Small Craft Advisory 806, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 PM AKDT Tue Oct 24 2017

.ANALYSIS AND UPPER LEVELS...
Water Vapor satellite imagery shows the parent upper low along
the Kuskokwim Delta this morning, is now moving north across the
Bering Strait. A sharp negatively tilted trough axis associated
with this low is lifting across the Kenai and central Gulf of
Alaska. A broad area of low pressure associated with this system
is supporting rain along the Gulf. This is bringing a mix of snow
and rain along the Kenai and snow along a sharp axis that
stretches along western Cook Inlet to western Susitna Valley.
Gusty winds developed late this morning with a tighter pressure
gradient along this axis.

To the west, the extratropical storm system across the western
Bering picked up eastward momentum early this afternoon with gusty
winds and rain along the leading boundary which moved across to
the central Bering and eastern Aleutians.

&&

.MODEL DISCUSSION...
Models are in good synoptic agreement with the overall pattern
shift. The NAM was primarily used to update the short term
forecast with some use of the GFS across Southcentral and the
Gulf. The impacts through Thursday include precipitation amounts
and type, gusty winds, and warming trends.

&&

.AVIATION...
PANC...Some snow showers will persist in the area through the
night but generally VFR conditions will exist through the night
and into tomorrow. Winds will shift to the southeast by morning
with gusts developing as the pressure gradient increases along
Turnagain Arm.

&&

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

A negatively-tilted trough over the Kenai Peninsula will lift
northeastward tonight, spreading precipitation northward. The
heaviest precipitation associated with this system will fall along
the northern Gulf Coast as a rain/snow mix and as snow in the
western and northern portions of the Susitna Valley through the
evening hours. The western Kenai Peninsula, Anchorage area, and
Matanuska Valley will continue to see gusty southeasterly winds
this evening that will downslope out precipitation, keeping
conditions mainly dry. Overnight, expect winds to briefly taper
off, opening up Anchorage to the possibility of some overnight
snow showers.

A gale force front will move into the southwestern Gulf this
evening and move to the central Gulf/northern Gulf Coast by
Wednesday afternoon. This system has a long, southwesterly fetch
of moisture and warm air associated with it, so expect snow to
transition to rain throughout the day for lower elevations. The
northern Gulf Coast will see a prolonged period of rain Wednesday
morning through Thursday. Slightly further inland, gusty
southeasterly winds will ramp up once again along the Chugach
Mountains throughout the afternoon hours Wednesday and Thursday,
subsiding during the overnight hours. This will once again have
the effect of downsloping out the majority of the precipitation
in the lee of the Chugach Mountains. In addition to the gusty
winds and prolonged period of rain along the coast, expect
temperatures over most areas to climb into the 40s by midweek with
snow levels rapidly rising.

&&

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

Wind and cloud cover will rapidly increase this evening as a
strong frontal system moves ashore this evening. Radar returns
show precipitation spreading NE towards the coast, already
showing up at Cape Newenham in the form of rain. As the warm front
continues to track inland snow will initially fall before quickly
changing over to rain along the coast. A quick 1-2" could
accumulate close to the coast before the transition while
locations further up the Kuskokwim River Valley could see 3-4"
through tonight. Higher accumulations will likely be seen in the
elevated terrain above 2000' through tomorrow morning before snow
levels further rise throughout the day. Another impact from this
system will be in the form of strong southerly winds over Bristol
Bay and along the coast, with gusts up to 50 mph expected before
midnight tonight. Winds and precip will subside Wednesday as the
warm front pushes off to the east. As the low center moves into
Bristol Bay Thursday morning, southerly winds will pick up again
and another round of precip will move inland, though warm air will
keep precipitation in the liquid form for all low elevation
locations.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A very strong 940 mb low currently centered near Attu will
continue to wreck havoc across the Bering and Aleutians over the
next 24-36 hours with heavy precipitation and very strong winds.
A stout warm front well out ahead of the low is currently racing
east across the Eastern Aleutians and Bering bringing strong winds
and hurricane force gusts through channeled terrain, hence a high
wind warning is still in effect through this evening for the
AKPEN. The low will weaken quite a bit as it tracks east
following the Aleutian chain and loses some of its upper level
support, though a tight pressure gradient will remain intact
keeping strong winds across most of the region through Thursday.
The next system approaches the Western Aleutians by Thursday
morning bringing another round of rain and winds, though this
system is expected to be much weaker than the current one.

&&

.LONG TERM FORECAST (Saturday through Wednesday)...

The weekend forecast begins with a continuation of the very active
and progressive pattern, bringing more wet and unseasonably mild
conditions to most of southern Alaska. There remains considerable
model spread on timing, intensity, and placement of significant
weather systems, but sometime in the Sunday or Monday timeframe a
final storm will come in from the southwest and bring another round
of rain to the area.

By the middle of next week, there is surprisingly good model
agreement in the development of a large upper level high pressure
system developing somewhere over the western half of interior
Alaska. This should definitely dry things out across most of the
area, but it remains to be seen whether this weather pattern will
allow a plunge of cold air down into southcentral and the Copper
River Basin along with strong offshore winds (GFS and Canadian
models showing this) or much lighter winds and warmer conditions
and potential for widespread fog across the area (ECMWF solution).
A middle-ground solution is the most prudent at this point, which
shows a cool-down to seasonal norms but not a plunge of cold air
hinted at by some models.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 181.
		 Winter Weather Advisory 145.

&&

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