Small Craft Advisory 853, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
520 PM AKST Sun Nov 8 2020

.ANALYSIS AND UPPER LEVELS...

A major winter storm is impacting the southern Alaska mainland
today. There were two distinct surface lows this morning, one
tracking northward across interior Southwest Alaska, and a newer
triple point low near the southern tip of the Kenai Peninsula.
However, as the day has worn on, the Gulf low has become the
dominant low while the southwest low appears to have dissipated.
This will be key to the wind and precipitation in Anchorage this
afternoon. When lows pass to the west of Anchorage, Turnagain
winds tend to bend into Anchorage and warm temperatures. In this
case, the Gulf low will work against that or at least limit the
strength and duration of southerly winds. In any case, Anchorage
has seen a wintry mixture of precipitation types, with snow,
sleet, freezing rain, and plain rain all observed early this
morning.

Looking across the region, some of the greatest impacts include:
storm force winds along the north Gulf coast and Prince William
Sound as a front pushes onshore; widespread gale force winds with
storm force gusts from Bristol Bay across the Alaska Peninsula,
western Gulf, and Kodiak Island; and moderate to heavy
precipitation across nearly all of Southcentral Alaska. Anchorage
and the Matanuska Valley are near the transition zone for
rain/snow, with mostly rain to the south and snow to the north. In
particular, heavy snow continues across most of the Susitna
Valley, where snow totals (as of mid day) ranged from a foot to a
foot and a half. Moderate to heavy snow has also been observed in
Valdez, Thompson Pass, and the northern Copper River Basin.
Lastly, light snow continues across interior Southwest Alaska,
especially across the Kuskokwim Valley.

Out west, a deepening low is tracking northward across the western
Bering Sea, with a frontal system tracking north and east across
the western bering and Aleutians. This is bringing gale force
winds and rain to the western Aleutian Islands.

&&

.MODEL DISCUSSION...
Models are in good agreement with the current storm systems. The
main challenge will be precipitation type and snow accumulation
in Anchorage and the Matanuska Valley this afternoon due to the
aforementioned complexities of the local wind and temperatures.
Model spread does grow fairly large with handling of the next
set of storms moving into the Bering Sea on Tuesday. Despite the
differences in low track, they are grouped together and of similar
strength. Thus, have moderate confidence in the overall weather
and impacts from these systems.

&&

.AVIATION...
PANC...Today's forecast is quite challenging and forecast
confidence is low. Snow will continue to mix with rain this
evening with IFR ceilings/vis persisting. The mixed precipitation
is expected to change back to all snow later this evening. Cold
air advection and strong low level winds on the back side will
lead to improving conditions overnight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tonight
through Tuesday)...

Strong winds combined with mixed precipitation/snowy conditions
are expected to pass through over much of Southcentral Alaska
tonight and Monday. This is due to a 988 mb low pressure over
eastern Kenai Peninsula and the frontal boundaries attached to it.
These synoptic features will disperse a lot of snow/rain as it
progresses eastward, with heavier snow amounts over Prince William
Sound and Copper River Basin overnight tonight. Steady light snow
mixed with light rain continues to spread across Mat-Su and
Anchorage Bowl into this evening, and these conditions will
continue into late tonight before clearing by early Monday morning.
Strong breezes with hazardous gusts will develop for Thompson
Pass and higher elevations, with wind gusts reaching up to 70 mph
there on Monday. As the frontal boundary and low pressure exits
the region, breezy conditions will continue over Prince William
Sound on Monday and Monday night especially near Whittier and
through the Pass. Much of the windy conditions will stay east of
Kenai Peninsula, though it will be relatively quiet there for the
next couple days.

Over Kodiak, strong winds will gradually taper off after midnight.
However, breezy conditions will linger there until Monday
afternoon. Warmer temperatures in the mid to upper 40s
(Fahrenheit) will surround Kodiak Island, with overnight lows
hanging on to between mid to upper 30s the next couple nights.

Overall, the wet pattern and snowy conditions will diminish across
Southcentral Alaska through Monday afternoon, with the skies
clearing as the bulk of clouds and snowy weather departs from the
region Monday evening.

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

Partly clear skies will skies will gradually build over the
region this evening under a weak ridge. This combined with
onshore flow could potentially lead to patchy fog developing along
the coast by early Monday morning. The flow will be weakest over
Bristol Bay and thus the greatest chance of fog exists there. A
front will move onshore over the Kuskokwim Delta Monday morning as
well. Strong southwest flow ahead of it will likely keep
temperatures above freezing over non-mountainous areas, which
will thereby keep all precipitation liquid. The front will depart
over the Middle Kuskokwim Valley by Tuesday morning, brining light
rain there as well. The Bristol Bay area will remain dry over the
aforementioned time period under an upper level ridge.

A strong low will track into the Bering by Tuesday afternoon and
precipitation will overspread all of Southwest Alaska by the
evening. While there is still some disagreement on the low track,
moderate confidence exists that given the similar southwest flow
pattern, precipitation types will stay mostly rain except over
far interior and higher-elevation areas.

&&

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

A weak ridge will bring benign conditions to the central/eastern
Aleutians though Monday night. A front will extend from the
Pribilofs southwestward through the western Aleutians on Monday,
leading to gusty southwesterly winds and moderate rain. A strong
double-barrel low will gradually move west to east across the
Bering through Tuesday. While the timing and central pressure are
still somewhat in question, given the cold advection on the
backside of the low, there is moderate to high confidence of storm
force winds (along with rain) over the Pribilofs by Wednesday
morning.

