Small Craft Advisory 144, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 PM AKST Wed Feb 15 2017

.ANALYSIS AND UPPER LEVELS...
There is a nearly vertically stacked low centered near the Alaska
peninsula. There are several short-wave troughs associated with
this low. One is over the lower Kuskokwim Valley and Kuskokwim
Delta. There is also one over the eastern Bering Sea. Another is
just south of the Kenai Peninsula, and yet another south of the
Alaska Peninsula. There is an upper level high over the western
and central Bering Sea. There is also a high over northwest Canada
nudging into central Alaska.

&&

.MODEL DISCUSSION...
The numerical models are in fairly good agreement through the
short term portion of the forecast (Friday afternoon). There are
some differences in the handling of moisture and precipitation.
The GFS/ECMWF solutions are preferred with high QPF amounts for
inland areas and quicker arrival of precipitation.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. Some wind
shear is expected beginning tonight as the next weather system
works its way toward the area.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Even as the remnants of the low that brought widespread snow to
Southcentral today lifts northward and dissipates, the next round
of snow and active weather is quickly moving into the western
Gulf. Snow showers moving through the western Gulf will primarily
move over the extreme southern Kenai Peninsula (such as the Homer
area) and north Gulf Coast this evening. These showers may be
heavy at times due to their convective nature ahead of the
approaching cold upper level low.

Starting Thursday morning, widespread accumulating snow will
return to the area beginning on the southern Kenai Peninsula, and
then spreading northward into the Anchorage Bowl and MatSu
Valley's late Thursday through Friday. Confidence in the snow
accumulation forecast is highest along the Kenai Peninsula and
Western Prince William Sound where the heaviest and widespread
snow amounts are expected. There is a potential that system may
bring advisory level amounts to the Homer Bluff on Thursday,
however due to warm air associated with it amounts were kept just
under advisory level. This will need to be monitored further as
the front approaches late tonight. We also have good confidence
that higher elevations of the Kenai Peninsula including Moose and
Turnagain Passes will see the highest accumulations from this
event due to deep southerly flow but without significant warm air
advection. This should allow well over a foot of snow from the
entire storm (late Thursday-Friday) with temperatures remaining at
or below freezing even near sea-level. Over the Susitna Valley to
the west of the Parks Highway confidence is also increasing that
the area will see advisory level accumulations from the storm
beginning Thursday evening, with less confidence that it will
occur in a 12 hr period.

Confidence begins to diminish as the snow moves northward into
the Anchorage Bowl as there is significant uncertainty in the
exact timing, longevity, and role that downsloping may play in the
event. At this point it appears that the snow should begin
sometime late Thursday night with the front, but this is also
where the highest uncertainty in snow amounts exist due to
potential downsloping. A better chance of accumulating snow then
looks to move in Friday afternoon as the upper level trough axis
passes overhead, providing the strongest dynamics with very little
(if any downsloping)

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An upper level closed low is positioned over the eastern
Aleutians near Port Moller this evening before tracking into the
Bristol Bay region near Dillingham by Thursday midday. This
synoptic feature sends shortwaves through the Southwest Alaska
region through Thursday. These shortwaves will be accompanied by
snow as they move from east to west before exiting the Kuskokwim
Delta area. This will create blowing snow issues along the
Kuskokwim Delta coastline this evening. Therefore, a Winter
Weather Advisory for blowing snow was issued along the coast south
of Hooper Bay through this evening. While in the interior of the
Kuskokwim Delta the winds will be weaker but look for 3 to 6
inches of snowfall as the shortwave moves through this evening
before tapering-off midnight tonight. By Thursday, a surface low
pressure system tracks over King Salmon by midday before
eventually crossing the Aleutian Range near Iliamna by Friday
morning. This feature brings a swath of snowfall beginning over
the Alaska Peninsula before spreading northward into the Bristol
Bay interior, and the Lower Kuskokwim Valley before tapering-off
by Friday morning.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The Bering Sea/Aleutians is dominated by northerly flow resulting
in a showery weather regime. While a 967 mb low tracks along the
Alaska Peninsula before entering the Bristol Bay interior Thursday
afternoon. The northerly winds brings cooler temperatures to the
Bering accompanied by areas of heavy freezing spray as we head
into the weekend.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Beginning the long term forecast Friday evening, an upper trough
will be centered over northwest Alaska with a shortwave trough
along the southeast flank of the aforementioned trough moving into
the central interior. Bands of light snow will be ongoing from
interior southwest Alaska through Southcentral. Over the Bering,
there will be continued cold advection snow showers. Heading into
the weekend the upper trough will sharpen and dig south, bringing
in rather cold air into southwest Alaska (850 hpa temperatures -15
to -25C). Cold temperatures will last into Tuesday and will bleed
east into Southcentral as the trough finally weakens and moves
east. However, before that time, there will be continued light
snow chances for most of Southcentral Saturday through Monday
night mainly from the Chugach west.

By Monday, a deep low will move into the Western Bering Sea, and
this will result in a rather dramatic change in the pattern as a a
series of powerful shortwaves eject off a powerful North pacific
jet stream in a general southwest flow. Beginning Wednesday into
the end of the week, it is looking increasingly likely that a
barrage of lows and fronts will impact southern Alaska with
widespread rain and snow events. Temps will be tough as some
inland areas will remain snow while more prone coastal locations
may mix at times. Either way, expect very active weather by
midweek.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155
MARINE...Gales 130-132 150.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...JA

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1117 PM AKST WED FEB 15 2017

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

1115 PM     SNOW             SEWARD                  60.11N 149.39W
02/15/2017  E4.0 INCH                           AK   LAW ENFORCEMENT

            TOTAL OVER THE WHOLE DAY TODAY.

&&

JPW