Small Craft Advisory 580, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
444 AM AKDT Tue Aug 8 2017

.ANALYSIS AND UPPER LEVELS...
The synoptic pattern features broad cyclonic flow extending from
the Bering Sea to the North Pacific. In satellite imagery, there
are three distinct features including the remnants of a frontal
boundary that are south of the AK peninsula, a new system pushing
into the Western Aleutians and a cold front draped over the north
slope. The vortex south of the AK peninsula continues to kick
vort lobes out of it...ushering precipitation into the central
Aleutians...AK peninsula and south central.

&&

.MODEL DISCUSSION...
Models are in good agreement with the longwave pattern with a pair
of lows impacting the Aleutians and the ridge holding steadfast
over SE AK in the short term. However, a major transition is
gearing up as Arctic air begins to push south of the Brooks Range
this weekend. Both the GFS and the ECMWF depict this intrusion of
cold air.

&&

.AVIATION...
PANC...There is an area of disturbed weather that is moving
towards PANC...but the models keep the CIGs above 5,000 ft.
Cautiously anticipate VFR conditions to persist. The gradient
associated with this system merit keeping gusty winds in the TAF.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
With south to southeast flow aloft over Southcentral Alaska the
next few days there will be continued periods of rain along the
coast with some of it occasionally spilling over the mountains to
the inland areas. A front moving into the Gulf of Alaska will also
increase winds to Gale force around Kodiak Island, particularly
in Shelikof Strait and the Barren Islands. These strong winds will
go a long way to keeping the precipitation on the windward side
of the mountains with downslope protecting the leeward sides of
the kenai and Chugach ranges. As the winds diminish late Wednesday
into Wednesday night the possibility of rain making it into
Anchorage, the Mat-Su and western Kenai Peninsula increase. The
Copper River Basin very well may not see much if any precipitation
as the areas of deep moisture will be west of there. This will
help keep temperatures warm in the Copper River Basin but there
should be enough increased moisture to help increase the Relative
Humidity today.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The showery pattern over the mainland will diminish today as
instability wanes and a front approaches from the Pacific. An
upper jet will punch in with the front, carrying above average
moisture for this time of year. Widespread rain will affect the
Bristol Bay area this evening into the overnight hours. The low
pressure system that is responsible is very fall-like, and will
contain a moderate amount of cross-barrier wind behind the front.
The wind will create some downslope drying along the
Alaska/Aleutian Range. The main upper level disturbance will
continue to curve into the Bering Sea, taking the bulk of the
precipitation there as well. The mainland will be left in a deep-
layer southeasterly flow on Wednesday and Thursday, leading to a
showery/sprinkle pattern.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Two different low pressure systems will enter the Bering, one from
the northwest, another from the southeast. The front to the west
is weakening, hence no major wind or precipitation issues, while
the one entering from the southeast will be stronger, bringing
widespread precipitation in on late Tuesday night. Otherwise,
another normal day in the Bering...low stratus and fog.

&&

.LONG TERM FORECAST (Days 3 through 7/Wednesday night through
Monday)...
Beginning Wednesday night the large upper level low is lifting
northward in the vicinity of the Alaska Peninsula, with a short-
wave on the east side of the low lifting northward across the
Gulf and Southcentral. There are model differences in timing and
amplitude of the short-wave. In any case, with weakening low level
flow over Southcentral, rain looks likely during this period for
all except the Copper River Basin. Some of this rain will also
spread into interior Southwest Alaska. Otherwise, expect showery
conditions for the eastern Bering/SW Alaska region underneath the
upper low. As we head into the weekend this upper low begin to
exit across the Gulf/NE Pacific, with lingering clouds and showers
over the southern mainland.

Models are in quite good agreement with the large scale evolution
through the weekend and into early next week, though as is usual
there are some key differences in the details. A deep and cold
trough will drop out of the Arctic and interact with another
closed low along the Aleutians. The extent to which individual
short-waves phase (or not) will determine the mean trough position
and areas most likely to see rain. Models tend to take a while to
lock on to timing and phasing details for these types of scenarios,
so expect continued large variation with each new model cycle.
Based on the larger scale pattern, it is generally favorable for
deep south to southwest flow into mainland Alaska, so there is
potential for much wetter weather by early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

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