Small Craft Advisory 702, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
348 PM AKDT Tue Sep 24 2019

.ANALYSIS AND UPPER LEVELS...
An upper-level low near the Bering Strait is driving a shortwave
over Southcentral Alaska. Cloud cover is extensive as the wave
and its dying surface front are working through, rainfall in the
region has been minimal at best thus far.

Over Southwest, another shortwave connected to the Bering Strait
low is driving a stronger push of convection moving quickly into
the Yukon-Kuskokwim Delta where scattered showers are moving into
the Kuskokwim Mountains and radar returns over the far Western
Coast indicate snow at 15,000 to 16,000 feet, though sensible
weather below is overcast and with showers and mist. This is the
feature that is being tracked for potential high terrain snow over
Southcentral Wednesday.

Further west, a more powerful upper-level low over the Kamchatka
Peninsula is driving a vigorous jet and developing gale into the
western Bering Sea with a shortwave ridge move eastward ahead of
it.

&&

.MODEL DISCUSSION...
Models are in general agreement with timing and strength of main
pressure centers near and over Alaska at this point. Main timing
differences seem concentrated under the Pacific jet where low
centers and frontal positions get out of sync beginning Wednesday
into Thursday.

&&

.AVIATION...
PANC...VFR conditions and light winds continue. An upper-level
trough swinging in from the west will bring scattered showers this evening
which may briefly lower ceilings to MVFR in the vicinity, however
overall VFR conditions are expected to prevail through mid
morning Wednesday. The second round of precipitation Wednesday may
bring a period of more intense precipitation and lower ceiling and
visibility for a time Wednesday midday and afternoon.

&&

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

A weak upper trough is moving across the Alaska Range into
Southcentral as we speak, and this is spreading moisture northward
from Cook Inlet into the Susitna Valley. The low will strengthen
as it moves east, so the best precipitation amounts will be over
the Susitna Valley, Copper Basin, and the North Gulf Coast from
Prince William Sound eastward. Snow levels will not be
particularly low and will likely remain above 3000 feet across
most locations. There will be a break in activity before the next
shortwave trough tracks across the Alaska Range on Wednesday
morning, bringing more rain to the region. The low will favor
locations north of the Kenai Peninsula, although all areas should
see at least light rain. Given the forward speed of the trough,
precipitation will be rather quick hitting for locales west of the
Chugach and Talkeetna Mountains. Cold air behind the trough will
drop snow levels as low as 2000 feet, but any snow above that
elevation should not be significant. The Copper Basin may briefly
see rain/snow down to the surface Wednesday evening. Thereafter,
decent cold air advection and offshore flow/winds will rapidly dry
and clear the region out, allowing for some chilly temps in low
elevations Friday morning.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
through Friday afternoon)...

Gusty westerly winds and rain are currently impacting the
Kuskokwim Delta coast as a strengthening low moves inland. The
associated front currently extends over the eastern Bering and
will eventually push ashore, spreading gusty westerly winds along
the entire southwest coast. Winds are expected to reach Bristol
Bay and the Alaska Peninsula by Wednesday morning. In addition to
winds, this low will also bring precipitation as it tracks from
west to east. Snow is likely in higher elevations, including the
Kilbuck Mountains and Alaska Range. Scattered showers will
eventually taper off by Thursday as a weak ridge builds in briefly
ahead of the next front that approaches Southwest for the
weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Tonight through Friday afternoon)...

A ridge of high pressure over the central Bering will bring
generally fair conditions for the central and eastern Aleutians
Wednesday. Meanwhile farther west, a gale force front currently
over the western Aleutians will bring gusty southerly flow and
rain as it treks across the Bering, reaching the Pribilofs and
central Aleutians by Thursday. As the front continues moving
eastward, the parent low enters the western Bering Friday,
bringing the possibility for another round of gale force winds to
the western Bering for the end of the week.

&&

.MARINE (Days 3 through 5)...

A vertically stacked low over the western Bering will maintain south
flow over much of the Bering and Aleutians Friday, while sub-
advisory level west winds are expected in the Gulf of AK under high
pressure. The occluded front associated with the Bering low will
track eastward over the Aleutians Friday into Saturday. This may
bring near gale force southerly winds to the Kuskokwim Delta coast
Friday afternoon, and will bring a transition to southerly winds
over the Gulf Saturday into Sunday. Meanwhile, behind the front
expect broad westerly flow over the Bering Saturday into Sunday as
a ridge builds over eastern Siberia.

&&

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

A front ahead of an occluded low over the Bering Sea will track
from the central Aleutians to the Southwest mainland Friday.
Expect gusty southerly winds along the front, likely impacting the
Kuskokwim Delta coast Friday. Meanwhile, a ridge axis will stretch
from the North Pacific to the Arctic... and quiet, sunny
conditions are likely over Southcentral Friday. Saturday, a low
will develop south of the Aleutians and move northward, crossing
the AKPEN into the Bering Sea. While there's some model
differences in the timing/progression of this low, there is decent
agreement in the basic pattern. Thus, confidence is moderate that
conditions will turn wet over the Southwest mainland Saturday into
Sunday... then over Southcentral in the rough Sunday to Monday
timeframe. With a strong subtropical/tropical moisture tap, snow
levels will be on the high side and this will mostly be a rain
event.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 119 120 130 131 132 160 173 175 176 177 178 180 181 351
              411 412 413.
FIRE WEATHER...NONE.

&&

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