Small Craft Advisory 681, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
404 PM AKDT Mon Sep 3 2018

.ANALYSIS AND UPPER LEVELS...
The large upper level ridge now extends from the eastern Bering
Sea through Interior Alaska and into the Yukon Territory. The surface
pressure pattern largely reflects this with the high center near
the Pribilof Islands and extending over Southern Alaska. A weak
trough over the Gulf of Alaska is also helping to keep north-to-
northwest flow over Southern Alaska. The subsidence associated
with the high pressure keeping precipitation out of most areas but
also an 'all-or-nothing' for the cloud cover. The eastern Bering
Sea is under widespread stratus due to this subsidence which is
pushing onshore in the Kuskokwim Delta until it gets hung-up on
the Kuskokwim Mountains. Southcentral Alaska is also protected by
the Alaska Range and Aleutian Range which is keeping mostly clear
skies over that region.

&&

.MODEL DISCUSSION...
Models are once again in good synoptic agreement for a number of
days. However, there is a rather noticeable difference from
yesterday's runs in the upper levels over Southcentral Alaska
later in the week. Both the GFS and ECMWF are showing a cold-core
low digging southward along the Alcan border and into the Gulf of
Alaska Thursday night into Friday. This will bring a chance of
precipitation to the Copper River Basin and some clouds to other
parts of Southcentral before the ridge reasserts itself again by
Friday night. How widespread the cloud cover will be over
Southcentral will largely depend on how strong this feature is
and how far west it tracks. For the 12Z models, the GFS was both
stronger and farther west than the ECMWF.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. There is a
chance for fog to develop around the airport late tonight as
there will be a little more low level moisture available. For now,
going with VCFG and a low SCT layer in the TAF, but it could
easily become IFR visibility and ceilings for a few hours early
tomorrow morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

Dry and unseasonably warm weather will continue across
Southcentral as a blocking ridge remains over the Bering Sea. The
fog that has been patchy the past couple of nights looks to
become more widespread tonight. This is due to the ridge axis
extending across southern Alaska tonight and the development of a
lee side trough over the Cook Inlet and Susitna Valley. This
surface trough will draw some additional moisture northward and
help pool low level moisture. Some low stratus could also make its
way up the inlet and push across far western portions of the
Kenai Peninsula, helping keep overnight temperatures a couple of
degrees warmer than previous nights. The ridge axis will then
shift into the northern Gulf on Tuesday as a series of upper level
shortwaves dive across the eastern half of Southcentral Alaska.
The first will push across the region Tuesday afternoon spreading
clouds and some isolated showers mainly over the Copper River
Basin. As this system passes, the gradient between a quick-
building ridge to the north and the trough to the south will
increase the offshore winds Tuesday evening through early
Wednesday. A second, stronger wave will push into the area late
Wednesday into Thursday producing more widespread clouds and a
better chance for showers, especially over the mountains.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Thursday)

As a former colleague of mine would say "big bubble, no trouble",
and that best sums up the weather for southwest Alaska for the
next couple of days. Strong subsidence under the ridge will
maintain dry conditions across the area while decreasing cloud
cover, with the least cloud cover expected near the Alaska Range
and the highest near the coast from Goodnews Bay northward. The
light winds and partial clearing may allow some patchy fog to
develop during the morning hours.

Things will begin to change somewhat on Thursday from north to
south, as a weak short-wave trough drops across the region,
increasing cloud cover some and perhaps allowing a few light
showers to develop.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Thursday)

A somewhat persistent pattern will exist through the next few
days, as a strong ridge of high pressure across the central
Bering/Aleutians slowly migrates off to the east, while a series
of lows ride north across the western portions of the sea/Aleutian
chain. Each disturbance will flatten the western edge of the
ridge, with precipitation potential increasing while also inching
eastward as each wave passes by. Sustained winds will increase to
gales ahead of the first low, with small craft conditions likely
with the second system.

Further east, look for some patchy fog to develop across the
eastern portions of the sea/AKPEN where subsidence will be
strongest, with fairly widespread stratus dominating most of the
Bering Sea.

&&

.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...

The long term forecast remains relatively unchanged as models are
all still in decent agreement showing a highly amplified pattern
over the area. An omega block forms over the Bering Sea as the
northern jet stream rides over the top of the Bering Sea ridge
and descends south across mainland Alaska. On Friday, energy
dropping south across eastern Alaska will move into the Gulf
forming a surface low in the NE Pacific. The low will help to re-
amplify the Bering Sea ridge increasing the pressure gradient
across Southcentral. This will lead to gusty outflow winds for
typically favored locations along the North Gulf Coast, including
Seward, Whittier, and Valdez Friday into Saturday.

The persistent upper level ridge over the Bering will lead to dry
northerly offshore flow, continuing the streak of pleasant early
fall weather across most of the mainland through at least early
next week. It is interesting to note this pattern is nearly
opposite the pattern we saw last month, where an upper low set up
over the Bering bringing predominately cloudy and wet weather with
only a few short breaks. Conversely, upper level ridging over the
Bering will promote predominately dry/sunny weather with only a
few short periods of cloudy/rainy weather over the mainland. Enjoy
it while it lasts!

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales...175 176 177 413.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...KVP