Small Craft Advisory 748, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
452 AM AKDT Wed Oct 4 2017

.ANALYSIS AND UPPER LEVELS...

The large scale pattern features a trough over much of the
Mainland and extending well south into the northern Pacific, and
another trough entering the western Bering/Aleutians from the west.
At the base of the trough lies an upper level wave propagating
northward into the Gulf. Downstream of the negatively tilting
Mainland trough is a ridge that continues to holds steady over
western Canada. Between these two upper level features, an
atmospheric river continues to pump subtropical moisture to the
northeast Gulf coast. Radar imagery shows the remainder of this
moisture spreading inland over Southcentral during the early
morning hours.

Further west, showers over the Kuskokwim Delta associated with a
departing low continue to weaken and taper off into the early
morning hours. A strong (180 kt) jet streak aloft, and rather
impressive low-level baroclinicity associated with a surface front
entering the central Aleutians, is priming the western and
central Bering for the development of a strong low today. This
low will be the main weather-maker over the coming days.

&&

.MODEL DISCUSSION...
The models remain in general synoptic agreement, with some minor
differences in timing an intensity with the low and associated
front tracking through the Bering/Aleutians today through the end
of the end of the week. It seems reasonable as the two main
features which affect alaska develop under fairly strong upper
jets. The first feature traveling inland from the north gulf
earlier this morning, which was captured by the GFS models. The
next big storm moving across the Bering Sea today through the end
of the week is also best captured by the GFS. This system is
expected to bring strong winds and higher seas to much of the
Bering sea and Aleutians today, and eventually the southwest
coast and Alaska Peninsula Thursday into Friday.

&&

.AVIATION...
VFR to near MVFR conditions will continue through today, as rain
persists over the area. The rain will briefly taper off this
evening before the next front moves in from the west and brings
MVFR conditions back to the area overnight tonight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A negatively tilted trough over southwest Alaska will move through
Southcentral today. This will keep conditions showery and cloudy
across most of the area. The flow aloft then becomes stronger, and
the next frontal system will race into the area from the west
tonight. This will bring winds and more rain to the area. Winds
will diminish on Thursday morning as a triple-point low develops
near Kodiak Island, which will move into Prince William Sound in
the afternoon. The front will then move east of the area by
Thursday evening. Showery conditions are still expected as the
main upper low will be over western Alaska, and cyclonic flow will
be over Southcentral.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A weak upper level trough will dissipate as it slowly lifts out of
the area today, bringing a short break in precipitation this
morning before a strong storm force low rapidly approaches the
area. Heavy rain associated with a warm front out ahead of the low
will move over the coast today, and spreading well inland through
the evening. A brief shot of strong southerly winds will accompany
the warm front, bringing sustained winds between 30-40 mph with
gusts up to 60 mph along the coast. The storm's biggest impact
will be after the cold front passes tonight as a long fetch of
westerly storm force winds approaches the coast, leading to
increased surge and the potential for minor coastal flooding and
beach erosion. Right now, the highest potential for flooding looks
to be for locations along Kuskokwim Bay, including Quinhagak and
Goodnews Bay, where water levels could be 2-4 feet above high
astronomical tide (grass level) on Thursday. See the Special
Weather Statement for more information. The main core of winds
will shift south to Bristol Bay and the AKPEN by Thursday night
where gusts along the coast could be upwards of 60 mph, though
coastal surge and flooding are not expected to be much of an issue
here.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A low pressure system over the Western Bering will rapidly
develop today supported by a strong 190kt westerly jet paralleling
the Aleutian Chain. The low will draw up warm moist air north
from the Pacific this morning leading to the development of a warm
front accompanied by a very strong southerly low level jet
bringing a quick burst of strong storm force winds over the
Central Aleutians. The low will deepen to near 960 mb tonight and
track west across the Northern Bering. A core of strong storm
force winds will develop on the south side of the low center and
move over the Pribilofs tonight and tomorrow morning, with peak
gusts expected to be over 60 mph. The low will move over the
mainland by tomorrow night and slowly dissipate, leading to a
gradual decrease in wind speeds across the Bering and Aleutians.

&&

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

The extended forecast period begins Thursday night with the
remnants of a powerful Bering sea storm moving eastward into
Southern Alaska. This strong cyclone will still be impacting
Bristol Bay and the Alaska Peninsula with strong gale force winds
with the strongest core of winds moving into the Gulf of Alaska
Friday. Strong mid level westerly flow will be supporting a well
defined upslope/downslope pattern with most lower elevations areas
drying out while precipitation favors higher mountain slopes.
This pattern regime will persist through Saturday before the
pressure gradient weakens and a trough digs out over the Bering
Sea. Much uncertainty develops at this juncture as the global
models diverge rapidly with the development of a surface low along
the subtropical warm front. The Canadian GDPS and GFS prefer to
phase the upper trough with the entire warm front, creating a much
deeper and slower moving low while the ECMWF has a faster moving
low and a lower amplitude trough. The differences become stark by
Sunday as the ECMWF is racing the low across northern Gulf of
Alaska while the Canadian and GFS depict a deep low south of the
Eastern Aleutians. Long story short, we continue to favor a WPC
blend approach which favors more stable ensemble solutions showing
a weaker low moving north toward the AK Peninsula Sunday and
Monday with increasing chances for precipitation into the early
week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm 179 180 181 412 414.
         Gale 119 120 130-132 138 139 150 155 160 165 170
              172-178 185 351 352 411 413.

&&

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