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FXAK67 PAJK 222244

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
244 PM AKDT Tue Aug 22 2017

.SHORT TERM.../through Thursday night/ A slow moving cold front
will be east of the area by early this evening. Low pressure over
the far southern gulf will drift ESE to near Haida Gwaii by Wed
evening. An occluded frontal system will move into the eastern
gulf by Thu evening. Used mainly the 12z NAMnest to handle
adjustments through Thu night, but did use some of the 15z SREF
to help with POPs.

Heaviest rain will be exiting east of the area early this evening
as cold front moves out. More showery precip will move in as low
moves toward the area later this evening and overnight. Still
could be a few TSTMS near and east of the low center over the gulf
tonight into Wed with forecast CAPE values around 300-400 J/KG.
Think they should remain offshore though, so no mention for land
zones. Most of the showers will be over the southern half of the
area later tonight into Wed with low approaching, while areas
further N see less shower activity. Strongest winds will be in
band to east and north of low center with 25-30 kt likely over the
southeast gulf and far southern inner channels tonight into Wed.
The showers and winds should diminish Wed night as low moves over
Haida Gwaii.

Next frontal system should spread rain into the area Thu afternoon
and evening. Further ahead of the front, showers will likely
increase in coverage ahead of the main rain band as low level
convergence increases over the area and shallow instability
continues. This system looks fairly wet, but nowhere close to the
rainfall amounts that occurred with the current system is
expected with the Thu night front at this point.

.LONG TERM.../Friday through Tuesday/ Broad upper low tracks into
the AK gulf from the west moving high pressure ridge over the
panhandle eastward Friday into the weekend. Center of the low
will swing over Prince William Sound by Sunday with a second wave
just the west rotating into the south central gulf. Sunday night
into Monday expect the entire upper level low to push east with a
ridge building in behind, but some timing uncertainty. Another low
will move into the western gulf behind the ridge next week. At
the surface, occluded frontal boundary associated with the parent
low moving over the AK peninsula will cross the AK gulf then over
the panhandle through the day Thursday. This will end any brief
dry period for SE AK. As more surface short waves rotate around
the main low into the gulf bands of precip will move over the area
before finally beginning to diminish Sunday into Monday.
Potential for some areas of heavy rain with this system as
indicated by GFS integrated water vapor transport and NAEFS mean
precipitable water standardized anomaly of 1 to 1.5. However,
highest QPF depicted from these is mostly over the far southern
panhandle. Gulf winds around the waves increasing to at least
small craft to possible gale force due to barrier jet formation
along the front.

Still getting fair model agreement at least through early Sunday,
so used a blend of GFS/ECMWF through then. Model spread rapidly
increases with respect to the timing of the exiting low and the
system moving in next week, so kept with WPC guidance for rest of
the extended range. Forecast confidence is initially above
average for these next systems but drops significantly by the end
of the weekend.


.HYDROLOGY...With rain diminishing across the area, water levels
on area streams should drop off tonight. Some of the larger rivers
will continue to rise due to their slower reaction to the
rainfall, but none are expected to reach bankfull at this point.
Will most likely allow the Flood Advisory for the Ketchikan area
to expire at 4 PM this afternoon.


.AJK Watches/Warnings/Advisories...
PUBLIC...Areal Flood Advisory until 4 PM AKDT this afternoon for AKZ028.
MARINE...Small Craft Advisory for PKZ036-041-042.
     Small Craft Advisory due to Seas for PKZ043.




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