Gale Warning 115, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
532 AM AKST Fri Feb 15 2019

.ANALYSIS AND UPPER LEVELS...

Aloft, there is an area of broad and diffuse upper ridging
extending from the central Bering and across the Mainland of
Alaska. Interrupting this is a low amplitude shortwave trough, the
remnants of a once powerful front which crossed the Bering Sea,
which is currently tracking into the Gulf of Alaska. This feature
is dry, with the only impact being a brief increase of local
pressure gradients (and offshore winds) along the Gulf Coast.
Along the ALCAN, the remnants of the once strong upper trough,
which brought moderate strength offshore winds to Southcentral, is
slowly moving off to the east. However, the dry air and mostly
clear skies remain in its wake.

Over the western Bering Sea, it is the same story rehashed once
again...there is a low end storm force low lifting northward
towards Siberia. This compact low is depicting an impressive dual
warm conveyor belt structure in the cloud field. ASCAT satellite
derived ocean winds are already showing a narrow region of storm
force winds along the backside of this low (possibly a weak sting
jet, especially given the cloud "fingering" in the evaporating
cloud head). This storm force region is expected to expand as it
deepens along its northward trajectory.

&&

.MODEL DISCUSSION...

The numerical models remain in decent agreement with the "general"
overall synoptic flow through early next week, but they continue
to remain in rather poor agreement with respect to a complex
front/low which will move into the Mainland Saturday through
Sunday night. As is always the case with lows moving north from
the Pacific (especially when cold air is present), the devil is in
the details. Currently the NAM and GFS models are the most
aggressive in phasing a current North Pacific trough with a
subtropical warm front, which then amplifies and takes a westward
track as it lifts north into Southwest Alaska. These models favor
the Kusko Delta for landfall, while the Canadian, FV3, and ECMWF
have a weaker low and a farther eastward track. All these
differing solutions have varying impacts, some which vary widely,
for interior locations. With all that said, there has been a
couple trends from yesterday that all the numerical models have
moved towards. First, the surface low will likely move west of
Cook Inlet, so there will be some period and degree of downslope
winds which will likely result in periods of dry
weather/warming/potential precip type challenges for locations
down-wind of the Chugach/Kenai/Talkeetna Mountains. Moreover,
there is increasing chances for pretty much all of Southern Alaska
to see periods of snow this weekend, with the potential for
localized heavy amounts in favored locations (for instance, the
Susitna Valley). Stay tuned...

&&

.AVIATION...
PANC...VFR conditions and light winds are expected through Friday
night. MVFR conditions are likely sometime on Saturday morning as
light snow moves over the terminal.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

We are looking at another day of widespread sunshine over
Southcentral Alaska before the pattern changes tonight. The
outflow winds through channeled terrain and along the north Gulf
coast should begins to weaken today as the trough in the northern
Gulf weakens and dissipates this evening.

A front moving over Kodiak Island and the Western Gulf this
evening will continue to push northeastward overnight. Kodiak will
likely see precipitation start as snow and then change to mostly
rain at sea level by midnight. The Kenai Peninsula will be next
for precipitation and it should remain all snow there except for
possibly around Kachemak Bay.

The way the precipitation moves into Southcentral will bear some
watching as there should be snow over the Cook Inlet and Susitna
Valley regions with this first front tonight into Saturday
morning. There may or not be a significant break as a second
front pushes in to the area Saturday night into Sunday. Models
have been jumping around on this and the NAM gives a significant
break and significant period of downsloping while the GFS has
little-to-no break between these systems.

The pattern setting up will likely bring a enhanced band of snow
over either over the Western Kenai Peninsula near Kenai and
Soldotna, or it may be mainly over Cook Inlet. This is creating a
lot of snowfall uncertainty for Saturday night in particular for
this area.

