Small Craft Advisory 91, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 PM AKST Mon Jan 28 2019

.ANALYSIS AND UPPER LEVELS...
Radar trends through late morning kept persistent snowfall along
Cook Inlet coastlines and over the Susitna Valley. The full-fledged
southerly aloft and below puts Southcentral in mild and wet
pattern - as evident by this, the first snowfall received in
Anchorage since January 11. Despite the fresh powder, climate logs
indicate a monthly deficit of about 8 inches.

Rain and snow were falling around Prince William Sound. And West
and Southwest Alaska were seeing light amounts of snow, at times
mixed freezing rain and fog.

Of concern is the low-level warm layer present on Anchorage's
morning upper air sounding. With this warmth, the delicate dance
between snow and rain will hover over Southcentral through
Tuesday. Clear solutions won't be pulled from any magic forecast
hats this day.

At the surface, a cold front extends along the Yukon-Kuskokwim
Delta with a surface high in the Northern Bering. A warm front is
moving north and east in the Gulf of Alaska. And an occluded front
and attached storm-force low is moving north east toward the
West-Central Aleutians.

A wealth of Alaska is under the control of a full latitude upper-
level trough that stretches from the Last Frontier deep into the
tropics. This is moving warm, wet air into the region.

Two upper-level lows are moving through Bristol Bay and the North
Pacific with the former part of the extensive full-latitude
trough. The latter is powering a storm force low at the surface.
North and west of the trough is a quasi/tilted Omega block
centered over far eastern Russia and the Chukchi Sea.

This high's placement is and will be critical in monitoring cold
air advection and potential for gradient and gravity driven winds
for Alaska interests toward the end the week.

&&

.MODEL DISCUSSION...
NAM is the model of choice for the West domain today. In the
East, model consensus is poor. A lean toward the NAM will be
reflected in the forecast.

In general, the broad dynamics of the NAM and GFS do agree on
upper-level features and bring the vorticity maximum through
Southcentral midday Tuesday and keep the region unsettled through
midweek. In the west, the upper-level trough should reach the
Eastern Aleutians by Thursday morning.

&&

.AVIATION...
PANC...Light northerly winds will persist through late Tuesday.
Other factors are much less clear. Visibility will be greatly
dependent on temperature and resulting precipitation type, which
is very much in question. Based on the latest sounding, expect
precipitation this evening into the overnight hours to fall as
primarily snow. However, think there is enough warming overnight
to change to rain by morning. Precipitation intensity will also
diminish on Tuesday. Thus, expect lower conditions (IFR) this
evening, with improving conditions as we head through the taf
period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tonight
through Wednesday)...
The first of three storm systems to affect Southcentral is
currently clearing most of the area as a band of snow is now fully
into Cook Inlet and the Susitna Valley. The second storm system
is fast on its heels however, with rain overspreading much of
Prince William Sound in association. When the bulk of that
moisture moves fully onshore this evening, snow is expected to
resume again over much of the area initially, with the exception
of the Palmer and eastern Matanuska Valley which will remain
blocked by the mountains. Accompanying the precipitation will be
increasingly warmer air that will gradually lead to a changeover
in precipitation from snow to rain for Anchorage and much of the
Kenai Peninsula. The timing of that changeover is still uncertain
as it will depend on small changes in the wind speed across the
Chugach and how warm the warm air mass gets. In addition, an
uncertain storm track and exact positioning of the warm front
could keep some areas, like the Susitna Valley and possibly the
far northwestern Kenai Peninsula right on the Inlet holding on to
snow longer.

On Tuesday, the rain will diminish in intensity through the
morning and any changeover to rain should be helped along by
diurnal heating. Most of Tuesday afternoon and evening should
feature widely scattered rain showers with temperatures warming
into the mid to upper 30s. Another weaker system will move through
Southcentral Tuesday night, as temperatures fall back to at or
just below freezing. There is some potential once again for mixed
precipitation, especially since the intensity should be lighter.
With the nose of warm air in the mid-levels getting shunted off
to the east by then, for now the current forecast is for an
additional inch or two of snow starting after midnight and
tapering off during the morning commute.

On Wednesday, the early morning snow will clear from west to east,
persisting in the Copper River Basin through much of the day.
Meanwhile a surge of much colder air will invade the area
Wednesday night, which could make for strong winds and falling
temperatures. The strong winds are looking increasingly likely
through the gaps, namely the Matanuska Valley, Thompson Pass and
Valdez, the Copper River Delta, and Broad Pass. The cold air mass
is expected in the Copper River Basin initially Wednesday night as
the rest of Southcentral waits until Thursday for the colder air
to arrive.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Warm air advection ahead of a negatively tilting trough with its
axis crossing the AKPEN will continue to bring precipitation to
the AKPEN and Bristol Bay areas. Temperatures in Bristol Bay and
the southern AK Range are mostly a few degrees above freezing, but
temperatures should cool tonight as northeast flow at the surface
strengthens, and rain will change over to snow. Farther north,
patchy fog will persist tonight in the Kuskokwim Delta then scatter
out as northeast flow strengthens. Snow is possible in the lower
Kuskokwim Valley/Delta Tuesday but with dry northeast flow at the
surface, most precipitation will likely evaporate before hitting
the ground. Conditions will trend drier in Southwest Tuesday into
Wednesday as an upper high builds over eastern Siberia.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A powerful low that brought storm force winds to the western
Aleutians earlier today has started to fill. Gales will continue
tonight for the central and western Aleutians, and into tomorrow
over the Bering. An occluded front associated with the low will
bring areas of precipitation to most of the Aleutian chain through
Tuesday. Though a drying trend is expected Wednesday, this will
be relatively short-lived as the next system, a low that's
currently around 400 miles east of Japan, will reach the western
Aleutians late Wednesday into Thursday.

&&

.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...Gale Warning 119 125 130 131 139 173 174 175 176 177 178.
         Heavy Freezing Spray Warning 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...JR