Small Craft Advisory 1010, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
519 PM AKST Sun Dec 29 2019

.ANALYSIS AND UPPER LEVELS...
A very cold airmass is over the Bering Sea into Southwest Alaska
today while strong south-to-southwest flow aloft is heading into
Southcentral Alaska. Much warmer air is associated with the
airmass heading toward Southcentral with the Arctic front
retreating to near the Alaska Range. This sets up a very good
boundary for heavy snow over the Arctic front and west of it.

&&

.MODEL DISCUSSION...
While the models are in good synoptic agreement overall, they
differ on some very important features that will impact the
forecast in both Southwest and Southcentral Alaska over the next
few days. Therefore, at first glance they look good, but are being
difficult to use. The biggest impact to the forecast will be the
winds in the Cook Inlet area tonight into Monday morning and then
across most of Southcentral on Tuesday night into Wednesday. For
tonight, the NAM wants to bring in Southeast winds to Anchorage
for a period of time late tonight into tomorrow morning while the
GFS and ECMWF want to keep the winds all from the northeast at
the surface. NAM is usually correct in these scenarios, therefore
this solution was preferred for the Anchorage Bowl wind forecast.
Even more pronounced is the NAM on Tuesday night into Wednesday
where it is bringing in southwest cold advection up Cook Inlet and
southerly winds to Prince William Sound as well. Part of this is
due to the 500 mb low on the NAM tracking farther north each of
the past three model runs, but it also makes sense with the way
all the models are keeping a trough through the Susitna Valley and
the Copper River Basin at that time. Since there is no way to
split the difference in the forecast with this scenario, also went
with NAM wind directions for most of the forecast through
Wednesday.

&&

.AVIATION...
PANC...Wind shear will be in the vicinity of the airport through
the night. Surface winds will be a challenge as the southeasterly
Turnagain Arm winds will continue through the night. Until
midnight, they will bend down the Inlet and keep surface winds
northerly; however, late in the night, they will probably turn to
the Southeast and briefly usher in warmer temperatures which may
also bring in mixed precipitation. Winds will then go more to the
south-southwest for most of the day tomorrow which will also
bring in colder temperatures again and turn any precipitation back
to snow for Monday mid-morning. Ceilings and visibility will be
sensitive to the precipitation type with IFR likely with snow and
MVFR with a rain/snow mix.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Sunday night through Wednesday)...

The next round of weather is currently on the doorstep of
southcentral AK, with winds and temperatures just starting to rise
this afternoon. This latest round of weather is caused by an
occluded low pushing northeast through the Gulf of Alaska. The
low is quite broad, encompassing the entire gulf. Its center is
clearly depicted on satellite, currently over Kodiak Island and
moving northeast. The location and track of the low center staying
to the west of the Anchorage area is allowing for a strong
barrier jet setup and push of warm air to intrude into
southcentral. Models are having a difficult time of handling this
warm air and the low in general, offering a different solution
with every new run. Therefore, went with the best choice which is
the NAM today and adjusted the forecast accordingly. Winds are
also already coming down Turnagain arm, so adjusted the forecast
to move weather in a little earlier this afternoon/evening. This
means forecast details that affect local areas like the Anchorage
Bowl will be tricky, so moderate confidence in the particulars.

We do expect windy conditions and subsequent warming temperatures
to last through Tuesday. Winds will intermittently push into
south Anchorage and will also be strong on the hillside. The main
impacts of this will be slick roads and melting snow. Right now,
there is a low threat of freezing rain, but the possibility cannot
be ruled out. It should also be noted that the southerly push
will make it into interior areas of Southcentral AK over the next
couple of days. How warm it gets for interior areas remains in
question, but it will get a close look over the next couple of
shifts.

Fortunately, as this low moves north out of our area, strong cold
air advection will wrap in behind the low. This will cause a
steep drop in temperatures New Year's Day back to more normal
conditions. This means our latest wind event should leave some
usable snow pack behind. There is also a possibility of more snow
as it cools off, but with poor model performance it's too early
to to tell how that will play out. With rapidly changing and
fickle weather, be sure to stay tuned for your New Year's
forecast.

&&

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

An active pattern across Southwest Alaska starts tonight as a
North Pacific low moves over the Alaska Peninsula by Monday
morning and then into southern Mainland by Tuesday. Strong
northerly winds accompany this system as the front tracks
northward, which will lead to snow and blowing snow along the
AKPEN starting tonight. A Winter Weather Advisory has been issued
for these areas through Tuesday, with many locations expecting to
see about a foot of new snowfall with this system.

This same frontal system will move into Southwest Alaska on Monday
afternoon. Winter Weather Advisories have also been issues for
interior Bristol Bay (starting 11am Monday) and for the Lower
Kuskokwim Valley (starting 3pm Monday) as these areas are also
expecting large snow totals. There is still some uncertainty with
the exact track of the low, but models agree that the core of
precipitation will extend from Dillingham northward to Sleetmute.
Areas within this path could easily see more than a foot of
snowfall through Tuesday night, with surrounding areas seeing
around half a foot. This system will continue to be monitored as
new model runs come in to see how far west the front tracks and where
the highest snow totals will be.

Over the Kuskokwim Delta, cold temperatures are expected to
continue, along with strong northerly winds on the backside of
the low system moving into the area on Monday. A Wind Chill
Advisory has been issued from Monday night through Wednesday as
wind chills to -45F are expected across the region. Be sure to
check the forecast and bundle up before heading outdoors.

&&

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

Strong northerly winds will continue across much of the Bering
and Aleutians, shifting from the western Bering this afternoon
into the eastern Bering by Tuesday. With cold air temperatures
still in place, expect widespread snow showers to continue
throughout the forecast period, along with freezing spray for both
offshore and coastal waters. Heavy freezing spray will persist
over the eastern Bering and waters along the Southwest coast with
the northerly flow.

&&

.MARINE (Days 3 through 5/Wednesday through Friday)...

Confidence is high that Arctic air will pour southward across the
eastern Bering Sea, across the Alaska Peninsula and into the Gulf
Tuesday through Wednesday. Thus, expect the strongest core of
winds through here with widespread Gale to Storm force winds and
a good chance of hurricane force wind gusts through gaps in the
Alaska Peninsula. These winds will slowly diminish Thursday
through Friday, lingering longest along the south side of the
Alaska Peninsula and into the western Gulf.

Meanwhile, confidence is also high with a gale force front
tracking eastward across the western Bering and Aleutians
Wednesday through Thursday. Confidence rapidly breaks down on
Friday with widely varying solutions in the models. The front
could weaken or a new low could develop along the front.

&&

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

The focus on Thursday will be on a 990 mb low south of Kodiak
Island. Low confidence as to the track of this storm remains,
however, but it does look to bring snowfall and northerly winds
to areas of southern Alaska. The ultimate track of this low will
determine which parts of southern Alaska will be most impacted by
this weather pattern.

For Southcentral, Thursday keeps colder temperatures across the
Mainland. Chances of snow look to be within the realm of
possibility before Southcentral will dry out to end the weekend.
The active weather pattern will remain over the Aleutians as
another front associated with a Kamchatka low moves over the area.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 152, 161, 181.
         Wind Chill Advisory 155.
         Flood Advisory 145.
MARINE...Storm Warning 119, 125.
         Gale Warning 130-132, 139, 150, 155, 165, 170-181, 185,
         352, 411-414.
         Heavy Freezing Spray...160, 179, 180, 181, 185, 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...BB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...CK/SB

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