Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 231408
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
508 AM AKST Thu Nov 23 2017

.ANALYSIS AND UPPER LEVELS...
A deep low pressure system and associated front are bringing
strong southeasterly winds and snowfall to the Kuskokwim Delta as
well as the western Bristol Bay area with very strong west to
northwesterly winds pushing in behind it. The strong low, upper
level coupled jet stream and cold air advection are supplying
enough forcing for storm force winds to occur over much of the
central and eastern Bering as evidenced by scatterometer data. As
the low traverses south down the Southwest Alaska coastline, the
west to northwesterly gale to storm force sustained winds are
spreading over the eastern Aleutians and Alaska Peninsula as well
with hurricane force gusts through channeled terrain and mixing
down in areas of strong cold air advection.

While Southwestern Alaska is experiencing strong winds and
snow, the weather over Gulf and Southcentral Alaska is quieter if
on the chilly side. A triple point low to the east of Kodiak
Island is in the process of absorbing some of the energy from the
Bering Sea storm, causing it to further deepen in response to the
upper trough and cold air aloft crossing the Alaska Peninsula and
pushing into the western Gulf. Offshore flow and gap winds are
strengthening again in response both the low forming in the Gulf
and the cold air pool deepening inland.

&&

.MODEL DISCUSSION...
The models are in good agreement through around Friday night
bringing the next frontal system into the central Bering. Starting
Saturday however fast moving waves tracking along strong north
Pacific jet start introducing major model differences in the
timing and track of the series of strong lows tracking across the
Aleutians and southern Bering into the Gulf Sunday through next
week.

&&

.AVIATION...
PANC...Patchy low stratus moving in and out will keep conditions
bouncing between IFR and VFR this morning before increasing
offshore flow and breezy north winds clear things out with VFR
conditions persisting this afternoon through Friday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 2)...
A powerful storm force low over the eastern Bering Sea is moving
into the western Gulf of Alaska this morning as a triple point low
forms south of the Kenai Peninsula. Snow showers will impact parts
of Kodiak Island (mainly the northern and western parts) and the
southern Kenai Peninsula before the low quickly dives into the
central Gulf this afternoon.

Thereafter, the main story will be the rapidly increasing offshore
pressure gradient as cold arctic air over the interior and Copper
Basin pushes dense air through the Alaska Range and Chugach
Mountains along the coast. A potential vorticity anomaly along a
150 knot jet streak will track over Cook Inlet and Kodiak Island
Thursday night and Friday, and this will enhance the offshore
winds throughout Southcentral. Kodiak island will potentially see
some mountain wave trapping, so williwaw gusts may reach into the
high 60 MPH range with this event. Otherwise, Valdez and the
Matanuska Valley will also reach into the 60+ MPH range, so this
outflow wind event looks quite impressive. Otherwise, very cold
and dry looks to be the main story is 850 hpa temperatures drop to
-15 to -20 C by Friday. Folks who like to recreate in the
 mountains should plan for dangerous wind chill conditions,
 especially Friday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A strong area of low pressure easily seen in satellite and radar
data continues to slowly churn south southeast along the coast,
with the latest observations placing it just off the coast of
Kipnuk. The cold front stalled on top of Bethel as expected.
Further south, the front made more progress than though yesterday,
having easily cleared King Salmon before stalling earlier this
morning. This was likely due to a greater fetch of a marine aided
thermal push as the warm layer was very shallow based on the King
Salmon RAOB.

As the low continues its southeastward trek, colder air advecting
into the region will allow all liquid precipitation to switch
back over to snow showers. Both radar and satellite have a very
convective look to them, which is no surprise based on the
strength of the low, so a few briefly intense snow showers with
visibilities reduced to less than 1 mile may develop at a few
locations. Blowing snow further inland will add to this potential
as well.

Otherwise, look for improving conditions as we head through
tomorrow into Saturday, as short-wave ridging traverses the area,
bringing increased subsidence and drier air to the region.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The Gales (and Storms) of November are definitely blowing this
morning across the eastern half of the Aleutians/AKPEN and Bering
Sea, as a strong low moves from near Kipnuk towards the Gulf of
Alaska. Very strong cold advection behind a front that cleared the
region last night continues, with observations near Unalaska and
at Saint George Island having already reached warning criteria.
It`s only a matter of time before zone 181 gets in on the action,
as here too warning criteria wind speeds will easily be met.
False Pass and King Cove stand the best chance of seeing this
occur due to the more channeled terrain. Widespread showery
precipitation crossing the region may help mix down these gusts at
times, especially with the stronger showers due to increased
downward momentum transfer processes. We`re thinking mid morning
to mid afternoon will be the best time frame to see all of this
verify.

Further west, mainly gale force winds are expected through
tomorrow across the western Aleutians and Bering Sea, as these
areas remain on the edges of the strong storm further east. A
short-wave ridge aloft currently trying to build into western
portions of the chain from the south will amplify some during the
next 24 to 36 hours. This ridge will shift eastward with time,
bringing decreasing winds and clearing skies. However, another
cold front will sweep east across the region with widespread
precipitation developing in the Friday evening through Saturday
time frame.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term portion of the forecast begins Friday night with a
familiar outflow wind pattern in place across much of southern
Alaska, with a ridge over the Bering Sea and a longwave trough
extending along the Alcan border into the eastern Gulf. This will
keep most of the area cold and dry well into the weekend before
the ridge breaks down and the pattern begins to shift once again
Sunday into early next week.

Forecast confidence quickly diminishes by the beginning of next
week as a deep trough over eastern Russia sends a series of
shortwaves into the Bering Sea. Models continue to show poor
agreement with respect to the evolution of these waves and
subsequent development and placement of surface features. It
appears that a deep low will rapidly intensify over the Bering
Saturday night and then move to the general vicinity of the Alaska
Peninsula by late Sunday, sending a front near the southwest
coast and bringing the threat for another round of mixed
precipitation and gusty winds to portions of Southwest AK. An even
deeper low then looks to approach the Aleutian chain on Monday,
bringing stormy conditions to the southern Bering with the details
largely dependent upon the track of the low through the middle of
the week. Guidance is also hinting at another system moving into
the western Gulf in the Monday/Tuesday time frame, but confidence
is low at this time as specific solutions remain all over the map.
Generally speaking, those in southern Alaska can expect chances
for precipitation to increase through the first half of next week
(especially along the coast) as temperatures rebound closer to
seasonal normal values.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Coastal Flood Advisory 161
         Winter Weather Advisory 155 161
         High Wind Warning 181 185 195
MARINE...Storm Warning 150 155 165 170 171 172 179 180
         Gale Warning 119 120 121 125 126 127 128 130 131 132 136
               137 138 139 140 160 173 174 175 176 177 178 181 185
         Heavy Freezing Spray Warning 121 140

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JER/RA
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...CB



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