Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
602 am akdt Sat Aug 23 2014
Short term...an upper level trough currently passing over the
central Panhandle is causing scattered showers this morning. As
this feature progresses southeast the showers will move over the
Petersburg/Wrangell area then on to Hyder. Cloud cover will
decrease behind the trough from the northwest through the day and a ridge
of high pressure will rebuild over the eastern Gulf. Thus the
forecast for much of the northern half of the Panhandle reads
decreasing clouds. For this reason expect sea breezes to develop a
little later in the afternoon than the past two days.
Vorticity at the base of the upper level trough will enhance the
showers as they pass over the Hyder zone this afternoon. Models
have cape values of around 200j/kg at the same time so have
included mention of thunderstorms for this afternoon in Hyder. Limited
it to slight chance however because that area is already overcast
and should remain cloudy all day...without some extra surface
heating from breaks in the cloud cover there may not be enough
lift to spark a thunderstorm.
Winds will be rather light today with around 10kt most places.
Strongest winds will be around the ocean entrance of Cape Decision
where up to 20kt west-northwesterly winds can be expected this afternoon.
Have included patchy fog late tonight over the southern half of
the Panhandle. Expect that fewer clouds tonight over areas that
rain today will radiate enough for fog to develop. Did not mention
it for the northern half of the Panhandle due to how light the
precipitation is and that those areas will have more drying this
afternoon. Radiational cooling tonight will again bring lows into
the 40s over the northern areas including Mendenhall Valley.
Expect ample dew Sunday morning.
Preferred the European model (ecmwf) and NAM for pressure today. The European model (ecmwf) and sref
represented the precipitation going on this morning the best but limit
that slightly for the scattered/light nature of it. Overall
forecast confidence is average.
Long term...gradual transition to a more unsettled pattern will
continue late Sunday as fall begins its approach and upper ridge
over the Gulf gives way to troughing. First major wave west of
Aleutians crosses eastward into the Gulf on Monday as longwave
trough becomes entrenched again west of the Aleutians. This first
piece of energy is accompanied by a frontal system that the models
appear to handle well. Winds and precipitation will increase late
on Sunday in response to this system...and be enhanced by warm
temperatures currently in place...and this wave will gradually weaken as
it progresses eastward.
Second reinforcing wave appears likely to settle in west of Aleutians
on Tuesday....and will take a similar track cross the Gulf.
Another round of precipitation is likely with this wave over the
peninsula thurs/Fri. In the meantime a third area of troughing well west
of Aleutians appears likely to phase with polar vortex diving
southward into the state late in the week. Though too early to
specify timing this fall-like system has potential of creating
winds and weather next weekend. Used primarily European model (ecmwf) and GFS
guidance on these latter waves as in the extended...model agreement was
relatively strong. Few minor changes made to the extended period
but would not be surprised if model spread becomes a factor by
early this week.
Updates to pressure through Tuesday night made with a blend of
NAM, ECMWF, and Gem early on...then European model (ecmwf) and Gem. Pop and quantitative precipitation forecast
primarily from European model (ecmwf) and NAM with a little Gem and GFS through the
same time frame. Temperatures tweaked by a degree or two. Overall forecast
confidence is above average.
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