Small Craft Advisory 892, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 AM AKST Sun Nov 24 2019

.ANALYSIS AND UPPER LEVELS...
Broad upper cyclonic flow with embedded waves has brought a cold
pool of air aloft to Southwest and Southcentral Alaska. On the
radars at King Salmon and Middleton Island showers have been
detected. There is a faint ridge aloft in the central Bering/NE
Russia and then another potent low in the Sea of Okhotsk. On
satellite imagery there is a clearing over Cook Inlet and a zone
of cold air stratocumulus clouds from the Pribilof Islands to the
Gulf Of Alaska. Most of the jet stream is over the north Pacific,
however there is a strong jet core near Kamchatka.

&&

.MODEL DISCUSSION...
Models are in good synoptic agreement across the Bering, North
Pacific and the southern tier of the state through Tuesday.
There are some minor differences with the intensity of the next
Bering low. The ECMWF has it at 961 mb at 00z Tuesday while the
NAM and GFS both have it at 963 mb. Otherwise the track and the
orientation of the precipitation shield is fairly similar.

&&

.AVIATION...
PANC...VFR conditions will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper level low that has been in residence over Southwest
Mainland Alaska the last two days is now moving into Southcentral
today. A potent trough extending out from it helped to produce a
good amount of snowfall in the Copper River Basin overnight, but
that snow is now rapidly ending as that wave moves into the
Interior. There will be some snow showers around the region today
in association with this upper level low center though they are
not expected to be widespread or for any snow accumulations to be
very much at all.

What this upper level low really represents in the advection of
much colder air into the area for the next few days which will be
in stark contrast to the very mild week last week.

&&

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

A very active short-term period is expected across Southwest
Alaska. Today will be the most quiet of the next 3 days. A polar
air mass will continue building into the area today, though the
broad wind flow will begin shifting from the current northwest to
a westerly wind by this evening. This will continue to bring
Bering Sea moisture into Southwest Alaska. With most of the area
staying under cloud cover, the full potential of the air mass to
lower temperatures is unlikely to be realized, and relatively few
areas will get much below 10 degrees tonight. Plenty of moisture
in the form of widespread snow shower activity will continue to
impact much of the area today, especially along the coast. As has
been the case the past day or so, any snow showers have the
potential to bring visibilities down, but they will all be fast
moving, and thus, short lived. Therefore, accumulations in any one
area should be minor. The area getting the most moisture, the
Alaska Peninsula south of King Salmon, has seen the warmest
temperatures, hovering a few degrees above freezing, thus allowing
for some melting to counteract any snow accumulations.

On Monday, a strong warm front will begin moving into Southwest
Alaska, especially the second half of the day. The primary impact
from the warm front will be coastal flooding. About 24 hours of
gale force southwesterly onshore flow is expected to begin around
sunset Monday across the Kuskokwim Delta. The winds will
intensify overnight as well. These winds will push a storm surge
into the coast. Coastal Flood Watches remain in effect, but with
higher than normal tides due to the impending New Moon, which will
reach phase at the peak of this event Tuesday morning, it appears
probable that portions of the Kuskokwim Delta coast will
experience warning level coastal flooding at high tide Tuesday
morning. Further south, Bristol Bay will also experience onshore
flow, but since southwesterly winds and increased distance from
the front are not ideal, it appears the flooding threat is
significantly lesser for Bristol Bay's coastal communities.

While we will be starting this event with cold air in place,
unlike previous events, the parent low is expected to stay near
Kamchatka and will track north of the area. This will promote
southerly winds all the way to the surface over Southwest Alaska.
This will virtually eliminate any notable freezing rain threat
with no cold air moving in at the surface. Thus, the precipitation
type challenge with this system will be marginally warm air
causing a highly uncertain location of the rain/snow line. While
warm advection will be strongest over the Kuskokwim Delta with the
strongest winds, the highest amounts of precipitation over that
same area will potentially bring a dynamic cooling component to
the forecast. In other words, the snow may fall heavily enough to
overcome the marginally warm air and result in snow at the
surface, though surface temperatures will be above freezing over
the Kuskokwim Delta. Thus, the rain/snow line could set up
anywhere from north and east of Bethel to Sleetmute, and could
vary from anywhere in between depending on the precipitation
intensity. Between any snow being heavy and wet in consistency,
and the likely high variability in precipitation type between rain
and snow, no advisory level snowfall is currently expected
anywhere in Southwest Alaska. In the Lower Kuskokwim Valley, where
temperatures will be cooler as the cold air hangs on longer,
there will be less total precipitation, so even if snow is the
dominant precipitation type, it will be lighter in intensity and
storm total amount.

