Small Craft Advisory 642, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
859 PM AKDT Sun Aug 19 2018

.ANALYSIS AND UPPER LEVELS...

The North Pacific low below the central Aleutians has become
vertically stacked and is weakening. Strong winds are occurring
across Southcentral as the associated occluded front and
atmospheric start to lift northward. The atmospheric river is
bringing rain to coastal areas and Prince William Sound visible on
radar. Anchorage and surrounding areas have seen light rain this
afternoon, but not as much as coastal areas due to downsloping. To
the west, we have a front moving across the western and central
Aleutians and Bering associated with the low west of Shemya.

&&

.MODEL DISCUSSION...

Models are in good synoptic agreement throughout the forecast
period. Main forecasts challenges will be with the strength and
timing of the gap winds along the Copper River Basin as the
atmospheric river moves eastward. Precipitation amounts will also
be a challenge.

&&

.AVIATION...
PANC...Generally VFR conditions with occasional light rain and
patchy areas of lower stratus moving through this afternoon.
Winds will be fairly light at the surface; however, LLWS is
expected through this evening due to strong SE winds over the
airport complex.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Tonight through Wednesday)...

A classic atmospheric river situation is developing across the
Gulf Coast tonight. A low centered well south of the Alaska
Peninsula, associated with the remnants of former Hurricane
Hector, is drawing deep tropical moisture from near Hawaii, and
focusing it into a narrow corridor aimed at the Gulf Coast.
The atmospheric river is shifting eastward away from the Kenai
Peninsula and into Prince William Sound this afternoon. As a
result, less rain is expected in the Seward area from this event,
as rainfall rates are already diminishing. Thus, the Flood Watch
has been cancelled. On the other hand, that moisture is aimed
squarely at eastern Prince William Sound, particularly the
Cordova and to some extent, the Valdez areas. Over the next 48
hours, the atmospheric river will be nearly stationary through
Cordova, resulting in abundant amounts of rain falling through
that region. Due to the tropical connection, the rain will likely
be heavy at times, with as much as 6 inches of rain expected over
the next 48 hours, with most of it expected Monday night into
Tuesday. A Flood Advisory is now in effect for that area.

For the rest of Southcentral, a quick wave of rain is moving
northward into the Mat-Su Valleys and will diminish over the next
few hours with sprinkles or light rain expected. Much of inland
Southcentral, with the exception of the western slopes of the
Alaska Range should have a cloudy, but relatively dry day on
Monday, especially for the first half of the day. On Monday
afternoon and evening, a weak low moves northward across the
western Gulf and Kodiak and reaches the north Gulf Coast Monday
night. This will bring heavier rain back westward into the Kenai
Peninsula, Anchorage, and the Mat-Su Valleys sometime late Monday
afternoon or Monday evening. With the center of the low moving
over that area, any downsloping winds should diminish, allowing
the potential for heavier rain to spread westward into the
Anchorage Bowl Monday night. The rain will likely continue into
Tuesday morning, especially the further north and east you go, but
a trend toward drier weather will be ongoing Tuesday. By
Wednesday, look for showery conditions across the Copper River
Basin, with drier weather under mostly cloudy skies expected for
the rest of the area.

&&

 .SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Wednesday)

Wet and unsettled conditions will dominate your weather over the
next few days. Currently, a front seen in the 700mb RH fields
continues to lift north across the Kuskokwim Delta, with
precipitation becoming more showery in its wake. This front is
attached to a mid-level disturbance also ejecting forth from a low
in the Gulf. The low will slowly drift into the central Gulf,
while sending several vort maxes across the region over the next
few days. Given a deep southeasterly flow, abundant cloud cover
with periods of showers will be the common theme, helping to
keep afternoon highs seasonably cool.

&&

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

As one front weakened while trekking eastward today, another
front associated with a Kamchatka low will will trek from the
western to central portions of the Bering Sea and Aleutians over
the next 24 hours while undergoing frontolysis (the weakening of a
front). Ahead of this front, warm advection aloft over the cooler
maritime waters has resulted in widespread stratus with some
areas of patchy fog. This looks to continue overnight, thus, the
fog mention was extended and expanded eastward in the forecast.
Otherwise, the Kamchatka low will reach the western Bering by
Tuesday evening. Broad cyclonic flow around this feature will
keep the pattern unsettled for the next few days, with periods of
showers expected.

Further east for the eastern Bering and AKPEN, this region will
remain just east of a weakening short-wave ridge, while
maintaining a southeast flow aloft. This will keep the region in a
showery pattern.

&&

.LONG TERM FORECAST (Thursday through Sunday)...

Beginning Thursday, models are in good agreement regarding the
overall synoptic pattern as an upper level ridge over Eastern
Alaska is quickly replaced by a broad upper level low over the
Bering Sea. An embedded shortwave trough will lift NE across the
AKPEN and into southcentral during the day on Thursday bringing
another round of rain to the region, with the focus looking to be
along the Gulf Coast and the Susitna Valley. Unsettled weather
looks to continue into Friday as the flow becomes more zonal after
the passage of the shortwave.

Uncertainty creeps into the forecast by next weekend as model
solutions differ on a potential pattern shift with the building of
an upper level ridge. The EC shows a rapidly amplifying long wave
ridge building across the western mainland into southcentral
Saturday through early Sunday with an upper low digging south
across the western and central Bering at the same time. The ridge
pushes east rather quickly as the upstream low moves into the
mainland by Sunday. The GFS has trended in building an even higher
amplitude ridge and keeping it somewhat stationary over the west
coast through early next week. This would bring a prolonged period
of nice weather across most of the mainland, while the EC
solution would mean only a day or two of 'summer' like weather.
Ensemble and WPC solutions are taking a middle ground approach
building a somewhat muted ridge over the southern mainland for the
weekend. Either way, there is decent agreement that at least
portions of southcentral and eastern Alaska will see a break in
the rainy/unsettled weather next weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood advisory for the Cordova Vicinity.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...DEK/AH
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...KV