Small Craft Advisory 1004, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
503 PM AKST Fri Dec 27 2019

.ANALYSIS AND UPPER LEVELS...

The next storm in the line on its way to Southcentral is rapidly
developing east of Kodiak this afternoon. Blizzard conditions are
expected in Kodiak City and all across the eastern side of Afognak
and Kodiak Islands over the next several hours as the storm
continues strengthening and moving north across the Gulf. This
will be the storm that brings the next round of winter weather to
Southcentral tonight into tomorrow morning. Snow is beginning to
develop for Gulf Coast communities as well.

Across Southwest Alaska, cold temperatures and winds are the
primary forecast hazards. Wind chills have been between 40 below
and 50 below zero through much of the night and through the
morning across the Kuskokwim Delta and interior Bristol Bay. Wind
Chill Advisories remain in effect for these two areas through
tonight.

For the Bering, strong winds and snow showers are ongoing over
nearly the entire basin. This pattern will continue as the front
drawing the arctic air mass southward shifts toward the Gulf ahead
of the developing lows.

&&

.MODEL DISCUSSION...

For a strong low that is developing off Kodiak Island as I type
this, the model agreement with it remains downright atrocious.
Doing a poor man's measurement, the models remain up to 100 miles
apart in where the center of the low will be this evening.
Fortunately, and somewhat unconventionally, agreement actually
improves by Saturday morning, as the center tracks into western
Prince William Sound. This remains a favorable spot for snow to
get to the west side of the mountains and into western Kenai
Peninsula, Anchorage, and the Matanuska Valley. A secondary wave
will move through Saturday night into Sunday morning, which will
either enhance the snow across the Cook Inlet region, or allow it
to resume again after a brief break. Following that, the next
strong low moves to near Kodiak Island during the day on Sunday,
which will bring yet another round of precipitation to
Southcentral, with some likely on the Bristol Bay and Lower
Kuskokwim Valley side of the mountains as well. There is decent
agreement on this low tracking very near or over Kodiak Island
Sunday afternoon, especially its general existence and track,
though the location of the center will once again determine
temperatures and impacts. Forecast confidence remains very low.

&&

.AVIATION...
PANC...VFR conditions and northerly winds will persist late this
afternoon. This evening, northerly winds will pick up as the next
Gulf low moves closer. Snow will begin later this evening, picking
up in intensity early Saturday morning. IFR visibility and
ceilings are likely. The snow will diminish towards dawn, perhaps
down to flurries during the afternoon. Another round of snow with
good potential for IFR ceilings is expected again Saturday night,
but will begin after the current TAF period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)..
(Tonight through Monday)

Overall the forecast remains on track from previous thinking. We
upgraded Kodiak Island to a blizzard warning, based on a further
west trend seen in the satellite imagery this morning/early
afternoon regarding where the low was developing, with the latest
ECMWF and NAM models having trended in that direction as well. In
fact, the GFS and ECMWF both track the low over Hinchinbrook
Island Saturday morning, with the NAM being an outlier with its
further east track that it has maintained. Still, it too has
closed the gap and is now within 40 miles of the other two models.

Another change that we made was to increase snowfall totals for
the storm tonight into Saturday. This mornings sounding from
Anchorage showed a 35:1 snowfall to liquid ratio, indicative of
just how cold the airmass on top of us is. Granted, they'll be
some modification of this as the warm advection pushes in at 850
mb (~4000 feet), but we still expect ratios to be at least 20:1 (a
normal snow ratio of 10:1 would give you one inch of snow per 1/10
an inch of liquid. So...a 20:1 would mean 2 inches versus
one...and so on). Virtually all of the models give Anchorage 0.2
to 0.25 inches, which would support storm totals of 4 to 5 inches.
The northeast winds on the backside of the low may cut down totals
some, but not too much (maybe a half inch). Using the same
approach for Kodiak Island and the Copper Basin, the overall
forecast looked good as those locations weren't on the edge of the
precipitation shield.

Heading into Saturday night and Sunday, another low will lift
north towards the Copper River Delta, with a trough also moving
overhead from the west/southwest. This will foster increased lift
with good snow to liquid ratios being maintained...thus snow
totals were increased again, especially for the Kenai Peninsula
northward through Anchorage and into the Mat-Su Valleys. Here,
although moisture will be somewhat less than the Copper Basin,
the approaching upper trough and a favorable southwesterly flow
aloft should foster a few inches of accumulation, with this wind
direction aloft being a more favorable pattern for this to occur.

