Small Craft Advisory 369, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
501 AM AKDT Sun May 6 2018

.ANALYSIS AND UPPER LEVELS...A low over the northern Bering is
diminishing, while weak ridging is still present over eastern
Alaska. This setup is still bringing a southwest flow along with
precipitation over Kenai Peninsula and the north Gulf coast. It is
also advecting cold air over Bristol Bay and southwest Alaska and
is producing snow showers over the same areas. An intensifying
low over the western Aleutians is already bringing up warm air and
rain to the western and central Aleutians as its warm front moves
through.

&&

.MODEL DISCUSSION...
Models are in good agreement through Monday night as the next low
tracking from the North Pacific slowly tracks across the southern
Bering. Only minor edits were needed on the morning package with
good model consistency on the 00Z run.

&&

.AVIATION...
PANC...VFR conditions are expected to persist through Monday
morning...however a weak front moving across from the southwest this
evening may briefly lower ceilings to MVFR as scattered to
numerous showers develop along the boundary. Gusty southeasterly
winds taper down during the evening with light and variable winds
prevailing on Monday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Southwesterly flow aloft will continue to bring some showers and
breezy conditions to Southcentral today. An upper level short-wave
trough currently moving into southwest Alaska will reach
Southcentral this afternoon. This will increase the shower
activity, but the winds will diminish as the surface gradient
weakens. This trough will cross the area Sunday night and exit
into Canada Monday morning. High pressure will then build in over
the area aloft and at the surface, for a dry day with light winds.
Another front will approach the area Monday night. Precipitation
and winds will increase across Kodiak Island by Monday evening,
with some precipitation reaching the Kenai Peninsula after
midnight.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

The quasi-stationary low spinning over the Seward Peninsula this
morning will continue to produce scattered snow showers from
Norton Sound south through the Y-K Delta today. A shortwave
trough rotating around the southern flank of the main low will
act to further destabilize the atmosphere across SW Alaska today.
This will kick off another round of rain and snow showers from
the AKPEN through the Y-K Delta, especially in favored upslope
locations along the western Alaska and Kilbuck ranges. Clouds and
precipitation will diminish tonight as a weak ridge sets up in
advance of the next system currently strengthening south of the
western Aleutians. This low will move west Monday, swinging a
front across coastal SW Alaska Monday night. Precipitation is
expected to begin as all snow or a mix of rain and snow overnight
Monday into Tuesday Morning as cold air lingers at the surface
during the onset of the event. Warmer air surging northward behind
the front however, should quickly scour out this colder air
changing the precipitation to rain in valley locations.
Southeasterly winds will also increasing rapidly along and behind
the front, most notably along the coast and terrain gap areas.
This unsettled weather pattern will remain over all of western
Alaska as the low slows and stalls on the Bering side of the
eastern Aleutians.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

Scattered clouds and snow showers will continue through most of
today as colder air continues to be advected southward across
much of the Bering due in large part to two shortwaves, the first
moving into the western Alaska coast and the second dropping down
from eastern Russia.

The main focus, however, is on a deepening area of low pressure
south of Shemya. This system will continue to strengthen
throughout the day, pushing a front across the western Aleutians
and producing gale-force winds across the central and western
Bering by tonight. The leading warm front will occlude as it
surges northeast, moving across the Aleutians Monday and into the
AKPEN and western coast Monday evening. Southerly gale-force
winds of 45 kts will accompany the front as it works eastward.
Colder, northerly gales will also develop along the backside of
the low across the western Bering tonight. These winds will linger
through early Wednesday morning as the system stalls between Atka
and the Pribilofs. Long fetches on both sides of this system will
also increase swells to near 25 ft on the Pacific side of the
Aleutians and across the western Bering.

&&

.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...None.
MARINE...Gales 155 165 170 171 172 173 174 175 176 177 178 179
               180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...ALH/KH
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
LONG TERM...ML