Small Craft Advisory 399, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
447 PM AKDT Thu May 18 2017

.ANALYSIS AND UPPER LEVELS...
The main upper level feature over southern Alaska is the low
centered near Kodiak Island with a broad trough extending
northward into the interior of the state. The surface low
associated with this upper level low is weak and nearly
vertically stacked. However, it is still enough to give lift for
widespread rain over the Gulf region. Numerous shortwaves moving
in the weak south to southeast flow over southcentral. These waves
have been enough to keep rain over a good part of southcentral as
there has been little to no downslope to diminish the rainfall.

A strong jet stream which is peaking at over 190 kt is running
south of the Aleutians and feeding a strengthening low in that
area. This low will stay south of the forecast area.

&&

.MODEL DISCUSSION...
While models remain in good synoptic agreement into the weekend,
they are struggling with the nuances that are producing the
precipitation in the Cook Inlet region. Hi-resolution NAMnest
appears to once again best choice for localized gap winds.

&&

.AVIATION...
PANC...Predominantly VFR conditions will persist with an outside
chance for a brief MVFR ceiling Friday morning. Southeast winds
should increase some this afternoon and evening but not as strong
as the past few days.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Cloudy, cool, and moist conditions will continue through Friday as
a series of disturbances move through the area. Due to widespread
clouds/rain today over the Susitna Valley, there is some question
in how strong the southeast winds will be around the Anchorage
Bowl this evening, with recent guidance suggesting that it will
be much weaker and short lived than the last two evening. The
forecast was trended down to reflect that, however there is a
chance that the thermal low over the interior will not be strong
enough to support to bring winds into town at all. Otherwise rain
showers will continue over the Anchorage Bowl and Matanuska
Valley this evening before diminishing during the late evening as
downsloping ahead of the next low moving into the Gulf develops.
This will be a brief period of downsloping as another round of
showers will then move into the area Friday morning, followed by
generally cloudy and showery conditions that will persist into
Saturday. Slightly warmer and drier conditions will then spread
into most areas late in the day Saturday as weak high pressure
moves overhead.

&&

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

An upper level low south of Kodiak will continue transporting
moisture and a series of waves of precipitation into Southwest
Alaska over the next couple days. Thus, except for possibly the
northern portions of the Kuskokwim Valley and Delta, cloudy skies
will continue to be the norm. With each wave of moisture,
scattered showers are likely to develop over much of the region,
especially the closer to the Alaska Range you go. The upper level
low will drift towards the Bristol Bay region by Saturday
afternoon, which will only prolong this dreary weather pattern.

&&

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

Fog and low stratus will remain dominant across nearly all of the
Bering Sea from this afternoon right into Saturday. Continued
northeasterly flow throughout the Bering coupled with an upper
level ridge, will keep the weather pattern locked in place. The
warm air associated with the upper level ridge overriding the
cool, moisture-laden air near the surface promotes widespread
stratus and fog formation and maintenance. Elsewhere, a North
Pacific low is forecast to track well south of the Aleutians on
Friday, but the northern periphery of its wind field will kick up
small craft winds and seas as it passes by.

A strong front moving into the western Bering on Saturday will be
the next system to impact the region, and will dissipate the
fog/stratus as it moves through.

&&

.LONG TERM FORECAST (Days 3 through 7/Saturday night through Thursday)...
A highly amplified pattern with support from a robust subtropical
jet over the North Pacific will lead to active weather over the
next week. Beginning Saturday night, a short-wave and surface
front lifting northward across the Gulf of Alaska will spread rain
to the north Gulf coast/Prince William Sound region by Sunday
morning, with clouds and showers spreading inland during the
remainder of the day. With deep southerly flow aloft (up the west
side of a western North American ridge) expect abundant moisture
transport leading to moderate to heavy rain along the coast.

Meanwhile, the strong subtropical jet will drive a trough eastward
across the Bering Sea, with the leading edge reaching Southwest
Alaska on Monday. There are some differences in the handling of a
cut-off low out ahead of this as well as with track of individual
short-waves moving through the long-wave trough. This produces a
blurry picture of what may happen over Southwest and Southcentral
Alaska Monday through Tuesday. However, most solutions point
toward a pretty wet scenario, with potential for widespread steady
rain over Southcentral.

As the initial trough exits, medium range solutions are now
trending toward the ECMWF, which has been depicting amplification
along the Aleutians, with a ridge building into Southwest Alaska
Tuesday and Southcentral Alaska Wednesday. If this does verify,
would expect a day of really nice weather. However, the next
trough will quickly follow bring clouds and rain back into the
forecast.

For the Bering Sea and Aleutians, a continuous supply of short-
waves transiting the upper low/trough will produce unsettled
weather, with occasional rain and wind region-wide.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...SB