Small Craft Advisory 967, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
503 AM AKST Fri Dec 7 2018

.ANALYSIS AND UPPER LEVELS...
A very deep upper level trough is located over the Eastern Bering
and western mainland extending well into the Pacific, digging as
far south as 30 S. A strong southerly jet is tapping into
subtropical moisture and sending it north into the Gulf of Alaska
and Southcentral. Several surface lows are seen rotating around
the upper trough, with one located north of Nunivak Island and
several others over the North Pacific south of Kodiak. The
southerly flow is helping to keep temperatures well above the
freezing mark along the coast, with only locations well inland
across Southcentral remaining below freezing. Radar imagery shows
a wide swath of moderate precipitation filling in over the North
Gulf Coast as a warm front ahead of the next low coming in from
the south moves ashore. Winds are also beginning to ramp up
through the channeled terrain, including Turnagain Arm, as well as
the upper elevation spots. This is leading to some downsloping
helping to keep leeward locations mostly dry this morning.

Further west, a front is pushing east across the Kuskokwim River
Delta with a band of snow showers seen on radar moving past
Bethel as cold dry air filters in behind the front. Weak ridging
covers most of the Eastern/Central Bering with generally tranquil
conditions, though cold and dry air filtering down from the north
is helping to enhance northerly winds over the area. Over the
Western Bering/Aleutians, satellite imagery shows clouds pushing
north ahead of the next system, with strong zonal flow leading to
some gale force winds, as seen on the most recent scatterometer
pass.

&&

.MODEL DISCUSSION...
Models have come into better agreement regarding the surface low
moving north into the Gulf of Alaska today. There is still some
disagreement regarding specific details, including how much wind
will get into the Anchorage Bowl as well as precipitation type
and amount before downsloping kicks in this morning. Models are
also locking in on the next system moving up into the Gulf Friday
night into Saturday morning, with only some placement and timing
issues, though the Canadian seems to be the slower outlier. By
Saturday morning, guidance shows another low rapidly moving into
the Gulf, though there is still a fair amount of disagreement,
with the GFS/NAM generally showing the low moving over the Eastern
Kenai Coast early Saturday morning while the EC brings the low
into Prince William Sound later in the day.

&&

.AVIATION...

PANC...Some LIFR ceilings may linger for a few hours early this
morning before low level drying from downsloping completely
eradicates any remaining low level moisture in the Anchorage Bowl
by sunrise, keeping conditions VFR. The forecast challenge then
shifts to low level windshear as a warm front pushes into the
area, ramping up the low level winds through the Turnagain Arm and
upper elevations. Hi-res models indicate the Turnagain Arm wind
will bend south down through Cook Inlet as the low center pushes
between Homer and Kodiak. Stiff northwesterly surface winds
combined with southeasterly winds of 40-50 kt around 2000 ft will
lead to decent low level windshear over the terminal later this
morning through the afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Saturday night)...
A steady stream of storms will track northward from the Pacific to
Southcentral the next couple days. These will generally be powerful
storms, producing widespread strong winds and heavy precipitation
across the Gulf of Alaska and along the Gulf coastal zone. Expect
rapidly changing conditions as each of these storms passes through.

A strong surface front oriented roughly northwest to southeast
across the western Gulf this morning will lift northward out ahead
of a high amplitude negatively tilted trough approaching from the
northeast Pacific. This trough will induce cyclogenesis along the
front, leading to a storm force low tracking right across the
central Gulf tonight, then inland across the Kenai Peninsula
sometime early Saturday morning. Strong dynamic lift ahead of
the trough will maintain moderate to heavy precipitation for
coastal areas. Persistent low level southeast flow will favor
western Prince William Sound/eastern Kenai Peninsula for the
heaviest precipitation. The steady supply of warm air means this
will be all in the form of rain at lower elevations, with snow
levels running roughly 1500 feet or higher. Deformation and
upslope will also maintain heavy precipitation along the west side
of Cook Inlet and the Susitna Valley. Temperatures are a little
cooler here, so expect mostly mountain snow, with a wintry mix
closer to sea level. With the front sitting right near the
southern tip of the Kenai Peninsula, also expect steady rain on
the western Kenai Peninsula south of Clam Gulch. Elsewhere, expect
mostly dry conditions thanks to downslope flow across the coastal
mountains. There may be a bit of light rain making it over the
Kenai and Chugach Mountains this morning, but flow will strengthen
through the morning hours which should bring an end to this.

There will be very strong surface pressure rises behind the low
center, with the isallobaric max on the order of 4 mb/hour. There
is some uncertainty in the exact track of the low as it heads to
the Kenai Peninsula early Saturday, but would expect these
pressure rises to contribute to a very strong uptick in south to
east winds across the Prince William Sound region late tonight
through early Saturday, then continuing inland across
Southcentral during the morning hours. This will favor strong
southeast winds through Turnagain Arm into Anchorage, the Knik
River Valley into Palmer and Wasilla, and along the Copper River.
Would expect a nice jump in temperatures as these winds spread
inland.

Meanwhile, arctic air will advect from the Bering Sea into the
western Gulf beginning this afternoon then continue northward to
Southcentral during the day Saturday. While the air aloft looks
cold enough to support snow, the near surface layer will be well
above freezing most places thanks to the aforementioned winds.
However, as the next storm system moves up from the Pacific
Saturday and into Southcentral Saturday night would expect
precipitation to fall mainly as snow inland. The track of this
storm is very much in question, so it's hard to say at this point
in time which areas will see snow. Stay tuned as this very active
weather pattern continues.

