Small Craft Advisory 390, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 PM AKDT Sat May 12 2018

.ANALYSIS AND UPPER LEVELS...
The vertically stacked low that has been hanging out over the
Yukon Delta is beginning to weaken as it tracks to the north.
Southerly flow and showers are making their way up to southern
Mainland between this low and ridging over the eastern Gulf and
panhandle. A developing low at the base of the longwave trough
south of Kodiak is making its way towards Southcentral. An
occluded low west of the Aleutians is slowing tracking eastward
bringing warmer air and rain to the western and central Aleutians.

&&

.MODEL DISCUSSION...
Models continue to be in good agreement with the main features
through the mid term forecast. There is a bit of disagreement with
the timing of precipitation associated with the low moving into
the Gulf on Sunday. The GFS and EC have the rain moving in sooner
than the NAM, but there are no major differences of the impacts
that this system will bring.

&&

.AVIATION...
PANC...VFR conditions will persist. Gusty southeast winds will
continue through the evening, but diminish overnight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Broad upper troughing and southerly flow aloft is keeping the
region under clouds and the threat of showers this afternoon.
There is also enough of a low level gradient to keep gusty winds
in favored terrain gaps. The pattern switches overnight as a low
moves into the Gulf of Alaska and then inland Sunday. This will
bring a swath of rain to most of SOuthcentral including Anchorage
and the the Western Kenai. Lesser chances and lighter rain will
impact the Susitna Valley with the best rain coverage over the
Copper Basin and the Gulf Coast. The low exits Monday but there
will remain enough moisture for scattered showers, especially over
the mountains in the afternoon.

Another upper trough, this time crossing the Alaska Range, moves
in Tuesday with more chances for showers and light rain.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A broad, cyclonic circulation with deep, onshore flow continues
to advect moisture into the southwest Alaska, producing widespread
rain showers over the Y-K Delta and a few convective showers from
King Salmon to Levelock. The main low responsible for this
unsettled pattern will continue to slowly drift northward toward
Saint Lawrence Island tonight. As it does, the gusty southerly
winds will diminish from south to north. A stationary trough to
the south of this system will continue to produce scattered rain
showers and elevation snow showers on Sunday as winds shift to the
west.

An upper level trough with an associated weak cold front will
push toward the coast Monday morning. Cooler air will advect in
behind this system allowing for precipitation to mix with and
change to snow beginning late Sunday night. Moisture is limited
with this system, however, so little to no snow accumulation is
expected. Scattered rain and snow showers will continue over the
Y-K Delta into early Tuesday as this system also stalls.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The Bering is currently bookended by one low pressure system
drifting slowly north along the west coast and a 993 mb low 400
miles southwest of Shemya. A series of shortwaves sliding
underneath the first low will produce showers over the central
Bering on Sunday. These disturbances will slide east and allow
rain and snow showers to persist across the eastern Bering through
Monday. Gusty southerly winds along the west coast will subside
tonight and turn to more westerly.

The second low will slide southeast, grazing the western Aleutians
with clouds and rain through Monday morning. The ridge will
strengthen through Tuesday as the axis slides south and settles
over Shemya. With light winds and stable air trapped underneath
the ridge, patchy fog will develop over the western Bering.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Mid/extended guidance over the past 24 hrs has exhibited a slight
but notable eastward shift in the longwave trough over the
eastern Bering next week. This is primarily being driven by a
stronger shortwave dropping out of eastern Russia early in the
week before evolving into a compact closed low over the southwest
mainland and eventually tracking towards the north slope by
Wednesday. The net effect of this eastward shift of the trough
axis is to nudge the developing downstream ridge a bit farther
east into Canada and pave the way for a North Pacific disturbance
to track north across the Akpen into the southern mainland during
the middle of the week. This would keep Southwest AK in an already
expected rainy pattern through late week but would also point to a
much wetter and windier regime for Southcentral than had been
suggested in recent days as broad southerly flow sets up over the
Gulf on the western periphery of the ridge. The noted changes in
recent guidance lend decreased confidence to the long term
portion of the forecast, but the bottom line is to expect a
generally cloudy and wet pattern to remain in place across the
southern mainland through late next week. Meanwhile, high pressure
building near the Kamchatka Peninsula suggests a quiet period
over the western half of the Bering, with a Pacific front possibly
clipping the western tip of the Aleutians late in the week as the
upper trough maintains showery conditions across the eastern
Bering.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...DK/AH
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
LONG TERM...CB