Gale Warning 117, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 PM AKST Sat Feb 16 2019

.ANALYSIS AND UPPER LEVELS...
A broad upper trough anchored by a low center over eastern Russia
has set up broad southerly flow across much of the southern
mainland. Morning sounding showed a fairly pronounced 100-125 kt
300 mb southwesterly jet extending from the AK PEN to over the
southwest interior. This feature was helping to support a broad
surface front which extends from Southcentral to over the north
Pacific south of the eastern Aleutians. Fairly widespread snow was
occurring across the Kenai Peninsula, Cook Inlet, and then north
to the MAT-SU Valleys. Radar imagery shows snow spreading east
toward the North Gulf. Satellite imagery shows a surface wave
developing along the front south of the eastern Aleutians. A warm
front associated with this feature is pushing rapidly north
spreading rain and snow toward Bristol Bay. Further west satellite
imagery shows widespread open cell cumulus and showers associated
with cold unstable air spreading over the western and central
Bering Sea and ALeutians.

&&

.MODEL DISCUSSION...
The big change in the numerical models is the faster and more
southerly track of a storm force low heading into the eastern
Bering Sea Sunday night/Monday morning. Models are in general
synoptic pattern but struggle with the handling on the positioning
of waves and the QPF pattern.

&&

.AVIATION...
PANC...LLWS developing tonight as surface winds remain out of the
north while winds aloft become southeasterly. Periods of MVFR to
IFR conditions in snow can be expected through tonight, though
there should be some VFR breaks overnight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A moist, southerly flow and isentropic lift along a now-nearly
stationary warm front stretching from Kodiak Island east across
the northern Gulf is producing widespread light snow from the
Talkeetna Mountains south this evening. Areas of light snow will
continue as the first front stalls along the coast and a second
frontal system attached to a surface low just west of Bristol Bay
lifts into the northern Gulf. This next system will have plenty of
support with a continued moist, southerly flow in the lower to
mid-level and a jet streak passing over the southern mainland.
This second system will continue to produce a widespread snowfall
for all but the immediate coastline through Sunday night before
the occluded front sweeps across the Gulf and the trailing upper-
level trough lifts into the interior of the state. Multiple Winter
Weather Advisories have been issued for this event, including
locations along the western Kenai Peninsula, Susitna Valley, and
Portage Valley and eastern Turnagain Arm.

The biggest forecast challenge continues to be the possibility of
a downslope component for areas immediately west of the Chugach
and Kenai Mountains. Given the moist, southerly flow from 850mb
through 700mb however, chances of snow remain in the forecast for
these locations with the notion that the moist flow will
overwhelm the downslope component at times. Southeasterly gap
winds will also remain strong through Sunday as the front
approaches and the gradient tightens. These winds will then
diminish late Sunday as the system moves to the east.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Saturday
night through Monday)...
An advancing frontal system moves inland over Southwest tonight,
bringing southerly flow and advecting relatively warmer air inland
over Bristol Bay for Sun morning. While precipitation associated
with this system has begun to fall over the northern AKPEN as
snow, a quick change to rain is expected overnight as temperatures
warm rapidly. By Sun morning, the low is expected to track inland
over the Kuskokwim Delta, bringing measurable snow to the Lower
Kuskokwim Valley where temperatures remain colder. Coastal areas
along the Kuskokwim Delta may see gusty winds, which when combined
with snow, will limit visibility at times. As the low continues
to move north, conditions should gradually improve from south to
north by Sun afternoon.

Meanwhile, another system in the Bering approaches Southwest by
Sun evening, bringing another round of precipitation. Snow will
begin initially, however warmer air mixing in over the southern
AKPEN may allow for a change to rain Mon morning. More critically,
a prolonged area of westerly fetch brings a threat of coastal
flooding along the coast from Goodnews Bay to Port Heiden. Thus, a
coastal flood watch has been issued for Monday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A complex frontal system continues to advance over the eastern
Aleutians and AKPEN while a secondary low follows behind and
weakens into an open shortwave overnight. The front advances
inland over Southwest for Sunday, while the shortwave remains
south of the eastern Aleutians and advances eastward toward the
Gulf.

The real story for the Bering is an approaching storm force
low. Its leading front pushes west to east across the Aleutians,
arriving over Adak Sun morning. While a brief period of mixed rain
and snow is possible with this front, precipitation will likely
fall mostly as rain across the Aleutian chain, as southerly flow
advects warm air northward. Southerly gales with a wide swath of
embedded storm force winds are also expected with this front as
it traverses eastward, reaching Cold Bay and nearing the Pribilof
Islands by Sun afternoon. Contrary to the previous Bering lows
we've seen earlier this week, this 970 mb low will track through
the central Bering, bringing with it a large area of westerly
gales and embedded storm force winds on the south side of the low.
The strongest winds will pass over Atka and Amukta Island Sunday
evening as the low center nears the Pribilofs. As the low moves
inland Mon morning, gale force westerlies will continue across the
eastern Bering through Mon afternoon.

&&

.MARINE (Days 3 through 5)...
Monday night and Tuesday low pressure tracks across the southern
mainland creating gusty gale westerly winds across the gulf at the
base of this system. On Wednesday, this low exits to the east
with the next system advancing from west, so not much reprieve is
expected. There is still high uncertainty with the second system
through Friday with potential for gale to storm force winds as the
low deepens on an eastward track.

Across the Bering...two systems will keep gale force winds along
area waters with shifts in direction through Wednesday. Two
additional storms track northeast across the Bering, one crosses
over Shemya Wednesday night and moves to the west coast Thursday
afternoon. The following system tracks across Shemya Thursday
night and takes a northerly track toward Kamchatka.

&&

.LONG TERM FORECAST (Days 3 through 7)...
An active wave pattern continues throughout the next work week.
Therefore, higher chances for rain and snow will be advertised
with rain mainly along the southern Gulf and Bering. Low pressure
along the Bering will usher several fronts across the southern
mainland in a cyclical pattern. The system to watch during the
week for snow across the mainland moves to the West coast on
Wednesday, tracking east across Southcentral and the Gulf into
Thursday. The most organized storm system for the week develops in
the North Pacific and moves across the western half of the
Aleutians Thursday night. This system should slow the forthcoming
active low pattern, allowing a ridge to build back across the Gulf
and southern mainland bringing drier conditions for southern
Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 121 125 145 155 161.
 Coastal Flood Watch 161.
MARINE...Storm Warning 170 172>177.
 Gale Warning 119 120 125 130>132 139 150 155 165 171 178>180.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...KH