Small Craft Advisory 368, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
510 PM AKDT Sat May 5 2018

.ANALYSIS AND UPPER LEVELS...
A cold upper level low centered near Norton Sound is slowly
trekking northward. This is leaving mainland southern Alaska in
southwest flow aloft which is bringing in colder air to the
region. This overall pattern will persist through the weekend
before warmer air moves into the region. This has caused much of
the precipitation to be snow from the Kuskokwim Delta westward and
also from Valdez to the Copper River Basin. Heaviest snow amounts
from last night were in the Valdez area where over 8 inches fell
overnight in town and around two feet in Thompson Pass (with more
snow accumulations from what has been falling today). From Bristol
Bay through the Gulf coastal areas it has generally been warm
enough for rain.

&&

.MODEL DISCUSSION...
Synoptic agreement on the models remains very good into the middle
part of next week. The main model questions are related to more
subtle details such as the freezing level in the Valdez area and
timing of some short waves.

&&

.AVIATION...
PANC...VFR conditions will persist through Sunday afternoon. Gusty
southeast winds will also persist through Sunday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The firehose of moisture which brought copious amounts of rain
and snow to the eastern portions of Prince William Sound overnight
Friday into this morning has now shifted eastward and over the
Eastern Bering. However the wet conditions are not over yet, as
south to southwesterly flow and an approaching upper level
short-wave Sunday evening will keep more showery conditions over
much of southcentral Alaska through Sunday night. Areas prone to
these southerly flow/upsloping conditions, specifically the
Valdez and Thompson Pass areas, will see another round of snowfall
tonight into early tomorrow morning. We are expecting smaller
amounts of accumulation tonight in comparison to what was observed
overnight last night into this morning.

Showers will then taper off from west to east Sunday night behind
this short-wave as the flow aloft becomes westerly. Winds will
become fairly light as high pressure builds in over the Gulf
overnight Sunday into Monday, bringing a reprieve from the wet and
gusty conditions through the early part of next week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A surface low currently centered over the Kuskokwim Delta will
continue drifting north tonight as cold air wrapping around the
backside of the system will help promote scattered snow showers
over the SW mainland. Skies should remain mostly cloudy though
temperatures will again fall to at or below the freezing mark with
plenty of cold air aloft. On Sunday, a shortwave trough will
rotate east around the base of an upper level low and help to
kick off and area of rain and snow showers along the western side
of the Alaska Range, including Iliamna, during the afternoon.
Weak ridging builds in Sunday night bringing a brief break to the
precipitation before a large low pressure system swings a front
across the coast by late Monday morning. With enough cold air in
place at the surface, precipitation looks to start off as snow
before quickly changing over to all rain for lowland locations as
warm air surges north behind the front. Winds will also kick up
as the front moves ashore, most notably along the coast and
through the typical terrain gaps, where gusts could be in excess
of 40 mph.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Most of the Bering/Aleutians will continue to be under the
influence of cold northerly flow as an upper level low slowly
slides north over the Bering Strait leading to cold air being
pulled south off the Kamchatka Peninsula. This will lead to
scattered clouds and snow showers over most of the eastern and
central Bering through tomorrow afternoon.

For the western Aleutians, conditions will quickly deteriorate
overnight and into tomorrow as a rapidly strengthening low
pressure system brings another shot of gale-force winds, warm
air, and rain to the region. By Monday morning, this low will
deepen to 973 mb and swing a front east across the Bering and
Aleutian chain, reaching the western end of the AKPEN before noon.
Strong cold air advection on the backside of the low will lead to
a swath of upper end gale-force winds across the western Bering
and Aleutians, with storm force gusts likely reaching the surface.
Long fetch northerly winds will also increase wave heights in
this region with swells up to 25 ft.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Expect a continuation of near normal to slightly below normal
temperatures, an mostly cloudy conditions that break a bit toward
late week. The discussion for the extended begins in the short-
term. As a short-wave trough drops into the Western Bering from
eastern Russia, it will phase with another disturbance riding
through the east-Asian Jet. The phasing of these two features will
shape the long-wave pattern through the extended forecast as the
long-wave formed by those two features will anchor the pattern.
The big question with these types of setups how jet energy
interacts with the large-scale trough, specifically timing of
individual waves within the flow. By the middle of next week, the
long-wave looks to be centered in the southern Bering Sea with an
upper low pressure in the eastern Gulf of Alaska. The latter
system will partially phase into the Bering long-wave, bringing
rain to the western Prince William Sound area, along with likely
downslope condition for the Cook Inlet region. Toward the end of
the week is when uncertainty begins to really arise. The culprit
is a disturbance riding a stationary baroclinic zone in the
Pacific. Longer-term models are in agreement with rapidly
deepening this disturbance once it phases with the antecedent
trough, even acting to propagate the long-wave eastward. The
disagreement comes with what to do with the low pressure that
spawns, does it move inland, does it occlude and move southward,
or somewhere in between? These are the questions that need to be
answered as far as uncertainty goes.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisories 152 155.
MARINE...Gales 155 165 170-179 411-414

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...TP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...ML