Small Craft Advisory 646, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
518 PM AKDT Fri Sep 6 2019

.ANALYSIS AND UPPER LEVELS...
The upper level pattern has a more highly amplified look to it as
compared to yesterday. The main reason for this is that the ridge
over Southcentral Alaska has become more amplified as it has been
moved eastward by the trough in the eastern Bering Sea. A
secondary trough is digging southward through the western Bering
Sea today and will cut south of the existing Bering Sea trough.
This will have the effect of keeping the trough axis over the
Bering Sea and southern Alaska over south-to-southwest flow aloft.

&&

.MODEL DISCUSSION...
Models remain in good agreement even in the features that are
changing. The biggest feature to mention is the front southwest of
Kodiak. This front will move faster and farther east than what was
anticipated from earlier forecasts. All models are showing this
and reflect the current satellite data well.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.FIRE WEATHER...
The only area of any concern will be the Copper River Basin on
Saturday. While winds are still expected to be gusty along the
Copper River and through channeled terrain in the Chugach
Mountains, the temperatures and RH values look to be moderated by
the faster moving front into the area. Other areas with active
fires are expected to see rain by Saturday night if not sooner.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3;
Tonight through Monday)...

A front will slowly track into Southcentral on Saturday bringing
mostly cloudy skies through the day. Though it will gradually
weaken as it moves eastward, upper level forcing will support rain
continuing through early Sunday morning for much of the Kenai,
Mat-Su and Anchorage bowl. Total accumulations of up to half an
inch are possible in associated moist, southerly flow. Winds will
remain light for most areas, with the exception of gusty
southeasterlies near 30 mph in and around Turnagain Arm. Farther
east, a ridge is expected to hold in place through much of the
weekend, bringing partly cloudy skies and dry conditions. Expect
high temperatures primarily in the 60s, with gusty southeasterly
winds likely along the Copper River. The aforementioned front
will completely weaken just east of Anchorage on Sunday morning,
thus no precipitation is expected in this region and skies should
remain mostly sunny generally east of the Richardson Highway
through the remainder of the weekend.

As we head to the new work week, a broad upper level trough will
move in from the Bering Sea. While there may be some specific
short-waves embedded in the trough, at this point in time
everything looks pretty weak. Thus, expect increasing clouds
Monday with increasing chances of showers from Kodiak Island to
the Kenai Peninsula, Anchorage and the Mat-Su. Ridging to the east
should provide drier conditions for eastern Prince William Sound
and the Copper River Basin.

&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Tonight
through Monday)...

The weather pattern will remain quite active across Southwest
Alaska with periods of wet and windy weather over the next couple
of days, especially along the coast. The first of two surface
lows is situated just east of the Pribilofs this afternoon, and is
draping an occluded front southeast over the eastern Bering and
southern AKPEN. A widespread area of rain is falling ahead of the
front from Bristol Bay north across the Kuskokwim Delta and
Valley. This front, and its associated sub-1000mb low, is running
up against a strong ridge downstream as it tries to advance to the
east. This is creating a tight pressure gradient which is
producing gusty southerly winds along the frontal boundary, with
the strongest gusts from Port Heiden to Goodnews Bay.

Strong pressure rises on the back side of the front and newly
forming triple point low south of the AKPEN will allow strong
southerly winds to persist across the AKPEN overnight and the
Southwest early Saturday. Rain and winds will then diminish late
Saturday into the first half of Sunday as the low lifts north and
a weak, transient ridge moves in. A second surface low then lifts
northeast from the Aleutians pushing a front toward the Southwest
coast Sunday. This will bring another round of rain and gusty
winds to the region through Monday. By late Monday, the widespread
rain tapers to showers as the low lifts north and winds decrease.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through
3/Tonight through Monday)...

A low just east of the Pribilofs and its associated occluded
front dropping southeast across the AKPEN is producing widespread
rain and gales across the eastern Bering. The gusty southerly
winds will continue tonight before transitioning to westerly
small-craft winds on the back side of the low as it lifts north
toward Saint Lawrence Island. The core of small-craft winds
decrease by Saturday night as the low continues to lift into the
Bering Strait.

Farther west, a low tracking over the western Aleutians tonight
will sweep a front toward the central Aleutians through Saturday
as the low begins to advance east. This frontal system will bring
southerly small-craft winds along the front, reaching the AKPEN
and Pribilofs by Saturday night. The low then weakens as it lifts
north of the Pribilofs for Sunday, with winds becoming sub-small
craft by late Sunday through Monday. Behind the low, expect
scattered showers with weak, northwesterly flow across the western
Bering as an upper-level shortwave rotates through. A ridge
builds over Attu and slowly works east by early next week.

&&

.MARINE (Days 3 through 5)...
(Monday through Wednesday)

...Bering Sea and the Aleutian Coastal Waters...

A weak cold front will cross the Bristol Bay and AKPEN coastal
waters on Monday, with sustained winds remained below gales on
Monday, with these winds then diminishing for Tuesday and
Wednesday.

Out west, an approaching front will cause sustained winds to
approach low-end gales for the western portions of the chain and
adjacent waters on Tuesday, spreading eastward into the central
Aleutian coastal and offshore waters.

...Gulf of Alaska...

A weakening front will enter the western Gulf on Monday, with
sustained winds just shy of small craft criteria. Otherwise, look
for relatively light winds and waves to continue.

&&

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

A fairly active pattern looks to be in store for much of the
upcoming week, as a series of lows affect the region. Our first
feature of interest will be a north Pacific low lifting towards
the southern Gulf on Wednesday. An attendant inverted trough will
develop and extend across the Alaska Range, giving portions of
southwest and southcentral another potential for rainfall. We'll
have to watch later model runs with regard to precipitation
coverage and storm track, as moisture and the remnant circulation
from Tropical Storm "Faxai" gets entrained into this system,
perhaps boosting rainfall coverage and intensity somewhat.

Further west, things really could get interesting on Thursday, as
an ejecting Siberian Low absorbs moisture and energy from what is
currently Typhoon "Lingling" (currently west of the South Korean
coast). The GFS model is fairly aggressive with this as it shows
these two systems merging with bombogenesis (the rapid development
of a low characterized by a 24 mb or more drop in 24 hours)
commencing, as it bottoms out around 955 mb near the Pribilof
Islands. However, the European (ECMWF) and Canadian Global models
aren't as aggressive with the low deepening, as they show a system
in the 970 mb range after the systems phase.

In either case, abundant tropical moisture associated with the
former typhoon and a strong 1.5 PV anomaly down to 500 mb should
foster very good precipitation efficiency near the low pressure
center and ahead of its attendant surface front, as it treks from
the central Bering/Aleutians into Southwest and Southcentral
Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales 130 150 155 160 180.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...AP/SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...PD