Small Craft Advisory 476, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
412 AM AKDT Sun Jun 17 2018

.ANALYSIS AND UPPER LEVELS...

Widespread rainfall continues across much of Southern Alaska this
morning. Portions of the western Kenai Peninsula up to Anchorage
have some downsloping keeping that area dry, which also holds true
for the Copper River Basin. Nearly everywhere else in Southcentral
is getting rain. This is largely the result of a strong vertically
stacked low now over the southeastern Bering midway between the
Pribilofs and Bristol Bay cycling a nearly continuous feed of
moisture from the Pacific into southern mainland Alaska. This
moisture is moving in primarily from the southwest, which
parallels the axes of most of southern Alaska's mountain ranges.
The result is only small areas of downsloping winds, as mentioned
above, with most of the rest of the area either getting upslope
enhanced precipitation or the lift is being generated from the
Bering low itself, which is more so the case across Southwest
Alaska. Along with the downsloping comes the strong gap winds. The
Turnagain Arm Jet continues impacting Anchorage and the Hillside
with gusty southeast winds, the Knik River wind is gusting into
Palmer, and strong southerly winds continue in the Copper River
Basin near Glennallen as well.

The upper level pattern consists of a highly amplified upper
low/trough that is nearly stationary over the eastern Bering. With
the downstream ridging centered over the southern Panhandle, the
entire area of real estate in between, essentially covering the
entire Gulf Coast is under broad southwest flow aloft. The flow
is bringing tropical moisture northward from the Pacific, causing
the cloudy, wet pattern ongoing over southern Alaska.

&&

.MODEL DISCUSSION...

The models are in decent agreement through Monday night. As
expected with each low moving north into Alaska out of the data-
sparse Pacific, the models differences appear relatively quickly.
The model consensus once again has been to slow down the track of
the next low, the center of which will be approaching southern
Kodiak Island early Monday morning. The NAM has been faster with
this low and as such is far enough away from the model consensus
of EC/GFS that it was thrown out for winds along the coast and
marine areas. Further north in Southcentral however, that
difference disappears, so its high resolution solution was
preferred for the gap winds through Turnagain Arm and the Knik
River Valley. Forecast confidence is a bit above average as the
pattern is consistent, though small details as usual remain
uncertain.

&&

.AVIATION...
PANC...Continuous gusty south to southeast winds will continue for
much of the day with the Turnagain Arm Jet. As a front approaches
the coast, the Turnagain Wind will shift southward out of
Anchorage. However, strong southeast flow will continue aloft as
the surface winds turn more northerly in response to the front's
approach, resulting in LLWS. Rain is possible resulting in
potential MVFR conditions Sunday night as the front moves inland.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Southerly flow aloft will continue for Southcentral Alaska for the
next couple of days as the main upper low remains in the Bering
Sea, with a ridge to the east. An upper level short-wave trough
and associated surface low will move northward towards the Gulf.
The leading front will reach Kodiak Island and the Gulf coast this
afternoon bringing increasing rain to those areas through Monday
morning. Inland areas will also see an increase in rain chances,
but considerably less than along the coast as strong downslope
winds develop. This system will also bring stronger surface winds
for many channeled areas like Turnagain Arm late tonight through
Monday morning, as well as fairly strong winds to the Copper River
area Monday afternoon and evening.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
As the weakening low remains stationary over the eastern Bering,
the associated front over the southwest Mainland will also show
signs of weakening throughout the day today. Rain over the area
will briefly tapper down through this afternoon, as forcing
decreases over the area. Rain will pick back up over the southwest
Mainland later tonight through Monday night, as a reinforcing
north Pacific low tracks north and phases with the weakening
system over the southeast Bering. Mildly gusty southeast winds
will persist over the southwest Mainland through tonight, with the
strongest gusts through channeled terrain along the Alaska and
Aleutian Ranges.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The Alaska Peninsula and eastern Bering will continue to see an
active wet and windy pattern through Monday night, as low pressure
over the eastern Bering remains stationary over the area and is
reinforced by another north Pacific low tonight. A ridge builds
across the western waters behind the main low, with more showery
conditions expected over the central and eastern Bering/Aleutians
under slightly cooler northerly flow. Along with this ridging and
more stable conditions over the western half of the Bering and
Aleutians, low clouds and fog are expected to spread across much
of the western Bering and Aleutians through Tuesday.

&&

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

The long term forecast begins Tuesday evening with the dominant
upper air feature being a large and sprawling closed low center
over the eastern Bering/Alaska Peninsula. Cloudier and showery
conditions will me the dominant weather mode over Southern Alaska.
After Wednesday the low slowly weakens and falls apart as weak
upper ridging looks to build in. The ECMWF is more in favor of
this scenario, but the GFS model is now trending in that direction
as well. By Friday and into the weekend, slow warming should
ensue, but plenty of lingering moisture and the intense solstice
sun should still support rather widespread shower activity.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale: 119 125 351 352.

&&

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

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
357 PM AKDT Sun Jun 17 2018

AKZ101-125-131-135-145-190300-
Anchorage-Western Prince William Sound-
Northeast Prince William Sound-Southeast Prince William Sound-
Susitna Valley-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Whittier, Seward, Girdwood, Moose Pass, Valdez, Thompson Pass,
Cordova, Talkeetna, Willow, and Cantwell
357 PM AKDT Sun Jun 17 2018

...HEAVY RAIN TO BRING RISING WATER LEVELS TO THE NORTH GULF COAST
AND WESTERN SUSITNA VALLEY MONDAY AND TUESDAY...

Another wave of heavy rain will be impacting the north Gulf coast
starting with the southern Kenai peninsula early Monday morning,
moving east to Cordova Monday through early Tuesday. Highest
rainfall accumulations are expected for Prince William Sound with
greater than 5 inches possible through Tuesday.

Some of this precipitation will reach the western Susitna Valley,
bringing 2-3 inches to the headwaters of the Yentna and Skwentna
rivers.

In addition to heavy rain, unseasonably strong easterly winds
will accompany this storm with winds gusting up to 70 MPH for
Turnagain Arm and Portage Valley. Motorists with high profile
vehicles should exercise caution.