Area Forecast Discussion
Issued by NWS
FXAK67 PAJK 252302
Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
302 PM AKDT Tue Apr 25 2017
.SYNOPSIS...A developing low Tuesday night southwest of Haida
Gwaii is moving northward and will slowly weaken as the low moves
towards Cross Sound by Thursday.
.SHORT TERM.../Tuesday night to Thursday night/ Only minor
changes overall for the forecast package. Model solutions mainly
broke down into the track along the immediate coast line of the
panhandle or inland near southern Chatham Strait, or staying off
shore to Wednesday and then moving towards landfall around Cross
Sound Thursday. Early look at model initialization had me keeping
the same track which was the offshore track. The system was
looking to be shade weaker so lowered winds about 5 kts in the
marine forecast. Zone 27 kept the Strong wind for gusts to 40 mph
for the late evening to a little after midnight and then falling.
The PoPs also were slowed down a few hour across the panhandle
this evening. The low in the eastern gulf, should track north
with rain into the southern panhandle through mid week lifting
likely to categorical pops into the central panhandle.
There does seem to be some ridging over the western and central
gulf late thursday that will be in advance of the next system
being moved into as the zonal jet stream across the North Pacific
to a sharp trough at 250 mb Thursday afternoon.
.LONG TERM.../Thursday through Tuesday/ (Issued at 6 am Tuesday)
At upper levels broad area of low pressure over the western gulf
with a weakening low in the eastern gulf. Building ridge over the
eastern gulf and panhandle mid week. The pattern shifts to the
east with an upper low developing in the southeast gulf by
Saturday then tracking east. Meanwhile another upper low moves in
from the west near the Aleutian chain with ridging again over the
eastern gulf by Monday.
At the surface models continue to shift position of developing
surface lows in the gulf. The low moving up along the panhandle
coast from the south mid week expected to deepen to 994 mb off
the coast near Sitka and becoming quasi stationary. This has been
a shift from previous models with the low center now further to
the north and west. In addition new runs keeping the low intact
for a longer period of time before finally dissipating Thursday
night. Changes due to this new location include an increase in
winds over the southeast gulf and increase pops over the southern
panhandle. Surface ridging behind the low will keep onshore flow
and thus keep at least chance pops for the end of the week.
Similar pattern for the weekend with another low moving up the
panhandle coast from the south, but lower confidence on exact
track due to large model spread. Expect higher winds over the
southeast gulf, higher pops over the southern third of the
panhandle and as least a short dry period due to offshore flow for
the northern areas. For the rest of the extended range while
models are in poor agreement on details they are showing the
continued pattern of waves moving into the region with a few
Inner channel winds will see a number of speed and directional
changes as these surface lows move in and out of the region with
ridging over the panhandle in between. Due to the lows being at
peak strength while in the south highest winds, potential gale
force, will occur there. Temperature trend cools a bit for day
time highs due to the increased precip and cloud cover but warms
overnight for the same reasons. Only potential for late spring
snow is at high elevations.
Used a blend of GFS/ECMWF for Thursday into Thursday night as both
were showing a deeper low over the eastern gulf. These models were
close on strength but a bit off of exact position, hence the
blend. Overall this somewhat model agreement for the ECMWF/GFS
lasts through the rest of the forecast period. Decided to use new
WPC for its ensemble blend of these models. There were only minor
difference for this time frame between inherited grids and new
WPC. Forecast confidence is below average due to the the model
spread and run to run inconsistency.
.AVIATION...Change groups this evening bring the rain in for the two
southernmost TAF sites and lower channel/sea breezes and clear
skies for the most of the northern sites. Where rain and cloud
ceilings will not fall to any significance change groups for
Wednesday were omitted. The most complex TAFs were reserved for
the south where rain and ceilings fall from tonight into
Wednesday. We also added wind sheer for Ketchikan and Klawock. No
IMC conditions are expected with the exception of tempo in patchy
fog for Yakutat early Wednesday morning. Otherwise, conditions
will fall to MVMC Wednesday for the south.
.MARINE...A low moving to the southeast Gulf of Alaska by
Wednesday afternoon. Will spread a Gale force front into the
southern coastal waters Tuesday night. Seas will build through mid
week with 8 ft seas building to near Cross Sound along the outer
waters by late Wednesday. Over the southern Coastal Waters seas
are expected to to rise to the lower teens. North gulf waters
should remain under 8 ft wave heights.
PUBLIC...Strong Wind from 7 PM this evening to 1 AM AKDT Wednesday for
MARINE...Gale Warning for PKZ041.
Small Craft Advisory for PKZ022-032-033-036-042-043.
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