Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
316 PM akdt Wednesday Aug 20 2014
Short term...after a foggy morning across many parts of the
inside channels sunshine above and light offshore winds did their
best to make it a dry and clear afternoon across Southeast Alaska.
There have been exceptions. Fog associated low clouds hung out
over Angoon and especially the Frederick Sound vicinity through
midday. At this early afternoon hour...Petersburg is still
reporting some patches of fog and webcams confirm this. This has
limited heating in zone 26, and a repeat performance may keep
Wrangell, Petersburg, and Kake out of the 70s tomorrow.
The weather over southeast Wednesday night and Thursday will be
affected by two short-waves and one building ridge aloft. First a
weak short-wave is sliding down the mountains of southern Yukon
into northern British Columbia. This has been responsible for some
cloudiness over Lynn Canal but is finding it difficult to make
much progress southward...on the Alaska side of the Coast
Mountains. Some rain showers over Whitehorse are enough to leave
the scattered showers for the Skagway with the greatest chance
over White Pass. The weak vorticity ahead of this short-wave will
couple with some surface heating near Hyder Thursday afternoon
enough to cause US to offer a slight risk of a thunderstorm for
Thursday afternoon. Lifted indices are well negative and cape is
somewhat impressive by Alaska standards. In any event...Hyder will
more likely see some scattered showers passing through in the
afternoon. Another short-wave supporting a weak low over the outer
Gulf is exiting to our southeast and may increase some middle-level
clouds over Prince of Wales Island this evening before it heads
toward Vancouver Island. Behind this short-wave, the ridge aloft
will build squarely over the Gulf region warming and drying
temperatures in the middle-levels over the entire County Warning Area. This will dry
and warm our surface temperatures even more to mainly 70s with the
exception of the north/central coastal areas and the Frederick
Sound area again for the possibility of fog lasting longer here.
Haines Borough may approach 80 as ridge strengthens Thursday and a
thermal trough extends over the southern Panhandle.
Very little change in the winds...although did strengthen some
northwest winds in the south as thermal trough builds in the
afternoon. This is the main reason we followed the European model
for any changes on Thursday as with the warming temperatures, it
made sense to follow the model with the sharpest thermal trough
feature. Forecast confidence good.
Long term...somewhat active long range period as our weak upper
ridge over the Gulf starts to flatten over the weekend. Friday is
still looking dry as the surface ridge is still sharp enough to
keep the various weak systems to the south and west but that ridge
flattens by Sat with the first chance of rain returning. Into next
week the upper ridge gets pushed east as another upper low sets
itself up over the Alaska Peninsula. This pattern does not last
however as that feature also gets pushed east with mostly west to
east flow taking over late next week.
At the surface generally dry and clear weather remains for Friday
before the first weak system sends clouds and a chance of rain
into the Panhandle on Sat. This first system will not survive
trying to push through the ridge so there is little in the way of
precipitation or wind expected.
It's the next one in line that is more pressing. The next system
comes accompanying the upper low that moves over the Alaska
Peninsula early next week. The front from this one will also have
trouble getting over the ridge so rainfall and wind will not be
that spectacular. Only expecting around a quarter to half an inch
total as the front moves through on Monday and Tuesday. However timing is
still a bit of an issue between models. The GFS is the fastest
while the European model (ecmwf) is a bit slower but still close. Decided on a
blend between the two with wpc data to get a good middle ground
for precipitation. After that forecast becomes a bit more murky as recent
model runs have been bouncing around as far as what is where.
Generally favored wpc with its use of more ensemble solutions.
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