Small Craft Advisory 833, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 PM AKST Tue Nov 5 2019

.ANALYSIS AND UPPER LEVELS...
The upper levels are fairly convoluted today. A developing upper
level shortwave is diving down from the Bering Strait along the
Eastern Bering Sea. This is producing what looks like a baroclinic
leaf on satellite with lower pressure trying to establish itself
near the surface as almost an inverted trough. Inland over the AK
Range, a ridge of high pressure is helping to keep some colder air
over the interior portions of the state. A weak shortwave is
drifting from south to north across Southcentral. This led to
brief periods of mixed precipitation (sleet, rain, freezing rain)
over portions of the Cook Inlet and the Matanuska Valley earlier
today. The primary upper level 300 mb (30,000') jet is over the
Western Aleutians. It drives in from west to east at around 140
kts before turning sharply to the north at 115 kts. This serves to
support a weakening 970 mb low over the extreme Western Bering.
Storm force winds were reported overnight at Shemya.

Surface temperatures across much of SW AK remain well below
freezing. As warm air continues to flow over top of this cold
layer, widespread freezing rain has been observed. The 12z
sounding at Bethel was a classic representation of a thermodynamic
environment primed for freezing rain. As of noon Tue, the Bethel
Airport had already received a third of inch of ice accumulation. The
YK- Delta looks to be seeing the worst of this event, however,
many other places in the Lower Kuskokwim Valley and interior part
of Bristol Bay are also reporting periods of mixed precipitation.

&&

.MODEL DISCUSSION...In the short term, models remain in rather
good agreement and thus, forecast confidence in moderate to high.
All major synoptic features through Thu are handled well by the
major models. Some of the finer details that will have to be
closely assessed will be precipitation types through the area and
the end time of the freezing rain across the YK-Delta. This
package utilized mostly hand edits to address these details. By
Fri, the models diverage wildly across the Gulf of AK with a
developing surface low. The GFS appears to be an aggressive
outlier with this system, so the forecast trends towards the
ECMWF.

&&

.AVIATION...
PANC...VFR conditions and light north winds will persist. A front
will bring some gusty southeast winds to the Turnagain Arm and the
Hillside tonight into Wed, but these should remain clear of the
terminal.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Tonight through Thursday)...

A weakening warm front lifting northward over Southcentral is
bringing mixed precipitation to the region. The current freezing
line is just north of Wasilla. Thus, anywhere north will see snow.
Areas to the south will mostly stay dry in downsloped flow on the
windward side of the Chugach Mountains, however periods of drizzle
cannot be ruled out this evening. Accumulating snow is also
likely along the Parks Hwy in Broad Pass. Gusty southeasterly
winds up to 50 mph will continue through Turnagain Arm through
Wednesday. The same general precipitation pattern will persist
through Thursday with rain across Kodiak Island and coastal
Prince William Sound. Snow will continue further inland, with
3-5" possible along the Denali Hwy between Cantwell and Paxson.
Anchorage and the Matanuska Valley will generally remain dry and
any precipitation will fall as rain with temperatures near 40.

&&

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

The prolonged freezing rain event will continue across the
Kuskokwim Delta through the night as a front continues across
Southwest Alaska. Bethel, which has already reported warning
criteria ice accumulations (0.25 inch), will likely see more
freezing rain through the early Wednesday morning hours when the
precipitation will then switch over to snow. Areas north of Bethel
near the coast and across the Kuskokwim Valley could see freezing
rain hanging on for a few extra hours until their temperatures
warm up enough above freezing to make their transition to rainfall
as well. The exact time that precipitation will switch over to
rain is still a bit uncertain, but will be closely watched heading
into the overnight hours.

By Wednesday afternoon, showers across the Kuskokwim Delta begin
to taper off, with any precipitation confined to the coastal areas
as the front moves out of the area. However, the showery pattern
will return late on Wednesday through Thursday as a trough pattern
moves over Southwest Alaska.

&&

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

A gale force front stretched over the western Aleutians and Bering
associated with a Kamchatka low will weaken and lift northward out
of the areas. Small craft winds, extending from Bristol Bay to
the north central Bering will continue as a North Pacific low
center moves over the Pribilofs through Wednesday afternoon. These
stronger winds will continue to push northward as the low weakens
over the central Bering. The next system entering the region on
Wednesday afternoon will stay south of the western Aleutian Chain
continuing south of the central Chain through Thursday. This low
could bring a brief period of small craft winds to the forecast
area as it moves towards the North Pacific.

&&

.MARINE (Days 3 through 5: Friday through Sunday)...

A gale force low may move northward across the western Bering
starting with the western Aleutians Saturday afternoon, and
continuing in the western Bering through Sunday. Gale force winds
are possible over the Gulf on Sunday, but there is a high degree
of uncertainty with the timing and track of the low, and is
considered low confidence.

&&

.LONG TERM FORECAST (Days 3 through 7: Friday through Tuesday)...

More of the same in the long-term forecast regarding a high degree
of uncertainty and low confidence. Here are the aspects of the
forecast where there is some certainty: a trough over the central
Bering, the remnants of the low that is currently there, will
dissipate as a weakening North Pacific low sits south of the
eastern Aleutians/AKPEN. A second low will move into the western
Aleutians and Bering through Sunday, but will track north and
should not cause significant impacts. The weather across the
mainland should be relatively dry and quiet through Saturday. Then
depending on how fast another North Pacific low tracks north
across the Gulf, there could be more active weather Sunday into
Monday. Most of the models agree on somewhat colder airmass
settling into the Copper River Basin Friday and Saturday, which in
turn could make for windy conditions through Thompson Pass and
the Matanuska Valley.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Warning 152 155.
MARINE...Gale Warning 120 130 131 139.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MO
SOUTHCENTRAL ALASKA...AP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...JW