Heading into Sunday night and Monday, another strong low rides
northeast along the Alaska Peninsula, bringing more snowfall and
increasing temperatures to the region.

&&

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

With strong winds and frigid temperatures continuing throughout
the weekend, the wind chill advisories have been extended until
Saturday evening for interior Bristol Bay and until Sunday morning
for the Kuskokwim Delta when the winds begin to weaken. By Sunday
afternoon, temperatures begin to warm up across the region as the
Arctic trough moves out of the area, with high temps rising above
the zero degree mark for much of Bristol Bay. The clear skies and
dry conditions across Southwest Alaska come to an end on Sunday
as the next system approaches the region. There is still some
uncertainty in the track of the low position that will produce
snow across interior Bristol Bay, but areas from Dillingham to
Koliganek will likely see a few inches of snow from this system.

Meanwhile, the Alaska Peninsula will continue to see snow showers
throughout the weekend from Dutch Harbor to Port Heiden, along
with windy conditions, which could lead to areas of blowing snow.
By Monday morning, the new front moves across the AKPEN, which
will bring a few inches of new snow to these areas.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

Winds across the eastern Bering this afternoon will diminish
through Saturday morning as the associated low center moves into
the North Pacific. The next big system approaches the western
Aleutians tonight as its front spreads to the eastern Aleutians
through Sunday afternoon. Strong northeast gale force winds, with
gusts to storm force level, will persist along with this system.
Widespread snow showers and freezing spray will also be concerns
with the strong winds through the weekend.

&&

.MARINE (Days 3 through 5)...
(Monday through Wednesday)

...Bering Sea and Aleutian Coastal Waters...

It looks like a moderately strong low will be south of or over
the Alaska Peninsula Monday producing a broad area of northerly
gales across the majority of the Bering Sea. This low moves over
the southern mainland through Wednesday morning. The northerly
gales will be east of the dateline by Wednesday morning and
weakening to sub gale force through Wednesday. Weak ridging builds
into the central Bering Sea Tuesday behind the low over the
mainland and in advance of a front moving into the western Bering
Sea. This front may bring gale force wind to the western Aleutians
and western Bering Sea Wednesday.

...Gulf of Alaska...

Models continue to struggle to resolve the structure and track of
the storm moving into the southern mainland Tuesday. The storm
may remain a single low or it may split into two lows. This
results in either gales to minimal storm force winds across the
Gulf of Alaska Monday or much weaker wind and lower wave heights.
The current thinking is to maintain the single low solution that
supports low end gales across the entire Gulf of Alaska Monday and
early Tuesday.

&&

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

...Southcentral...

Active weather will continue into the new year as we remain in a
pattern of storms tracking from the north Pacific into the
southern mainland. As is typical of model performance with such an
active pattern, solutions vary considerably, especially in the mid
to long term time frame, thus confidence in model solutions
remains low. Beginning the work week, a storm is expected to pass
across the Alaska Peninsula into the southern mainland. The models
have now converged on a solution that keeps the low farther south
than previous runs. This means the warmer marine air will most
likely remain along the coastal side of the mountains and not make
much headway inland through Thursday. The models are consistent
about keeping the low along the North Gulf Coast Thursday and
Friday.

A front is expected to push into the western Aleutians Monday
night. Tuesday, a weak low is likely to form along this front
south of or just along the central Aleutians then dissipate. At
this point the models are not in good agreement at all other than
generally weak wind and benign conditions for the central
Aleutians and central Bering Sea Wednesday through Friday. The
western Aleutians and Bering Sea will be influenced by a low
remaining in the Sea of Okhotsk supporting a front or weak lows
transiting through the western Bering Sea.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 131 171
         Winter Weather Advisory 121 125 141
         Wind Chill Advisory 155 161
         Flood Advisory 145
MARINE...Storm Warning 119 130 174
         Gale Warning 120 121 127 128 129 131 132 137 138 139
                      165 170 172 173 174 175 176 177 178 179
                      180 181 185 351 352 411 412 413 414
         Heavy Freezing Spray 139 140 160 179 180 181 185 414
FIRE WEATHER...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JW
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...TM