Small Craft Advisory 836, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
426 PM AKDT Thu Nov 2 2017

.ANALYSIS AND UPPER LEVELS...
A strong upper level high is centered over the southwest Gulf of
Alaska. The high is flattening a bit as the jet stream to the
north of it is pushing southward as a trough is moving eastward
across the northern Bering Sea. This trough is picking up some
moisture that is streaming north across the central Aleutians and
central/eastern Bering. There is an upper level low over the
Kamchatka Peninsula. A front associated with this low is moving
into the far western Aleutians, which is weakening and moving
towards the north.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement through the short term
portion of the forecast (Saturday afternoon). There is some
disagreement beginning Saturday afternoon with a low south of the
Alaska Peninsula and eastern Aleutians that is moving in from the
southwest. Fog and stratus continue to be an issue that the
models do not handle real well.

&&

.AVIATION...
PANC...IFR to LIFR conditions in fog are expected through tonight.
Stronger flow and cooler air aloft will break things up Friday morning
which will bring in VFR conditions.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Fog will continue to be an issue along the Cook Inlet and
Anchorage Metro Area through tonight. Breaks in the fog were
observed earlier today. With the ridge breaking down over the
region and shortwave energy gliding across the Mat-Su Valley
tonight and tomorrow morning expect the fog coverage to be
reduced. Also as the shortwaves move into the area, the likelihood
of inversions or at least strong inversions wanes which also
plays into the notion of a reduced fog trend. That said, without a
definitive scouring mechanism in the boundary layer pockets and
areas of fog will possibly linger Friday. By Saturday,
anticipating less fog potential.

With the ridge over Kodiak, expecting mild conditions there and
over the southern portion of the Kenai Peninsula this forecast
period. Elsewhere, the energy bisecting eastern Alaska is
expected to bring some showers to the Copper River Basin with high
elevation snow possible.

&&

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

Increasing chances for precipitation and with potential fog
development continues to be the main focus for southwest Alaska.
Currently, a weakening front is moving towards the Kuskokwim
Delta, with the Bethel radar showing precipitation moving into the
upper portions of this region. This trend is expected to continue
overnight, with rain being the predominant precip type near the
coast. However, as one travels east, enough cold air will fill
into the area for rain to switch to snow for location generally
north and east of a line from Bethel to New Stuyahok.

Further south towards Hooper Bay and and upper portions of the
Alaska Peninsula, with the ridge aloft holding firm for another
12 to 24 hours, fog development looks likely once again tonight.
However, visibilities look to be higher compared to previous
nights, with dense fog likely not being an issue. Otherwise, look
for temperatures to be near seasonal values through the period.

&&

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

A weakening front currently stretches from just east of Adak
northward into the central Bering Sea. Widespread rainfall has
developed along this feature, and this will continue to be the
case through the next 24 hours as it slowly nudges east while
weakening. Further east, some fog development is expected from
Cold Bay east along northern portions of the peninsula tonight, as
850-mb temperatures aloft hold in the 6 to 8 degrees Celsius
range, maintaining the strength of the inversion.

Attention then turns to an approaching warm front for Friday into
Saturday. A moisture plume of tropical origin will surge northward
toward the chain, with widespread rainfall developing. Some of the
rainfall will be locally moderate in intensity at times, and some
locations could easily see between 1/4 and 3/4 of an inch of
rainfall by Saturday evening. Fog development looks likely as
well, given the moist airmass surging north while aided by
orographically-induced effects.

&&

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

Active weather is on tap for this weekend as a trough over the
west coast of Alaska digs into the Mainland. A weak surface low
over the southern Alaska Peninsula Saturday night will track
northeastward into the southwest Gulf, where it will then phase
with a strong shortwave dropping south. This shortwave along with
favorable jet dynamics aloft will provide the upper-level support
for the weak surface low to rapidly strengthen over the southern
Gulf Sunday. At this time, the shortwave will break off and form a
closed low atop the surface low. Models are struggling coming to
a consensus on the track and strength of the now vertically-
stacked low, however expect a stormy Sunday and Monday in the Gulf
with winds generally subsiding Tuesday and Wednesday.

The upper-level shortwave dropping down from the north and the
surface low moving in from the southwest will make for a tricky
precipitation forecast this weekend. QPF amounts and timing will
be highly dependent on the track of the low and the timing of the
wave. However, the setup for a quick shot of fairly widespread
precipitation over Southcentral Alaska and the Gulf Saturday night
to Sunday morning continues to look increasingly favorable with
subsequent model runs. With cold air quickly pushing southward and
rapid height falls aloft, expect the majority of the
precipitation over inland areas to fall as snow, with some mixed
precipitation and rain possible along parts of the northern Gulf
Coast.

Over Southwest Alaska and the Bering Sea, it looks like a pattern
change is in store starting late Sunday. The high amplitude upper
level ridge that had been present over the past week will finally
break down as a series of shortwaves begin to erode the longwave
ridge. As this happens, colder air will begin to filter out of
Russia and move back over the Bering Sea which should re-introduce
convective showers to the Bering Sea while a series of
disturbances will push across the Bering Sea. It's safe to say the
docile period highlighted by fog and warmer weather will be
coming to end by early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...PS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...RD