Small Craft Advisory 821, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
800 AM AKDT Sun Oct 29 2017

.ANALYSIS AND UPPER LEVELS...
A longwave trough extends down from northeast Russia through the
Central Aleutians and into the North Pacific. Two embedded waves
are moving through the trough with the first on the eastern side
moving across the Alaska Peninsula towards Southcentral Alaska.
The second wave is supporting a low in the base of the trough
south of the Central Aleutians. This low is trending north towards
the southern Bering. Both of these waves are bringing more warm
air to the southern mainland.

&&

.MODEL DISCUSSION...
The models are in good agreement through the short term. However,
they do struggle on Monday night as the low troughs out over the
southern mainland and how much rain will linger over the area.

&&

.AVIATION...
PANC...Surface flow will switch to northerly as a front
approaches the area from the south this morning. During this time
Turnagain Arm winds will also increase and move over the airport
out of a southeast direction bringing wind shear to the airport.
With northerly winds expected to continue through the TAF period,
wind shear should also continue through the TAF period.

&&

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

A warm front and associated shortwave trough is quickly moving
toward the Gulf Coast with rain overspreading Kodiak Island. Radar
echoes are already showing up on the Kenai radar and this activity
should quickly move into Southcentral this morning. Rain is
expected for inland locales, but snow levels will drop as low as
2000 feet along the Chugach mountains. One area where low
elevation snow is likely is across the northern Susitna Valley
near the Alaska Range where 2-4 inches of snow is possible. This
system will quickly move east before a stronger warm front moves
into the region. This system will bring in more warm air and
should change any snow over the northern Susitna Valley back to
rain. The bulk of the precipitation will move east Monday morning,
but light rain will continue to be a threat through Monday night
as the upper low passes over.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A wet and unsettled pattern is on tap for the southwest Mainland
as impulses of energy embedded in southwest flow move onshore. A
deep fetch of moisture spans from the Y-K Delta and south of the
AKPEN. A new low will close in on southwest Alaska bringing more
diffluent southwest flow and more dynamics to the region. Monday the
jet core will track directly over Bristol Bay and careen into
southwest Alaska with strong winds mixing down to the surface.
More warm air advection will accompany the next low and the
predominate p-type will be rain.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A 973 mb low south of Adak will continue to track north-northeast
bringing strong winds to the Aleutians. The surface low will move
east of the Pribilof Islands early Monday. The pressure gradient
with this storm is fairly concentric, anticipate gusty winds in
the warm sector and the back side of the low. This will bring
stratiform precipitation to the area, as well as gale and storm
force winds. The ensembles ping into some interesting anomalies
with the wind field relative to climatological norms. But with the
low slowly filling as it migrates north-northeastward, expect to
stay below the NPW threshold. In a nutshell, this storm will bring
wet and windy conditions to much of the Aleutian chain and Bering
Sea as it progresses northward the next two days.

Further west, Post Tropical Cyclone Saola rears its head west of
the Aleutians late in the period.

&&

.LONG TERM FORECAST (Days 3 through 7: Tuesday through Saturday)...

The quick succession of waves of precipitation continues into
Tuesday morning as the low that moves into Southwest Alaska on
Monday continues to impact the area. There is wide model
disagreement on where the low goes, but the most consistent
solution takes the upper low south across the Alaska Peninsula
into the North Pacific. Regardless, its presence near southern
Alaska should mean scattered showers for much of Southcentral
early in the day, then clearing out as strong ridging associated
with an anomalously strong upper level high moves over Southwest
Alaska by Wednesday morning. This will begin a prolonged period of
dry weather for nearly all of Southern Alaska.

The upper level high and associated warm air will keep winter on
hiatus through at least Thursday. The caveat with all the warm
air around is that it will effectively cap any low level moisture
in place which will make for widespread fog concerns, particularly
Thursday morning when temperatures aloft are at their warmest.
Beyond that, the upper level ridging looks to flatten out,
allowing a somewhat more seasonable air mass to return to
Southcentral, especially towards the Copper River Basin.
Meanwhile, the ridge and very warm air associated with it will
persist through Saturday and beyond over Southwest Alaska and the
eastern Bering. Thus, a continuation of warm daytime highs with
morning fog concerns is expected.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales...130 131 150 160 165 170 171 173 174 177
				 178 180 181 185 411 413 414.
		 Storms...155 172 175 176 179 412.

&&

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
LONG TERM...JW