Small Craft Advisory 740, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 AM AKDT Sun Oct 1 2017

.ANALYSIS AND UPPER LEVELS...
High pressure at the surface with an upper level ridge over
southern AK remain the dominant weather feature this morning.
This is leading to overall very quiet weather across the mainland.
The biggest issue this morning is some patchy fog and low stratus
along the north Gulf Coast and in Kachemak Bay. Over the Bering,
a weakening front is starting to stall out along 176W. But...the
bigger story is the rapidly developing area of low pressure near
52N to the SW of Kodiak. This system is moving north on a 130 kt
piece of the subtropical jet and it is bringing some of that
subtropical air with it. This means it will bring a good slug of
rain to Kodiak Island and much of the Gulf Coast. There have even
been a few lightning strikes associated with it over the North
Pacific overnight, indicating a healthy amount of instability as
well. Over the Western Aleutians, another finger of the jet is
moving from west to east at 150 kts. This energy will eventually
make it towards the Gulf of AK and will help to keep the weather
pattern rather active this week.

&&

.MODEL DISCUSSION...
Models remain in decent agreement on most major features. The most
important item of interest is that they have been playing "catch-
up" to the storm moving towards the Alaska Peninsula. They were
slow to resolve it a few days ago. Now they are struggling to keep
up with its speed and strength. There is a better consensus this
morning on timing and placement, but there is still some
uncertainty with how strong it will get and when. The higher
resolution NAM Nest was used early on to best capture the stronger
winds and heavier rain. The GFS was relied upon towards the
middle of the week.

&&

.AVIATION...
VFR ceilings and visibilities will persist through the TAF
period. However, north winds at the surface will increase through
the day. By this evening, expect strong southeast winds to develop
at mountain top and over the Turnagain Arm. This will lead to
some strong wind shear in and around the aerodrome.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The strong weather system south of Kodiak Island will move rapidly
north today. This system will reach Kodiak Island around midday,
and the Kenai Peninsula by late this afternoon. Gale force winds
are expected throughout the Gulf. Some areas of storm force winds
are also expected over Shelikof strait, near the Barren Islands,
and over the open Gulf southeast of Kodiak. This system will push
inland over Southcentral Alaska this evening through Monday.
Strong winds are expected through Turnagain Arm, the upper
hillside of Anchorage, and Portage Valley. Gusty winds are also
expected through the Knik Valley and Copper River Valley. Even
though this is a fast moving storm, considerable rain is expected
on the eastern Kenai Peninsula, with upwards of two inches
possible tonight through Monday. Plenty of moisture and some
showers will continue over the area through Monday night as the
system pushes further inland.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The main focus over the next several days will be a strong low
pressure system rapidly approaching from the south today. A warm
front out ahead of the low will rapidly spread rain across the
AKPEN and into the Lower Kuskokwim River Valley beginning this
morning through Monday night. Strong upslope flow will lead to
periods of heavy rain on the east side of the Aleutian and Alaskan
Ranges while downsloping will limit the amount of precipitation
seen on the leeward side, though precipitation will redevelop
over Bristol Bay and move up the Kuskokwim River Valley. However,
the biggest impact from this system will be from high winds as a
southeasterly low level jet moves over the area Sunday night into
Monday morning. Winds between 30 to 50 mph will be likely across
the AKPEN with gusts as high as 70 mph through gaps and passes,
including near Kamishak Bay and west across Lake Iliamna.
Locations above 2000 ft along the Alaskan and Aleutian Ranges
could see winds as high as 80 mph on Sunday night as well. Rain
and winds will taper off throughout the day on Monday and the low
center weakens and moves over Bristol Bay.

&&

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

An elongated frontal boundary stretched across the Central Bering
will stall out today and weaken in places before dissipating
Monday evening. A short wave ridge temporarily moves across the
Western Bering and Aleutians before a low coming off of Russia
rapidly moves in Monday night bringing a wide swath of rain and
gale force winds to the region.

&&

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

Fall storm activity will be on the rise, despite a predominantly
zonal upper level pattern. The biggest area of concern revolves
around a rapidly developing and deepening low pressure system that
will track from near Attu, northeast towards Toksook Bay. Models
have been trending deep over the last day or so and have really
locked in on a 960mb, or deeper, low. An area of storm force
winds, and rather widespread area of gales. Given the long west to
southwesterly fetch of this system, coastal flooding problems
could loom for parts of the SW coast Wednesday night and Thursday.
Secondly, guidance is now agreeing on a low rapidly developing
south of the Gulf of Alaska and shooting north near Prince William
Sound. The rapid pressure rises along the Gulf Coast and through
the Turnagain Arm, could lead to other potential wind problems
closer to the Anchorage metro.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 119 130 131 138 352.

 Gale Warning 120 125 132 136 137 139 351 150
155 160 165 176 178 180.

&&

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