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FXAK67 PAJK 182305
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
305 PM AKDT Fri Aug 18 2017

.SHORT TERM.../Tonight through Sunday/As of 1400 Friday afternoon,
a weak ridge of high pressure is currently building out of the
gulf into southern portions of the SE Alaskan panhandle as a
shortwave exits the region. Current visible satellite imagery
even shows what almost appears to be an agitated CU field over the
coastal waters of the gulf near the panhandle, with cells of
heavier precipitation moving over land into the panhandle. A few
lightning strikes have also been witnessed along the coast near
Icy Bay and Dry Bay.

According to WV satellite, the next shortwave
that will impact the panhandle is currently positioned near
Kvichak Bay of mainland Alaska. This shortwave should begin to
progress towards the area Saturday, bringing additional
precipitation, with possible heavier areas in and around Yakutat
Bay. We can also see an additional shortwave currently positioned
near the Aleutian Island chain that contains the remnants of what
was Typhoon Banyan. Current precipitable water analysis shows a
possible atmospheric river setup being initiated with this system
as we head into next week. Models also hint at a coupled jet
situation with this shortwave of interest, including a subtropical
jet advecting additional moisture, with the left exit region
being positioned over the panhandle during the Monday timeframe.
This should lead to additional lift for precipitation due to the
divergence aloft. Since this system originated in the tropics
upper air temperatures will also be warm aloft. Which locations
will receive the heaviest areas of precipitation is still to be
determined.

Most changes to the forecast were made following the GFS model.
PoPs were increased into the weekend and early part of next week
using primarily the SREF, with additional forecaster edits, since
the precipitation will be the big story going into the early part
of next week.

.LONG TERM.../Sunday through Friday as of 10 pm Thursday/ The
extended forecast remains wet and cool. Upper level southwest flow
is advecting large amounts of moisture toward southeast Alaska.
A surface low pressure system over the central gulf Sunday will
send a weather front toward the Panhandle. While the period
leading up to Sunday will be wet, the Sunday`s weather front will
likely reinforce and enhance precipitation over the southern
Panhandle. However, a brief break in the rainfall is possible over
the northern Panhandle Sunday before rain begins again Sunday
night. Between Sunday and Tuesday rainfall totals range between
about 1 to 4 inches, with heaviest rains near Ketchikan and
Annette Island; the heavy precipitation over the southern
Panhandle is expected due to the remnant moisture from tropical
storm Banyan directly impacting the area.

Beyond this system, conditions appear to dry out slightly with
only a chance of rain toward the middle of the week; however,
there is still significant spread among solutions.

Primary changes in the extended were confined to Sunday and
Monday, where winds were increased over the southern Gulf and
Panhandle and decreased over the northern Gulf and Panhandle
associated with better model agreement handling the low pressure
system. Daytime humidities were increased due to the large amounts
of moisture that will be present over the Panhandle as a rainy
period begins. QPF totals were also increased between Sunday and
Tuesday especially over the southern Panhandle. A blend of the
ECMWF and GFS was used for Sunday and Monday with minimal changes
beyond that timeframe.

&&

.HYDROLOGY...Still keeping a wary eye on heavy rain potential
Sun-Mon. System developing south of Adak has abundant moisture
and warm air. It will likely interact with with colder air moving
south from Bering Strait which will help it strengthen into a
potent gale as it loops into the Gulf late in the weekend. Latest
models seem to indicate the associated surface low will be south
of Prince Wm Sound by Mon morning. A broad swath of WSW flow under
the low center will steer exceptionally moist air into the
Panhandle or just south of the Panhandle starting late Sun. We are
collaborating with National Weather Prediction Center on how this
system will evolve and it will be the main focus of subsequent
discussions and forecasts into the weekend.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-013-052.
     Small Craft Advisory due to Seas for PKZ041>043-051.

&&

$$

Voveris/Byrd

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