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Southcentral and southwest Alaska forecast discussion 
National Weather Service Anchorage Alaska
509 am akst Tuesday Mar 3 2015

Analysis and upper levels...

The highly amplified pattern continues with the base of the
longwave trough centered around 39n 162w. The ridge axis over the
western Bering Sea has begun to shift eastward as a developing
upper level low over the Kamchatka peninsula and its surface
reflection have begun to shift eastward toward the Bering Sea.
Toward southcentral Alaska...a potent shortwave has rounded the
base of the upper level trough and quickly pushed into southcentral
Alaska overnight. This brought widespread rainfall to the Kenai
Peninsula and portions of the northern Gulf Coast...that has now
pushed into Anchorage and the mat-su valley. Toward southwest
Alaska...a broad area of low pressure has moved onshore and has
brought an extremely moist air mass over Bethel and King Salmon
with precipitable water values (derived from the 00z March 3 soundings) of 0.54
inches and 0.68 inches respectively. These values are in the 99
percentile for this time of the year...however it appears as if
cross barrier flow over the kilbucks...the ahklun...the Alaska
Range and Aleutian Range has been eating up a majority of the
precipitation and keeping ceiling MVFR or greater. With the upper
level trough shifting east...cross barrier flow is beginning to
weaken and precipitation has begun to fill in the previously
downsloped locations.


Model discussion...

Models remain in remarkably good synoptic agreement through the
majority of the short term. The difficulty as usual lies within
the details...especially with precipitation type this morning over
southcentral Alaska and the lower Kuskokwim valley. While model surface
to 850 mb thicknesses hint that precipitation should be falling as
rain across Anchorage this has actually been falling
as light snow. Thicknesses are expected to continue to warm which
will pave the way for the snow to steadily transition to rain
during the morning hours. The next big question is the surface
temperatures. With a warm front now along the northern Gulf
Coast...temperatures should begin to warm and limit the potential
for freezing rain. Other than the aforementioned precipitation
type issues...models are in very good agreement with an upper
level low pushing off Kamchatka this afternoon and moving into the
northern Bering Sea where it is expected to bring a blast of much
colder air from Russia. A second Kamchatka low will push into the
Bering Sea again on Friday and bring a reinforcing shot of colder


Short term forecast southcentral Alaska (days 1 and 2)...
the upper level wave that is beginning to move northward onto the
north Gulf Coast will bring a wintry mix of precipitation to the
Gulf Coast and southcentral today. The biggest forecast challenge
continues to be in determining precipitation type over
southcentral. This mornings Anchorage sounding does not have a
significant warm nose...however there is a deep saturated
supercooled liquid water layer that may allow for some light rain
or drizzle to mix with the snow this morning. A limiting factor to
this precipitation will be weak downsloping from the Chugach Mountains as
the front associated with this system continues to slowly weaken
from the north Gulf Coast into Prince William Sound. Weak warm air
advection from this system and daytime heating should allow the
low levels to warm enough to change any remaining precipitation
over to rain by this afternoon...however as the upper trough moves
northward through the Mainland these should also become more

Conditions over the southern Mainland will then continue to slowly
improve into this evening as the upper trough moves out of the
area and weak ridging slides in from the southwest coast. The Gulf
Coast on the other hand will continue to see rain and snow showers
into Wednesday as another low moves into the southern Gulf on
Wednesday morning. There is good confidence that this system will
stay far enough south to keep southcentral cloudy and
dry...however a few slop over snow showers cannot be totally
ruled out on Wednesday.


Short term forecast southwest Alaska...

Areas of rain across southwest Alaska will gradually weaken in
intensity and mix with or change to snow as cooler air filters
south and east across the southwest Mainland through tonight. Some
light snow will remain mainly across the Kuskokwim valley and
Bristol Bay Wednesday into Wednesday night...with an approaching
weather front moving into the Kuskokwim Delta Wednesday afternoon
into Wednesday night.


Short term forecast Bering Sea/Aleutians...

Scattered rain showers will end from west to east today across
the southeast Bering and adjacent land areas including the akpen
and eastern Aleutians. This will be immediately followed by
arrival of mainly cold rain with some wet snow from a decaying
front moving across the Bering. This gale-force front is currently
beginning to move into the western Bering and western Aleutians
and will reach the central Bering and central Aleutians as
weakening begins this evening before reaching the eastern Bering
and eastern Aleutians early Wednesday...then reaches Bristol Bay
and the akpen by Wednesday night.


Long term forecast (days 3 through 7)...
shower activity will linger over the southern Mainland on Thursday
as weak low pressure remains over the Gulf. Thursday evening into
Friday morning a sharp trough from the west will bring precipitation
over the Cook Inlet region. Precipitation type will depend on how
much warm air is left at the surface but should change to snow as
colder air filters in from the west. An active pattern will then
continue through the weekend as a series of waves track quickly
eastward across the region in zonal flow beneath a strong polar
jet centered between 50n and 60n latitude. A sprawling low will
move into the Norton Sound region early Saturday and spread
precipitation over much of the Mainland for the weekend.
Precipitation type will remain a challenge as temperatures hover
near freezing...with mixed precipitation likely for many
locations. After that...models are in reasonable agreement that a
more amplified pattern will begin to take shape by early next week
as as a deep upper trough digs over the Mainland and high pressure
builds over the western Bering. This scenario would result in a
return to much colder temperatures across a wide swath of the


Afc watches/warnings/advisories...
Marine...gale 175 176 177 178
heavy freezing spray 180 185.
Fire weather...none.



Synopsis and model
southcentral Alaska...dek
southwest Alaska/Bering Sea/Aleutians...cirrocumulus
long term...cb/mtl

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