Small Craft Advisory 160, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
1034 AM AKST Wed Feb 22 2017

.ANALYSIS AND UPPER LEVELS...
One front is moving from Southwest to Southcentral AK this
morning, while another system is pushing through the Central
Aleutians. The front across southern AK is bringing widespread
snow to the area. The heaviest snow overnight was over parts of
the Kenai Peninsula and Southwest AK. But radar clearly shows that
more is in store for Anchorage and the Matanuska and Susitna
Valleys. This snow is being aided by a fairly sharp trough that is
sweeping quickly to the east this morning. In between the two
systems, a 110 kt jet is moving over top of a short wave ridge
and keeping the pattern very progressive.

The second system is located over the Western Bering with a cold
front between Dutch and Adak and a warm front stretching towards
the Alaska Peninsula. It is being supported by a 150 kt piece of
the subtropical jet. This jet is carrying a healthy amount of warm
air northward with 850 hPa temperatures surging near 5C. This warm
air will cause precipitation type issues (snow changing to rain
and possibly freezing rain) over the Aleutians first, then into
the Pribilof Islands, and then into Southwest AK. It will also
allow the ridge to build over the southern mainland over the next
24 hours.

&&

.MODEL DISCUSSION/Uncertainty...
The NAM relented on its slower timing and further west solutions,
so all models are in solid synoptic agreement this morning. This
is leading to average forecast confidence with regards to synoptic
features. There is still uncertainty with snow amounts from this
morning's system. Higher resolution models were used to tweak
those values. Out west, the GFS was preferred due to its
consistency, especially with winds. The area of uncertainty in the
mid-range (Fri-Sat) will be with precipitation types. How much
warm air will there be and how far inland will it go? At this
point, think at least a mix of rain and snow is in store for the
Cook Inlet by on Fri.

&&

.AVIATION...
PANC...Snow reducing visibility to IFR conditions will
continue through mid-morning, then should quickly end by late
morning. After that, VFR conditions are expected with light
winds.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Strong south to southeast flow across the Kenai and Chugach
Mountains has thus far limited snow intensity in much of Anchorage
and the Mat Valley. However, winds will rapidly diminish over the
next couple hours as the surface low tracks south of Seward. This
will allow for a quick shot of heavier snow before the upper
trough moves through between 10 am and Noon. The short-wave will
go from a negative tilt to neutral tilt as it continues eastward
over Southcentral and a lot of the moisture will be pulled south
over the Gulf with the surface low. Thus, expect generally light
snow amounts for the Copper River Basin on south to Eastern Prince
William Sound.

A warm front will then ride a building upper level ridge northward
across Southwest Alaska tonight. The overrunning precip associated
with this will largely stay out of Southcentral, though there
could be some very light precip along the Alaska Range. Southcentral
will warm as the ridge passes overhead on Thursday and southerly
flow behind the ridge increases. There is some uncertainty as to
exactly how much the airmass will warm. This will be key to
precipitation type as a weak front and upper trough approaches
from the west Thursday night into Friday. The best chance of
precipitation Thursday night will be across the Cook Inlet-Susitna
Valley corridor. There is potential for freezing rain so will have
to closely monitor the trends in temperatures with this front.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Last evening's trough is pushing out of the southwest, but the
progressive pattern remains. Some light snow will persist this
morning along the Alaska range as cold advection runs into the
mountains, otherwise there is a short break between systems.
Pressure rises behind the departing low are bringing a much
warmer(relatively) air mass over the mainland, however, the next
warm(red) front is quickly approaching from the Bering Sea. It is
not nearly as strong as the previous front, but will spread light
snow over the most of the mainland, save eastern areas of Bristol
Bay.

Being the second warm front in as many days, this one is bringing
some above freezing air with it. Snow could change to freezing
rain just as the precipitation is ending, but shouldn't amount to
any accumulation. Deep layer southerly flow should bring surface
temperatures above freezing by late tonight/early Thursday morning
as well. The cold(blue) front with this system(a Kamchatka low
800+ miles away) follows on Thursday into Friday with a west to
east progression over the mainland. Precipitation will likely
begin as rain and transition to snow as it ends on Friday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Storm-force easterlies transition to gale-force southerlies. The
warm front over the northern Bering will transition snow to rain
as the southerly flow kicks in. The cold front will feature mostly
rain as it moves from west to east over the Bering but could
transition to snow on the back edge. The front slowly moves toward
the Alaska Peninsula/mainland late Thursday into Friday.
Scattered rain/snow showers will encompass much of the
western/central Bering on Thursday before yet another strong low
pressure enters late Thursday night featuring another warm front
and storm-force southerlies.

&&

.LONG TERM FORECAST (Days 3 through 7)...
There remains good confidence that a shortwave moving over the
Mainland on Friday will force the highly amplified ridge well
south of the area, allowing for widespread precipitation to move
into the south mainland. It still appears that precipitation will
move into Southcentral sometime Friday afternoon or evening, and
most likely as a rain/snow mix. Afterwards an active pattern,
especially for the Bering Sea and Southwest Mainland, will
continue into this upcoming weekend as a gale-to-storm force low
moves through the Northern Bering. This active pattern then
continues into early next week and we continue to have good
confidence that an arctic trough will descend through the
Mainland. There still remains some uncertainty in the exact track
and how much cold air will be associated with this trough, but the
general trend has been towards stronger solutions through the
Mainland. This would allow for cooler weather, chances of snow,
and a generally active pattern for the Mainland into the middle of
next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 111 145
MARINE...Storm 178 185 Gale 119 120 131 155 165 170 172 173 174
175 176 177 179 351 411 412 413 414 Heavy Freezing Spray 185

&&

SYNOPSIS AND MODEL DISCUSSION...MO
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MTL
LONG TERM...DEK

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
932 AM AKST WED FEB 22 2017

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

0522 PM     BLIZZARD         TOKSOOK BAY             60.53N 165.10W
02/21/2017                                      AK   ASOS

            HEAVY SNOW WITH VISIBILITY LESS THAN 1/4 MILE AND WINDS
            GUSTING UP TO 47 MPH. BLIZZARD CONDITIONS LASTED BETWEEN
            220 PM AND 745 PM.

0900 AM     SNOW             ANIAK                   61.58N 159.54W
02/22/2017  E6.0 INCH                           AK   LAW ENFORCEMENT

            ESTIMATED 4 TO 6 INCHES OF SNOW FELL IN LAST 20 HOURS.

&&

RDUELL

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
956 AM AKST WED FEB 22 2017

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

0900 AM     SNOW             NINILCHIK               60.07N 151.63W
02/22/2017  M12.0 INCH                          AK   PUBLIC

            SOCIAL MEDIA REPORT OF 12 INCHES OF NEW SNOWFALL
            OVERNIGHT.

&&

RDUELL