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Southeast Alaska forecast discussion 
National Weather Service Juneau Alaska
551 am akdt Friday Aug 22 2014

Short term...a ridge of high pressure remains over the eastern
Gulf...however the satellite image looks a lot different this
morning compared to yesterday. Flow at the middle and upper levels has
turned onshore as the ridge flattened some allowing clouds to flow
over the now "dirty" ridge. Most of the clouds are thin high
cirrus that will still make for a mostly sunny morning for the
Inner Channels. Along the coast there is a lower deck associated with a
marine layer. The high clouds are obstructing The View of the
marine layer on GOES infrared satellite however a polar orbiter that
passed over early this morning...the modis-viirs fog product
showed that the marine layer still has large breaks in it and is
largely over the western Gulf. This coincides with the
observations from Middleton Island which has had a deck of low
clouds between 800-1200ft for more than 24hrs. Because the marine
layer is not fully developed on the eastern half of the
Gulf...have left mention of fog to this morning in places along
the outer coast. Think that it will back off/lift through the
day...although remaining mostly cloudy due to the high clouds. Tonight
if the marine layer moves back into the coastal areas think that
it will be more similar to what Middleton Island has seen with low
clouds instead of fog with visible restrictions.

Models show vorticity moving over the ridge through the day and an
upper level trough passing over the northern Panhandle late
tonight. Expect the wave of vorticity to cause increasing clouds
through the day then the trough to introduce a slight chance of
rain later tonight. The NAM/GFS/Gem keep the area dry while the European model (ecmwf)
and sref have higher probability of precipitation. The best possibility for precipitation is as
the vorticity passes over the thicker marine layer deck some
drizzle could be squeezed out...but think that at most the Inner
Channels would see is a few sprinkles.

More cloud cover today along with slightly cooler temperatures
moving in aloft will make for high temperatures a few degrees
cooler than yesterday. Min temperatures for Friday night will on
the other hand be warmer due to the clouds holding in the radiation.

Used an ensemble approach for most forecast fields which resulted
in only minor changes. Most other adjustments were for local
effects and based on adjustments compared to yesterday and current
conditions. Forecast confidence is average.

Long term...upper level model agreement on the synoptic scale
very good through early Thursday morning. At that time, European model (ecmwf)
develops an upper low on an upper trough and moves it into the
Gulf. GFS depicting a weaker version of the same this about 36
hours later. By Saturday, GFS has a fairly compact upper low over
the northern Gulf with significant jet support at the base of the
trough. European model (ecmwf) by this time is depicting a much more broad upper
low with weaker jet support farther south and west and very
little steering flow over Southeast Alaska. At the surface,
European model (ecmwf) run to run consistency breaks down by Wednesday evening, so
no confidence builder there. Technical problems preventing a look
at the previous GFS run.

Extended range forecast begins with a broad ridge bisecting the
Gulf from south to north and a weak thermal low lies over western
Canada. To the west, a low near 160w and south of the peninsula
will be pushing a fairly strong and well developed front to the
east. As it presses up against the middle Gulf Ridge, much of its
energy and precipitation will dissipate. However, models
suggesting the development of a triple point low south of the
Kenai Sunday evening as the parent low tracks north over Bristol
Bay and into the southwest interior. Jet strength and positioning
not optimal for said development. GFS and NAM certainly weaker
with the low and associated frontal precipitation reaching the
Panhandle. Gem much more robust with the precipitation and jet support
for the triple point. European model (ecmwf) is the most progressive with the
triple point front and has most of the Panhandle under precipitation by
middle day Sunday. Previous forecast heavily weighted towards European model (ecmwf)
and given the model divergence, opted to not make any major course
corrections with this forecast.

Few changes in the extended range forecast. Overall confidence is
average to below average.



&&

Aviation...the marine layer discussed in the short term section
is causing some IFR conditions along the coast this morning at
paya...pasi and pags. Expect this deck of low clouds to move in
and out through the early morning then back offshore more in the
afternoon with a return to VFR conditions. The marine layer will
likely cause ceiling restrictions again along the outer coast late
Friday night/early Saturday morning.

&&

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

&&

$$

Ferrin/fritsch

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