Small Craft Advisory 71, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 AM AKST Tue Jan 22 2019

.ANALYSIS AND UPPER LEVELS...A vertically stacked low situated
over the central Aleutians is gradually shifting northwestward,
while an Arctic upper low sits over the northern Bering. A warm
occluded front associated with the low is wrapped from the north
side of the low towards the southwest mainland. This front has
brought precipitation, and temperatures are increasing in south
flow behind the front. Precipitation has changed over to rain over
the central and eastern Aleutians, while ahead of the front the
primary precip type continues to be snow. Another front associated
with that Aleutian low is tracking from the eastern Aleutians
northeastward, bringing another band of increased winds and
precipitation. Over Southcentral, light precipitation in warm air
advection continues along the Gulf coast and in the Prince William
Sound, with the Anchorage bowl shadowed by the Chugach.

&&

.MODEL DISCUSSION...Models are resolving the current weather well
and are in general good agreement through mid day Wednesday.
Thursday, model to model and run to run differences continue to
increase uncertainty regarding the setup of a series of shortwave
lows in the North Pacific/Gulf of AK, and behind this feature, a
stronger low that will approach from the North Pacific. While
there's confidence in the basic pattern of a series of fronts
bringing increased winds, precipitation, and warmer air from the
Pacific to Southwest and Southcentral, the exact trajectory of
that North Pacific low will have a significant impact on the
extent of warming. While we haven't gone as extreme as the latest
run of the EC with warming Southcentral later this week, should
that solution verify temperatures could reach into the 40s as
early as Thursday.

&&

.AVIATION...PANC...VFR conditions and light northerly winds will
persist into Wednesday. Wind shear will develop this evening as
southeasterly winds aloft increase and surface winds remain
northerly. At this time the 2000 to 3000 ft level looks like the
most likely level for this shear to occur. Beyond that, a few
flurries will be possible at times, but otherwise weather will
remain dry.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Wednesday night)...
A weak upper wave is being overtaken by increasing anticyclonic
flow aloft as it heads inland across Southcentral this morning.
Thus, don't expect more than a brief period of flurries or very
light snow out of this. Low level instability and upslope flow
will maintain some showers right along the Gulf coast and Prince
William Sound. Meanwhile, a weakening occluded front will lift
northward across the western Gulf and Kodiak Island today then
dissipate as it reaches the Kenai Peninsula tonight. A slow
warming will continue as each of these fronts/upper waves moves
up from the Gulf. So while the air mass in place is currently
cold enough to support all snow across Southcentral, the southern
Kenai Peninsula (including Homer) and locations near sea level
and along the Gulf coast will transition to rain by this
afternoon.

Although the front will lose a lot of its structure as it reaches
the Kenai Peninsula, do expect an increase in winds ahead of the
front this afternoon and evening. Southeasterly winds will then
ratchet up more quickly overnight tonight through Wednesday as a
deep low tracks from the Pacific into Bristol Bay, pushing a very
strong occluded front northward across the Gulf. This will produce
widespread strong winds and heavy precipitation along the coast.
While do not expect to reach high wind warning criteria, do expect
Turnagain Arm winds and ridge-top winds across the Kenai and
Chugach Mountains to peak at 50 to 70 mph on Wednesday. These
winds will produce downslope flow in the lee of the mountains,
keeping the bulk of the Western Kenai Peninsula, Anchorage,
Matanuska Valley, and eastern Susitna Valley dry.

As the front pushes onshore late Wednesday afternoon through
Wednesday evening, flow will shift around to south to southwest.
During this transition, there is some potential for some spotty
freezing rain for wind protected areas such as the Anchorage bowl,
but do not expect enough qpf to cause any significant problems.
As the front continues onshore, expect all but inland areas of
the Susitna Valley and Copper River Basin to be above freezing,
so the predominate precipitation type will be rain. Fortunately,
it looks like just a quick chance of rain behind the front as
the upper trough quickly exits to the north.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Tue and Wed)...
The pattern change is underway. A series of fronts will push
through the area over the next couple of days. Each one will take
a bite out of the deep cold airmass that has been in place. The
initial front moved through last night. Now, the next one is
queued up near the Alaska Peninsula. It is stretching from NW to
SE and will slowly move north through Bristol Bay this morning.
Southeast winds are expected to increase in front of it. These
winds will help to create some additional local warming as they
downslope off the Aleutian and Alaska Ranges. They will also cause
some localized wind shear conditions that could impact aviation
interests. Cold air will do its best to hold on in parts of the
YK-Delta and the Lower Kuskokwim Valley. Northeast winds in
advance of the front could create some blowing snow conditions
along the coastal areas of the Delta. While visibilities could
briefly drop to one half mile this evening, these conditions
should be limited in the area they impact and length of time they
persist.

