High Wind in Alaska, April 24, 2018

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
502 AM AKDT Tue Apr 24 2018

.ANALYSIS AND UPPER LEVELS...
The synoptic pattern early this morning includes a large upper
level low extending over the Bering, Aleutians and portions of the
North Pacific and a very strong southwesterly jet stream
currently aimed at the Alaska Peninsula. A developing triple
point low currently west of Cape Newenham and along the left exit
region of the jet lifts north toward the Kuskokwim Delta coast as
it rapidly strengthens into a compact yet powerful feature. The
associated frontal system is currently spreading precipitation
across Southwest Alaska, Kodiak Island, the Kenai Peninsula and
the western Gulf. Further north in southcentral Alaska
southeasterly pressure gradients and winds continue to increase in
advance of the approaching frontal system.

&&

.MODEL DISCUSSION...
Models remain in good synoptic agreement regarding the current
system and subsequent cool upper level trough that sets up over
mainland Alaska Wednesday and Thursday. Forecast challenges
include the fine scale details of the spatial extent of the
strongest of southeasterly gap winds and temperature and
precipitation type questions for the Susitna Valley.

&&

.AVIATION...
PANC...The primary concern for PANC today is the increasing
southeasterly flow across the Chugach Mountains. Rapidly
increasing pressure gradients ahead of an incoming frontal system
will bring increasingly strong Turnagain Arm winds to area this
morning through this afternoon. The pattern looks favorable both
for the increasingly strong southeast surface winds to continue
bending north and over the airport and much strong winds not far
off the surface to create significant wind shear concerns as well.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A Fall-like storm is moving into Southcentral Alaska today
bringing strong winds and rain, with snow at higher elevations.
The strong winds will increase across the area and a high wind
warning remains in effect for Turnagain Arm and the higher
elevations around Anchorage. Strong winds will also increase in
town in Anchorage as well as the Palmer-Wasilla area with peak
winds likely occurring from late-morning through mid-afternoon.
The Copper River basin will also see these increasing winds with
gusts to 45 mph along the Copper River.

Rain will mostly downslope out on the lee of the mountains in the
strong southeast flow, but as the front passes through late this
afternoon, it will likely bring some rain into the lee side as
well. Rain will be heavy along the north Gulf coast and Seward may
see around 2 inches of rain from overnight Monday night through
Tuesday evening.

Thompson Pass is an interesting scenario as temperatures look to
remain cold enough for snow and there is also some stiff southeast
wind in the 20 to 30 mph range. However, the warmer southeast
winds tend to both lessen the precipitation rate and keep the
blowing snow from reducing visibilities as much as cold advection
winds from the northeast. Therefore, while there will be some snow
and wind through Thompson Pass, the direction of the wind (and
being that it is late April) is expected to mitigate the impact
to the highway. The uncertainty is due to the potential for
increased precipitation to help cool the air and slightly colder
air may bring some increased snow accumulation to the highway and
start to interfere with visibility.

North of Talkeetna this evening through overnight is also a region
that will see some accumulating snowfall as the colder air moves
in behind the front. Overnight snow accumulation for this area is
expected to be around 2 to 4 inches, but the southerly flow aloft
is a direction that could cause enhanced snowfall and therefore
could lead to higher amounts than currently expected.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A deep and unseasonably strong upper level trough is situated
over the Bering Sea and will continue to dominate the pattern and
sensible weather through the forecast period. The upper trough
will dig further south this morning as a very strong 150 kt
southwesterly jet pushes north over the AKPEN and SW mainland.
This upper jet will support the development of a rapidly deepening
surface low over Bristol Bay and the Kuskokwim Delta coast. An
occluded front extending out from the low will sweep NE across
the SW mainland this morning through the evening bringing a large
swath of moderate to heavy precipitation as well as strong
southeasterly winds, which will switch to southwesterly as the
front passes through. Warm air ahead of the front will keep
precipitation rain at the surface but colder air filtering in
behind the front this afternoon and evening will lower snow levels
mixing in some snow at the surface, especially along the
Kuskokwim Delta coast. The upper low will continue to spin over
the Bering on Wednesday as several shortwaves rotating around the
base of the low swing into the SW mainland, bringing scattered
rain and snow showers throughout the day and continued strong
southwesterly winds. A long gale force fetch will materialize over
the Kuskokwim and Bristol Bays leading to high surf with water
levels at or slightly above highest astronomical tide levels
around Tuesday afternoon and early Wednesday morning, though any
coastal flooding and beach erosion are expected to be minimal.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A large upper level low with numerous embedded shortwaves and
individual surface circulations will continue to bring widespread
rain showers to the Bering and Aleutians today through tomorrow
afternoon. Gale force west/southwesterly winds will spread across
most of the southern Bering and Aleutians today as a surface low
shifts slightly north bringing the core of strongest winds
parallel along the island chain. By Wednesday the upper low will
slowly drift to the north and east allowing a shortwave ridge to
momentarily build in over the western Bering/Aleutians. This will
bring a brief break in the weather as drier air filters in
leading to some clearing as well as a decrease in the wind
speeds.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long range forecast beginning Wednesday night is a
progressive pattern with a low spinning near the Bering Strait.
Embedded short waves rotating around the low brings continued
chances for precipitation across the southern mainland through
Friday, then lifts quickly north Friday night as the next strong
short wave races across the Bering. This short wave initially
moves a surface low across the Bering Thursday night, pushing to
the Southwest coast Friday night.

There is high uncertainty in the extended forecast beyond Friday
as models greatly differ on the track and timing of fast paced
storm systems moving across the Bering. Overall, the synoptic
pattern is dominated by a broad trough over the Bering and an
eastern North Pacific ridge which vies for northward extent into
Southcentral. The ECMWF and its ensembles offered the best
continuity which maintains the ridge over the gulf through the
beginning of next week which keeps Southcentral in a drier regime.
There is better agreement for The Southwest and Bering to have a
front moving across Saturday and Sunday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning for 101.
MARINE...Gale Warning 119 120 125 130 131 132 139 150 155 165 170
            171 172 173 174 175 176 177 178 180 351 352 414.

&&

SYNOPSIS AND MODEL DISCUSSION...JER
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KV
LONG TERM...KH