Small Craft Advisory 622, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
446 PM AKDT Tue Aug 22 2017

.ANALYSIS AND UPPER LEVELS...
The upper level longwave trough remains over the mainland part of
the state as it extends southward out of the Arctic. The cut-off
low that broke off this trough is in the central Gulf of Alaska
and will be moving eastward overnight. The upper level ridge is
over the eastern Bering sea and Eastern Aleutians today producing
a tight NW gradient and jet streak over Southwest mainland Alaska
and the Alaska Peninsula. The next upper level low is just north
of Attu and providing more moisture and lifting in the Bering
Sea. This system will merge with the digging trough in the
Interior to bring in solid southwest flow aloft to Southwest
Alaska by tomorrow morning and Southcentral Alaska late tomorrow
afternoon.

&&

.MODEL DISCUSSION...

Models remain in good agreement over the next few days. The ECMWF
is slightly slower and more southerly with the way the low exits
Bristol Bay into the Gulf of Alaska on Friday. A GFS/NAM blend was
the preferred guidance.

&&

.AVIATION...
PANC...Skies should mostly clear this evening and there is a
chance for some patchy fog to develop overnight before the cloud
cover moves back in. Conditions are expected to remain VFR
through tomorrow. Southeast Turnagain Arm winds are expected to
move over the airport Wednesday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3, through
Thursday)...

As the current surface low in the Gulf of Alaska moves off to the
southeast, weak ridging is in place over south central Alaska
resulting a break in the rainy weather. We have a low now over the
Bering Sea pushing a front east toward the mainland. This front
will be approaching Kodiak Island by tomorrow morning. As it does,
a sharp ridge will build in along the North Gulf Coast resulting
in accelerating in the wind along the coast and through Turnagain
Arm by Wednesday afternoon and evening. Moist southwest flow aloft
in support of this incoming front will return the southern
mainland to the rainy pattern Wednesday afternoon. The low
supporting the front will move over southwest Alaska by Thursday
morning then drift to Kodiak Island through Friday. This will keep
the region under the cool wet conditions through Friday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A weak disturbance traversing the northern part of the area will
bring light rain showers to parts of the Kuskokwim Delta and
Valley tonight. Otherwise, most of Southwest AK will remain dry
through late Wednesday morning as a transient upper level ridge
shifts east of the region. Steady rain will then move back into
the area during the afternoon, spreading from southwest to
northeast as the Bering front arrives along the coast and pushes
inland. The front will quickly move east of the area by Wednesday
night, however widespread stratiform rain is expected to continue
as an upper trough racing across the Bering partially phases with
the next trough digging down from the Arctic. This should keep
the rain going through Thursday night as a closed upper low forms
along the Southwest coast and slowly tracks towards the Alaska
Peninsula.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Rain and gusty winds will impact much of the Bering through
Wednesday night as the broad low pressure system rapidly tracks
eastward from near Attu to Bristol Bay over the course of the
next 24 hours. A more tranquil pattern will settle over the region
on Thursday in the wake of the low, with northerly flow perhaps
generating a few showers over the eastern Bering on the backside
of the trough digging over the southwest mainland. Meanwhile,
benign cloudy conditions will return to the western Bering as high
pressure builds over the area.

&&

.LONG TERM FORECAST (Days 4 through 7)...

Beginning the extended period forecast Friday night, there will be
a large upper trough carved out across Alaska with a thin by
amplified upper level ridge axis over the Bering Sea. Another
arctic trough will be digging across Southwest Alaska and
reinforcing another low over the Gulf of Alaska. Rain will be
along the coast with showery, cooler, and cloudy conditions
inland. Eventually, the low in the Gulf of Alaska will weaken and
move east by Sunday, with diminishing precipitation through about
Monday. In the meantime, the upper level pattern will be in
transition as now all global numerical guidance undercuts the
Bering ridge with a series of low amplitude shortwave troughs as
they cross the Aleutian Chain and amplify somewhere over the
North-central Pacific Ocean. This evolution will begin to unfold
Monday night with the development of a larger low approximately
south of the Alaska Peninsula. As usual, there is uncertainty on
the track, strength, and orientation of the eventual low, but the
possibility exists for heavy coastal rain (since this low will be
tapping subtropical moisture) and strong/gusty southeast winds
impacting inland areas. Rain is also likely inland, but the extent
of which east of the Alaska Range is more uncertain as the global
models differ in how strongly the Yukon ridge axis builds
westward. Either way, more active weather looks likely by midweek
across parts of Southern Alaska and the eastern Bering Sea.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...SA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...JA