Small Craft Advisory 745, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
439 PM AKDT Mon Oct 2 2017

.ANALYSIS AND UPPER LEVELS...

A fairly active pattern continued over much of Southcentral and
western Alaska today. A stacked surface low entering Bristol Bay
and its associated frontal band extending across Southwest Alaska,
Southcentral, to south over the Gulf, spread rain across many
areas. The west and northern Gulf Coasts received the majority with
some locations reporting over an inch. Gusty winds were also
evident across the Gulf and Southcentral. Winds were particularity
strong with values in the 50-60 mph range across Turnagain Arm.
Further west over the eastern Bering Sea, a weakening frontal band
extended north from a retreating low south of Unalaska. Another
frontal trough over the western Bering Sea was heading east and
had gales along its leading edge.

&&

.MODEL DISCUSSION...

The 12z model runs are in decent synoptic agreement. Differences
in timing and intensity seem reasonable as the two main features
which affect Alaska develop under fairly strong upper level jets.
The first feature traveling north in the Gulf on Tuesday/Tuesday
night appears best captured by the NAM/GFS models. The next big
storm moving across the Bering Sea Tuesday and Wednesday are
handled similarly by the 12z model runs.

&&

.AVIATION...
VFR conditions will continue through today, but will likely drop
to low VFR conditions (3500ft) as winds diminish Tuesday morning.
LLWS will likely continue through the late afternoon hours. Gusty
southeast surface winds should impact the airport complex late
this afternoon and continue into the early morning hours Tuesday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The gale-to-storm force winds that have blown through the north
Gulf coast into Prince William Sound will diminish by late this
evening as the barrier jet and associated low weaken. The pressure
gradient along the Chugach Mountains will be much slower to
weaken, forcing the gusty winds that have been blowing all day
along Turnagain Arm to shift northward into south/west Anchorage
as the down-inlet flow dissipates. This will bring gusty southeasterly
winds into the Anchorage Bowl for most of overnight hours. This
typical active fall-like pattern will then continue into Tuesday
as another low quickly moves into the Gulf, bringing another round
of gale force winds to the area. This low will take a much more
westerly track into Southcentral, producing another round of
southeasterly winds with widespread rain late Tuesday-Wednesday.
This system will also bring rain to much of the area east of
Kodiak Island including the Gulf Coast, however as the jet will
remain well off to the east heavy amounts are not expected.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The forecast period commences with the remnants of a potent low
near Lake Iliamna and a re-enforcing lobe of vorticity just off
the west coast of Alaska near Nunivak Island. The predominate
sensible weather impact this evening will be gusty winds.
Anticipate widespread unsettled weather to continue overnight.
Lingering showers expected on Tuesday. Expecting a brief hiatus
between systems for most of the Yukon-Kuskokwim Delta on
Wednesday.

&&

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

The progressive pattern continues for the Bering. First, there is
a negatively tilted trough extending from NE Russia to the
Pribilof Islands and a cyclone east of Kamchatka. Both of these will
propagate eastward this forecast package. Meanwhile south of the
Aleutians there is a closed low with an occluded front impacting
the Central Aleutians which has wrapped moisture into the region
and this moisture will be ushered southeastward as time elapses.
In its wake, a short wave ridge builds in briefly clearing out the
Western Aleutians. Wednesday, another round of cyclogenesis is
advertised by the models and expect to hear more about this system
as it barrels its way across the Bering during the latter half of
the week.

&&

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

The extended forecast period begins Wednesday night. The dominant
weather maker will be a strong storm force low over the Northern
Bering Sea with the associated gale force front moving inland
across Southwest Alaska and into Southcentral during the day
Thursday. Eventually strong gale to low end storm force winds are
expected to move toward the Southwest coast on the back side of
the strong surface low. There is the potential for some coastal
erosion and high surf conditions across parts of the Southwest
coast given the strength of the low and the length of the wind
fetch. See the Special Weather Statement regarding this aspect of
the latter week Bering Sea storm.

Eventually the low and associated front will quickly weaken and
de-amplify, with the general upper air pattern becoming a fast
westerly regime aloft. What this means is by Friday, precipitation
will generally become more favored along the mountains with more
showery/drier conditions for lower elevation regions. By Saturday
evening, another low moving toward the Gulf of Alaska will bring
another shot of rain to parts of Southern Alaska, but much
uncertainty exists with this low as the ECMWF favors a faster and
farther east solution compared to the GFS and Canadian models. For
now, a WPC blend approach was favored by Sunday and Monday as
model uncertainty regarding a potential re-amplification of the
upper flow pattern becomes disparate among the global models.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 119 125 175 176 177 178.

&&

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