Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
325 PM akdt Thursday Aug 28 2014
Short term...another day another low. This one...already over the
central Gulf...will slow its momentum as the main jet on its
backside will continue diving to the southeast across the western Gulf.
This low will approach Baranof Island by Saturday afternoon. There
does appear to be a subtropical connection for good moisture
spreading into the Panhandle in the form of rain already having
begun in Yakutat last night and slowly spreading coastward this
afternoon. We cannot deny that much of the rain around Yakutat is
a little showery in nature. The air mass has changed little and
even considered going with showers for the entire time period
southeast-wide. But the shape of the system and its behavior fell
in line just enough with classic Norwegian model of cyclones. The
moist conveyor belt will be stratiform ahead of the system...even
though temperatures aloft only moderate slightly...while the areas
behind and underneath its center will be convective with the
slightly colder air. Continued and expanded risk of thunderstorms
from zone 43 south. Considered adding Sitka but sref probabilities
for lifted indices less than negative one seemed to back off from
Baranof Island...although cape from GFS and NAM does reach the
southern portions of the island. Even with parameters
indicating positive environment for thunderstorm development, they
are notoriously difficult to predict in the marine environment of
Southeast Alaska. We kept the idea of spreading the convection
onshore in all but the northern interior Panhandle for Friday
morning as the cold core's proximity increases with the low's
approach. Finally Friday afternoon we paint everything as showers.
Precipitation will start to draw southward Friday afternoon as the
focus directs more towards the southern Panhandle. Yakutat may
indeed dry out a little due to more offshore component of middle-
Employed a lot of NAM 850 winds to dictate wind flow...especially
in spots where gradient is parallel or even slightly offshore.
This ensures most places remain south. A front will push into the
southern Panhandle early Friday morning causing winds to
accelerate near Hydaburg, Lincoln Rock, and even Baranof Island overnight.
Winds will rise with the frontal passage north to Juneau by
Friday morning. This is not a very strong front as there is no
significant baroclinic zone, but small craft will likely be met in
Frederick Sound, and Clarence may approach gale.
Used NAM/European model (ecmwf) for pressure/wind updates. Some help from sref as
Long term...models continue to show a series of short wave
troughs moving across the Gulf this weekend into next week. The
first will be passing over the southern Panhandle Friday night.
Instability will decrease Friday evening as the upper low and
associated cold pool aloft move inland. Marginally unstable
atmosphere will support scattered showers through the day Sat
before precipitation becomes stratiform as the next wave
approaches the region heading into sun. Precipitation chances will
continue into early next week as the third wave impacts the
region. Refreshed the inherited forecast package with NAM/ec
through the weekend with minor overall changes. Zonal flow
persists through Tuesday before models trend towards a building upper
ridge over the Gulf. 00z ec showed troughing middle to late next week
while 12z solution is ridging. Given the poor continuity...trended
forecast towards wpc guidance. Overall...above average forecast
confidence into early next week with confidence decreasing to
below average by middle next week.
Marine...Small Craft Advisory for pkz022-033>036-041>043-051-052.
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