Small Craft Advisory 446, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
548 PM AKDT Tue Jun 5 2018

.ANALYSIS AND UPPER LEVELS...
Generally easterly upper level flow with weak embedded waves
continue across Southcentral Alaska. The axis of the thermal
trough extends from the Tanana Valley to the Lower Yukon Valley.
As of noon, numerous lightning strikes had already been detected
from the Tanana Valley to the upper Yukon Valley. Visible
satellite imagery showed convection initiating along the Alaska
Range and Talkeetna Mountains this afternoon. Low stratus and fog
are evident this morning over the lower Kuskokwim Valley is
rapidly dissipating. A wave moving west across the northern and
western gulf is bringing light rain to the coast and southern and
eastern Kenai Peninsula.

Patchy flow in interior areas of the Bristol Bay have dissipated.
Many areas in Southwest Alaska was under generally fair skies
setting up prime conditions for solar heating and convection. A
weakening trough extends over much of the western and central
Bering Sea/Aleutians holding clouds and areas of light rain.

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.MODEL DISCUSSION...
Models are in generally good agreement through the short term.
Tracking weak upper waves and timing and location of convection
will be the main forecast challenge.

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.AVIATION...
PANC...VFR conditions with showers are expected through the
overnight hours. Gusty southeast winds across the airport complex
will end late this evening.

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.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3...Wed
through Fri)...
For tonight, southeast winds will remain gusty through the usual
areas like the Knik Arm and west Anchorage. Several showers are
tracking from northeast to southwest across the area, with some
tracking right along the Glenn Highway. This could lead to a wet
commute for many folks. Some models show that the convection from
the Copper River Basin consolidates overnight and moves into the
Cook Inlet Region as steadier rain. We opted to increase PoPs
overnight to account for this. There is likely to be some
fog/stratus that develops after this precipitation moves through
the Susitna Valley, but trying to nail down exactly where it will
be remains challenging. Went with a persistence forecast for now.

Otherwise, the overall pattern will remain much the same into the
end of the work week. Days will generally start out with partly
to mostly sunny skies and temperatures at or slightly below
normal. By afternoon, expect convection to develop along the
higher terrain, generally over the Copper River Basin and Susitna
Valley. As the sun helps to warm the surface and build
instability, convection will increase in coverage with showers
blossoming over the Talkeetna Mountains and parts of the Chugach.
The best chance for afternoon/evening thunderstorms still looks to
hold further north towards the AK Range, but there should still
be some isolated cells further south.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
With strong instability pushing into Southwest Alaska
accompanying the thermal trough this afternoon and evening as well
as plenty of sunshine, things look good for scattered
thunderstorm development through the Kuskokwim Valley and northern
portions of the Kuskokwim Delta. Tonight's easterly wave will
track further south than yesterday's, pushing across the Greater
Bristol Bay area and turning the showers and thunderstorms to more
of a stratiform sort of rain overnight and further expanding the
expected coverage of wetting rains beyond what this evenings
storms will put down. With the thermal trough further strengthen
Wednesday afternoon and evening, the accompanying strong
instability will push even further into southwest Alaska for
another afternoon of numerous showers and scattered thunderstorms.
While instability will diminish somewhat Thursday and Friday,
there will still be plenty to continue a pattern of
afternoon/evening convection producing scattered showers and
isolated thunderstorms.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
A ridge gradually moving south into the Bering will push the
weaken trough stretching from the Kamchatka Peninsula east
southeast across the eastern Aleutians, Alaska Peninsula and
western Gulf to the south as it shears apart and dissipates. A
North Pacific low will track past the western and central
Aleutians but now looks to remain south of 50N. A second
developing frontal system will track northeast towards the central
and eastern Aleutians Friday and Friday night but likely brush by
just to the south.

&&

.LONG TERM FORECAST (Days 4 through 7)...
The long term forecast begins on Friday night with an upper level
ridge moving into the Gulf of Alaska, bookended by two low
pressure systems, one in the Southeastern Gulf and the other just
south of the Central Aleutians. A weak to no-flow pattern will set
up over most of mainland Alaska leading to pleasant and warm
weather for the weekend. Rising 1000-500 mb thicknesses will
promote above average temperatures for most of the southern
mainland Saturday and Sunday, with Sunday looking to be the
warmest day of the weekend. A thermal trough will develop over the
interior promoting diurnally driven convection over the higher
terrain in the afternoon.

One kink to the forecast is the possibility of an
upper trough digging south across the state from the North Slope
over the weekend. Models are consistent in the fact that there
will be a trough sweeping across the northern half of the state
but are not in agreement as to how far south the trough will get.
The GFS has the trough splitting west as it reaches the AK Range
while the EC is more bullish and keeps the system fairly intact as
it crosses the mountain range. If the EC solution verifies,
widespread thunderstorms will likely develop over the Northern
Susitna and Copper River Valleys Saturday night, possibly
exasperating recent fires that have popped up in those areas.
Those with a keen interest in the fire weather conditions should
pay close attention to the forecast this weekend as the situation
develops.

Looking into next week, models are in agreement with an area of
high pressure building over the mainland, resembling a typical
pattern associated with very warm summertime temperatures.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...RMC
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...KVP