Small Craft Advisory 343, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 AM AKDT Thu May 2 2019

.ANALYSIS AND UPPER LEVELS...

The main upper level feature is a digging trough extending south from a
low center over the northern Bering Sea to over the north pacific
along 180w. This digging upper trough has setup broad
southwesterly flow across the North Pacific which is starting to
encroaching on the AK Pen/Bristol Bay and western Gulf. Satellite
imagery shows abundant clouds and moisture streaming north under
the upper flow. Observations and radar data indicate rain over
the AK Pen, Bristol Bay, and southern Kodiak Island. The cirrus
shield is also moving north across Cook Inlet and the Kenai
Peninsula.

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.MODEL DISCUSSION...
Models overall have a similar handling of the synoptic pattern.
Differences in sensible weather will resolved by a combination of
the higher resolution models and hand edits.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist today. Rain
moves in Thursday and should bring MVFR ceiling and possibly
visibilities. Gusty southeast winds are expected across the
airport complex late Thursday afternoon/early evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
There remain a bit of forecast uncertainty for tonight through
Saturday in regards to the precipitation moving through the area.
While the rain with the front moving into Southcentral Alaska
from the Southwest today looks good, there is a significant
difference between the GFS and NAM models in how far north the dry
slot will move in the upper level southwest flow. Basically, the
NAM is mostly ending the rain in Anchorage and the Matanuska
Valley by midnight while the GFS keeps a lot of shower activity
going into Friday. Both solution bring rain into the Susitna
Valley, but the GFS drops a lot of precipitation in the Copper
River Basin while the NAM is projecting only a fraction of the GFS
amount. Basically went with the higher POP values and mitigated
the QPF amounts for the morning forecast.

By Friday night, a short wave associated with some colder air
aloft will move into Southcentral and bring another good shot at
rain across the region. The thing to watch with this system is how
the 500 mb wave behaves. The wave may become a cut-off low over
the Prince William Sound area and then head eastward. With this
situation, the rain will be more along the North Gulf coast with
limited amounts in the Susitna Valley and Copper River Basin
Friday night into Saturday. Should the wave not cut-off it should
bring rain (and snow at higher elevations) into these inland
areas with less precipitation along the coast.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and
2)...Rain continues over the mainland from Bristol Bay into the
Kuskokwim Valley and Delta, supported by warm south flow ahead of
a surface low near Nunivak Island. As the low shifts to the
northwest, cold air wrapped around the backside of that low will
bring a changeover to colder conditions and westerly winds. Friday
morning lows will be 5 to 10 degrees cooler than this (Thursday)
morning. Absent the southerly moisture source, precipitation will
diminish later today into Friday, though it's unlikely to
dissipate completely. While the main moisture draw will be cut
off, the cold front will be a source of lift to trigger a few
showers. In addition, as the dominant flow veers from southerly to
west/southwesterly, expect stronger gusts to mix down
particularly over the Kuskokwim Delta later today into Friday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...Benign
weather will continue across the Bering and Aleutians today, with
a weak low in the western Bering potentially generating some
showers, and some cold air advection showers possible over the
Aleutians. A developing low roughly 500 miles northeast of Japan
will drift eastward today into Friday, pushing a front across the
western Aleutians. This will likely bring some southeasterly
gales, and precipitation from Shemya to Kiska, then to Adak and
Atka later in the day.

&&

.MARINE (Days 3 through 5)...
A low will move along the Aleutian Chain Friday through Sunday,
gradually weakening as it moves eastward. This will bring gale
force winds into the Central Aleutinas by Saturday, but winds will
diminish through Sunday. In the Gulf of Alaska, the biggest
concern is on Friday, with winds just shy of gale force thorugh
the Barren Islands and along the north Gulf Coast. By Saturday
these winds will have diminished and winds will remain sub-gale
force through Sunday, with clearing skies overhead.

&&

.LONG TERM FORECAST (Days 3 through 7)...
On Saturday, showers will linger through the eastern half of the
state, while a more substantial low pressure system moves into the
Central Aleutians. Between the two systems, a weak ridge will
form, bringing a shortlived round of sunny skies to western
Alaska. However, as we move into early next week, the longwave
trough will begin to take over the weather pattern, and rainy,
unsettled weather will expand to include the east side of the
state as well. Expect the most steady precipitation to remain
along the coast, with showers moving further inland. By that time,
the low will have weakened enough that it won't be windy in most
areas, just cloudy and rainy across the bottom portion of the
state.
&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...177 176.
FIRE WEATHER...NONE.

&&

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