Heavy Freezing Spray Warning 13, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
419 PM AKST Tue Jan 10 2017

.ANALYSIS AND UPPER LEVELS...

A large blocking ridge continues over the mainland. Generally
fair skies were observed over much of them mainland. However
there were areas of low clouds and stratus across the Anchorage
Bowl, Knik Arm, and Western Kenai Pen of Southcentral. Areas of
stratus and fog were also observed along the Southwest Alaska
Coast. Cloudy conditions prevailed across much of the Bering Sea
and Western ALeutians. With the exception of weak outflow
conditions across the eastern Kenai Pen and North Gulf Coast,
light winds were observed across much of the mainland. A weak
front boundary skirting the central and western Aleutians brought
a gusty easterly winds.

&&

.MODEL DISCUSSION (through Thursday afternoon)...
Models overall continue to be in very good synoptic agreement.
Some minor differences arise in snow potential for the North Gulf
Coast Thursday afternoon.

&&

.AVIATION...
PANC...Some breaks in the fog are expected this evening. However,
IFR/LIFR ceilings and visibilities will be the general rule
through Wednesday afternoon.

&&

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

High pressure over the mainland will provide one last night of
fairly stagnant flow across Southcentral. This will allow
continued fog formation along Knik and Turnagain arms, northern
Cook Inlet, and Prince William Sound. As an upper level trough
sweeps across the Aleutian range on Wed, there will be sufficient
air movement near the surface to finally eliminate the fog by
afternoon. As this weak trough continues across Southcentral, its
remnants will provide sufficient lift for a chance of snow
showers late Thu afternoon along the coast from Prince William
Sound east. Some outflow winds will be expected as these system
moves east, and temperatures will remain below normal through Thu.

&&

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

High pressure remains dominant this evening, but will slowly
begin to break down as a trough moves into the region from the
Bering Strait Wednesday. This will result in increasing north
winds which will help weaken the stout inversions in place. Fog in
the Kuskokwim Delta and Bristol Bay region will also diminish with
the northerly winds. Initially this will result in warming
temperatures even though colder air aloft will be moving in
through Thursday before temperatures begin to fall again Thursday
night.

&&

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

East to northeast winds will continue over the Bering through
Thursday as a ridge of high pressure aloft weakens and an upper
trough starts to dig into the Northern Bering Sea Thursday
evening. This will tighten the pressure gradient allowing for
widespread 20-30 knot winds over the Bering along with increasing
showers as colder air overspreads the warmer waters.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Since Monday, the models have made a relatively small but very
important change. The arctic trough that will dig through the
Southwest Mainland on Friday has trended further west, with
improving agreement that this will bring the surface trough
towards the north gulf coast. As a result the chance of
precipitation for the North Gulf from Whittier-Cordova was
increased significantly on Friday night, mostly likely falling in
the form of snow. Over Southcentral, confidence is much lower in
the snow forecast, but chances are increasing in at least a brief
period of snow. This is due to significant model spread in the
amount of snowfall reaching into Cook Inlet. The GFS has been by
far the most aggressive in bringing snow into Cook Inlet. On the
other side of the spectrum, EC and NAM are much further to the
east in bringing only a brief period of light snow but are
beginning to trend slowly west.

Once the chance for snow moves out of the area late Saturday-
Sunday, there is very good confidence that this arctic trough will
bring a deep layer of extremely cold air throughout most of the
area. Since this is coming from a trough there remains some
uncertainty as to how much cloud cover will be associated with it.
If enough clearing occurs over interior areas to create strong
inversions this system has the potential to bring some of the
coldest temperatures in recent years, especially to the lower
elevations of the Southwest Mainland. Once the cold air sets up
there is good agreement that it will continue through early next
week.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...BC
SOUTHCENTRAL ALASKA...DS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...DK