Small Craft Advisory 529, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
429 PM AKDT Fri Jul 19 2019

.ANALYSIS AND UPPER LEVELS...
In the upper levels a broad ridge encompasses most of the Bering
Sea and Aleutians. A second ridge over western Canada is building
westward into south central Alaska. To the north a deep closed
low is spinning over the Beaufort Sea with a weaker closed low to
the south over the Gulf of Alaska. A weak short-wave trough is
dropping into the col area between the two ridges, across the
Kuskokwim Valley and toward the western Alaska Range.

As far as the sensible weather at the surface, stable conditions
are producing widespread low clouds across the Bering Sea,
Aleutians, and much of southwest Alaska. There are some clouds
and a few showers along the Gulf of Alaska coast, but the bulk of
rain with the low remains offshore. South central Alaska started
out the day mostly clear, but airmass instability (and perhaps to
a lesser extent some weak upper level waves embedded in upper
flow) is leading to development of showers and thunderstorms along
mountain ranges, particularly from the Talkeetna Mountains north
to the Alaska Range.

&&

.MODEL DISCUSSION...
Models remain in pretty good agreement with the evolution of the
large scale pattern. One of the biggest challenges through the
weekend will be the cloud cover forecast for south central
Alaska. The atmosphere will trend more and more stable, with some
favorable indicators of potential for formation of low clouds in
the Cook Inlet region (including Anchorage). The cloud forecast
will of course affect the temperature forecast as well. Therefore,
confidence in these forecast parameters is low. The overall
forecast confidence for the remainder of the area as well as for
precipitation over south central Alaska is above average.

&&

.AVIATION...
PANC...The forecast for the next 24 hours is complicated. First
off, an increase in southerly flow, with both a Turnagain wind and
strong up-Inlet flow, will likely transport more smoke into the
Anchorage area tonight through Saturday morning. With the presence
of the Turnagain wind for much of the night, any surface inversion
which does form should be weaker than the past couple mornings, which
works against really low visibility from smoke. We do think there
will be a period of visibility reduction from smoke sometime
Saturday morning.

There are also some favorable signs for development of a marine
layer, with sub-VFR ceilings. The air mass is already fairly
moist and we will continue to further stabilize the low to mid
levels of the atmosphere. Working against this is the presence of
the Turnagain wind for much of the night and potential for some
mid level clouds from afternoon/evening convection over the Kenai
and Chugach Mountains to drift over Anchorage. For now, we have
left out any indications of low clouds in the TAF and focused on
the aforementioned smoke.

&&

.FIRE WEATHER...
Fire weather concerns over southern Alaska will continue to
diminish through the weekend as the atmosphere becomes more and
more stable with increasing ridging aloft, increasing clouds, and
cooler temperatures and increasing humidity. Isolate thunderstorms
will be focused today over interior south central Alaska, particularly
from the Susitna Valley to the Copper River Basin. A few storms
will also be possible over the Copper River Basin on Saturday,
while Sunday looks stable enough to preclude storm development.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Not much change to the forecast today. Showers and thunderstorms
will continue to form over the ranges through the evening. By
tomorrow the main source of instability will move further eastward
over the Copper River Basin. The main question for tonight is
whether the southwest flow in the low levels will really cloud in
Southcentral. Current thinking is that a midlevel deck will form
overnight, but dissipate by tomorrow afternoon. This should give a
cool start to Saturday. On Sunday, skies look a little sunnier
and this should push a return to warmer temperatures again.

Smoke also remains a concern around Southcentral. The main smoke
source continues to be the Swan Lake Fire. Southwesterly flow in
the evening will become more southerly overnight, pushing smoke up
into the Anchorage area. Anchorage area residents should not be
surprised to wake up to the smell of smoke again tomorrow morning.

&&

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3; Tonight
through Monday evening)...

Mostly cloudy skies are expected across southwest Alaska through
Monday ahead of a weakening ridge. Weak onshore flow will likely
continue the fog along coastal areas, potentially moving further
inland Saturday and Sunday mornings.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3;
Tonight through Monday evening)...

Gusty northwesterly winds are presently occurring in bays and
passes generally east of Unalaska. These will gradually diminish
through Saturday with generally light winds and low seas
persisting through the forecast period across much of the region.
Drizzle and fog are likely through Monday under a ridge for the
AKPEN and eastern Aleutians. A weak low will move near Adak on
Sunday and track west to east across the western and central
Aleutians through Monday, bringing widespread showers.

&&

.MARINE (Days 3 through 5; Monday through Wednesday)...

Sub gales are expected to persist through Wednesday over the Gulf
of Alaska waters. A front moving into the western Bering Sea
Monday may result in near Gale force wind west of 180.

&&

.LONG TERM FORECAST (Days 4 through 7; Tuesday through Friday)...

Long range models are generally in good agreement through
Wednesday then begin to diverge. The overall general concept of
low pressure in the north central Bering Sea with a trough over
the Bering Sea and moist onshore flow into western Alaska is
consistent between the models. The Bering Sea and Aleutians will see a
continuation of lows tracking across the region.

The mid to long range for the Gulf and southern mainland looks to
be under generally weak low pressure over the Gulf with weak
ridging over the southern mainland. The models are in good
agreement with the upper level high moving into the Yukon
Territory with an upper level low over the Gulf of Alaska. This
pattern is indicative of a return to more seasonal temperatures
and precipitation over the southern mainland for the middle to
later part of next week.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...LF
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...SA