Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
344 PM akdt Tuesday Oct 6 2015
Short term...satellite imagery is showing upper level onshore
flow this afternoon. At the surface, observations and high
resolution model data are indicating a continued offshore flow
pattern. Breaks in the higher cloud cover in visible imagery are
revealing an offshore motion of the low level clouds as well. This
situation will change over the short range forecast period as low
level winds along the outer coast become more southeasterly and
increase in intensity. As this happens on the outside, wind over
the east-west Inner Channels will become offshore the northern
Inner Channels will take on a northerly component while slowly
increasing. Cross Sound is already easterly at 20 kts and will
likely increase through the night. Clarence Strait will become
southeasterly and increase to small craft intensity early tomorrow
afternoon. Somewhere in the middle, the northerly gradient will
meet the southerly gradient and this represents the main
difficulty in forecasting the wind directions for Stephens Passage
northern Chatham Strait, and the northern portions of southern
Chatham Strait. On the outside, small craft conditions will
develop over the coastal marine zones with a Gale Warning out for
offshore zone 310 and coastal marine zone 43 tomorrow afternoon.
Pop currently not a factor, but am expecting categorical probability of precipitation
associated with an approaching front over the southern zones by
tomorrow afternoon with chance probability of precipitation as far north as Angoon. North
of there, look for one more day of dry weather. Rainfall amounts
through tomorrow afternoon totaling only a third of an inch, so
no Hydro issues through Wednesday afternoon.
With the increasing cloud cover associated with approaching front,
overnight lows will be warmer than last night, getting down to the
middle 30s in the Juneau area and only down to the upper 30s
elsewhere north of the Icy Strait corridor. Farther south, expect
lows in the middle 40s to low 50s.
Used a blend of NAM and Canadian New Hampshire for updates to pressure with
little to no change over the previous forecast. Therefore, little in
the way of changes to wind grids. Temperatures adjusted per GFS MOS since
the NAM looked a little too cold considering the increasing cloud
cover. Overall forecast confidence is average.
Long term...it will be getting more active for the area as
multiple systems will be affecting the area Wednesday night into the
weekend. Still some model differences on timing/track and strength
of individual systems so confidence in the details is on the low
side. Due to the detail issues...included "possibly" caveat to
heavy rain mention. However...the general pattern is favoring wet
and mild weather for the area. After coordination with weather forecast office
pafc...decided to go with the 12z GFS for Wednesday night-Friday
evening...then latest wpc for Friday night Onward.
Rain will continue spreading northward across the area Wednesday night as
occluded front moves in. The highest rainfall from this front will
likely be over the S due to the amount of Ely low level flow
further north. Gale force winds are expected ahead of the front over
the Gulf and parts of the Inner Channels...with the east-west channels
likely having more wind than the north-S ones.
Then a low pressure wave looks to move into the southeast Gulf Thursday then
to the north-central Gulf Thursday night. This should lead to more rain and
wind for the area...with gales likely ahead of the low.
Friday is getting trickier due to model differences handling remnants
of hurricane oho. European model (ecmwf) and Gem were suggesting that the remnants
could come into the southeast Gulf...while the 12z GFS kept them further
southeast. The 18z GFS and NAM were trying to trend this system further north
so later shifts may need to bring this into the forecast for
For the weekend...may see one or more lows affect the area with
models suggesting they will likely track over the southeastern part of
the area. This may allow more breaks in precipitation over the northwest part of
the area especially daytime Sat. Do have probability of precipitation remaining at chance
levels for the paya area westward for that time frame. The upper trough
will be slowly shifting east over the weekend into early next week so
subsequent systems will tend to remain further southeast sun into Monday.
Marine...Gale Warning for pkz043.
Small Craft Advisory for pkz022-033-036-041-042-051-052.
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