Small Craft Advisory 510, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
410 PM AKDT Sun Jun 28 2020

.ANALYSIS AND UPPER LEVELS...

Water vapor satellite shows deep moisture and clouds spreading
across Southcentral and into the Alaska Interior as a result of
an upper level short-wave embedded within a long fetch of moisture
transport. Weather observations from across Southcentral indicate
widespread measurable rain this morning. Anchorage NWS received a
wetting rain (as of 10am) with 0.14" from 4am-10am. This feature
has shifted beyond the forecast area this afternoon.

Another interesting weather feature is an upper level trough
which extends northeastward from the North Pacific through an
upper low near the Alaska Peninsula. This is tracking toward
Southcentral, bringing another round of rain for this
afternoon/evening. Another upper level trough runs south-
southwest from the Kuskokwim Valley through Bristol Bay. Steady
rain and clouds have spread over most of Southwest Alaska,
tracking eastward toward the Alaska Range. Lastly, low level
clouds are seen over much of the Bering Sea and Aleutian Islands.
With a brisk westerly flow, there are some breaks on the northern
to eastern side of islands.

&&

.MODEL DISCUSSION...Models have some significant differences in
both the short and the long term. Right out of the gates, there
is some discrepancy in how long the SE winds around Cook Inlet
persist this afternoon/evening. If they hold on a bit longer, it
would take a bit of a bite out of the QPF with some downsloping.
However, all models depict rain coming back up the Inlet for late
this afternoon/evening. The gusty winds across some of the marine
areas (Kachemak Bay, Valdez, etc) should also abate this evening.
For Mon, models have trended a bit drier with some weak ridging
and westerly flow. That said, cloud cover will still be widespread,
but this forecast tones down some of the PoPs, especially early
Mon.

In the extended portion of the forecast, models now vary
significantly from the more consistent picture they were painting
yesterday. Most of the trends are toward a ridge of warmer and
drier weather. However, there are now a few flies in that
ointment. Please see the extended discussion for details.

&&

.AVIATION...
PANC...The Turnagain Arm Wind should continue through the evening.
However, there is uncertainty in the exact strength and timing of
when it dies off. As the next round of rain arrives, the winds
should slowly diminish. These factors will lead to deteriorating
ceilings and the chance for lower vis in the heaviest precipitation.
Low stratus looks to develop overnight and linger through Mon AM.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

An upper level trough quickly moves eastward over the Gulf of
Alaska tonight allowing for precipitation to taper off late this
evening into the overnight hours. An upper level shortwave
arrives over Southcentral for Monday, bringing additional showery
precipitation to northern areas of the region, including the
Susitna Valley and the northern Copper River Basin. Precipitation
with this feature will taper from west to east with only residual
showers in place over the northeastern Copper River Basin late
Tuesday. By Wednesday, there is increasing confidence a ridge of
high pressure will resume its place over Southcentral bringing
beautiful clear skies and warmer temperatures across the area
midweek.

&&

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

Tuesday and Wednesday are going to be the dry days for Southwest
Alaska this forecast period. The radar at Bethel is broken and the
radar at King Salmon has widespread stratiform precipitation.
However, on satellite imagery the AMSU (Advanced Microwave
Sounding Unit) and the SSMI (Special Sensor Microwave Imager) are
pretty juicy over the Bethel/Kuskokwim area which is seconded by
the weather models. Onshore flow and moisture advection will
persist for the remainder of today and tomorrow and a drying trend
will begin Tuesday.

&&

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

The remnants of a frontal boundary are pushing across the Eastern
Aleutians. The low weakened and it became an open wave near Sand
Point. As time elapses, expect the remnants of this system to push
eastward and get absorbed into the longwave pattern. There is a
broad dome of high pressure over the Central and Northern Pacific,
so this will result in zonal flow over much of the Bering tonight
and tomorrow. All the while, a new low is developing east of the
Kuril Islands and will really intensify between Kamchatka and Attu
Island. Very confident that this storm will have a tight pressure
gradient with it. It's a slam dunk that the Western Aleutians and
portions of the Bering will see gale force winds. Looking at
satellite imagery (specifically the CIMSS MIMIC layer Precipitable
H2O), the fetch of moisture associated with this new system
actually extends back to the Okinawa (Ryukyu Islands). This system
too will have modified tropical air entrained in it. This will be
another juicy system.

&&

.MARINE (Days 3 through 5: Wednesday through Friday)...

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

There is high confidence in small-craft southerly winds over the
Eastern Bering/West Coast of AK to start this period. These winds
could push seas as high as 15 feet over the Central Bering. As
those winds shift north and dissipate on Thu, a new small-craft
level front will move into the Western Aleutians on Thu. This
system has the chance to bring some gale force gusts, but
confidence is moderate that sustained winds will not reach
gale-force.

...Gulf of Alaska...

Westerly flow with a ridge of high pressure will keep the
sustained winds below gales (and potentially not reaching even
small craft), with waves below 10 feet expected.

&&

LONG TERM FORECAST (Days 4 through 7: Thursday through
Sunday)...

Models in fairly good agreement on Thursday with the ridge building
over mainland Alaska and a trough skirting to the northeast across
northwest Alaska. But for the rest of the period the GFS and
ensembles keep the ridge strong over the area with a slight
retrograde to the west Friday and Saturday. The ECMWF is much
different from previous runs with a low near the ALCAN spinning
off easterly waves through much of the long term. We believe
this is a one model run issue as all the other models continue
to agree upon a ridge building through the long term.

The ridge is expected to keep most of mainland Alaska dry and
warmer. The only exception is the trough that will skirt the YK
delta region Thursday and early Friday with some areas of rain.
A few showers may begin to develop over the Alaskan and Chugach
ranges over the weekend mainly in the afternoon and evening due
to surface heating.

Over the Bering Sea and Aleutians it will be dry on Thursday but
a low pressure and frontal system will move into the area Friday
and Saturday producing rain. Another system will bring some
showers to the western Aleutians Sunday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 177 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CB/MSO
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
MARINE/LONG TERM...MSO/RC