Small Craft Advisory 339, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
437 PM AKDT Wed Apr 25 2018

.ANALYSIS AND UPPER LEVELS...
A pool of cold air aloft from the nearly stationary low just south
of the Bering Strait continues to bring widespread rain and snow
showers as well as widespread clouds to much of the southern
Alaska Mainland. The most widespread shower activity has occurred
over the Northern Susitna Valley where persistent and moist
southwesterly flow has produced rain and snow since the front
passed through that area yesterday evening, allowing for about
4 inches of snowfall in the Cantwell area since yesterday
evening. Over the Southwest Mainland showers (mostly in the form
of snow) continue to move through the area and are beginning to
intensify with daytime heating. This may allow for the development
of a few isolated thunderstorms this evening (especially over
Eastern Bristol Bay). Conditions further west over the Bering
Sea/Aleutians are much quieter behind the upper level trough as
weak ridging is allowing for the development of stratus over the
Western Bering and diminishing winds.

&&

.MODEL DISCUSSION...
The models remain in great synoptic agreement through the work
week with the timing of a series of waves that will rotate around
the quasi-stationary upper low this week. Even with this good
agreement, there continue to be differences in the models in the
placement of shower activity as is often the case in these
southwesterly flow events. As a result, details in the precipitation
forecast into Friday focus on where there is agreement, such as
upslope prone areas of the Susitna Valley and Gulf Coast, with
widespread chance POP's elsewhere. Looking into Friday, agreement
begins to improve significantly as a much deeper upper level low
moves through the Bering into the southern Mainland. This is where
much of the focus was for forecast updates today.

&&

.AVIATION...
PANC...Gusty south to southeasterly winds are expected to
continue into Thursday with little change the pressure gradients
along the Chugach. There will be a uptick in the winds this
evening, producing gusts to around 35 kt, which should then
fall to around 25 kt after sunset as daytime heating diminishes.
Otherwise conditions should remain mostly VFR as rain and show
showers pass through the area. Forecast confidence is however
low in the timing and track of these showers, so there is a
possibility that conditions may intermittently drop to MVFR if
heavier showers move through the vicinity of the airport. The
greatest risk of showers moving near the airport will be from
convective activity along the Chugach this afternoon/evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Persistence will be a great forecast for the next couple days.
The driver will be the upper level long-wave trough, but more
importantly, the cold air contained within. There is a weak short-
wave moving up Cook Inlet tonight, which looks to focus convection
along the western Kenai Peninsula, keeping it out of the
Anchorage Bowl. However, it won't be far away. These showers
should be mixed with rain and snow, possibly turning to all snow
in the Susitna Valley. Instability will be increasing as the cold
core aloft moves overhead tomorrow. Scattered rain showers will
increase in coverage during the afternoon, especially along the
higher terrain of the Chugach. These showers should be mostly
rain, but could see some rogue flakes or even graupel mixing down
to the surface in the more intense showers. The back edge of a
short-wave will pass late in the evening, bringing an end to the
showers, but the break will be short-lived.

A much stronger shortwave will be dropping out of the Bering
Strait region late Thursday evening. The effect will be first to
tighten the pressure gradient, returning stronger southeast gap
winds to Turnagain and Knik Arm, second to return rain and snow to
most areas. There is not any strong temperatures advection, either
warm or cold associated with this system, so precipitation type
should be strongly tied to the diurnal surface/low-level
temperatures. It likely will be rain during the day, mixed with
or changing to snow at night depending on location/elevation.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Cool and wet weather continue to plague Southwest Alaska through
the forecast period as an upper level low remains in place over
the Bering, and showery onshore flow persists over the area. A
series of waves will rotate around the base of the upper low
through Friday, bringing enhanced shower activity to the Bristol
Bay coast and up through the Kuskokwim River Valley. Models are
currently indicating these enhanced showers to line up over the
Bristol Bay area overnight tonight into Thursday morning, with the
next stronger wave tracking into the Southwest Mainland Thursday
night. There is potential for these showers to become more deeply
convective in nature as limited surface heating combined with cold
air aloft will lead to steep mid-level lapse rates and decent
instability through this evening. Guidance continues to show a
line of enhanced showers forming from a line between King Salmon,
Iliamna, and Sparrevohn through this evening. Thus, embedded
isolated thunderstorms may be possible in this area.

With the secondary system tracking in Thursday night, another
round of enhanced showers and gusty winds are expected to spread
from the southwest coast inland through Friday afternoon. Colder
air associated with this system as well as plenty of antecedent
cold air at the surface should keep precipitation as snow for most
locations, though expected accumulations remain minimal at this
time.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A broad upper low over the northeast Bering will remain stationary
through Thursday morning before shifting northward over the
Bering Strait by Thursday afternoon. Cool and showery conditions
over the eastern half of the Bering/Aleutians are expected to
persist through Thursday afternoon, under gusty westerly flow.
Whereas over the western half of the area, shortwave ridging
begins to build over the area tonight, bringing drier conditions
to the western/central Aleutians through Thursday afternoon. With
the shortwave trough dropping southward over the eastern Bering
Thursday, we are expecting colder temperatures and snow showers
to persist over the area through Thursday night.

By Thursday evening the next system enters the western
Bering/Aleutians, and tracks eastward through much of the Bering
and Aleutians by Friday night. This system will spread gale force
winds and rain along much of the chain, as it pulls warmer Pacific
air northward and tracks eastward across the Aleutian chain
through Friday night.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A system over Southeast Alaska Friday evening will quickly move
east and dissipate Friday night. A fairly significant upper low
will be over the central/eastern Bering Friday night. The leading
front will push across Southwest Alaska early Saturday morning
and then into the Gulf and Southcentral during the day Saturday.
The upper level low will move eastward across the Gulf Saturday
night. A fairly strong surface low will move into the Bering on
Saturday. After this, the model guidance begins to diverge. The
models agree on continued fast zonal flow aloft, along with the
general idea of an upper low in the Bering and upper ridging over
the northeast Pacific into eastern Alaska. However, the location
and timing of various systems has a large spread in the guidance.
Therefore the fairly active weather pattern will continue, with
the details not well defined. Thus, the WPC guidance begins to
lean fairly heavily on the ensemble means after Monday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 175 176 177 178.

&&

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