatures cooling throughout the day and
over the next couple of days.

&&

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

Low pressure continues to spin the western Bering, with the low
center south of Attu Island. This low will continue to fill and
spin with winds gradually weakening. Storm force winds have
diminished and gale force winds will hang on for today in the
western Bering. An occluded boundary stretches east across the
Aleutian Island chain and will produce mainly rain showers through
tomorrow. The Bering will then see a brief break until Thursday,
when the next low moves in from the north Pacific. This low is
anticipated to be another storm force low for the western Bering.

&&

.MARINE (Days 3 through 5)...

A very active weather pattern will continue for the western
Aleutians and Bering for Wednesday night through Saturday with
stronger winds spreading progressively further to the east
through the time period. Confidence is high in a Gale force front
stretching over the western Aleutians Wednesday afternoon/evening
with a stronger developing low tracking north Wednesday night.
While the there is still some uncertainty regarding the location
of the strongest winds associated it is likely that either the
western Aleutians or western portions of the central Aleutians
will see sustained storm force winds out of the front late
Wednesday night or early Thursday morning. As the frontal system
continues north and east Thursday through Thursday night it will
spread a broad area of Gales along with it, to then be reinforced
by another front/upper trough swinging through Friday.

Another system that bears particularly close watching is a
powerful low forecast to track across the central or eastern
Aleutians Friday night or Saturday then continue to strengthen as
it tracks to northeast across the southern Bering, towards
Southwest Alaska. With a low this strong and still continuing to
deepen, rapidly strengthening winds of at least storm force are
likely, although significant run to run changes in the track of
this low makes forecasting the location of the strongest winds
extremely challenging at this time.

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

Upper level ridging over the central Aleutians will shift east
Wednesday night to come into phase with a strong upper level high
centered over the Bering Strait. These features will merge into a
high amplitude upper level ridge centered over western Alaska on
Thursday. To the east of the ridge, an arctic low digging south
through the Yukon and Northwest Territories will drag a cold upper
trough south through eastern Alaska Wednesday night and Thursday
bringing a surge of strong northerly winds to Southcentral Alaska
with it. In channeled terrain where outflow gaps align to support
more of a north or northwesterly wind direction winds will peak
Thursday afternoon with northeasterly oriented gaps becoming more
favored Thursday night through Friday morning. The cold offshore
flow will ease gradually through Saturday when flat ridging ahead
of an incoming frontal system moves over southcentral Alaska. The
associated front will track into the southwestern Gulf Saturday
afternoon and lift north to the northern Gulf coast through
Saturday night.

Out to the west, a front will lift north across the far western
Aleutians Wednesday afternoon while a second frontal wave further
to the south rapidly deepens and tracks north along 180. The
strong front associated with the second low will reach the western
and central Aleutians late Wednesday night through Thursday
morning and then lift north into the southern Bering Thursday and
the parent low curves back to the west. With the support of an
upper level trough rotating north and over the central Aleutians,
the front will continue east to reach the eastern Aleutians
Thursday afternoon and Alaska Peninsula Thursday night. Another
strong and rapidly deepening low pressure system will track across
the central Aleutians Friday night and then continue to the
northeast to reach to reach Southwest Alaska Saturday or Saturday
night bringing strong winds and possible coastal erosion concerns
with it. With a very large upper low stretching across the Bering
and strong southwest to northeasterly oriented jet stream over
the north Pacific it looks likely that the strong southwesterly
storm track will continue to bring a very active weather pattern
to much of the area through midweek.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
MARINE/LONG TERM...DK