Small Craft Advisory 624, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
525 AM AKDT Wed Aug 23 2017

.ANALYSIS AND UPPER LEVELS...
An upper level longwave trough is centered over the northern part
of the state and an upper level ridge is located over west and
southcentral AK. To the south a cut-off low pressure system over
the southern Gulf weakens as it moves east, though moisture
wrapping around the top of the system is currently producing rain
showers along the Gulf Coast and into the Prince William Sound.
The upper level ridge is quickly transiting off to the east as
another upper level low over the Bering moves in rapidly behind
it. A frontal system associated with this low is currently
approaching the southwest coast with a band of rain moving ashore
as seen on radar.

&&

.MODEL DISCUSSION...
Models are in good agreement regarding the low moving in from the
Bering Sea into western and southcentral Alaska today. There are
some differences regarding mesoscale features including the
amount of downsloping seen on leeward side of the Kenai and
Chugach mountains, with the NAM being more aggressive in drying
out the low levels across Anchorage and the Kenai Peninsula this
afternoon. Sided mostly with the GFS in keeping light
precipitation in these areas throughout the forecast period.

&&

.AVIATION...
PANC...VFR conditions will prevail over the terminal throughout
the day today. Mid and high level clouds will begin to move into
the area this morning ahead of the incoming system to the
southwest. Rain will move into the area by at least 00Z which
looks to coincide with the development of southeast Turnagain Arm
winds over the terminal. Slight downsloping should dry out the low
levels some keeping the terminal VFR through the forecast period.
Possible LLWS conditions may develop tomorrow night as a strong
southeasterly low level jet moves over the area. Hi-res models
show strong ridgetop winds over the front range in the 30-40 kt
range with light northerly flow setting up over the Anchorage Bowl
at the same time. We will watch the development of this event
as more recent model runs come in today and update the forecast as
necessary.

&&

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

Radar imagery this morning indicates a broad line of showers that
stretches from the Kuskokwim Delta to across Bristol Bay. This
rainfall is associated with a front that will progress quickly
across the Kenai Peninsula and towards the Anchorage Bowl this
afternoon and evening. While models are showing fairly decent
consistency in the progression of the front and intensity, there
is still uncertainty in conditions ahead of the front. Currently
models are indicating a low level southwesterly jet forming along
the Inlet ahead of the main front. This may result in a round of
showers along the mountains and within the Anchorage Bowl this
afternoon, and a convergence band may form as a southeasterly
Turnagain Arm jet forms. However, each model indicates a
different strength and location of this convergence and
precipitation amounts are uncertain.

Wet conditions across much of Southcentral will continue into
Thursday evening as the front lingers along the north Gulf coast.
Pressure gradients will favor a persistent low level jet along the
gulf coast and into Prince William Sound which will support gusts
well into the Gale category. Gusty winds are also anticipated
through Turnagain Arm and higher elevations as cross barrier flow
persists.

 &&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Cloudy and rainy conditions will begin to spread from west to east
this morning over the southwest mainland, as the front over the
eastern Bering tracks inland. Gusty winds are also expected to
pick up along the southwest coast and Aleutian range this
afternoon, as the pressure gradient remains tight over the area
through Thursday. As the parent low pushes inland over the Bristol
Bay area Thursday morning, rainfall will continue and gusty
northerly winds will ramp up along the Kuskokwim Delta coast
through Friday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Rain and gusty winds will impact much of the Bering through
tonight as the broad low pressure system rapidly tracks eastward
from the central Bering 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 but
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...Gale warning 119 125.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KVP
SOUTHCENTRAL ALASKA...ELN
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PEPE
LONG TERM...JA