The Susitna Valley looks like it is in a good position for a
prolonged snowfall starting Saturday afternoon or evening. While
there will be some snow Saturday morning with the front, the main
snow event looks to be with the second front so this may become an
emerging weather item of interest.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

The next Bering front will reach the Southwest Alaska coast this
afternoon for another brief round of snow and strong winds. The
front will weaken rapidly as it pushes inland this evening. A
developing wave to the south will stall the front over the lower
Kuskokwim Valley and eastern portions of the Greater Bristol Bay
area tonight and pump more moisture in over the area. The next
front will lift in over the area from the south Saturday afternoon
through Saturday night with the associate parent low bringing
cooler air in aloft pushes into Southwest Alaska around Sunday
morning. Another front from the Bering will reach the Southwest
Alaska coast Sunday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

A storm force low in the western Bering will continue to track
north today while its associated frontal system, currently
stretching across the central Bering and eastern Aleutians,
swings east to reach the Southwest Alaska coast this afternoon.
The next developing low in the north Pacific will lift north
towards the central Aleutians tonight then turn more easterly to
push through the eastern Aleutians Saturday afternoon and evening.
The low will then split with the main closed surface feature
continuing northeast towards the Kuskokwim Delta coast Saturday
night while a trough continues east along the Alaska Peninsula. A
rather large low and strong front will approach the western and
central Aleutians from the south Saturday night and cross into the
Bering on Sunday. The eastward moving portion of the front will
reach the eastern Aleutians Sunday and Alaska Peninsula and
eastern Bering Sunday night.

&&

.MARINE (Days 3 through 5/Sunday through Tuesday)...

A strong ~970mb low will move into the Bering Sea Sunday and
confidence is growing for southwesterly storm force winds and
high seas across the western Aleutians. These winds are expected
to gradually diminish as they track west to east as the associated
front weakens, however gales will be possible as far east Bristol
Bay. Winds will gradually diminish and seas subside through
Monday as the low weakens and departs to the north. Generally
light winds and calm seas are expected Monday evening into early
Tuesday morning under a ridge. Although model agreement begins to
degrade Tuesday, it appears another low will track into the
Bering, though looks much weaker than Sunday's low and as of now
models are indicating gales for the western Aleutians and sub gale
for eastern regions.

A weak low will track into Bristol Bay Sunday and bring the
potential for marginal gales to the western Gulf, mainly through
the Barren Islands. Generally sub gales expected Monday under a
ridge. The ridge will remain in place Tuesday, however as models
are indicating a strong pressure gradient the gale potential will
return and seas will slightly build through the day, especially
over the southern Gulf.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Strong winds and moderate rain are likely over the Aleutians
Sunday into Monday as a strong low tracks across the Bering. As
this low tracks to the north and east across the Bering, a strong
fetch of southwesterly winds will impact the southwest coast by
Monday afternoon. If current guidance comes to fruition, there may
be potential surge and rough surf issues along the coast,
especially where shorefast ice is absent. A weaker low will
develop over the western Aleutians Tuesday bringing more light
rain over the region. Precipitation will continue moving west to
east Wednesday with partial clearing beginning to occur Wednesday
evening into Thursday as a ridge builds in behind the front.

Most of the latest model runs now indicate a weak low moving
across western mainland Sunday. Given the strong southerly flow
and subsequent moisture advection into the region, significant
snow could occur around the center of circulation near the
Iliamna area. Accumulation is much more uncertain for inland
areas in Southcentral area with some models splitting the low over
Prince William Sound which in turn would affect the flow pattern
over the Chugach Mountains and subsequently how much downsloping
occurs and how much snow makes it into the typical leeside
locations. Model agreement becomes poor into early next week. The
general pattern appears to look mostly benign with party to mostly
cloudy skies, seasonal temperatures and possible scattered show
showers through Wednesday. Both the ECMWF and Canadian model
solutions indicate a relatively strong cold front tracking over
southern Alaska on Thursday which would bring the potential for
snow, though confidence remains low at this time.

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155.
MARINE...Gale Warning 130 131 172 173 174 175 176 177 178 179 180
                      181 185 414.
         Storm Warning 411 412 413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JER
MARINE/LONG TERM...AP