&&

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

The primary active weather across Southwest Alaska today is the
expectation of a deformation band developing within the broad
moisture plume streaming northward out of the Gulf and across the
Alaska Range. This area of precipitation will be focused along the
Alaska Range, but the western side of it is likely to result in
accumulating snow for areas north of Iliamna and from Sparrevohn
eastward. The snow will begin as early as this morning, picking up
in intensity this evening, then slowly drift back eastward and
weaken overnight tonight. As such, by far the greatest
accumulations are expected for communities nearest the Alaska
Range, especially Port Alsworth. The exact location of where the
western edge of the snow will set up will be the determining
factor as to which communities get how much snow. Unfortunately
that is very rarely a certain thing before the snow is actually
falling. Winter Weather Advisories remain in effect, but lower
elevation communities should expect to be closer to the 4 inch
side of the forecast accumulations.

An Arctic cold front currently over the Kuskokwim Delta will
slowly push eastward today, and will be what ultimately pushes the
deformation band back eastward across the Alaska Range tonight.
The arrival of the cold air behind the front will likely end most
of the precipitation activity for Bristol Bay and the Lower
Kuskokwim Delta. However, a small area of low pressure in western
Norton Sound will keep a steady onshore flow continuing across the
Kuskokwim Delta. This means occasional snow showers and patchy
fog are expected for the Kuskokwim Delta area into the day
Saturday, since the low is barely moving. Otherwise, quiet and
cold weather will persist through Saturday night.

On Sunday, the next area of low pressure off the Bering will bring
the next round of wintry precipitation and warmer temperatures to
Southwest Alaska. There is a large degree of uncertainty as to
where the low center tracks. The GFS/EC keep the low center off
the coast and the low's warm front will be the primary driver of
precipitation. Meanwhile the NAM and Canadian have more easterly
tracks and the center of the low itself provides the
precipitation. Either way this marginally warm air will be
overtopping a cold air mass, which are significant ingredients
for somewhere in Southwest Alaska to get freezing rain. However,
if the low center tracks eastward then it's likely to be mostly a
snow event. Windy conditions will also be a concern, so stay
tuned as more winter weather products may be needed with this
event.

&&

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

The Arctic air mass over the Bering producing widespread snow
shower activity will slowly move eastward into mainland Alaska
today. Meanwhile a warm front with associated rain will be moving
into the western Aleutians tonight. As the deepening low tracks
over or just north of the western Aleutians Saturday afternoon,
its intensification and westward movement are likely to create a
rather rare phenomenon called dynamic fetch. In this case, strong
westerly winds in the area south of the low center will persist
along with the low, which itself is moving eastward. Thus, strong
winds with essentially unlimited fetch will be impacting the same
area of ocean waves for an extended period of time. The result
will be a region of very high waves, exceeding 40 feet high in
places on the Pacific side of the western Aleutians. The primary
time period of this dangerously high swell will be Saturday
afternoon through Sunday morning. Mariners are urged to avoid this
area through the stated time period. As the low moves into the
Bering, it is forecast to begin weakening and elongating eastward
as new triple-point lows develop closer to the greatest
temperature gradients. This will effectively end the threat of 30
foot and higher waves by Sunday night. Warmer air associated with
the low should change the precipitation type across the Aleutians
to mostly rain until the low center passes and more cold air
follows behind the low.

&&

.LONG TERM FORECAST (Days 4 through 7)...
(Monday through Thursday)

A very active pattern will continue through the week, with
forecast confidence growing as model continuity continues to
improve. Monday will feature a rapidly deepening low which
undergoes bombogenesis (defined as a rapidly deepening cyclone
with a pressure decrease of 24 mb in 24 hours), as its pressure
lowers from 970 mb southeast of Sand Point late Monday morning, to
947 mb south of Seward by early Tuesday morning. This storm will
also draw up tropical moisture, with current water vapor values
forecast to be 300 to 400 percent above normal. Thus, some of the
precipitation will probably be of a moderate to heavy intensity at
times. Another important aspect associated with this system is
wind speeds, with current indications suggesting some hurricane
force winds possible for the coastal waters, as a strong barrier
jet develops.

Next in the active pattern will be another area of low pressure
that will track along or just south of the Aleutians in the Monday
through Tuesday time frame, This system will also undergo
bombogenesis, as mid-level energy drives into the backside of a
central Bering trough, with current indications showing this low
dropping from the around 983 mb to 949 mb (34 mb drop in 24 hours)
as it moves to just east of Kodiak Island by Wednesday morning.
This storm will also tap the tropicals with a similar moisture
feed in terms of percentage above normal. Similar to its
predecessor, warm air will be pumped north with very strong
winds, especially for the marine waters, a good bet.  This storm
will weaken as it fills across the northern Gulf, with yet another
rapidly deepening and moisture-laden storm moving into the west
central Aleutians on Thursday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 152 161.
MARINE...Storms 119 125.
Heavy Freezing Spray 185.
Gales 120 128 130 131 132 150 177 178 185 351 352 411 413.

&&

SYNOPSIS AND MODEL DISCUSSION...KVP
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...PD