FXAK67 PAJK 271201

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
401 AM AKDT Tue Jun 27 2017

.SHORT TERM...Upper ridge will sharpen up over the far E gulf
today then push onshore tonight. Weak shortwave trof will move NE
across the N gulf and NE gulf coast this afternoon and tonight. A
weak occluded front will move into the SW gulf today, then stall
there as low pressure develops and moves N to the SW gulf by Wed
night. For the most part, current forecast was still looking
reasonable, although did make some tweaks based on current
conditions and model trends.

Main forecast concerns will be precip potential, cloud cover, and
winds. With upper ridge sharpening up over the far E gulf, the
shower activity over the panhandle will decrease today. However,
along the NE gulf coast, will see an increase in precip threat
late this afternoon and tonight as weak shortwave moves in. Have
POPs rising to categorical by early this evening in the NE gulf
coast area. This shortwave will also increase precip threat over
the N panhandle tonight, but POPs will generally remain at chance
levels or less over the N area. After shortwave moves by the area
Wed, precip threat will decrease again along the NE gulf coast
and N panhandle. The S half will see showers diminish in coverage
through this evening, with the most coverage remaining along the
Coast Mtns.

Cloud forecast is a bit trickier as it will depend on how much
moisture remains trapped under subsidence inversion especially
over the far SE. Think the N and central areas will keep enough
moisture trapped under the inversion for stratus or stratocu to
persist, although a few small breaks may occur late in the
afternoon as low level heating could cause shallow cumulus to
form. This cumulus should flatten out to stratocu this evening
once temps begin to cool. Going with mostly cloudy to cloudy skies
for the N and central areas through Wed. Over the far SE part of
the area, the stratus/stratocu layer appears to be thinner, so
some cumulus formation there could allow for larger breaks in the
lower clouds. There will be some mid-level clouds moving through
the area in warm advection pattern ahead of building ridge this
morning, but these clouds should diminish this afternoon and
tonight as ridge axis pushes onshore. Am going with mostly cloudy
for the far SE, but short duration of partly cloudy could occur
late this afternoon.

As for winds, low level ridging offshore is causing a SCA level tip
jet near Cape Spencer, and a lesser jet near Cape Decision. As
low level ridge moves onshore today, these jets should weaken. For
the N inner channels, moderate strength southerly pressure
gradient will remain into tonight, with some diurnal enhancement
during the afternoon and early evening. Going with SCA level S
winds during the afternoon and evening hours for N Lynn Canal,
with 15-20 KT for S Lynn Canal. Over the gulf, SCA level winds
will be likely over the SW part of the offshore waters tonight
and Wed, with SCA possible in the far NW part of the gulf coastal
marine area Wed. This is being caused by approach of occluded front
from the SW through tonight and developing low moving N toward the
SW gulf Wed and Wed night.

.LONG TERM...Friday through Tuesday...As of 1000 PM Mon...An
upper level ridge located east of the southeastern Alaska region
with an associated surface high pressure will remain over the
area, influencing conditions for Friday into Saturday. Deep layer
onshore flow and plenty of moisture will fuel a sustained cloud
low level deck. Multiple shortwave troughs will then progress east
out of the gulf, originating from a broad area of low pressure
located over the central gulf, impacting the region through this
weekend. Long range models are in general agreement for
accumulating amounts of precipitation through this weekend into
Monday. As the systems make their approach, timing of
precipitation will become clearer. PoPs were analyzed using mainly
the GFS model ensemble and then trended more with WPC guidance.
Pressure and wind updates blended towards GFS through Friday, then
towards WPC through the remainder of the long range period.
Overall confidence is average.


.AJK Watches/Warnings/Advisories...
MARINE...Small Craft Advisory for PKZ012-022-041>043.
Small Craft Advisory due to Seas for PKZ052.




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