Small Craft Advisory 659, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
500 AM AKDT Sun Aug 26 2018

.ANALYSIS AND UPPER LEVELS...
A vertically stacked low over the Bering continues to move
northeastward toward Norton Sound. This low, coupled with a large
upper level high over the Gulf, has created strong southwest flow
aloft over the eastern Aleutians and AKPEN. Moisture continues to
stream into the southwestern part of the state this morning. As a
result, radar imagery shows rain falling across the Bristol Bay
coast, northern AKPEN, and southern parts of the Kenai Peninsula.
The leading edge of the front associated with the Bering low has
pushed eastward into the central Gulf, allowing for rain showers
fill in behind it.

&&

.MODEL DISCUSSION...
Model solutions remain in generally good agreement through the
next 24 hours. Differences arise with the timing of a surface low
developing over the AKPEN Mon morning and quickly moving into the
Gulf by Mon night. Currently, the NAM has the fastest solution,
while the GFS and EC both show a more westerly position with the
location of the low by Mon night. The GFS/EC solution was
preferred at this time due to recent run to run inconsistencies
with the NAM.

&&

.AVIATION...
A front moving north this morning will bring rain for a majority
of the day today. Ceilings will likely go MVFR with rain earlier
in the morning before lifting to VFR as the Turnagain Arm winds
and downsloping kick in. Gusty southeasterly winds are expected to
impact the terminal later this morning through this evening as
the low position and track are favorable for a northward bend of
the Turnagain Arm jet into the Anchorage Bowl. Expect strong
crosswinds on the 07 runway for most of the afternoon/evening
hours.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Today through Tuesday)...
Rainfall from a low pressure system centered over the Eastern
Bering will spread northward into Southcentral this morning,
bringing widespread rains to most areas by this afternoon. With
deep southwesterly flow, the heaviest rainfall amounts are
expected along the Talkeetna Mountains where strong orographic
lift will be present for much of the day. This will also allow for
continued rainfall in the lee of the Chugach even as strong winds
along Turnagain Arm and the Knik River Valley spread into the
Anchorage Bowl and Matanuska Valley this afternoon.
While these winds are not expected to be strong, the usual windy
areas along the south/west Anchorage as well as the hillside will
see gusts from 30-40mph this afternoon and early evening. Rainfall
will then diminish from west-east late tonight as the front makes
landfall, however the pressure gradient from the weakening low as
it moves through the interior will keep gusty southerly winds in
the area into Monday morning.

Fortunately for those looking for a break from the persistent
rains over Southcentral, the next low that develops in the Gulf on
Monday will remain well south of the Mainland. This will allow
for clearing skies for most areas beginning Monday afternoon and
persisting through Tuesday as much drier offshore flow develops
behind the Gulf low. Expect outflow winds in the usual areas
along the North Gulf Coast on Tuesday, but with very little in the
way of available cold air to work with these are not expected to
be significant.

&&

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

A low just south of Saint Matthew Island will continue its trek
northeast towards Norton Sound today. A warm front associated
with this system is currently moving across Bristol bay and will
continue to push into the interior of Southwest Alaska today. As
this feature moves north, the widespread rainfall over the
Southwest will taper off and become more showery behind the now
occluded front. With the low north of the region on Monday,
residual moisture from the cyclonic flow will produce some
scattered shower activity, especially during the afternoon and
evening hours and over the higher terrain when instability
increases slightly due to daytime heating. A secondary low
skirting the southern coast of the AKpen may produce some light
rain and gusty southwesterly winds for region overnight Sunday
night through Monday morning before the low moves across Kodiak
Island and into the western Gulf.

High pressure over the Bering then builds across the region for
Monday night and Tuesday. This will produce a gradual drying trend
progressing from the AKpen into the Southwest Mainland. Patchy
valley fog is also possible during the overnight Monday into
Tuesday morning under this ridge. a product of radiational cooling
due to the clearing conditions combined with saturated grounds
from the recent rains.

&&

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

Low pressure moving into the northern Bering this morning will
continue to advance towards Norton Sound. Its trailing cold front,
which had been making some eastward progress across the central
Aleutians last night, is now becoming stationary just west of
Dutch Harbor this morning. This will keep clouds and scattered
showers in the picture from the eastern Aleutians north into the
eastern Bering through Sunday afternoon. A secondary low will form
well south of the Aleutians along this boundary today and began
to push northeast toward the AKpen. This will bring a second round
of precipitation and gusty southwest winds across the southern
coast of the AKpen for late Sunday into the first half of Monday
before the ridge upstream moves east and kicks the low out into
the western Gulf.

Farther west, the aforementioned ridge will dominate the western
and central Bering and Aleutians. The ridge axis will slide east
through the forecast period, reaching the eastern Aleutians by
Monday. Areas of low stratus, patchy fog, and drizzle are likely
across the Bering as moisture continues to rotate around the
northern half of the ridge. Clearer and drier conditions should
take hold across the central and eastern Bering Sunday afternoon
through Monday as drier air advects in along the southern flank
of the ridge. Clouds will return to the western and central
Aleutians beginning Monday night, however, as a low southwest of
Attu and a triple-point low south of Adak begin to move north
towards the Aleutian Chain.

&&

.LONG TERM FORECAST (Days 3 through 7)...

By Tuesday, ridging over Southwest Alaska will have pushed much of
the precipitation out of Southern Alaska for a brief drier midweek
break. The upper level ridge will amplify slightly Tuesday night
as a strong north Pacific low crosses the western Aleutians and
tracks into the Bering. The ridge axis will shift east over
Southcentral Alaska on Wednesday as the frontal system
accompanying the Bering low approaches the Kuskokwim Delta coast.
The front will push inland over Southwest Wednesday evening and
then be followed by the associated upper level trough on Thursday.
The front will continue east into Southcentral Alaska on Thursday
with the upper trough following Thursday night and Friday.
Additional frontal systems with moist tropical origins will likely
follow over the weekend, although model uncertainty high
regarding their timing and track.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 130 131 132 138.

&&

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