Small Craft Advisory 520, NWS Anchorage, AK

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

.ANALYSIS AND UPPER LEVELS...
A ridge extending from a Russian Arctic high bisects Mainland
Alaska from the Northwest Capes through Southcentral. This ridge
continues to push warmer temperatures over many parts of the Mat-
Su valleys and Kenai Peninsula. Some low stratus/fog is trapped
underneath it along the North Gulf Coast, but should dissipate as
the ridge moves east through today. A short wave brought showers
and some thunderstorms overnight to sections closer to the AlCan
Border, Eastern Alaska Range and Copper River Valley. A front in
the Central Gulf decays through today. In the West, the extra-
tropical remains of TS Prapiroon is starting to merge into the
trough over the area. The result is low clouds, rain and gale-
force winds from the Western Aleutians into the Bering. There were
even thunderstorms and lightning observed over the Western Bering
last evening. Marine stratus and areas of dense fog continue to
plague the Southwest Alaska coastal zones.

&&

.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 first is with the
marine layer/stratus hanging around the coastlines, especially
over the North Gulf. Hand edits were mostly to handle it. Then there
is some slight disagreement in the amount of convection that will
develop around Southcentral this afternoon, especially over the
Kenai and Chugach Mountains. Models have trended slightly drier as
the ridge builds in, so we have trended PoPs down. Finally, there
are some slight timing differences with the large front pushing
from west to east late Sat-Sun. The NAM is slightly slower than
the other models, but in general, all signs continue to point
towards a major pattern shift.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
High pressure will build over the area today. This will bring weak
offshore flow at the low levels and some warming of the
atmosphere. This will also suppress convection, with what few
afternoon showers and thunderstorms develop confined to the
mountains. On Saturday the ridge will begin to slide southeast as
a deep upper low approaches from the west. The surface flow will
become onshore and strengthen, with increasing southerly winds
across the area, and afternoon temperatures will cool a bit. The
thermal trough will shift inland, and some cooling aloft will
allow for the development of some afternoon and evening
thunderstorms for portions of the Susitna Valley and Copper River
basin.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3...Fri
through Sun)...
Today and much of Sat will see dry conditions with some sunshine
before the front brings the pattern change late Sat. Some low-
mid clouds continue to linger around interior areas with some low
marine stratus and patchy fog holding to the coastlines. These
should slowly break up today giving way to a fair amount of
sunshine. But then all the focus will shift to the approaching
front over the Bering.

Clouds will start to spill in through the day Sat with southeast
winds starting to ramp up through the afternoon. Rain will arrive
in earnest at the coastline mid-day Saturday and makes its way
quickly from west to east. By Sunday, the pattern change will be
felt all the way from the coast through the AK and Aleutian
Ranges. Expect markedly cooler temperatures, gusty SE to SW winds,
and widespread steady rain.

We are monitoring the potential for some high surf and possible
coastal erosion late Sun into Mon. As the low pressure system
moves inland near the Kuskokwim Bay, it will leave much of the
coastline exposed to long duration west-southwest flow. Tides are
relatively low during this time period, so this should help to
mitigate impacts. However, if timing aligns correctly, this could
bring high surf into coastal marine zones and shorelines Mon.
Mariners and coastal residents should continue to monitor the
forecast.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3...Fri
through Sun)...
The Bering Sea will be where the pattern change for all of AK
begins. First, marine stratus and fog continue to hold on across
the Eastern Bering this morning. But rain and winds are already
starting for the Western and Central Aleutians. This will only
continue to increase as the upper 990mb low plows into the area.
Winds will climb to gale-force levels later today. The low will
sling the entire front across the domain by dawn Sat morning.
Winds will be southeast ahead of it, then shift to southwest.

By late Sat, another piece of energy will drop into the system
from Siberia. This will reinvigorate both winds and precipitation.
Expect a second round of gales to develop over the Central Bering
and track over the Eastern Aleutians by Sun afternoon.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. There could
be some cumulus build up and showers developing over the
mountains this afternoon, but they should remain away from the
terminal.

&&

.LONG TERM FORECAST (Days 3 through 7)...(from Thu afternoon AFD)

By Sunday, the large western vertically stacked low pressure
system will have moved over the eastern Bering with an associated
frontal system stretched across Southwest Alaska and into western
portions of Southcentral Alaska. Out ahead of the front, the
southeasterly pressure gradient will continue to increase through
Sunday afternoon and evening, bringing windy conditions to the
Copper River Basin, Knik Arm, Portage Valley and Turnagain Arm,
Eagle River Valley, Anchorage Hillside and other locations
favoring terrain channeled southeasterly winds. Precipitation will
continue to spread east Sunday night and Monday, impacting all
but the most downsloped portions of the Copper River Basin.

Numerous troughs rotating around the upper low along the Southwest
Alaska coast will continue to reinforce precipitation across
southern Alaska through midweek. By around Wednesday, model
differences begin to increase with the 12Z GFS and Canadian models
building a transient ridge into Southwest Alaska (though with
different timings), while the ECMWF keeps the pattern wet and
troughy. Tend to think persistent troughing is the more likely
outcome, though either way all the models return to a more active
west to southwesterly storm track through the end of the week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 174-177.

&&

SYNOPSIS AND MODEL DISCUSSION...KZ/MSO
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
LONG TERM...JR