Small Craft Advisory 518, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
449 PM AKDT Thu Jul 5 2018

.ANALYSIS AND UPPER LEVELS...
A weak shortwave descending through the Mainland is bringing
rainfall to areas of the interior, however dry low levels to the
south of the Alaska Range have prevented any showers from reaching
the ground until early this afternoon. Cloudcover from this
feature has been much less impressive than initially expected,
which has provided areas from Anchorage southward with a generally
sunny and warm day. This is a different story for both the
Southwest Mainland and all of the Bering Sea which continue to be
taken over by widespread fog and stratus, and even some dense fog
across inland areas of the Southwest Mainland early this morning.
There is a second area of fog and stratus over the Central/Northern
Gulf that also brought dense fog into Cordova this morning, but
is beginning to shift eastward and dissipate ahead of a weak
developing lee side low in the Western Gulf. The remnants of
Tropical Storm Prapiroon are beginning to enter the Western Bering
with widespread rain, gusty winds, and even some lightning
strikes well to the south of the Aleutian Islands. This system
will continue moving to the east over the next few days and become
a dominant feature for the South Mainland towards the end of the
weekend.

&&

.MODEL DISCUSSION...
The models remain in generally very good synoptic agreement
through the short term, with most of the challenges surrounding
the placement of the weak low developing in the Gulf today. The
NAM was generally avoided for its placement of this low, but
otherwise there is very good agreement that the low will remain
very weak and not significantly impact forecast confidence. Over
the Bering the models continue to be in much better agreement
than just a few days ago with the remnants of Prapiroon as it
progresses through the Bering this weekend. As a result there
remains generally good confidence in the forecast through this
weekend before agreement begins to break down early next week.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. Shower activity
should develop primarily along the mountains to the east of the
terminal this afternoon, and with west-east movement of any
showers conditions at the airport should remain dry through this
evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A shortwave trough is trying to move south this evening, but the
progress is slow and it seems to be encountering a deformation
zone which is weakening the trough. That said, there is enough
forcing and speed shear to support more widespread shower and
thunderstorm activity over the interior mountains and Copper Basin
and into the Matanuska Valley. This feature will slowly move
south overnight then completely weaken, but not before it induces
weak offshore flow which will help warm and dry the low levels
for Friday. Friday looks quite warm as interior locations soar
into the upper 70s to low 80s. Any afternoon convective threats
will remain over the mountains (and will be rather isolated) as
high pressure aloft builds in. On Saturday, ridging will shift
east and the surface pressure gradient will build which will
support increasing southeast winds moving inland. This will also
help support afternoon/evening thunderstorm development over the
interior as the thermal trough is impinged northward. Otherwise,
Saturday looks to be the last "nice" weather day before the
pattern shifts dramatically Sunday and beyond towards cooler and
wetter (see extended forecast).

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
Two weakening shortwaves, one working south from central Alaska
and the second sliding east along the Alaska Peninsula, continue
to produce clouds and a a few widely isolated showers across the
southwest mainland this afternoon. These isolated showers will
remain relegated to the higher terrain this evening before
dissipating overnight (along with the cloud cover) as both systems
move away from the region. The bigger issue overnight will be the
redevelopment of patchy fog from the Kuskokwim Delta south along
Bristol Bay as the moist, westerly flow moving over the flattened
ridge south of the eastern Aleutians persists. The fog should burn
off in most locations by mid-morning, but may linger into the
early afternoon along the northern AKpen due to a developing
northwesterly flow. For the rest of the southwest mainland, weak
ridging should bring a warming trend, along with clear and drier
conditions, for Friday and Saturday before the next front and
chance for widespread rain approaches from the Aleutians beginning
Saturday night. Ahead of the front, southeasterly winds will
increase, especially in the favored gap locations.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
Low stratus and patchy fog will continue as the dominant weather
features over almost the entirety of the Bering and Aleutians for
another 24 hours before a shift in the weather pattern takes hold.
Tonight, a deepening low wrapping in remnant tropical storm
energy will begin an advance on the western Aleutians. By Friday,
widespread rain will develop from Atka west with gale-force winds
from Shemya to Adak. Rainfall will shift east as this low moves
into the western Bering by Friday night and over the Pribilofs by
Saturday night. Gale-force winds will also continue for the
southern Bering Sea and central Aleutians through Sunday before
slowly winding down. By the end of the weekend, the ridge of high
pressure from this past week will be but a memory as a long-wave
trough sets up over the Bering with a vertically-stacked low near
Nunivak Island. This will keep the weather unsettled over the
entire Bering Sea region well into next week.

&&

.LONG TERM FORECAST (Days 3 through 7)...
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...None.

&&

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