Gale Warning 51, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 PM AKST Thu Jan 10 2019

.ANALYSIS AND UPPER LEVELS...
There is a large upper low over the state, centered near Galena.
This is associated with a very cold air mass, with surface
temperatures across southern Alaska generally between 10 and 25
degrees below normal this afternoon. Surface high pressure is
producing generally dry offshore flow over the mainland. Some
moisture and a few showers are over the Copper River basin
extending south into the north central Gulf associated with a weak
short-wave trough. A skinny and negatively tilted upper level
ridge extends from the eastern Alaska Peninsula to the northern
Bering. There is a strong vertically stacked low centered south of
the western Aleutians. The central surface pressure is around
941 mb, and Storm force winds are currently across the central
and western Aleutians, just extending into the southern Bering.
There is a weak and dissipating trough stretching from the western
Alaska Peninsula through the Pribilofs.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement through the first 24
hours (Friday afternoon). After that, they begin to diverge
significantly with a low (or lows) moving into the Aleutians, and
another low moving into the southwest Gulf. For the Aleutians
low(s) the timing of the front is similar, but with the central
low pressures there are and east-west differences. For the Gulf
system, the models vary on the timing, especially as the low
"opens up" into a front late Saturday morning. At this point it is
difficult to pick a "favorite" model and the forecast confidence
for Friday night and Saturday is below normal.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

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

A broad upper-level low continues to anchor itself over mainland
ALaska this evening. Ejecting from this low, an upper- level
shortwave undercutting and its associated weak surface low near
Prince William Sound will slide northeast and bring scattered
clouds and a few snow showers from Valdez and Cordova north over
extreme southern portions of the Copper River Basin overnight. For
areas north and west of these locations, cyclonic flow around the
main low will continue to allow cold, dry air to spill south and
east from the interior. This intrusion of arctic air will bring
the coldest temperatures of the season for many locations,
especially areas near the coast as the outflow winds begin to
diminish this evening. The coldest temperatures overnight are
expected across the Susitna Valley and throughout much of the
central and northern Copper River Basin.

The upper-level low then begins to lift north on Friday as a
change in the weather pattern begins to take hold and more
predominant southerly flow develops over the Gulf and a series of
fronts moves toward the southcentral coast. The first arrives late
Friday as a weak warm front that will produce some light snow from
Kodiak Island northeast to western Prince William Sound as warmer
air from the Gulf rides overtop the colder air entrenched nearer
the surface. This front stalls then dissipates Saturday as the
next front associated with a low south of the AKPEN approaches.

This front moves over Kodiak Island early Saturday, bringing snow
initially before a changeover to rain as warmer air pushes across
the western Gulf. Later on Saturday, the front moves closer to
the southcentral coast with snow along the eastern Kenai Peninsula
and Western Prince William Sound. This initial front then weakens
as a second, stronger front trails behind it with another shot of
rain/snow for Kodiak Island and snow for the eastern Kenai
Peninsula. This second front will bring another surge of warmer
air across southcentral, with snow most likely changing to rain
along the immediate coast early Sunday, but not before a few
inches of snow accumulates. Southeasterly gap winds will also
increase out ahead of the front as well as warmer downslope winds
for locations in the lee of the Chugach and Kenai Mountains (e.g.
Anchorage, Kenai, Palmer).

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Fri and Sat)...
More very cold temperatures and wind chills are on tap before a
pattern change starts to develop over the weekend. First, the cold
airmass will only get colder overnight into Fri as an upper level
low pushes Arctic air across the region. As the front over the
Bering draws slowly closer to the coast, this will only serve to
strengthen the surface pressure gradient. The net effect will be
continued gusty offshore conditions. Thus, we have elected to
expand the Wind Chill Advisory to include the inland portions of
Bristol Bay in addition to the YK-Delta. Conditions across the
YK-Delta could actually approach Wind Chill Warning levels on Sat
morning if skies stay clear and winds remain gusty. We will
monitor this situation closely.

As we head into Sat, the aforementioned front over the Bering
finally starts to bring some warmer air to the region. Winds will
shift to a south-southeasterly direction and start to slowly erode
the cold dome as we go through Sat. High temperatures from Fri
afternoon to Sat could jump as much as 20 to 30 degrees across
Southwest AK. This front will also try to bring some light snow
with it, but the best chances will remain along the coastal areas
and Western Capes. It will take some time to saturate such a dry
airmass, so we are not expecting much in the way of accumulations
at all.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Fri and
Sat)...
A 941 mb low continues to dominate the scene over the Bering. The
low has occluded and is starting to weaken just south of Shemya.
Its associated front will stretch out across the Central and
Eastern Aleutians over the next 24 hours. Storm-force east winds
can still be expected along the boundary as it pushes up against
the cold air over the Northern Bering with primarily gale- force
along the Aleutian Chain. This front will also bring a modest
amount of precipitation in the form of snow mixing with rain to
the islands.

This frontal boundary will then become the foundation for layers
of chaos to unfold heading into the weekend. The "parent low" will
remain over the far-Western Bering. But along the front itself,
several new lows will attempt to develop in the left exit of the
upper-level (30,000') jet stream. Each models depicts a different
solution of multiple surface lows developing along the front and
tracking to the east/northeast. However, each model also has a
different location and intensity for each low. Thus, we held the
forecast mostly as is for this package until there is higher
certainty. Either way, weather looks to remain active over the
Bering with the potential for several (at least) gale-force lows
passing quickly through the region.

&&

.MARINE (Days 3 through 5)...

Beginning the marine extended period forecast on Sunday, there
will be a gale force low moving into the eastern Bering Sea. This
low will move inland Monday, leaving behind weaker and broad
cyclonic flow over the Bering Sea, which will be a reduction in
winds and seas compared to the preceding two days. There is
uncertainty with the low tracks by Tuesday, but the storm track
will shift east into the Gulf of Alaska with a high likelihood of
a series of gale force fronts/low tracking in succession toward
the North Gulf Coast by Sunday and potentially lasting into
midweek. Confidence on timing or exact track by that juncture is
very low, but the pattern will be more active than it recently has
been for the Gulf.

&&

.LONG TERM FORECAST (Days 4 through 7)...

The state of the atmosphere will be rapidly changing on Saturday
evening as the deep upper level arctic trough over the interior
shears apart and retreats to the north. This will allow for a
barrage of fronts/lows to assault the Southern Alaska mainland.
This pattern will be messy to say the least, with warming
temperatures and threats for the entire spectrum of wintery
precipitation. The signal in the noise of this pattern is that a
very high amplitude negative tilt ridge axis will build over the
mainland through Wednesday. Low tracks are uncertain, but this
pattern will be active through at least middle of next week
before the storm track relaxes. However, that is still uncertain
with some guidance (the GFS, most specifically) keeps the
southerly flow onslaught through Thursday before upper ridging
tries to build. Key takeaway: expect warming temps and threats for
rain/freezing rain/mixed snow for most of Southern Alaska
beginning Saturday night and Sunday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 155 161.
MARINE...Gale 130 139 165 170 172 173 174 176 179 185.
		 Storm 175 177 178.
		 Heavy Freezing Spray 121 127 139 140 160 179 180 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
MARINE/LONG TERM...JA