Small Craft Advisory 353, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
429 AM AKDT Wed May 3 2017

.ANALYSIS AND UPPER LEVELS...
Our strong shortwave trough from yesterday has grown from a
compact disturbance into a beautiful sprawling low pressure
system stretching from the Canadian Rockies through the Yukon and
into the Gulf of Alaska. The potent disturbance acted to
propagate the whole longwave trough eastward, becoming the new
center of circulation of the longwave. The cloud shield associated
with the system has spread over all of Southcentral Alaska and
into the southwest mainland as well. At the surface, a late
evening scatterometer pass confirmed widespread gale force winds
along the front as well as a smaller embedded area of storm force
winds. Rain along the front is mainly confined to coastal areas,
but some echoes are appearing as some hydrometeors try to sneak
over mountains. They are contending with some pretty dry low-level
air around the Cook Inlet region. Down-Inlet and offshore flow
dominate the region while cloud cover and wind keep overnight
temperatures elevated in the 40s.

&&

.MODEL DISCUSSION...
Guidance is in great agreement with the current system as we are
beyond peak intensity, entering the weakening phase of the system.
There is still a little discrepancy with exact location as the
system weakens and slides east, but guidance paints a similar
picture. The next dynamic system to affect the pattern is entering
the western Bering/Aleutians currently. The major difference
between models with this system is triple point location, but
frontal position and strength are well agreed upon through late
week.

&&

.AVIATION...
PANC...Uncertainty in the TAF today will revolve around the
Turnagain wind and predominant wind direction. While it won't be
very strong, it will be pulled from both south and north as it
exits the Arm. The strong down-inlet gradient toward low pressure
will bend most of the wind southward, but clearing and heating up
in the Susitna Valley will promote the pulling of light southerlies
from the Arm this afternoon into the evening. With the two
competing forces, the predominant direction could flip around a
fair amount.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The deep low over the Gulf of Alaska will remain nearly stationary
as it gradually weakens through Thursday. Winds will remain
strongest on the northern side of the low center, across the
northern Gulf and into southern Prince William Sound. Easterly
flow combined with a series of upper waves rounding the low will
provide a steady supply of rain to western Prince William Sound,
with lighter and more occasional rain for the rest of the Gulf
coast and Kodiak Island.

For the rest of Southcentral the combination of elevated
instability and upper level waves rounding the low will trigger
showers, primarily over the mountains. The low levels are quite
dry and will remain so. Thus, as showers advect over the low lands
of Southcentral expect most of the moisture to evaporate before
hitting the ground. Most valleys will stay dry with just a few
sprinkles or an isolated shower over the next couple days. The low
will maintain fairly extensive mid to high clouds over the
region, though do expect some breaks of sun, particularly on
Thursday and over the Copper River Basin.

Lastly, with tight pressure gradients in place do expect localized
gap winds. Most notably and very typical for the season, expect
winds through Turnagain Arm, the Knik River Valley, and along the
Copper River.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Northerly flow will continue to increase across Southwest Alaska
this morning as the deep surface low in the Gulf tracks north.
Conditions will remain dry and breezy for much of the day with the
strongest north to northwest winds across the greater Bristol Bay
area and the higher elevations around the Kuskokwim Valley. A
north to south running band of showers will form in the late
afternoon/evening to the east of the Aleutian and Alaska Ranges in
response to deformation band associated with the low in the Gulf
rotating west across the mountains. Showers will persist in this
general area tonight through Thursday morning then shift to more
orographically enhanced showers along the mountains Thursday
afternoon and evening.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Northwesterly winds will increase through the gaps and passes in
the Alaska Peninsula today, continue tonight and then diminish
Thursday as the low in the Gulf fills. A ridge over the central
and eastern Aleutians will shift east to the Alaska Peninsula and
eastern Bering Thursday and Thursday night. A surface low south of
the Aleutians will send a front into the central Aleutians
Thursday morning with a secondary frontal wave swinging into the
eastern Aleutians Thursday night and across Alaska Peninsula
Friday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The models remain in good agreement with the vertically stacked
weakening low over the Gulf of Alaska Friday weakening and exiting
to the southeast through Saturday. The big fall-like storm is
handled similarly by the models through the weekend, weakening the
low as it moves over the Alaska Peninsula and southwest of the
Gulf of Alaska. As we move into early next week the remnants of
the low drift into the Gulf of Alaska through Tuesday.

We expect a ridge to build in along the Gulf Coast through Friday
with the interior trough of weak low pressure fueling the somewhat
typical south and southeasterly gap and cross barrier winds across
the Chugach Mountains and Alaska Range.

Further west another low is on tap to enter the western Bering
Sea Sunday and Monday, although the models diverge on resolving
the location of the low and the associated front. Generally this
will still result in continued cloudy and wet weather for the
Central and Western Aleutians.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 119 120 125 130 131 132 139 150 155 160 172
					  173 174 175 176 177 178.
FIRE WEATHER...NONE.

&&

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