The second piece of energy will come in as more of a closed area
of low pressure than just a front early Wed. This low looks to
track right up the coastline of SW AK and bring another push of
warm air and precipitation. A few inches of snow are expected over
the Delta and the Kuskokwim Valley with some blowing snow possible
early Wed before the warmer air arrives.

Another system will pass offshore of the coast on Thu bringing
another shot of warm air.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Tue and
Wed)...
An anchor low over the Central Aleutians will be the dividing line
for weather across the Bering. The low, currently sitting near
Atka, will drift north today. To the west, a cold Arctic trough of
low pressure is digging to the south. To the east, several pieces
of energy will traverse quickly from south to north bringing warm
air from the North Pacific. For the Western Bering, expect
widespread cold air advection snow showers and gale force winds
(at least through Tue). The low will pivot back towards the colder
air on Wed, shifting the battle front farther west. This will
open the door wider for systems from the North Pacific to clip the
Central Aleutians as they track through the Alaska Peninsula
toward the Bering Strait. Each system will pack a round of
precipitation (likely a rain/snow mix) and at least some gale
force winds.

&&

.MARINE (Days 3 through 5 Thursday through Saturday)...
The marine outlook remains quite challenging with poor run to run
model consistency. The general trend continues to be the
development of a ridge of high pressure extending across the AK
Panhandle and a broad trough over the central Aleutians and
Bering. This pattern will allow a series of lows to track from the
central and eastern Aleutians north into the Bering through
Saturday. How fast this occurs, along with the strength and
position of the low pressure centers remains unclear due to
guidance uncertainty.

Given this synoptic setup, periods of westerly and southerly
gales are possible over the Pacific side of central and eastern
Aleutians, respectively, from Thursday through Saturday as a
series of lows parallel the Aleutian Chain before lifting north
into the eastern Bering. Easterly gales are also possible from
Bristol Bay to the Pribilofs Thursday night into early Saturday.
The trend through the weekend is then for winds to weaken to a
broad area of cyclonic small-crafts across almost all of the
Bering as the main low weakens and lifts north. For the Gulf, the
biggest issue is the possibility of widespread southerly gales
developing on Friday as a low lifts north toward the southcentral
coast. These should begin to weaken by Saturday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Model uncertainty with the shortwave features between both
deterministic and ensemble runs remains quite high in the long
term with guidance diverging quite quickly by day three.
Specifically, the NAM and GFS begin day three solutions in rather
good agreement with a trough over the northern Gulf and a closed
surface low south of the eastern Aleutians with a triple point low
spinning up well south of Kodiak Island off of an occluded front
draped east from the main low. The EC, on the other hand, broad-
brushes a larger, weaker low south of the AKPEN. All three models
do build a ridge along the AK Panhandle during this time, so
guidance produces a northerly track for the low(s) to the west.
However, with the GFS and NAM developing a closed low south of
Kodiak, this center of circulation lifts into the western and
northern Gulf and eventually along the southcentral coast Friday
as a stronger, more organized system than what is depicted by the
EC.

The bigger picture, however, is that guidance is holding onto the
longwave solution of a developing ridge along the west coast of
Canada and a deep trough over the western Bering. This will favor
a trend for moderating temperatures as we head through the long
term with prolonged southerly flow from the central Bering east
to the Gulf allowing for various shortwaves bringing
precipitation across the forecast area from Friday through early
next week. Precipitation type issues for interior sections of
southcentral and southwest may also be a concern due to the
influx of warmer air aloft with the increased southerly flow.
However, as each wave moves through, more of the colder air at the
surface will scour out. Areas along the coast may see a mix of
rain and snow with the system on Friday, but mostly rain for
subsequent waves over the weekend.

Guidance is also hinting that the main low over the Bering may
begin to lift north beginning early next week. If this holds
true, expect a return of cooler temperatures from west to east by
the start of next week as colder air from the northern Bering and
eastern Siberia begins to wrap in behind the departing low.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm: 119 130
         Gales: 120 125 131 132 136 137 138 139
                150 175 178 185 351 352 411 412 413
         Heavy Freezing Spray: 185 412 414
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MM/JA
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
MARINE/LONG TERM...TM

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
327 PM AKST TUE JAN 22 2019

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0250 PM     FREEZING RAIN    ANCHOR POINT            59.77N 151.81W
01/22/2019  E0.00 INCH                          AK   PUBLIC

            STEADY -FZRA

&&

KUTZ