Small Craft Advisory 873, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
514 AM AKST Wed Nov 7 2018

.ANALYSIS AND UPPER LEVELS...
The upper level ridge that was in place over the state has drifted
east and is now over the Yukon. The broad upper level trough that
was in the Bering is moving in behind it, so we are transitioning
from a high pressure regime to low pressure influence in the
synoptic flow. This is allowing low pressure at the surface to
once again work its way into southern Alaska.

IR satellite clearly reveals this storm, with the center in the
Gulf of Alaska slowly moving northeast. The occluded boundary
wrapping around it is pushing into Southcentral Alaska and a
deformation zone over Southwest Alaska. Both areas are seeing
precip as a result. Even with some warming due to the maritime
influence of this low, temperatures in place from the previous
high pressure system will stay cold enough for snow in most inland
areas.

&&

.MODEL DISCUSSION...
Models are in good agreement with each other overall. One major
change from yesterday is that models are now bringing precipitation
over Anchorage, whereas yesterday the area was largely dry. While
this is a significant overhaul from yesterday, the models are in
solid agreement on this change now. The lone exception is the
Canadian Regional keeps the system much further to the east, so we
will watch to see how later runs handle the system. We adjusted
the forecast overnight to bring snow into Anchorage from this
system.

&&

.AVIATION...
PANC...VFR conditions will persist for today. Offshore winds will
steadily increase. Light snow and low ceilings will cause MFVR
conditions this evening into tomorrow.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
The weather concerns in the short term revolve around the system
moving into the Gulf from the North Pacific. The front associated
with this system looks to bring precip to eastern Kenai Peninsula
throughout today and all along the north Gulf coast by this
evening. There is enough warm air being pulled up with this system
for most locations near the water to at least be mixed with rain
if not transitioning entirely over to rain. Interior locations and
higher elevations will stay primarily as snow.

With uncertainty of the track of the main low center as it moves
through the Gulf there is also uncertainty with how far beyond the
coastal ranges snow will progress. Models are hinting at a
deformation zone setting up over the Anchorage area extending
north towards the Talkeetna Mountains tonight ahead of a shortwave
approaching from the west. However, there is disagreement with
timing and placement amongst the models. It seems likely that
Anchorage will see some snow tonight (2 to 4 inches) but it does
not look like there will be amounts of 4 to 7 inches as some of
the models have indicated. Thursday morning will see precip
diminish throughout Southcentral Alaska before a weak front pushes
through from the west Friday morning bringing intermittent light
snow to the area.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A north to south running deformation band running from the middle
Kuskokwim Valley into the Greater Bristol Bay area will bring a
mix of rain (south) and snow (north) to the area today. The band
will shift to the east this afternoon and evening with
precipitation changing to scattered mountain snow showers and then
tapering off overnight. With ridging building over the area
tonight and partial clearing expected, areas of valley fog are
likely Thursday morning. The next frontal system tracking rapidly
in from the west will reach the Kuskokwim Delta coast midday
Thursday and then swing onshore Thursday afternoon and evening.
Cold air aloft and a couple of shortwave troughs will follow the
front late Thursday night through Friday bringing snow showers and
brisk west to southwesterly winds. Precipitation will taper off
Friday night with winds turning to the south and then east in
advance of the next frontal system lifting north towards the
Alaska Peninsula.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The frontal system currently tracking into the western Bering will
continue east to reach the central Bering this evening and eastern
Bering Thursday morning. Southerly gales along the initial front
will be followed by a stronger push of westerly storm force winds
supported by strong cold advection as the parent low tracks into
the Bering following the front. With a long westerly fetch
following the area of storm force winds, combined seas will
quickly build up into the 30 to 40 ft range in the western and
central Bering offshore waters. The large low will become
vertically stacked and curve northeast into the northern Bering
Thursday night. Another upper level low supported by abundant cold
air aloft will track from the Kamchatka Peninsula into the
western Bering Thursday night and then swing east into the central
Bering Friday. In response, a developing north Pacific low will
track north to the Alaska Peninsula Friday Night.

&&

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

An active pattern will be on tap for the Bering, Aleutians and
Southwest AK Friday to Tuesday. The longwave pattern features
trough bisecting the Bering and a high amplitude ridge from SE AK
to the Yukon Territory. At the surface there is a low East of
Kamchatka and another low south of Kodiak. As time elapses, this
energy will shift eastward. In the beginning of the period there
is good clustering between the latest runs of the
GFS/ECMWF/Canadian Regional. The latest runs of the GFS and ECMWF
have converged compared to the solutions from 24 hrs ago. However,
given the vagaries models past 96 hrs, blending and hedging is
still required. Saturday looks like it will be driest day this
forecast period for the Anchorage metro area.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 175, 177, 178, 179, 180, 185, 413, 414, 120,
130, 131, 132, 138, 139, 351, 352.
Storm Warning 411, 412, 119.

&&

SYNOPSIS AND MODEL DISCUSSION...BJB
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...PS