Small Craft Advisory 499, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 AM AKDT Wed Jun 28 2017

.ANALYSIS AND UPPER LEVELS...

A nearly stationary vertically stacked low continues to spin
across the central Bering this morning. Across mainland southern
Alaska, the low is drawing moisture and precipitation northward
into the Gulf and the southwest. Frequent impulses of energy
embedded within the southerly flow have been developing into weak
low pressure centers over the North Pacific. Upon nearing the
Gulf, the lows have been the cause for the extensive cloud cover
over the area with precipitation along the coast.

The next low is heading towards Kodiak Island and the
southeasterly winds have been picking up across the Gulf out ahead
of it. The proximity of the low to southern Alaska will keep the
area socked in mainly cloudy skies with precipitation along the
coast and windward sides of the mountains through the end of the
week. By the time these lows reach southern Alaska, any
baroclinicity has been thoroughly mixed out and the extensive
cloud cover already set up over the area have been keeping
temperatures across the area relatively constant.

&&

.MODEL DISCUSSION...

The models are in good agreement through Thursday, with large
enough disagreements developing beyond that regarding the
evolution of the next low beyond the current one to move towards
the Gulf. Through Thursday, the NAM was largely favored for its
better handling of local effects when all the models agree. After
that time, a transition to the GFS was used as the middle ground
solution keeping a tighter circulation with the second low through
Friday. Through Thursday, forecast confidence is high, then
diminishes beyond that.

&&

.AVIATION...

PANC...VFR conditions will persist through Thursday morning. Off-
and-on rain showers will diminish early this morning, and a mainly
dry day is expected Wednesday before rain showers return Wednesday
night, which will persist into Thursday morning. The Turnagain Arm
Jet will persist south of Anchorage through Thursday morning, with
the best chance of stronger winds and gusts associated with it on
Wednesday afternoon. Stronger down-Inlet flow will develop
Wednesday evening and night, which will push the jet further
south away from the airport. Confidence in stronger winds and
gusts at the airport Wednesday afternoon is low.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Weak high pressure will build over south central Alaska this
morning, with shower activity diminishing along the coast and
ending for inland areas. A low moving north into the western Gulf
of Alaska today will push another round of rain and showers to the
coast this afternoon, and then inland this evening and tonight.
The upper low will weaken as it pushes towards Kodiak Island on
Thursday. This will stall the moisture over south central Alaska,
with a continuation of showers over the area through Friday
morning. At the surface, the coastal ridge will strengthen
today, with breezy conditions expected through the Copper River
basin, the Knik valley and Turnagain Arm. Anchorage may see some
winds this afternoon and evening as the Talkeetna to Anchorage
gradient becomes neutral to near negative. The coastal ridge will
then weaken a bit on Thursday.

&&

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

A series of shortwave troughs associated with a surface low
anchored over the Bering Sea are moving through the western part
of the state leading to widespread rain across most of southwest
Alaska today. Persistent southerly flow will provide a continued
influx of moisture leading to cool, cloudy, and wet conditions
through tonight. As the surface low begins to drift northwest
across the Bering Strait on Thursday, southerly flow weakens
leading to a brief break in precipitation late Wednesday night
through Thursday morning. A low pressure system in the southern
Gulf of Alaska will move north during the day on Thursday
bringing a subtropical influx of moisture to the region. Strong
easterly cross barrier flow will develop as the system moves in
leading to downsloping and decreased precipitation chances on the
western/leeward side of the AKPEN, though heavier amounts of rain
are expected on the eastern/windward side. Moisture is still
expected to make it into mainland southwest Alaska during the day
on Thursday leading to another round of precipitation for the
area. Winds will be gusty as easterly flow becomes funneled
through terrain gaps and down into the Kuskokwim River Valley.

&&

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

The low pressure system over the Bering will persist for the next
couple of days before slowly weakening and drifting off to the
northwest. Winds along the Aleutian chain will subside some as the
low moves away and the pressure gradient weakens. Light rain
showers/drizzle should persist along the Aleutians as weak
shortwave troughs move through along the base of the parent low. Another
low pressure system develops south of Attu by Thursday morning
and begins to track east along the chain bringing periods of rain
to the western and central Aleutians throughout the day, though
winds associated with this system should remain relatively light.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The weather pattern for the long term continues to trend towards
wet and rainy for both the Gulf and the Bering Sea. By Sunday, the
lows currently in the Gulf and the low moving into the Bering will
weaken, but strong ridging is not expected to set in and clear
things out. Aloft, by Sunday the jet stream will be in a zonal
pattern through the Gulf of Alaska, with a stronger jet streak
moving southward from the Kamchatka Peninsula into the western
Bering Sea. This pattern will allow for development of some
surface lows which will move eastward into the Gulf of AK and
southward from Russia into the Bering Sea. These low pressure
systems are not expected to be unseasonably windy, but will bring
low clouds and rain to most of the region, with some land areas
perhaps getting a short break with downsloping. Although all the
models agree on a generally unstable pattern, the timing and
strength of lows moving into the Bering and the Gulf vary quite a
bit. Therefore, the current forecast trends toward little change
with some blend of the WPC solution, especially as we head into
Day 7 and 8. All in all, the outlook for 4th of July weekend
across the southern mainland and Aleutians is rather cloudy and
damp.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE... Gale: 120 130 131
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...LF