&&

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

Gulf of Alaska:

Storm force winds are likely across the southwestern Gulf waters
south of Kodiak Island Wednesday. As a low in the Bering tracks
eastward, a triple point low will form in the vicinity of Kodiak
Island late Wednesday. This low will track eastward across the
Gulf bringing gusty gale force winds winds to the Gulf waters
through Friday. There is high confidence gusty northwesterly winds
will develop through Kamishak Gap and over the Barren Islands.

Bering Sea:

There is increasing confidence a strong low will bring northerly
storm force winds to much of the central Bering Wednesday. As the
low tracks eastward, it will weaken, however gales, and possibly storm
force winds will linger over the southeastern Bering through
Wednesday night. Westerly flow will fill in across the Bering
behind this system for Thursday with mostly small craft winds
expected. Gale force winds are possible over the western Bering
Friday as a colder airmass arrives from the north.

&&

.LONG TERM FORECAST (Days 4 through 7: Thursday through Sunday)...

A broad longwave trough is expected to persist over the AOR with
the jet stream centered over 50N. Two upper level waves will be
the focus for upcoming forecast packages, one centered over the
Bering Strait and the other the northern Gulf. At the surface, a
broad region of low pressure is expected from the Norton Sound
into the Gulf of Alaska on Thursday. By this time most of the
southern mainland appears to be in the cold sector of the low over
the Gulf, which will bring snow to areas of Southwest and
possibly Southcentral. There is some model discrepancy on the
placement of the low in the northern Gulf, which results in
varying precipitation potential for Southcentral on Thursday.

On Friday the through over the Gulf should exit to the southeast,
which will bring colder, drier air into Southcentral. To the
west, the low over the Norton Sound should begin to move south,
which will likely bring snow to coastal Southwest Alaska, the
AKPEN, and the central and eastern Aleutian Chain. As the low
pushes south, stronger winds are also expected over the central
Bering through Saturday. By Sunday, cold and dry air is expected
to move over the southern mainland while the next system
approaches the western Bering from the Kamchatka Peninsula.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 131
		 Winter Storm Warning 145
		 Winter Weather Advisory 101 141.
MARINE...Storm Warning 120 130 131 175 176 351.
		 Gale Warning 119 121 127 129 132 136-139 141 150 165
		 			  170 173 174 177-181 185 352 411-414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...KO/CJ

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE ANCHORAGE AK
942 PM AKST SUN NOV 8 2020

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

0840 PM     SNOW             4 N SUTTON              61.78N 148.87W
11/08/2020  M18.3 INCH       MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

            CORRECTS PREVIOUS SNOW REPORT FROM 4 N
            SUTTON. STILL SNOWING, NO SIGNS OF RAIN AND
            NO WIND.

0359 PM     SNOW             7 MILES SE OF TALKEETNA 62.14N 150.03W
11/08/2020  M18.0 INCH       MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

            STORM TOTAL OF 18 INCHES SO FAR AND STILL
            SNOWING HEAVILY.

1055 AM     HEAVY SNOW       OFF MILE 6 PETERSVILLE  62.32N 150.23W
11/08/2020  E15.0 INCH       MATANUSKA-SUSITNA  AK   CO-OP OBSERVER

            13.5 INCHES OVERNIGHT. SNOWING HEAVILY. 31F.

0905 AM     HEAVY SNOW       25 WSW WILLOW           61.60N 150.76W
11/08/2020  M17.0 INCH       MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

            SINCE 230PM 11/7. 28F NW 5 MPH.

0630 AM     HEAVY SNOW       6 MILES ESE OF TALKEETN 62.14N 150.03W
11/08/2020  M10.0 INCH       MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

            12 INCHES STORM TOTAL.

1106 PM     SNOW             7 SE TALKEETNA          62.27N 149.94W
11/07/2020  M2.0 INCH        MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

            SMALL FLAKES, TEMPERATURE IS 22F.

0944 PM     SNOW             3 E KENAI               60.57N 151.08W
11/07/2020  M4.0 INCH        KENAI PENINSULA    AK   TRAINED SPOTTER

0915 PM     SNOW             EAST EAGLE RIVER        61.30N 149.53W
11/08/2020  M3.6 INCH        ANCHORAGE          AK   NWS EMPLOYEE

            STORM TOTAL OF SNOW AND DRIZZLE.

0452 PM     SNOW             NWS ANCHORAGE OFFICE    61.16N 149.98W
11/08/2020  M5.0 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0213 PM     SNOW             EAGLE RIVER             61.32N 149.55W
11/08/2020  M1.5 INCH        ANCHORAGE          AK   TRAINED SPOTTER

            SNOW CHANGED TO RAIN, 32 DEGREES.

0903 AM     SNOW             DEBARR AND BONIFACE     61.21N 149.78W
11/08/2020  M1.7 INCH        ANCHORAGE          AK   NWS EMPLOYEE

            1.7 INCHES IN LAST 24 HRS. CHANGED TO
            FREEZING RAIN.

0508 AM     SNOW             EAGLE RIVER             61.32N 149.55W
11/08/2020  M4.0 INCH        ANCHORAGE          AK   NWS EMPLOYEE

            CORRECTS PREVIOUS SNOW REPORT FROM EAGLE
            RIVER ISSUED AT 7AM. STORM TOTAL OF 4
            INCHES.

0334 AM     NON-TSTM WND GST POTTER HILL             61.08N 149.78W
11/08/2020  E51 MPH          ANCHORAGE          AK   TRAINED SPOTTER

&&