Small Craft Advisory 523, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
507 PM AKDT Fri Jul 6 2018

.ANALYSIS AND UPPER LEVELS...
An extra-tropical low pressure system has moved into the western
Bering Sea and is making its way towards the western coast of
Alaska. The negatively tilted, vertically stacked low is sitting
in the left exit region of the jet stream where jet streak winds
are reaching 130 kts. This is deepening the low as it is being
supported by the jet stream and the enhanced 500 mb vorticity in
the area. Winds are approaching gale-force levels along the
Aleutians where Atka is already experiencing 35 mph winds at the
base of the trough. Marine stratus, produced by the front
extending from the low, is becoming more widespread as the front
traverses eastward. Aloft and at the surface, the low has acted
as an anchor which has allowed a wide ridge to build in the east
over much of southern Alaska. This ridging pattern is continuing
the clear skies and dry conditions in Southcentral and the
surrounding areas that we have experienced this week.

&&

.MODEL DISCUSSION...
Models remain in good agreement which results in fairly high
forecast confidence through the weekend. There are a few detailed
elements where there is some uncertainty.The main one is some
disagreement in the amount of convection that will develop around
Southcentral this afternoon and evening, especially over the Kenai
and Chugach Mountains. Models have trended slightly drier as the
ridge builds in, but it may not be enough to suppress convection
over the mountains. There remain some slight timing differences
with the large front pushing from west to east this weekend, but
in general, all signs continue to point towards a major pattern
shift.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through midday
Saturday. A pattern shift will then ensue, bringing a return to
gusty southeast winds coming off of Turnagain Arm. These winds
will develop saturday afternoon then steadily strengthen as we
head through Sunday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
A weak ridge aloft and some low level offshore flow was enough to
push temps back into the 80s across the interior this afternoon.
Today will be the last "hot" day before slowly cooling off
Saturday and Sunday as a front approaches from the Bering Sea.
Tomorrow, ahead of the front, Southcentral will begin to
experience increasing southeast winds as the pressure gradient
builds along the coast. This surge of low level winds may help
support thunderstorm development over the interior mountains and
Copper River Basin. This will also be the last warm and nice day
before transitioning to cloudier and cooler as the Bering front
moves into the Gulf Saturday night. Southeast winds look to ramp
up dramatically across the interior including Anchorage. Expect
steady rain along the coast with shower threats inland through
Monday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
Clear and dry conditions will continue for the interior of the
southwest mainland through the first half of Saturday as a ridge
of high pressure continues to slowly build across central Alaska.
Along the coast, marine stratus will begin to diminish in scope
this evening as both drier air works in from the interior and an
onshore flow transitions to a southeast wind ahead of an advancing
front.

The ridge over the central mainland will quickly slide east
Saturday morning as a strengthening area of low pressure moves
quickly across the Bering Sea. Clouds and a band of light to
moderate rain will advance along the Alaska Peninsula Saturday
morning and push into the southwest interior by Saturday evening.
By Sunday morning, as the low moves to a position just south of
Nunivak Island, effects from this storm system will be felt all
the way from the coast through the Alaska and Aleutian Ranges.
Expect markedly cooler temperatures, gusty SE to SW winds, and
widespread steady rain with this system through at least Tuesday
as the low slowly drifts onshore.

High surf and minor coastal erosion from an increase in storm
surge is expected Sunday afternoon through Monday as an
unseasonably strong mid-summer storm strengthens over the Bering
Sea. As the low pressure system moves slowly east toward Kuskokwim
Bay, it will leave much of the coastline exposed to a long
duration west-southwest flow. Tides are relatively low during
this time period, so this should help to mitigate impacts.
However, with building seas and small craft southerlies, expect
high surf along Kuskokwim Bay and Bristol Bay coastlines. A
Special Weather Statement has been issued to highlight this
threat.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
Patchy fog and low stratus will remain over the central and
eastern Bering tonight ahead of the low crossing the western
Aleutians. Elsewhere, rain continues to spread east along the
Aleutians today as the low pushes a front into the southern
Bering. Winds to 30 knots today will increase to gale-force for
Saturday and Sunday along the central Aleutians and southern
Bering Saturday as a second shortwave dives south from Siberia
and is absorbed into parent low, allowing it to further
strengthen as it tracks east. By Sunday afternoon, the low will
begin to fill in over the eastern Bering as it becomes vertically
stacked. Expect plenty of clouds and showers across the eastern
half of the Bering through early next week - with a reprieve from
the unsettled weather over the western Aleutians - as the trough
axis remains near the coastline of western Alaska.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Model forecasts remain consistent with a trend toward a
significantly wetter and stormier pattern for next week. By
Monday, the rather large vertically stacked low will be centered
along the Kuskokwim Delta coast with broader upper level troughing
extending over the Bering, Southwest Alaska and the western Gulf.
In addition to a series of shortwave troughs rotating through
Southwest Alaska (bringing abundant precipitation to that area),
southeasterly flow and frontal systems in the Gulf will keep the
coastal areas quite wet with cool, cloudy and periodically damp
conditions inland as well. The entire upper low will shift gradually
north and east Tuesday and Wednesday, opening to more of a broad
and less consolidated trough with conditions transitioning to a
cool, showery regime. After a possible short break as a flat ridge
moves through southern Alaska Wednesday night or Thursday morning,
a Siberian upper low and north Pacific trough will phase and
merge to bring the next rather wet system into Southwest Alaska
Thursday and Thursday night and then into Southcentral through
Friday.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...RA/EZ
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
LONG TERM...JR