A cold front will begin to move into the Kuskokwim Delta at the
end of the day Tuesday. The precipitation should remain entirely
ahead of the front, so the reinforcing cold air is not expected to
result in accumulating snowfall. A low may track south of the
Alaska Peninsula into the Gulf Tuesday night, which may intensify
the precipitation over Bristol Bay and the Lower Kuskokwim Valley.
However, there are major differences in the models with the
location of this low and moisture influx, so that remains a very
low confidence aspect of the forecast that will be refined in the
coming days.

&&

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

Across the Bering, the main story will be the winds, as a strong
front sweeps across the Bering today. Widespread gales are
expected on Monday over much of the basin, with embedded storm
force winds from the central Aleutians over the offshore waters,
then up to near Nunivak Island by Monday night. Another influx of
Siberian air will move into the western Bering on Tuesday, which
will eventually push the front to the east. Widespread snow
showers will characterize the cold air masses, which will continue
over the eastern Bering today, and will move into the western
Bering behind the cold front on Tuesday.

&&

.MARINE (Days 3 through 5)...
Offshore flow tapers off Monday night, with small craft winds
expected across much of the Gulf. Locally elevated outflow winds
through channeled terrain may continue longer. A gale force front
moves into the Gulf by Wednesday morning.

For the Bering, storm force winds in the central Bering Monday evening
will spread eastward through Tuesday morning as the front moves
inland. Expect gales through Tuesday in the eastern Bering before
winds taper to small craft by Wednesday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
An active pattern is expected to continue next week as an upper
level trough over the southern Mainland slowly moves eastward
Monday night. Meanwhile a large frontal system in the Bering lifts
over Southwest Monday evening into Tuesday bringing gusty winds
and heavy precipitation. As this system weakens, the remnants move
eastward and reach Southcentral Alaska by midweek. By this time,
a wave over the northern Pacific lifts northward and merges with
the remnant front. This will produce the next round of active
weather for the southern Mainland late Tuesday into early
Wednesday. A warm front reaches the Alaska Peninsula and Kodiak
Island Wednesday morning and continues moving northeastward into
Southcentral by Thursday morning. With an ample moisture supply
from the Gulf, high precipitation rates are likely. The biggest
forecast challenge will be precipitation types and where the
rain/snow line sets up. By the end of the week, another strong
Bering low looks to usher in colder air over Southwest.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
Public...Coastal Flood Watch 155 161.
Marine...Storm 411 412 178 413 177. Gale 414 170 173 175 176 179
180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JPW
MARINE/LONG TERM...KO

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE ANCHORAGE AK
324 PM AKST SUN NOV 24 2019

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

0318 PM     SNOW             3 SW ANCHORAGE          61.19N 149.95W
11/24/2019  M2.0 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0317 PM     SNOW             12 NNW HOPE             61.06N 149.76W
11/24/2019  M3.7 INCH        ANCHORAGE          AK   TRAINED SPOTTER

0314 PM     SNOW             4 SW ANCHORAGE          61.16N 149.94W
11/24/2019  M3.0 INCH        ANCHORAGE          AK   TRAINED SPOTTER

0312 PM     SNOW             15 N HOPE               61.11N 149.72W
11/24/2019  E1.0 INCH        ANCHORAGE          AK   TRAINED SPOTTER

&&

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
340 PM AKST Sun Nov 24 2019

AKZ125-251600-
Western Prince William Sound-
Including the cities of Whittier, Seward, Girdwood,
and Moose Pass
340 PM AKST Sun Nov 24 2019

...Potential for Heavy Snow Wednesday...

A frontal system moving over Southcentral may bring moderate to
heavy accumulating snow to the eastern Kenai Peninsula from
Seward to Whittier, including Turnagain Pass. Snow is possible
beginning late Tuesday night and lasting through Wednesday.

Be sure to check your local forecast at weather.gov/Anchorage for
updates.

For road conditions and travel info, visit 511.alaska.gov.

AKZ145-251600-
Susitna Valley-
Including the cities of Talkeetna, Willow, and Cantwell
340 PM AKST Sun Nov 24 2019

...Snowfall Possible Wednesday...

A frontal system moving over Southcentral Alaska may bring
accumulating snow to the northern Susitna Valley. Snow is possible
beginning late Tuesday night and lasting through Wednesday.

Be sure to check your local forecast at weather.gov/Anchorage for
updates.

For road conditions and travel info, visit 511.alaska.gov.