Small Craft Advisory 349, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 PM AKDT Sat Apr 28 2018

.ANALYSIS AND UPPER LEVELS...
Fairly fast zonal flow continues over the Bering Sea and southern
Alaska. There is an upper level trough along about 155 degrees
west longitude. A surface low associated with this trough is
centered over the Kuskokwim Delta and in the process of
dissipating. Another surface low is forming over Kodiak Island at
the triple point. The main front with this trough is over the
western Gulf of Alaska and extends into Southcentral. There is
weak ridging over the eastern Bering. A strong and deepening low
is over the southwest Bering and moving rapidly towards the
northeast. The leading front is negatively tilted and approaching
the Pribilofs and eastern Aleutians.

&&

.MODEL DISCUSSION...
The numerical models are in fairly good agreement through the
short term portion of the forecast (Monday afternoon). The models
have settled on a solution for the low crossing the Bering through
Sunday night and reaching the Seward Peninsula Monday afternoon.
The exception is the Canadian model, which has a faster and
further south solution compared to the other guidance. Forecast
confidence is a little above normal today.

&&

.AVIATION...
PANC...VFR conditions will persist. Breezy southerly winds are
expected through Sunday evening, generally stronger in the
afternoon/evening hours.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Wet and windy conditions will persist over a good portion of
southcentral through early next week, as the active pattern
continues over southcentral Alaska. The weakening low tracking
northeast over the Kuskokwim Delta will rapidly dissipate, as a
weak triple point low develops near Kodiak Island this evening.
This newly developed low and lingering frontal system will
continue to keep the bulk of precipitation and gusty winds along
the northern Gulf coast through Sunday. Warmer air will accompany
this system, so lower elevations not shadowed by cross- barrier
flow will trend toward rain. That said, higher elevations and
northern valley sections will likely see some snow or mixed
precipitation through Monday, specifically across the northern
Susitna Valley, higher elevations along the Copper River Basin,
and along the Chugach and Talkeetna mountains.

Strong gusts are expected pick up along Turnagain Arm, Knik River
Valley, and along the Copper River early this evening then
diminish slightly overnight. Winds pick back up again late Sunday
morning through Sunday night. Winds in many areas will peak in
the late afternoon to early evening hours Sunday before
diminishing Sunday night. Winds will slowly diminish into Monday
morning.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Sun and Mon)...
Active weather will continue as we move into next week. For this
evening, the last vestiges of an upper level trough will continue
to produce some light rain and snow showers, primarily over
interior portions along the Alaskan and Aleutians Ranges. Some
patchy fog will also linger across the area through the evening
with onshore flow. By early Sun morning, the next front is already
on the door step ready to bring another round of rain to Southwest
AK. This front will bring another round of mild air with gusty
southwest winds. As it sweeps quickly to the east by Sun
afternoon, steady precipitation will come to an end. The parent
low will trail over the north-central Bering into Mon. This will
serve to pull some significantly colder air into the area. 850mb
(5,000') temperatures will drop from near 0C to near -8C by Mon
morning. This cold air combined with moist southwest flow will
help to excite some instability Mon afternoon. The end result will
be widespread snow showers.

The broad trough over the Eastern Bering to start the week will
lead to long-duration moderate to strong southwest to west flow.
As this flow continues to act on the waters, it will help to build
a potent surge near the West Coast. An SPS has been issued to
address this potential hazard.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Sun and
Mon)...
The Bering will continue to give birth to several robust storms. A
low currently moving north of the Western Aleutians will deepen to
approximately 975 mb as it tracks towards St. Matthew Island by
Sun morning. It will sling a warm front through the Aleutians
tonight and into the Alaska Peninsula early Sun morning. As this
low deepens, a swath of storm-force westerly winds will develop
on its southern flank Sun. Behind the front, the upper-level jet
and cold air advection will allow gusty westerly winds to envelop
the whole area.

Monday another piece of energy drops down the western side of the
low. This will serve to consolidate the snow showers and drive
them from NW to SE through the Pribilofs. Then by Mon afternoon,
yet another (much weaker) low is taking shape near the Western
Aleutians. This will track towards the Central Bering into Tue and
serve to keep the pattern quiet active.

&&

.LONG TERM FORECAST (Days 3 through 7: Tuesday through Saturday)...

The largely east to west weather pattern currently ongoing across
Alaska will continue for the start of the long-term forecast
through the first week of May. A weak low over the central Bering
Tuesday morning will quickly track eastward and move into Bristol
Bay by Wednesday morning. While the surface energy dissipates,
the upper level energy will continue into Southcentral, resulting
in Wednesday afternoon showers for much of the area. Meanwhile,
another stronger low strengthens across the central Bering
Wednesday evening. This low will flip the pattern to a more
amplified one, which in turn will result in the weather remaining
more consistent day-to-day going into the weekend.

While there are large model differences in the details, such as
where precipitation will be or how strong the low or trough will
be for example, the one consistency between the models is that the
low or deep trough anchors itself along the Bering coast of
Southwest Alaska. That would keep Southwest Alaska continuously
chilly and unstable from Thursday onward. For Southcentral, the
weather will be somewhat warmer, but cloudy and wet as continuous
southerly flow off the Gulf keeps a steady stream of moisture
moving into the area. The wet pattern would be quite unusual for
this time of year given that this is typically the dry season.
Strong ridging builds over the western Bering by Saturday which
will further amplify and slow down the pattern.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...
GALE...173 174 175 176 177 178 179 180 181

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...TP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
LONG TERM...JW