Small Craft Advisory 307, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
432 PM AKDT Sat Apr 14 2018

.ANALYSIS AND UPPER LEVELS...
An amplifying upper level ridge continues to build over the
Southwest mainland and Southcentral Alaska today. The 250 mb jet
stream is running up and over this ridge (and down the other side)
which is causing very different weather for areas east and west of
it. The upper-left exit region of this jet is over the central
Bering Sea and continues to feed the strong surface low in the
western Bering which has strengthened to under 970 mb today.
Conversely, the strong northerly flow and subsidence over Southcentral
Alaska is bringing warm and sunny weather to that region. There
has been enough warming the past couple of days in Southcentral to
start some afternoon and evening sea breezes in the typical
coastal locations.

Southwest Alaska mainland, while mostly under the surface ridge,
is seeing some widespread fog and stratus due to the southwest
flow associated with the pressure rises and subsequent onshore
flow under the upper level subsidence.

&&

.MODEL DISCUSSION...
It is actually a little difficult to discuss much with the models
today as they are in staunch agreement though the middle of next
week. About the only reoccurring question is the strength of the
sea breeze which is beginning to develop in the coastal areas
during the afternoons and evening. Even the hi-resolution NAMnest
is having a difficult time picking these out as the effect is
still weak.

&&

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

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A weak shortwave will continue to bring some areas of light snow
to the Copper River Basin, which will be reinforced and become
more widespread this evening as the stronger shortwave moves
overhead. The most widespread shower activity should occur near
the Wrangell Mountains. Another impact from this shortwave will
be from cooler air and increased gap winds out of the Copper River
Basin beginning the evening. The strongest winds are expected to
blow out of the Matanuska Valley and through Thompson Pass tonight
and into Sunday morning, though we are confident that these winds
will remain well below advisory criteria. The coldest
temperatures into the teens from this system are still expected to
occur in the low elevations of the Copper River Basin where
little wind is expected, though overnight lows in the 20's are
also expected in the Susitna Valley this weekend.

This shortwave will dive south into the Gulf on Sunday, leaving
much of the area under the influence of a shortwave ridge ahead of
an incoming front to the Western Gulf/Kodiak Island into early
next week. This front is expected to primarily impact the western
Gulf and extreme southern Kenai Peninsula, leaving the remainder
of the region with typically dry spring time conditions and
seasonable temperatures.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Monday)
The front along the west coast remains stationary through the
evening hours bringing steady light snow, rain and gusty
southeasterly flow to the coast before moving inland Sunday
morning. Southeasterly flow is expected to gust along the frontal
boundary as it moves slowly to the Alaska and Aleutian Ranges on
Monday. Precipitation will mainly be rain for inland locations,
however brief snow is possible during the early morning morning
hours on Sunday when temperatures hover near the freezing mark.

Moderate rain develops across the Western Capes of Bristol Bay
Sunday afternoon through Monday night as a strong upper level wave
rotates through and stretches along the west coast. Rainfall
amounts up to one half of an inch are possible along the coasts
with localized higher amounts. The upper level wave shears apart
Monday afternoon as it runs into the interior ridge, leading to
rapid dissipation of the front during the evening.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Monday)
The stacked storm system currently over the western Bering deepens
through this evening as it lifts across the northwestern Bering,
then begins to fill after midnight. A broad swath of southerly
gales across the eastern waters continues as the associated frontal
boundary stalls along the west coast through early Sunday. Across
the western waters, strong westerly gales increase tonight on the
south side of the low core due to a colder air mass wrapping
around the base of the storm system. A few storm force gusts are
likely to mix down to the surface as a strong short wave rounds
the base after midnight. Winds should taper down quickly as the
front pulls inland Sunday afternoon.

Precipitation will primarily be rain with the current storm
system mixed with snow at times as cooler air wraps around the
storm core. A pulse moving through the upper trough brings
moderate rainfall tonight across the Central Aleutians to the
Pribilof Islands and then across the Eastern Aleutians and Alaska
Peninsula on Sunday.

This system weakens as it continues on a northward track late
Sunday with the next system quickly following into the western
Aleutians Sunday night.

The second storm system takes a faster track as the upper jet
stream ridge pattern de-amplifies and allows a 150+ knot
jetstreak to nose across the Aleutian Chain. This storm system
should be monitored as it is likely to be stronger than currently
projected as it moves up from a data void area across the North
Pacific.

&&

.LONG TERM FORECAST (Days 3 through 7: Tuesday through Saturday)...

Tuesday: Tuesday's weather picture starts out with a strong,
sprawling area of low pressure centered to the west of the
Pribilofs over the central Bering Sea. The remnants of the
occluded front are primarily over the waters of the eastern
Bering, extending southward from the central Alaska Peninsula into
the Pacific. With broad, moist southeasterly flow over Southwest
Alaska, there is steady rain and snow from about Koliganek
westward into the Kuskokwim Mountains. Downsloping off those
mountains will be keeping the Kuskokwim Delta dry. Meanwhile all
of Southcentral except Kodiak and possibly the southernmost areas
of the Kenai Peninsula are dry under strong ridging. As the front
moves into Southwest Alaska on Tuesday, precipitation will break
out further and further north in Southwest Alaska. It also moves
into the southern Kenai.

On Wednesday, the front will swing across the remainder of the
Gulf Coast, but makes very little progress moving north with both
the parent low slowly moving southeastward and the high topography
of the coastal Chugach blocking it. Strong southeast to east winds
will leave most of the Cook Inlet region downsloped, as is very
typical in this pattern. With a lack of northward progress, the
precipitation, perhaps heavy at times, will remain confined to
both the Gulf Coast and the east facing slopes of most of the
mountains of southern Alaska. This pattern will favor the
development and maintenance of the Turnagain Arm wind.

Thursday will largely be a continuation of Wednesday as far as
Southcentral's weather is concerned. Another disturbance moves
northward through the Gulf from the Pacific, nearing the coast
late in the day. The models disagree primarily on the strength of
this disturbance, from the GFS which is fastest and weakest, to
the EC which has a concentrated closed low moving through the
northern Gulf Thursday afternoon. Other than differences in the
strength and wind direction however, the end result weatherwise
regardless will be a reinvigoration of the precipitation rates at
the coast. For Southwest Alaska, Thursday should be a drier day,
though there are model disagreements on that as well.

By Friday and beyond, the model disagreements only further
increase, both at how the low behaves, and how a new strong low
along the Aleutians behaves. However, the pattern remains
unsettled, so a cloudy, seasonable period for much of southern
mainland Alaska appears on tap going into next weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 155 165 170 172-175 177-181 185 411-414.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...TP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...JW