Small Craft Advisory 529, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 AM AKDT Sun Jul 8 2018

.ANALYSIS AND UPPER LEVELS...
The previously advertised pattern change is well underway across
mainland Alaska this morning as a sprawling low pressure system
tracks eastward across the Bering Sea and brings cooler, cloudier
conditions to southern AK. The low is nearly vertically stacked
and presently located just north of the Pribilof Islands, with the
upper low in the process of phasing with a low near 80N and
forming a well defined, positively tilted longwave trough which
extends from the Canadian High Arctic into the eastern Bering. The
associated front has pushed well inland and is bringing steady
rain to the Kuskokwim Valley and Greater Bristol Bay. Widespread
showers are also present across the Delta in the wake of the
front as difluent onshore flow becomes focused over Southwest AK,
further aided by favorable jet dynamics. Meanwhile, winds
continue to increase through favorable southeast gaps (Turnagain
Arm, Knik River Valley, etc.) as the pressure gradient responds to
the approaching low. Light rain has begun to fall along the Gulf
coast as moist onshore flow increases ahead of the front.

&&

.MODEL DISCUSSION...
Despite some minor differences on frontal timing for Southcentral
AK today, guidance remains in generally good agreement through the
short term part of the forecast as the Bering low shifts eastward
to the Kuskokwim Delta by Monday. This will keep the cooler and
much wetter pattern focused over the southern mainland through the
start of the week as a series of waves round the base of the
trough and brings abundant rain to the Gulf coast region.
Continued to favor mostly minor, NAM based adjustments for the
morning forecast package. Model agreement begins to diverge a bit
more by the middle of the week, the details of which can be found
in the Long Term portion of the discussion.

&&

.AVIATION...
PANC...Southeast winds out of Turnagain Arm will persist at the
terminal through the TAF period. Wind may diminish somewhat late
tonight as the down inlet component of the pressure gradient
marginally strengthens. Rain will spread into the terminal by
noon today and persist through Sunday morning. Ceilings may drop
to 5000 ft for a time this afternoon, but are nonetheless
expected to remain VFR throughout the TAF period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A significant change in the weather pattern is beginning today.
This new pattern will be wetter, cloudier, windier, and cooler
than conditions have been for the past week. A weather front
will push into Southcentral today from the west, and make slow
progress eastward before exiting the area Monday afternoon. This
front is associated with a significant low in the Bering Sea,
which will reach the Kuskokwim delta Monday morning and then
slowly shift northward. Another front will move to Kodiak and the
western Gulf Monday morning, then into the Southcentral mainland
Monday afternoon through Monday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3...through
Tue)...
The pattern change is underway. The well-advertised storm is
making its way through the Bering. It continues to push several
pieces of energy from west to east. The leading front is visible
on satellite and radar just clearing through the Lower Kuskokwim
Valley. Behind it, widespread showers continue this morning with
another round of steady rain queued up offshore of the Bristol Bay
coast. All of the low-level moisture that developed over the
Bering the past week or so is also moving onshore with this
system. This will continue to make for very low ceilings and
challenging aviation forecasts.

The gusty southeast winds will subside and yield to moderate
southwest winds (especially along the coast) as the low draws
closer. These winds will continue to act upon the open ocean and
serve to push higher waves and water levels into SW AK. Please see
the Special Weather Statement for further details on coastal and
marine impacts.

The other aspect to this system is the colder air. Freezing levels
across the area were about between 10,000-15,000' Sat afternoon.
Now they have fallen as low as 4,600'(St Paul) and 5,300' (Cold
Bay). So in addition to the clouds and precipitation certainly
making things feel cooler, the actual airmass is undergoing
significant modification as well. So high temperatures will swing
from running about 10 degrees above normal to 10 degrees below
normal for the next few days.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through
3...through Tue)...
The Bering will be split into two different weather regimes for
the next couple of days. In the east, a large low will continue to
spin as it slowly works from west to east. This will make for
widespread gale-force winds, abundant showers, and much cooler
temperatures. Some of the worst conditions will be across the
Pribilof Islands and the Bering-side of the Central and Eastern
Aleutians. As shortwaves rotate from north to south around the
broad low, the strongest winds and heaviest showers will take aim
on these locations.

Meanwhile, over the Western Bering, some rather warm air will
slide down from Siberia. As this warm air rides over the cooler
ocean waters, expect widespread stratus and fog to form. The
contrast in temperatures looks to actually even be enough that a
weak low spins up in the westerly flow Mon afternoon. This
could bring some light rain to Atka and Adak overnight Mon into
Tue. But the main story will be the fog across the area.

&&

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

The long term forecast periods begins Tuesday evening with a deep
upper level low over the state of Alaska. This low will be rather
impressive from an anomaly standpoint with 500 hpa heights around
-2 to -2.5 sigmas. This low will be in the process of weakening
 with a continued moist onshore flow supporting clouds and
 rain/shower threats through Wednesday.

The global numerical models diverge thereafter with quite a bit of
noise muddling the Thu-Sat timeframe as arctic upper level energy
drops through the Bering and phases with a migratory cutoff low
over the North Pac. All models show an eventual phasing, but
differ greatly on timing. The net result is there probably will be
a brief break in the wet/cloudy weather sometime late Wed into
Thursday, but eventually the trough will simply replace the
weakening low and bring more rain and clouds to Southern Alaska by
Friday and into the weekend. Long story short, expect a typical
mid-summer pattern for Southern Alaska consisting of mostly clouds
and chances for rain along with seasonal temperatures.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None
MARINE...Gale Warning 165 170 171 172 173 174 175 185 413 414

&&

SYNOPSIS AND MODEL DISCUSSION...CB
AVIATION...CB
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
LONG TERM...Ahsenmacher