Area Forecast Discussion
Issued by NWS
FXAK67 PAJK 270014
Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
414 PM AKDT MON SEP 26 2016
.SHORT TERM...Surface analysis and satellite imagery indicate a
weakening low over the NE gulf this afternoon with another low
moving into the SW gulf. A trough with the dissipating low
continues to generate convection as it slowly pushes east.
Although thunderstorm activity is much less than previous days, a
few lightning strikes have been detected just outside of Cross
Sound and off Prince of Wales. Modeled convective indices
continue to support TSTM development as this trough pushes inland
tonight, so continued mention of SCHC TSTMs through this evening.
Next forecast challenge is potential for fog development overnight
into Tue. Areas that experienced clearing Sun night had fog
develop this morning with PAYA seeing the best clearing and
thickest fog. Best chance for clearing will again be along the
north gulf coast, but with dissipating low sliding SE, a weak
offshore gradient is expected to develop inhibiting fog
development. Elsewhere, sky cover is expected to remain mostly
cloudy which will inhibit development, so kept mention out of
forecast at this time, but confidence is below average. Plenty of
open cell cumulus stretching across the gulf with continued
onshore flow Tue, so expect scattered showers to persist Tue as
.LONG TERM...Low tracking across the southern gulf will keep
showers in the forecast for most of the panhandle into Wednesday.
The orientation of the pressure pattern will shift the winds
through the inside waters from SE to N-NE Tuesday night. Showers will
tapper off from north to south as flow turns more offshore and the
low settles south of Haida Gwaii through Wed night. On Thursday
high pressure will build over the Yukon and increase the northerly
pressure gradient with the inverted trough over the inside waters.
As a result have winds increasing to small craft level over the
northern inner channels. The pressure packing increases further
on Friday as low pressure pushes back into Hecate Strait and
deepens. Expect strongest northerly outflow winds to occur Friday
night with northern Lynn Canal reaching gale force of 35kt.
Meanwhile an upper level low will drop south across the area
during the day Friday. Will have to watch for Taku wind
ingredients during this timeframe. The upper level low in
conjunction with the surface low over Hecate Strait/Dixon Entrance
will increase the cloud cover and potential for precip as well
over the southern panhandle on Friday. Will need to watch for
weather coming in from the East on this day as well, but dry
northerly outflow should limit precip from making it over the
Coastal Mountains over the northern areas.
Other big story of the week will be temperatures. 850mb temperatures
early Wednesday morning will be around -3C over the northern
panhandle which will make for lows in the low to mid 30s along the
Canadian borders and some places could see frost. The low
pressure moving up from the south looks to bring up warmer
temperatures over the southern panhandle on Thursday, increasing a
thermal gradient along the Coast Mtns. Then Thursday night into
Friday the cold air from Canada pushes south in a big way with the
upper level low. Early Friday morning 850mb temps drop to around
-5C across the north then the below freezing airmass pushes south
into the gulf through the day.
Used an ensemble approach for changes today as models were in good
agreement, but with an emphasis on the 12Z GFS and ECMWF. Forecast
confidence on drying trend with a switch to north winds is above
MARINE...Small Craft Advisory due to Seas for PKZ041-042.
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