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Southeast Alaska forecast discussion 
National Weather Service Juneau Alaska
349 PM akdt Thursday Sep 18 2014

Short term...complex gale force low over the central Gulf will
drift to the northern Gulf by late tonight. Main occluded front will
move across the area this evening...followed by a second wrap
occluded front pushing onshore tonight. This second wrap front
should weaken and lift north through the northern Panhandle late tonight and
Friday morning. Used blend of the 12z NAM and Gem as they had best
handle on placement of the main features.

Main forecast concerns are winds...precipitation potential/amounts...and
potential for thunderstorms. Right now strongest winds are just ahead of
the main occluded front and on S side of the second wrap front.
Both areas have gale force winds. Main frontal wind band will
diminish as it GOES inland this evening. Stronger winds with
second wrap of front should hit the central coast beginning this
evening...then move into the central and part of the northern Panhandle
later tonight. Expect 35-40 knots winds with this band...and have
gales/strong wind headlines with the band as it comes through. The
winds should slowly diminish over much of the area Friday...but the
far northern Panhandle will see strong winds by middle-morning Friday after
second wrap front passes north of them and a strong southerly pressure gradient
develops. Have gales/strong wind headlines for that area for much
of the day Friday.

As for precipitation...should be wet area wide. Precipitation character may go
back and forth between showers and stratiform as the various
frontal bands move through the area. Have decided to stay with
stratiform for this forecast period for simplicity. Think heaviest
rain will be associated with second wrap of front as strong SW flow
will favor the northwest-southeast oriented mountain ranges that dominate the terrain
of Southeast Alaska. Strongest SW flow will occur from the pasi-pajn
area...so will mention heavy at times for those areas for at least
part of the time tonight and Friday morning. Will need to watch some
of the area streams and rivers for potential fast rises later
tonight and Friday morning. Cannot rule out potential for minor
flooding especially Friday morning in the Juneau area.

As for thunderstorm potential GOES...500 mb temperatures are only around -20-22c
in the cold pool. Like to see sea surface temperature-500 mb temperature diffs of
40+c degrees for thunderstorms to occur...and the diff is in the 35-37c
range based on current sea surface temperatures over the southeast Gulf. Think this
will keep threat for thunderstorms on the low side at best. Models do show
cape values in the 200-400 j/kg range as vorticity maximum moves onshore
this evening just ahead of second wrap front...but think that is
overdone as current convection over the southeast Gulf does not look too
impressive. Still...system has some strength and a narrow band of
deeper convection is possible just ahead of second wrap front.
Will not mention any thunderstorms with this package but will let next
shift know my concerns.

Long term...active weather pattern continues through the
weekend. Forecast confidence remains fairly low for the long term
forecast period.

For the start of the long term period on Friday night, a low
will move in from the south. Right now, have gale force winds for
marine zones 41, 42, and Clarence Strait. Wind gusts of 40 miles per hour are
expected for Prince of Wales Island, Ketchikan, and Skagway areas.
There will be a brief break, before another low comes into the
Gulf from the subtropics for Sunday. This low should be capable to
bring in gale force winds to the outer coast. The active pattern
should quiet down a bit for next week.

With predominantly strong southerly flow, did adjust
temperatures upward for Saturday and Sunday. Do expect highs in
the upper 50s to the low 60s for the weekend, and lows in the
upper 40s to around 50 degrees.

Rain will continue through the weekend. Do expect half an inch
to an inch of rainfall per day. Streams and lakes in the area
should see rising water levels, with some getting to action stage.
It is tough to pinpoint areas that will experience heavy rain at
this time, since models are still going back and forth on the
storm track.

Forecast confidence is below average through the long term
period. Numerical models are struggling in the track and intensity
of the Friday night low and the Saturday-Sunday low. These lows
are originating from the subtropics, and they are barotropic in
nature without much temperature gradients. When these lows move
poleward and reach the Gulf of Alaska, they become increasingly
baroclinic as there are sources of colder air on the back side of
these systems. Because of the changing nature of these lows,
models are having some difficulties in predicting the evolution of
these systems. Gem is suggesting stronger lows, but Gem tends to
have bias on bombing out systems. GFS/ECMWF/NAM all suggest lows
moving up into the southern part of our cwa, but are much weaker.
Right now, go with an ensemble of all available models. Forecast
confidence is below average through end of next work week.

&&

Ajk watches/warnings/advisories...
public...strong wind from 1 am akdt Friday through Friday morning for
akz026.
Strong wind until 4 PM akdt this afternoon for akz023-027.
Strong wind from 1 am akdt Friday through Friday morning for
akz024.
Strong wind from 1 am akdt Friday through Friday afternoon for
akz021-025.
Strong wind from Friday morning through Friday afternoon for
akz022.
Strong wind from 7 PM akdt this evening through Friday morning
for akz023.
Strong wind until 10 PM akdt this evening for akz028.
Strong wind from Friday morning through Friday evening for
akz019.
Strong wind from Friday morning through Friday evening for
akz018.
Marine...Gale Warning for pkz012-013-022-031>034-036-041>043-051-052.
Small Craft Advisory for pkz011-021-035-053.

&&

$$

Rwt/rcl

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