Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 231339
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
539 AM AKDT SUN OCT 23 2016
.ANALYSIS AND UPPER LEVELS...
The early morning weather map is dominated by a deepening area of
low pressure moving across the far western Bering Sea and the
associated gale force winds currently pushing through the central
Bering Sea. Latest satellite imagery appears to show the frontal
boundary slowing down as a weak surface low has begun to spin up
near 50N 168W. High pressure over the interior of Alaska continues
to build further south and west across the mainland this morning.
This has allowed outflow winds to continue across much of
Southcentral Alaska. However, the outflow winds have weakened
quite significantly overnight as low pressure over the northern
Gulf of Alaska has retreated into the North Pacific. Looking at
the upper levels, a near zonal jet stream is observed across the
Bering Sea and Aleutian Islands with longwave ridging building
across the Alaska mainland. Several upper level lows are trapped
in the Bering Sea due to the downstream ridging over the Alaska
mainland which has and will continue to keep the weather quite
active across the Bering Sea while much calmer over the interior.
Models are in overall pretty good agreement through Monday night
but are struggling a little bit with how the ample amount of
energy over the Bering Sea is utilized by a series of low pressure
systems. Two areas of low pressure crossing from the North Pacific
into the Bering Sea will be the main players the next couple of
days as high pressure remains well entrenched across the Alaska
mainland. While the models are in great agreement for the most
part with the timing of the low pressure systems crossing into the
Bering Sea, placement of the lows remains challenging due to the
position and strength of the jet stream and amount of cold air
filtering in from Russia. The first low will move near Shemya
tonight and bring a round of storm force winds to the western
Bering Sea but the second low on Monday has the potential for
hurricane force winds. For now, the ECMWF seems to have the best
grasp on the situation based on the 00Z initialization and trend
toward the energy consolidating into one low over the northern
Bering Tuesday morning.
PANC...VFR conditions and light winds will persist.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A strengthening upper level ridge over Southwest Alaska will slowly
shift eastward over Southcentral the next couple days providing
for dry conditions under mostly clear skies. Localized outflow
winds will persist out ahead of the ridge, with fluctuations in
speed as supporting upper level flow changes. The coldest air will
remain over the Copper River Basin, favoring gap and drainage
winds through Thompson Pass down into Valdez and Northeast Prince
William Sound as well as the Copper River Delta. With winds dying
down across most of the rest of Southcentral, a dry air mass along
with snow cover in many areas will lead to chilly night-time
lows, with single digits likely across interior valleys. As the
ridge moves overhead on Tuesday outflow winds will completely
die down and low level temps will begin to warm.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Ridging across the Southwest keeps dry conditions through Monday
evening. Winds will mainly be south to east with gusts developing
along the Kuskokwim Delta this morning as the pressure gradient
tightens along the coast as the Bering low lifts north. A warm
front moves swiftly across the Bering on Monday, making it to the
southwest coast overnight bringing increased chances for rain. The
associated low pressure system is a much stronger system and will
bring strong winds early Tuesday for the Kuskokwim Delta coast.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Two low pressure systems will track from the North Pacific into
the western Bering through Tuesday. The main challenge through
Tuesday will be the second system which is expected to bring
hurricane force winds to much of the western waters. The first
stacked low, currently about 200 miles north of Shemya, lifts
north this afternoon and fills. Abundant moisture streaming from
the Pacific supports periods of heavy rain along the central
areas of the Bering with lighter rain along the eastern Aleutians
and Alaska Peninsula where the boundary is more diffuse. Gale
winds are expected to increase this morning across the central
Bering then across the eastern Bering in the afternoon.
Winds slightly slacken ahead of the next system tonight, and then
rapidly increase as the next low deepens on its track across the
Bering on Monday. The jet streams set up a south to north flow
across the Central Aleutians by Monday afternoon with 140 kt core
supporting a strong dynamic set up for high winds as it noses into
the Bering. The high wind watches for the Central Aleutians and
the islands east of Shemya were upgraded to warnings with the
morning package. This storm system is expected to maximize wind
gusts across the Bering and Aleutians through Monday night before
tapering down on Tuesday.
.LONG TERM FORECAST (Days 3 through 7)...
An amplified upper level ridge continues to build over the
Southwest/Southcentral region as we head into Monday time-frame.
Therefore, northerly offshore flow influences the Southcentral
region accompanied by colder air filtering into the region
through the upcoming midweek. By Wednesday morning this synoptic
feature will exit the region into the Yukon Territory. This
allows southwesterly flow with warmer temperatures to advect into
the Southcentral region through the next several days. Meanwhile,
several storms track through the Bering with the first storm force
low tracking just west of Shemya with an associated weather front
moving through the Western Aleutians. While upstream a 150 knot
northwesterly jet streak digs into the base of the longwave trough
located just west of the Western Aleutians. This synoptic feature
progressively intensifies a rapidly developing storm force North
Pacific Low located just south of Shemya by Monday morning. The
GFS model has been the most consistent, and depicts a 974 mb low
near Kiska by Monday afternoon. This low will rapidly intensify
to a 941 mb hurricane force low as it tracks west of Saint Matthew
Island by Tuesday morning. As this system tracks through the
Bering and the Western Aleutians look for strong gusty
south/southwesterly winds accompanied by widespread rain/stratus.
Therefore, look for warmer temperatures in the aforementioned
regions through next week. By Wednesday this system exits the
region into the Eastern Siberian region. Then, another storm force
low enters the Western Aleutians with the same storm track as the
previous storms. This new storm exits the region into the Eastern
Siberian region by Friday morning. Therefore, in between these
systems the Bering/Aleutians will have a showery weather regime.
The NAM/GFS models are in better agreement with the strength of
the intensifying hurricane force low pressure system as it tracks
into the Western Bering by Monday afternoon. The ECMWF has
briefly come into better agreement with these models, but then
tracks this system way faster than the previously mentioned
models. The GFS continues to be the preferred model of choice.
This system will remain under close scrutiny for possible weather
impacts as it tracks through the Bering.
PUBLIC...High Wind Warning...187 191.
MARINE...gale force...170 173 174 179 181.
storm force...175 176 178.
SYNOPSIS AND MODEL DISCUSSION...mc
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...kh