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Southcentral and southwest Alaska forecast discussion 
National Weather Service Anchorage Alaska
427 PM akst Wednesday Mar 4 2015

Analysis and upper levels...
water imagery vapor shows a weak upper level low over the Gulf
with a weak trough stretching south across the Copper River basin
with a fairly sharp trough evident over the central Bering
Sea/Aleutians. Light snow was occurring across the Copper River
basin. Low clouds(stratus) was observed across much of Cook
Inlet...Anchorage bowl...and portions of the Susitna Valley.
Fairly widespread fog under a weak inversion was evident across
the Anchorage bowl. Light rain and snow...associated with a weak
low over the Gulf...was observed along the north Gulf Coast. A
lower 990s low was apparent on satellite imagery southwest of the
Gulf of Anadyr (russia). A cold front associated with this low
extended south across the eastern Bering Sea...just east of the
Pribilof Islands...to over the eastern Aleutians. Satellite imagery
shows open cell cumulus behind the front which is a good indicator of
cold advection.

&&

Model discussion...
models initialized fairly well today. Timing of the upper trough
moving across the Mainland Thursday through Friday morning is
fairly similar. The NAM solution continues to show a much sharper
upper trough moving across southcentral and a stronger southerly
jet with a more robust vorticity field. While models differ in quantitative precipitation forecast
amounts...the pattern and timing are similar.

&&

Short term forecast southcentral Alaska (days 1 and 2)...
light winds across the southern Mainland and a weak temperature
inversion just off the surface will be another set up for areas of
fog to develop overnight...especially around area waters. Fog
conditions are expected to improve by late Thursday morning.
The surface low currently in the Gulf is spinning southeast of
Middleton Island with rain showers developing north of the low
center. Low level southeasterly flow is pulling a few showers into
the Prince William Sound and along the coast. This weak pattern
continues through Thursday afternoon and then a pattern change
begins as a front moves into the western Alaska Range Thursday
night. An upper level short wave gains momentum after midnight
pushing across to the Copper River valley and eastern Gulf Friday
morning. Snow will develop along the wave of energy bringing
accumulations across much of southcentral with the highest amounts
along west facing slopes.

&&

Short term forecast southwest Alaska (days 1 and 2)...
an upper level shortwave will continue to push into the southwest
Alaska region from the eastern Bering Sea tonight. This synoptic
feature will exit the region by Thursday morning. This will bring
periods of snow through tomorrow morning before tapering off to
scattered snow showers. By late Thursday night the next weather
front will begin to approach the southwest coastline. This will
bring a swath of moisture with gale force southwesterly winds to
the coastline. The temperatures will continue to warm above
freezing by Friday with the onshore flow and coinciding with the
warm air advection.

&&

Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the gale force low west of Saint Matthew Island will move through
Norton Sound before moving inland Thursday morning. The
associated weather front with this system is moving inland along
the southwest Mainland this evening. On the backside of this low
pressure system the central Bering/Aleutians will continue to have
northwesterly winds through Thursday afternoon. By Thursday
morning the western Aleutians will have a weather front pushing
through near Shemya associated with a storm force low positioned
along the Kamchatka peninsula. This will bring gusty southerly
winds with a large swath of moisture and warmer temperatures to
the western Aleutians...then spreading into the center Bering
Sea. As the low pressure system moves to the northeast this will
usher in cooler temperatures to the western Bering
Sea...destabilizing the lower atmosphere and helping to mix down
storm-force southwesterly winds that will weaken and move into the
central Bering Sea by Friday evening. During the same time frame
the eastern Bering Sea will have gale force winds. This will bring
showery precipitation throughout the area overall as the low
pressure system moves into the Bering Strait by the end of this
week.

&&

Long term forecast (days 3 through 7)...
a progressive pattern will continue to bring precipitation to a
wide swath of Mainland Alaska from Friday night through the
weekend as a strong polar jet spans from the western Aleutians
across the southern Mainland and a broad surface low over the
northern Bering tracks inland near the Seward Peninsula on
Saturday. Precipitation should change over to mostly snow across
southcentral Alaska on Saturday as colder air begins to filter into
the region. After that...confidence continues to increase that
significantly colder temperatures are in store for the start of next
week as zonal flow gives way to a more amplified pattern with an
upper trough digging toward the eastern Gulf. This will also
usher in a period of drier weather across the southern
Mainland...with gusty outflow winds developing along the north
Gulf Coast.

The exception to the overall strong model agreement in this time
period is over the western Bering...where there are significant
discrepancies with the placement of a surface low in the vicinity
of the western Aleutians. Most of the guidance keeps this feature
south of the chain...whereas the GFS brings it into the central
Bering by Monday afternoon. Sided with the more southern solution
for now which would keep the bulk of the impacts south of the
Bering. Models are in better agreement by the middle of the week
as high pressure builds over the Bering...which would help to re-
enforce the trough over the Mainland.

&&



Afc watches/warnings/advisories...
public...none.
Marine...gale 170 172 173 174 175 176 177 178 179 185
heavy freezing spray 185
fire weather...none.

&&

$$

Synopsis and model discussion...British Columbia
southcentral Alaska...kh
southwest Alaska/Bering Sea/Aleutians...period
long term...cumulonimbus

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