Small Craft Advisory 48, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
448 AM AKST Mon Jan 14 2019

.ANALYSIS AND UPPER LEVELS...A deep, vertically stacked low is
nearly stationary over the southern Bering Sea. Strong north winds
and cold air advection on the backside of the low persist over the
central and western Aleutians, with mountain-wave clouds evident
on satellite. An occluded front associated with that low continues
to lift northeastward and is bringing areas of precipitation to
Southcentral, primarily to the Prince William Sound and Kenai
Mountains in upslope flow. A series of weak disturbances rotating
around the main Bering Sea low are bringing showers to the Bering,
Aleutians, and AKPEN, with some showers making it into Southwest.
The precipitation type is primarily rain, due to warm conditions
and warm air advection from the Gulf. The exception is on the west
side of the low over the western Aleutians, north flow will
facilitate snow as the primary ptype.

&&

.MODEL DISCUSSION...Models are resolving current weather well and
are in modest agreement in the short term. The main challenge in
the short term is the GFS and NAM project a shortwave moving into
the eastern Bering later today bringing strong winds to
Southwest, while this feature is much weaker in the EC and CMC.
HiRes guidance was used for the wind fields with this forecast
package, which is more in line with the GFS/NAM. Model agreement
diminishes further Wednesday. The most recent runs of the GFS and
EC both project a series of lows developing in the northeastern
Pacific bringing potential gales to the Gulf the second half of
the week. The GFS shifts a low into the eastern Bering...similar
to the previous run of the EC. While confidence is moderate in the
general trend, it's low in the details given run to run and model
to model discrepancy.

&&

.AVIATION...
PANC...VFR conditions and light northerly winds will persist.
Strong easterly flow up above the surface will continue until the
occluded front finally passes through Anchorage Monday morning.
Thus, low level wind shear will persist through the early morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Warm, southerly flow is impacting Southcentral Alaska due to high
pressure to the east and a low over the Bering. This pattern has
resulted in higher than normal temperatures that will hold through
Tuesday. The setup is also bringing showers to areas around
Southcentral Alaska as several shortwaves move through the area
from the south. The majority of the showers will be around western
Prince William Sound with low lying areas seeing rain and higher
elevations seeing snow. This evening shower activity will diminish
but increase again Tuesday afternoon as another trough tracks
through the Gulf. There is some discrepancy with the models on the
track of this feature and whether it will stay as an open wave or
form a closed low.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Southerly flow aloft behind the frontal system that lifted north
across Southwest Alaska on Sunday and breezy east to southeasterly
winds at the surface continue to keep the area warm with most
locations in Southwest Alaska remaining above freezing. The next
frontal wave moving quickly north through the southerly flow is
currently centered south of the Alaska Peninsula at around 50N. As
this frontal system lifts north today, southeasterly winds will
increase out ahead of it across the Alaska Peninsula and Southwest
Alaska with strong easterly winds increasing through the gaps and
passes in the Alaska/Aleutian Range. Winds will gradually ease
Tuesday and Tuesday night with remnants of the frontal system
shifting offshore and dissipating. The next frontal system will
rotate in from the east Wednesday afternoon or evening.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A deep vertically stacked low to the west of the Pribilof Islands
will gradually fill through Monday evening before tracking north
into the northern Bering Monday Night and Tuesday. A cold upper
trough over eastern Russia will rotate slowly east over the
western Bering through Tuesday, continuing east into the Central
Bering Wednesday. A frontal system will approach the western
Aleutians Wednesday night.

&&

.MARINE (Days 3 through 5)...

Beginning the marine extended period forecast on Wednesday, a low
over the northern Bering will continue to weaken as it drifts
north allowing winds to diminish to small craft or below. A
frontal system approaches from the west on Thursday which may
bring a short period of southerly gale force winds to the far
western Aleutians and Bering, but models show the system rapidly
shearing apart and weakening as it stalls out over the western
Bering. Other than the frontal system, winds are expected to
remain below gale force through the end of the week.

A low south of the Gulf of Alaska will lead to easterly small
craft winds over the southern Gulf on Wednesday, before subsiding
through the rest of the week as the low drifts south and west.

&&

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

Beginning Wednesday, a broad upper low will be located over the
eastern Bering while a high amplitude ridge will extend from the
Pacific West Coast north over Eastern Alaska and into the Arctic.
Models have been trending towards a more SE-NW tilt of the ridge,
nudging a bit further west over the eastern mainland. This will
result in more of a southeasterly flow over the Gulf and into the
southern mainland, leading to generally wet conditions over the
windward coastal mountains, with drier conditions further inland.
Models are still struggling with the specific features embedded
within the main flow, but are trying to key in on a short wave
trough pushing west across the northern Gulf and into the Prince
William Sound Tuesday night into Wednesday, with a good shot of
precipitation expected over the coastal mountains during this time.
Warm surface temps will likely keep precip as rain at sea level or
slightly higher.. By the end of the week, the upper ridge will
try to nose southwest towards Southcentral bringing somewhat drier
conditions, though precip will likely still be seen along the
coast with continued onshore flow. Models show a broad surface low
somewhere between Bristol Bay to south of the AKPEN on Thursday
with ridging building in over the central Bering. A frontal system
will approach the western Bering on Thursday but looks to stall
and weaken as it runs into the upstream ridge. By the weekend,
there are signs that a stronger surface low will move up from the
North Pacific into the Northeastern Gulf. Right now, the system is
expected to impact the Panhandle and stay east of Cordova.
Overall, temperatures across the southern mainland are expected to
remain above normal through the end of the week, with a possible
slight cooling trend by the weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales: 132 150 155 160 175 180 181 185
         Heavy Freezing Spray: 185
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MM/JA
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
MARINE/LONG TERM...KVP