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Southeast Alaska forecast discussion 
National Weather Service Juneau Alaska
320 PM akdt Friday Aug 22 2014

Short term...the transitions from dry to wet and how they
transpire often present the toughest challenges for weather
prediction in southeast. The early part of this weekend has been
presenting such a case over the course of the last week. With this
package we decided to increase rain chances for the northern
Panhandle in particular tonight.

Looking at the upper levels a shortwave is denting the long-wave
ridge over the northern Gulf this afternoon and will lie over
Yakutat Bay this evening and over the Haines Highway late tonight.
We expect the 00z Yakutat sounding to come in with some
atmospheric cooling for sure. With this trough, definite cold air
will pass over the Panhandle. The cold air will not be apparent at
the surface as this is such a fast moving system. But it should
add enough significant lift in the atmosphere over the northern
Panhandle to spawn scattered showers. Where the showers meet the
higher terrain on their southwestern path such as Juneau...they
will become more numerous. NAM and GFS quantitative precipitation forecast were not impressive,
but quantitative precipitation forecast isn't the important thing here. Relied on the wetter sref
and its true pop field along with the wetter quantitative precipitation forecast of the European model (ecmwf) to
raise the probability of precipitation over the Panhandle overnight. The trough will begin
exiting into Canada Saturday and behind it middle-level winds will
once again turn offshore and dry our skies...at least until the
next storm arrives early next week. The central and southern
Panhandle though will see their dose of showers develop mainly
early Saturday morning and stick around longer during the day.

While their is a layer of marine stratus out over the Bering Sea
it has generally retreated from the coastline this afternoon. With
this passing short-wave and no real reports of fog since early
this morning, we think the marine layer will remain elevated
tonight with some low stratus decks potentially threatening
aviation concerns for the immediate coastal airports. Some fog may
form late tonight on Prince of Wales Island from radiation but
confidence not great.

Temperatures were mainly warmed a little tonight for increasing
shower chances and cloudier skies. The northern areas were
slightly cooler than forecast today. For Saturday...cooled the
southern Panhandle for the showers moving in.

Wind edits were generally small and mainly lowered wind speeds as
gradients will remain very light and mesoscale heating differences
will not be great this evening...nor Saturday.

Confidence in forecast good overall. Used mainly European model (ecmwf) for any
changes needed. Lesser Reliance on GFS/sref.



Long term...long range models are still having difficulties for
middle week and beyond as they try to understand what will happen
with a upper low as it crosses the Gulf late in the week. GFS was
quite possibly the worst off in this regard as it went from a
upper ridge over the Gulf in one run to an upper trough in the
next in the same place and time. European model (ecmwf) and Gem had a better time
of it, starting off with something close to the current run but just
a bit off with position and timing. Generally the overall and Middle
Range pattern has not changed much with a broad upper trough over
much of Alaska for much of the week.

At the surface, Middle Range forecast has not changed much with
another front approaching the Panhandle by Sun night. Indications
still paint a wet start to the week as first the front moves
through on Monday followed by what is left of the surface low by middle
week. Highest rainfall amounts will be Monday into early Tuesday
associated with the front. However, with a weaker tropical
connection and the fact that it is moving through at a decent
clip, total amounts should only be around half an inch through Monday
night. Rain will continue through Wednesday as the remnant surface low
crosses the northern Gulf following the upper low but amounts
should be light. Wind will not be much of a factor either. Highest
expected winds are out in the Gulf with frontal passage but even
then it will only be 25 knots at most.

Like the upper level features, the surface features past middle week
are also in a state of flux. Systems that yesterday's runs were
moving through the northern Gulf have shifted farther to the
south. Again this is especially apparent in the GFS which has
dropped the system all the way into the northern Pacific leaving
most of the Panhandle dry. Meanwhile the more consistent European model (ecmwf) as
the low going through the southern Gulf and painting the entire
Panhandle with rain. Overall adopted a more ensemble approach for
the extended period using a combo of wpc and European model (ecmwf). This still
resulted in more likely and chance probability of precipitation for the extended for the
Panhandle.

&&

Ajk watches/warnings/advisories...
public...none.
Marine...none.

&&

$$

Jwa/eal

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