Small Craft Advisory 166, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 PM AKST Mon Feb 17 2020

.ANALYSIS AND UPPER LEVELS...
There is a large vertically stacked low centered over the eastern
Bering Sea. An associated front stretches from the lower Kuskokwim
Valley south across western Kodiak Island into the Pacific. The
"bent back" occlusion wraps all the way around the low, curling
from the northern Bering, around the Pribilofs then north to
Nunivak Island. Strong surface and low level winds associated
with this low cover almost all of the Bering and southern Alaska,
with the exception Bering the western Bering and Aleutians. An
upper level high is over northwest Canada. This puts all of
southern Alaska under strong southerly flow aloft, which is
bringing plenty of moisture to the area, and a warmer air mass.

&&

.MODEL DISCUSSION...
The numerical models are in reasonable agreement through Wednesday
afternoon. The exception is the NAM, which is much faster and
weaker with a low moving north through the Gulf to the Gulf coast
Tuesday and Tuesday night. With a very fast and active pattern,
the details of the forecast are quite tricky. Therefore the
forecast confidence is a little below normal today.

&&

.AVIATION...
PANC...LLWS is expected this evening with rather strong winds
aloft. Gusty southeast winds are expected to develop early this
evening, then become south. Winds will diminish late Tuesday
morning. VFR conditions will persist into Tuesday morning. MVFR to
IFR conditions are expected late Tuesday morning into the evening
in snow.

&&

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

A challenging forecast involving gusty winds and precipitation
tonight through Wednesday as two fronts move through the region.

Snow and rain will continue to develop this evening as a weak
boundary remains stalled along the Gulf coast. Light snow along
the western areas of the Susitna Valley will also be ongoing as
moisture along low level southeasterly flow over runs a cold pool
along the base of the Alaska Range. The next impactful front moves
across the southern Gulf, bringing gusty winds and rain to Kodiak
Island this evening. Gusty winds will accompany the front peaking
during the early morning hours on Tuesday. The high wind warning
issued during the morning packing for Turnagain Arm and Higher
Elevations was updated late this morning to start sooner as
observations were already reporting speeds greater than 75 mph at
McHugh Creek and Bird Point.

The advancing front spreads precipitation across the Kenai to
Prince William Sound overnight and then continues north through
Anchorage, Susitna and Matanuska Valleys by Tuesday early morning.
Precipitation will continue through Tuesday night as two upper
level storm systems interact. Precipitation type remains the
biggest challenge on Tuesday as surface temperatures quickly
climb above freezing along the frontal boundary, meanwhile
temperatures aloft remain cool enough to support snow. There is
high uncertainty on snow amounts as models are struggling to
handle this highly active pattern. Updates were made today to cool
temperature trends Tuesday afternoon and expand snow coverage.
The highest snow amounts are expected along higher elevations just
inland from the coast. Issued new Winter Weather Advisories for
the Western Kenai, Matanuska Valley and Anchorage area and updated
the Susitna Valley advisory to begin this afternoon.

The front pulls across the Copper River Basin on Wednesday with
the upper level jet shifting its nose eastward. The second upper
low that quickly organizes over the Gulf Tuesday night, weakens
into an open wave as is moves inland. Gusty gale force winds were
extended across the northern Gulf Tuesday night. Widespread snow
is expected across much of Southcentral Tuesday afternoon through
Tuesday night as the low level jet pumps subtropical moisture
toward the interior. Forecast details on snow amounts were
adjusted to reflect higher amounts; however,model uncertainty
remains high through midweek.

&&

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

An occluded low centered roughly 75 miles south of Nunivak Island
continues to be the focus in the short term. Blizzard conditions
have started returning to the Kuskokwim Delta coast, with Chevak
back down to a quarter mile visibility in heavy snow and gusty
winds. While there's been a lull in snow today, conditions will
likely continue to deteriorate tonight as the low steers eastward
and pushes the cold second wrap toward the coast. Precipitation
has started to change back over to snow in the Bristol Bay area.
As for snow amounts, expect up to three inches from mow through
tomorrow afternoon, mainly from Dillingham north. Of course, much
more is expected in upslope flow in the Kilbuck and Ahklun
Mountains. The low will continue to weaken and track northeastward
toward the Yukon Delta tonight into Tuesday. This setup will
continue to set the stage for gusty southeast winds from Kamishak
Bay to the Nushagak Hills this evening. Gusts to 47 mph have
already been observed in Iliamna today. Tuesday, the trailing
cold air mass will push inland, bringing a distinct cool down over
most of the Southwest Mainland Tuesday into Wednesday.
Precipitation will greatly diminish Tuesday afternoon into
Wednesday. That said, there will probably be scattered "ocean
effect" showers over the mainland into Wednesday. By Wednesday
afternoon, attention turns to a low expected to traverse the
Aleutians during the day Wednesday. Since this low is currently in
its infancy as a wave off the coast of Japan, there's a bit of
uncertainty in its progression. But, as it stands now expect a
transition to south flow aloft with wetter, windier weather at the
surface. Stay tuned for updates.

&&

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

An occluded low between the Pribilof Islands and Nunivak Island
will continue to track towards the coast and inland tonight.
Gales over the Bering and eastern Aleutians/AKPEN will diminish
tonight, though widespread areas of small craft advisory level
winds/seas will persist through Tuesday. Expect broad cyclonic
flow over the Bering Tuesday, and ocean-effect snow showers are
likely over the Bering Sea and Pribilof Islands into the
Aleutians... mainly the central Aleutians and east. Attention
quickly turns to a gale force low expected to reach the western
Aleutians Tuesday night. Expect another round of active weather
across the Aleutians and areas of gales over the Aleutians and
southern Bering, with an embedded swath of storm force winds
Wednesday.

&&

.MARINE (Days 3 through 5 Thu through Sat)...
Some Gales are expected on Thursday and Thursday night to the
south of the eastern Aleutians and Alaska Peninsula as a system
moves east into the Gulf. Gale to near storm force winds are
expected along the north Gulf coast on Thursday associated with
this same system. On Friday night and Saturday, Gale force west to
northwest winds are expected over the Gulf and south of the Alaska
Peninsula. A storm system moving into the western and central
Aleutians on Saturday should produce gale force winds.

&&

.LONG TERM FORECAST (Days 4 through 7 Fri through Mon)...
On Friday morning the pattern will consist of an upper level
trough over southwest Alaska extending southeastward into the Gulf
of Alaska. The trough will very slowly slide northeast across
southern Alaska through Sunday as the main upper low retreats
northward. A weak ridge will then build over southern Alaska
Sunday night, sliding eastward on Monday. An upper level low just
south of the western Aleutians on Saturday will move eastward
through Sunday, possibly moving north into the central Aleutians
on Monday. The exact path and timing of this feature is somewhat
uncertain at this time.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WSW 101 111 121 131 145 155. NPW 101 NONE.
MARINE...Storm 119 165 170. Gales 125 128-132 150 155 160 171-173 178-180
                                  185 351 351 412-414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...BL