Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
337 PM akdt Friday may 22 2015
Short term...afternoon satellite image shows a weak upper level
low near haida gwaii and a weakening inverted surface trough of
low pressure over the central and southern Panhandle. Convective
clouds developing over British Columbia slowly propagate westward
closer to coast mountain range. Daytime heating once again
triggers numerous thunderstorms across British Columbia. High
pressure over the offshore waters has been weakened a little and
slowly drifting southeastward. Enhanced northwesterly low level
winds continue at near small craft level from Cape Spencer south
over the outside waters with rough combined seas up to 8 feet.
These winds are expected to diminish late tonight. No significant
winds are reported across the Inner Channels. Marine stratus still
linger over Cross Sound and Elfin Cove areas and holding their
temperatures in the upper 40s. Lows clouds moved back offshore
near Yakutat around noon and daytime high remains in the lower 50s.
Elsewhere...afternoon temperatures mostly in the upper 60s to the
lower 70s...except Haines where Mercury reading soaring up to near
Models remain in fair agreement for the weakening inverted trough
of low pressure over the southern Panhandle dissipates by late
tonight...replacing with a weak ridge of high pressure expanding
from the west as the high pressure over the Gulf slowly weakens
and moves southeastward. Refreshed the inherited forecast with the
nam12 for pressure and wind fields. Used the sref and nam12
guidance for probability of precipitation and quantitative precipitation forecast fields. The weakening upper low near
haida gwaii will continue to weaken and become an open trough by
Saturday morning...while the upper ridge continues to expanding
north to Yukon territory. As the surface ridge of high pressure
approaches from the west...expect the marine layer once again move
back to onshore late tonight with increasing areas of low clouds
and fog nearshore areas of the Panhandle through Saturday morning.
Isolated thunderstorms developing over British Columbia will
continue to propagate west across coast mountain range and will
likely to affect eastern and southeastern Panhandle this evening.
Around the weakening upper features...a bit better organized
showers are expected...but maintain generally isolated showers for
south central Panhandle and scattered probability of precipitation for southeastern areas.
Overnight temperatures will be mild in the middle 40s to the lower
50s. With increasing clouds and shower chance...daytime highs will
be a bit cooler to the lower to middle 60s...except in the upper 50s
along the Panhandle coast. As the ridge moves closer to the
Panhandle...expect southerly winds increasing to small craft level
over northern Lynn Canal by late Saturday afternoon. Small craft
level winds will also affect near ocean entrances of north to
south oriented southern Inner Channels by late Saturday afternoon.
Long term...still no sign of the large blocking upper high
breaking down in the next week. The best we can show is that it
will narrow in scope early next week as a weak trough tries to dig
into the western side of it but it just rebounds to its previous
size by the end of next week. In any case the dry and relatively
warm weather continues for most places with the exception of
afternoon showers and thunderstorms in places.
The main exception to the dry rule comes early in the long range
as a weakening trough moves into the central Gulf from the west on
sun and Monday. This feature will stay together enough that it will
bring a chance of rain to the Yakutat area Sun night into Monday.
Otherwise the surface ridge over the Gulf combined with the upper
level ridge will cause the trough to weaken away before it gets to
the rest of the Panhandle. The only effects that the rest of the
Panhandle might see is increased cloud cover on Mon, with lower
high temperatures as a result, and some slightly increased winds in the
northern Panhandle from increased pressure gradient.
Into the rest of the week, a weak upper level low is getting
forced into undercutting the ridge. How far east that low gets is
up for debate right now. The GFS is the farthest west having it
stall over the Panhandle. This results in higher chances of
showers and thunderstorms over the Panhandle middle week into late
week. However the GFS has not been very consistent run to run with
this change so far. The European model (ecmwf) has a more consistent solution but
it sends the low much farther east leaving US much drier. Decided
on the European model (ecmwf) solution because it so far has been the more stable
solution. However the new 18z run of the GFS has continued the
trend that the 12z run showed so will wait to see if there is any
substance with the GFS.
Through all of this the marine layer clouds will continue their
usual dance in and out of the outer coast and Inner Channels.
Current thinking is that with more onshore flow in the long range
the marine layer will be making further inroads into the Inner
Channels than it has recently. Some areas in the Inner Channels
that have not seen the marine layer clouds yet may finally start
to see them later this week.
Fire weather...various inland locations have gotten rather warm
and dry this afternoon with the general offshore flow. The Haines
Highway, taiya River Valley, Hyder, and various other areas away
from the channels have all reached the 70s with some low to middle
80s recorded. Relative humidity values have fallen into the 20s
to middle 30 percent range this afternoon. Thankfully winds have
remained at 10 knots or less across the Panhandle so red flag
conditions are avoided for now. Similar conditions but slightly
improved are expected for Sat afternoon as the offshore flow
weakens and turns more onshore by evening. Min relative humidity
values will be a bit higher in the middle 20 to 30 percent range but
there may be a bit more wind in the extreme northern Panhandle as
pressure gradients increase in response to the trough coming
inland. Fire weather conditions will be watched on Sat. Improving
conditions come on sun and Monday as the trough comes further inland
bringing more clouds and onshore flow increases as well. Min
relative humidity values should be up in the 40 and 50 percent
range on sun and Monday as a result. Dry weather continues into next
week with little if any rain, except what comes from showers or
Marine...Small Craft Advisory for pkz012-033-041>043.
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