Small Craft Advisory 523, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
834 PM AKDT Sun Jul 9 2017

Updated to include extended discussion

.ANALYSIS AND UPPER LEVELS...
The easterly wave that brought some locally y heavy rainfall to
Southcentral Alaska yesterday has moved into the Kuskokwim Valley
and Bristol Bay zone today. The low in the Bering Sea near the
Pribilofs has a front extending along the Kuskokwim Delta and into
Bristol Bay which is making and interesting squeeze on the ridge
between these features. They are actually close enough to each
other that the King Salmon Radar shows both the front and the wave
on it with a small break between them. This is slowing the
progress of both systems and is causing the front to begin to
regress back southward.
Southwest Alaska's airmass is supportive of stratiform rainfall
and not convection with these systems. Southcentral Alaska has
some instability at lower levels which will allow some showers to
develop where there are breaks in the clouds or forced by
mountains. However there is not enough support for thunderstorm
development so only showers are expected.

&&

.MODEL DISCUSSION...
Models are in good synoptic agreement through the middle of the
week leaving the pattern of upper level disturbances the main
issue to watch. Of particular note is the disturbance currently
over Southeast Alaska that will be heading into the Copper River
Basin tomorrow. The GFS and Canadian continue to be faster than
the NAM and ECMWF. Part of the issue is the ridge being squeezed
over the Susitna Valley into the Gulf of Alaska which will push
this wave a little farther northward before entering the Basin and
then the question becomes how far west does it make it? The GFS
and ECMWF have it reaching the Susitna Valley Tuesday and possibly
into northern Cook Inlet, but the NAM and Canadian are holding it
farther east.

&&

.AVIATION...
PANC...VFR conditions will persist. Southeast winds will increase
this evening and through the overnight hours before diminishing
again early Monday morning.

&&

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

A beautiful day has taken shape across Southcentral this afternoon
as the area lies in the subsidence in between weather systems. The
easterly wave that tracked across the area and brought substantial
wetting rains to the area has continued to push westward into
western Alaska. However, a portion of that energy has merged with
a slowly moving front pushing into Kodiak Island. The additional
atmospheric lift from those features, still plentiful atmospheric
moisture, and sunshine through the morning and early afternoon has
resulted in shower activity, briefly heavy across parts of the
western Kenai Peninsula, which happens to be the only non-
mountainous terrain in the vicinity of the front. The remainder of
the area, including Anchorage, the Mat-Su Valleys, and parts of
the Copper River Basin don't have as much lift, and as such are
just seeing fair-weather cumulus clouds.

The front near the southern part of Kodiak Island will stall along
the middle of the island, so the entire area will get a steady
soaking rain from tonight through Monday evening, at which point
an upper level low and its attendant surface low racing eastward
out of the Bering will quickly take all of the precipitation along
with it into the Gulf.

Across Southcentral, the next features to watch will be additional
easterly waves tracking in from Yukon. There remain considerable
differences between the forecast models as to how much rain will
fall over the Copper River Basin, which portions of the area will
get rain, and when it will start and end, but the general
consensus has it starting Monday night and ending late in the day
Tuesday. Furthermore, more models than not confine the
precipitation to the Copper River Basin, with little "slop-over"
onto the west side of the Talkeetnas and Chugach Mountains. Thus,
from the Kenai Peninsula north through the Susitna Valley, look
for a mostly cloudy, but dry day Tuesday.

By Wednesday, weak ridging building eastward will make for much
more sunshine and more summer-like temperatures, though with that,
instability increases markedly. That means at least a chance of
afternoon/evening thunderstorms returns to the forecast since less
in the way of lift and forcing will be needed in that expected
environment to trigger convective development.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An easterly wave aloft will continue to bring widespread rain from
the Kuskokwim Delta through Bristol Bay. The disturbance will
merge with a front approaching the coast tonight before both
dissipate tomorrow afternoon. As both systems slide into the Gulf
on Tuesday, the mainland is left in a drier northwesterly flow
with decreasing clouds and precipitation chances. On Wednesday, an
upper level ridge builds overhead, bringing above average
temperatures and clear sky to the region.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A dissipating front and low pressure system will continue to
weaken and slide into the Gulf on Monday leaving the eastern
Bering in northwesterly flow. A new front and low pressure system
moves into the western Bering early Monday morning, while slowly
pushing the front into the Central and Eastern Aleutians through
the remainder of the week.

&&

.LONG TERM FORECAST (Days 3 through 7...Wed through Sun)...
Models are continuing to come into agreement regarding a high
amplitude ridge building across most of mainland AK by the middle
of this week. A low pressure system over the Central Gulf of
Alaska will move east towards SE AK allowing a closed high over
Northern AK to merge with an amplifying ridge over the Eastern
Bering. This will lead to a large SW/NE oriented ridge building
across most of the state by Wednesday. Sunny skies and dry
offshore flow will lead to very warm temperatures across most of
Western and Southcentral AK during the later part of this week,
which could produce the warmest temperatures so far this season.
In fact, high temps will likely approach or break the 80 degree
mark for locations in the Susitna and Copper River Valleys on
Thursday with mid to upper 70's seen in other locations across
southcentral AK, including the Anchorage Bowl. However, with these
warm temperatures, there will also be the potential for some
afternoon showers and thunderstorms to develop. These will mainly
be over the higher terrain and very isolated in nature, but can't
rule them out entirely at this time.

A deep trough over the Bering will keep rain chances high over the
Aleutians. It will also bring a healthy slug of warm air which
will continue to produce widespread marine stratus and fog.

Confidence decreases towards the end of the week regarding how
long the ridge will persist over the state. A low over the
northern Bering and Gulf of Alaska move towards and act to break
down the ridge by the weekend. The GFS is more aggressive in
bringing in the two systems with precipitation moving into SW and
southcentral AK by Saturday. The EC is more optimistic with the
ridge staying intact over the state through the weekend.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...JPW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MTL
LONG TERM...KWV/MSO