Small Craft Advisory 339, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 AM AKDT Wed May 1 2019

.ANALYSIS AND UPPER LEVELS...

A surface low over Bristol Bay this morning is the main weather
driver over Southwest and Southcentral this morning as its front
pushes inland.  Rain showers have been observed extending from
the AKPEN eastward towards Iliamna and Sleetmute, with a few wind
gusts across the area.  An upper level ridge continues to be
positioned over Southcentral, resulting in high pressure and fog
and stratus across the northern Gulf coast. Yesterday, the high
temperature was 60F at Anchorage and 62F at Palmer and Wasilla.
Over the western Bering, a mature low continues to weaken north of
Shemya.

&&

.MODEL DISCUSSION...
The GFS and the ECMWF are in decent agreement with the placement
of the major synoptic features from the Gulf of Alaska, across the
Bering and to Kamchatka in the short term period. The GFS
ensemble members are tightly grouped through 12z Thursday.
Sensible weather-wise there are some vagaries in the models
regarding precipitation distribution over western Alaska and the
AKPEN but the differences are negligible.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

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

A high amplitude upper level ridge extends out of the Gulf of
Alaska and over the Copper River Basin this morning. This ridge
will weaken and move eastward through Thursday as a series f
fronts moves into Southcentral from the southwest. The first of
these fronts is dissipating over Cook Inlet this morning where it
is bringing mostly just cloud cover. A stronger front will move
over Kodiak Island and into southern Cook Inlet tonight and then
into the Anchorage and Mat-SU areas Thursday. The timing of this
front has been a bit difficult to nail down as the model runs are
fluctuating with the speed of this front. Therefore the morning
forecast has been updated to show rain arriving a little earlier
in the day tomorrow over most areas close to Cook Inlet due to
this. By Friday the front will have moved through the area with
much colder temperatures aloft which may leave the area more
susceptible to showers on Friday especially over the mountains.

&&

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

A developing low off the coast of Cape Newenham this morning and
its associated front will be the main weather-makers across
Southwest Alaska for the next couple days. The low is expected to
slowly drift north to near St. Lawrence Island by Friday morning.
Due to the low's slow movement and it's slow deepening, unsettled
weather will be the norm for much of Southwest Alaska. The low
will move north of Nunivak Island by Thursday. As it parallels the
coast today and tonight, rain will redevelop for all of the region
west of a Sleetmute to King Salmon line by this afternoon.
Tonight, the low will wrap around itself, which in turn will cause
strengthening onshore westerly winds to allow the rain and the
front to advance eastward. Thus, most of Bristol Bay and the Lower
Kuskokwim Valley will get some steady rains tonight into Thursday.

Meanwhile, cold air wrapping around the low will move in from the
Bering. The air mass will be cold enough that snow will be
increasingly likely to be mixing in with the rain, especially
across the Kuskokwim Delta. Fortunately, with the steadier rains
further east, these should be the form of temporary snow showers,
and is unlikely to result in much accumulation, especially during
the day, when the May sun should help any snow that reaches the
ground to melt on contact.

By Thursday afternoon, the front and its associated rain will move
into the Alaska Range and Southcentral, allowing some drying to
take place. However, the colder air moving in and steady onshore
flow will make fog and low ceilings increasingly likely Thursday
night. Another weak upper level wave may swing through during the
day Friday, causing snow and rain showers over much of the
Southwest. A brief break is expected Friday night with weak high
pressure building in, but without any removal of the moisture, fog
may be a concern again.

&&

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

The developing low off the coast of Cape Newenham will drift
northward through Friday, reaching St. Lawrence Island by Friday
morning. Most of the Bering should continue to deal with low
clouds and stratus as a colder air mass is drawn south and eastward
around the low. The low may briefly produce gales north of Nunivak
Island Thursday night as it peaks in intensity. Elsewhere, the
typical low stratus will continue to persist over much of the
Bering.

Another strong front will approach the western Aleutians Thursday
evening, and will continue moving eastward into the central
Aleutians by Friday evening. Gales are expected with this front,
particularly from the Aleutians south. However, since the front
will be fairly fast moving, the gales should not last too long at
any one location. The front will approach the Pribilofs and
eastern Aleutians Friday night as the parent low approaches
Shemya.

&&

.MARINE (Days 3 through 5)...
A low to the west of the Y-K Delta has the potential to produce
west to southwesterly winds in the 30 to 35 kt range along and
near the Kuskokwim Delta coast on Thursday where the cool,
unstable air mass behind the low could allow stronger gusts to
mix down. A front approaching the western Aleutians Thursday night
and central Aleutians Friday also has the potential to bring
Gales along with it. For the Gulf, winds are likely remain in the
20 to 25 kt or lower range from Thursday through Saturday with
only localized areas of stronger winds possible through favorably
oriented terrain gaps.

&&

.LONG TERM FORECAST (Days 3 through 7)...
By Thursday, the upper level pattern will have progressed to a
having a large upper level trough stretching across the Bering and
into Southwest Alaska with ridging extending north across the
Gulf. A frontal system over Southwest Alaska will swing east into
Southcentral Alaska Thursday night and Friday as the upper level
ridge flattens and shifts further east. The same general pattern
of upper level troughing over the Bering and flat ridging over
the Gulf and Western Canada will continue through the weekend and
the first half of next week. The persistent southwesterly flow
aloft over southern Alaska will allow a series of upper level
troughs and accompanying Bering/North Pacific lows to swing across
the area and keep conditions generally on the cool and damp side.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AH/PJS
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
MARINE/LONG TERM...JER