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FXAK67 PAJK 162256
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
256 PM AKDT Wed Aug 16 2017

.SHORT TERM...A weakening short-wave with an associated low has
moved into the northern Panhandle and we should begin to see
pressure rises shortly this evening as ridging develops behind.
This will translate into small craft winds that due to begin
shortly in northern Lynn Canal, peaking at 30 kt southerly later
tonight. Rain will all but transition to showers as cooler air
migrates aloft behind the front.

A longwave trough deepening across western Alaska will spark a
slight chance of thunderstorms spreading east from the zone 52
later tonight into Thursday. Yakutat and Sitka have been included
in the threat area.

Examining satellite imagery stretching from the gulf westward
across the North Pacific, one can detect a virtually uninterrupted
stream of deep moisture. From afar, it aligns into an even arc of
some sort. But upon closer inspection, one can pick out kinks
within the arc. These are what the models are desperately trying
to detect. We look to have another low weaker low embedded in the
southwest flow on Thursday, again tracking very similarly to the
low today.

Friday, more showers follow within broad onshore flow. While
models indicate more concentration east of Yakutat, a low due to
form across the northern gulf will likely keep showers numerous
over the northeast gulf as well.

High and low temperatures will be less interesting than the tiny
diurnal changes with this pattern. Dress no differently daytime
than nighttime, just take the rain jacket.

NAM/GFS were used for through Friday with a nudge toward GFS
Friday night. Forecast confidence is good for showers and
temperatures.

.LONG TERM...Beyond Thursday`s storm, a weaker shortwave
approaches the southeast gulf on Friday. With solutions
indicating varying degrees of intensity, we drew a moderately
weak system from which we can build if we need to strengthen as
it moves into the Panhandle. If anything, the system will help
reinforce some cooler air aloft and keep showers entrenched
across Southeast in the zonal flow that sets up through Saturday
night. The short-wave`s passage across the southern Panhandle
leaves weak ridging over the Inner Channels and weak troughing
over the gulf. This pattern begins to amplify later in the weekend
as a western gulf system arrives, possessing the deeper tropical
moisture from former Tropical Storm Banyan. Therefore we have
increased southerlies and southeasterlies over the gulf at this
time, but both ECMWF and GFS raises winds even further to 25 kt.

The system pressing into the western gulf will have originated
within a very moist region of the western Pacific. Satellite
imagery already indicates Banyan showing a slight eastward
component within its current trajectory. This will increase as it
approaches the main jet region. GFS ensembles are showing a high
probability of Integrated Vapor Transport in excess of 250 kg/m/s
across the gulf and southern Panhandle late Sunday into Monday.
However, just as quickly this feature ejects to the east. Thus,
this setup is showing signs of atmospheric river development and
will indeed cause rivers to rise. However, at this stage, even if
it does develop, any flooding impacts are still to be determined.
In any event, what we can say with certainty is that these
successive storms are priming soils in the Panhandle for potential
impacts later in the season. In line with its tropical origins,
we raised snow levels up to around 10K feet by late Sunday as
well.

Beyond this, we lowered temperatures over the waters in line with
the mainly mid to upper 50s we have across the land areas. We used
a blend of NAM, ECMWF, GFS Friday, followed by ECMWF and GFS for
the weekend. Confidence was too low to make changes beyond Sunday.
Forecast confidence high for wet weather and high that Monday`s
eclipse will not be viewable from Southeast AK.

&&

.AVIATION...Very moist lower level conditions for all of SEAK
except Yakutat area. South of the Yakutat area, widespread MVFR to
IFR conditions through the day with the lower clouds and rain and
areas of mist/fog. Think the ceiling which have lifted slightly
during the day will lower again as the next weather front moves
inland late Wednesday night. Expect the lower cloud heights up to
about 2000 feet, and visibilities lower to the 2 to 5 mile range
as well.

&&

.HYDROLOGY...Rivers slowly continuing to rise with rains
continuing for the remainder of the week. No flooding hazards at
the present time, however the development of an atmospheric river
aimed mainly to southern half of the panhandle late Sunday then
being drawn northward Monday to Tuesday by a potential secondary
wave in the flow will bear watching. Rain totals over 2 to 4
inches possible across the panhandle through Sunday night and that
is missing the AR`s return northward to the southern panhandle.
Will be watching the river levels and rain rates for the rest of
the week.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-013-042.
     Small Craft Advisory due to Seas for PKZ022-041-043-051-052.

&&

$$

Adkins/Bezenek

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