Small Craft Advisory 811, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
1104 PM AKDT Wed Oct 25 2017

.ANALYSIS AND UPPER LEVELS...
The upper level low along the Kuskokwim Delta yesterday has lifted
north of the Bering Strait today. In its wake, the Bering low
currently across the central Aleutians is now the main driver of
weather across the southern mainland and surrounding waters. Warm
air advection has intruded across the southern mainland, except
for a small cold pool along the western Alaska Range. At the
surface, the most notable feature on radars across Southcentral
is widespread rain across the northern Gulf and along Cook Inlet
to the Susitna Valley. Typical downslope holes along the Chugach
Range are evidenced by the lack of radar reflectivities which is
classically due to strong southeasterly flow across the region.
The lighter reflectivity along the northern Susitna Valley is
representative of the snow that was observed at Talkeetna this
afternoon. Along the southwest mainland, southerly flow is
streaming showers inland ahead of the main low along the central
Aleutians.

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.MODEL DISCUSSION...
Models are in good agreement through the end of the week on the
main synoptic pattern which is a slow shift of the Aleutian low
tracking eastward. The main challenges in the forecast through
Friday are winds across Southcentral and precipitation along the
Gulf of Alaska.

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.AVIATION...
PANC...Gusty southeasterly flow tapers down slightly this evening,
however another gradient squeeze along the Turnagain Arm Thursday
morning will increase winds through the afternoon to levels near
what they have been today. VFR conditions are expected through
tomorrow, however some periods of below 5000 ft (but above 3000
ft) will come and go through the night.

&&

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

Numerous forecast challenges were part of the afternoon package
today. One of them is the chance of high winds through Anchorage.
It looks as though the front causing the strong winds is still on
track to move through during the day Thursday. Winds could gust as
high as 75 MPH through Turnagain Arm and higher elevations and up
to 60 MPH over the Anchorage Bowl. The strongest winds look to be
Thursday morning into early Thursday afternoon.

A winter weather advisory has been issued for the Susitna Valley
north of Talkeetna. The warm air that has moved into areas further
south has struggled to continue advancing northward, and
Talkeetna and points north continue to get snow this afternoon.
Slow warming may continue overnight, which may allow Talkeetna and
its environs to mix with or completely change over to rain.
However, for Broad Pass, significant mixing with rain is not
expected. Over an inch of liquid equivalent is expected overnight,
which with it falling as snow should lead to additional snow
accumulations overnight up to 10 inches.

With the same front causing the winds in Anchorage, a brief-
duration barrier jet is expected to develop along the north Gulf
Coast Thursday. As the front moves northward into the coast, there
is potential for winds gusting to 65 mph in Cordova near the boat
harbor as the barrier jet winds funnel between the nearby hills
and mountains. The prolonged onshore flow of subtropical moisture
will also lead to rainfall amounts near the coast approaching 4
inches in Seward through Thursday afternoon and over 5 inches near
Cordova, causing rising river levels.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Unsettled weather will continue across the region for the next
couple of days as a weakening low pressure system tracks east
towards the SW coast. Relatively warm and moist flow will continue
out of the south on the front side of the low keeping temperatures
rather mild for this time of year. Showers will continue to
spread NE into the mainland tomorrow morning as the surface low
enters Bristol Bay. Cold air aloft and an upper level wave moving
in at the same time will enhance precipitation throughout the day.
Warm air at the surface will keep the precipitation type as rain
below 3000 ft. Colder air momentarily gets drawn south behind the
departing low on Friday morning before another wave of warm moist
air from the south pushes in ahead of the next approaching system.
Precipitation from this system looks to move into the Kuskokwim
Delta by Friday afternoon and could start off as a rain snow mix
before transitioning over to rain by Friday evening.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A 970 mb surface low will continue to weaken as it skirts the
Aleutian chain before moving NE into Bristol Bay and eventually
north along the western coast. The biggest impact from this
system will be from strong winds as the system is still hanging
on to a relatively low minimum pressure leading to a tightened
pressure gradient. Gale force southerly winds will transition to
northerly winds across the Eastern Aleutians and AKPEN as cold air
is filtered down around the backside of the low. A brief reprieve
in the weather will be seen Thursday afternoon and night as a
transitory shortwave ridge builds in across the Central and
Eastern Aleutians. The next system rapidly moves in behind the
ridge bringing another round of winds and rain to the Bering and
Aleutians tomorrow afternoon into Friday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The warmer pattern will continue through the weekend as broad
southwest flow remains focused over the southern mainland between
the Bering Sea trough and a stout northeast Pacific ridge. Friday
night into Saturday will also bring a break in precipitation for
Southcentral Alaska as high pressure briefly builds into the
central Gulf behind the departing system. Widespread rain will
then return to the area on Sunday as the next North Pacific low
moves into the eastern Bering accompanied by a front over the
Gulf. This will provide a reinforcing shot of warm air, with the
heaviest rain focused along the north Gulf coast through Monday
morning before that system also departs to the north.

There is lower confidence in how the pattern unfolds going into
the middle of the week as models continue to struggle with the
evolution of high pressure building into the Bering and western
Alaska behind the departing trough. The GFS/GEM both eventually
develop a closed high that drifts well to the north into the
Chukchi Sea. This would allow another trough to undercut the
ridge and move back into southern Alaska, keeping a chance of rain
in the forecast through the middle of the week. Meanwhile,
European guidance depicts a much more coherent blocking ridge
over the western mainland, which would keep most of the area dry
through much of the week in a weak offshore flow regime. Given
the uncertainty, the current forecast attempts to split the
difference in keeping a chance of rain over the southern mainland
into the middle of next week while gradually trending temperatures
down closer to seasonal normal values.

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.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 145.
MARINE...Gale 125 129 155 172-177 185.
 		 Storm 119.

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SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...MTL