Gale Warning 51, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
354 PM AKST Sun Feb 11 2018

.SYNOPSIS...The eastern Gulf of Alaska is transitioning into a
progressive pattern due to our upper ridge weakening and moving
eastward. A pretty strong SW jet at 30K FT with a core speed as
high as 165KT will be over Prince William Sound at noon Monday,
and although weakening after that, will be over the northern
Panhandle by early morning Tue.This whole change is supported by a
short wave trough moving from the west with only moderately cold
temperatures aloft, say -34C at 500 HPa. A 998 MB surface low will
end up off Ocean Cape by early Tue as warmer temps and mostly rain
bands move through the Panhandle.

&&

.SHORT TERM...These forecasts are a bit tricky due to big wind
shifts on Monday with a warm front to switch the precipitation
type. One challenge will be to get the snow amount just right
before the rapid switch to rain. Look for any snow accumulation -
all forecasted to be under advisory, say 1-4 inches at most, to
become wet heavy slush. As usual, water on packed snow and ice is
going to be a hazard. We are expecting 0.5 to a little over 2
inches of rain for storm totals through Tuesday. Confidence only
moderate in these values, and no real significant changes in the
forecasts from the previous shifts due to uncertainty. 11/12Z NAM
was judged to be the best ops model for this set of forecasts.

.LONG TERM.../Tuesday through Sunday as of 10 pm Saturday/
Models in good agreement through midweek, then diverge somewhat by
the next weekend. Occluded front will move through the area Tue.
Weak ridging will move in behind the front Tue night, then onshore
flow will increase for Wed as another system moves in. Later in
the week, may be some cooler air moving in on NW flow aloft, but
degree of cooling differs somewhat amongst the models. There may
be another system moving in during the Fri-Sat period but models
differ on how far N it will get. Ended up using a blend of the 12z
ECMWF and 18z GFS to handle Tue into Tue evening, then went with
mainly WPC further out.

Precip should start to diminish Tue afternoon as front moves past
the area. Weak ridge will help break clouds more and allow for
decent cooling Tue night. Weak onshore flow returns for Wed which
will increase threat for some light precip then. Weak shortwave
moving through the area will boost the precip threat some for Wed
night and Thu, but it is not looking particularly heavy at this
point. Better precip threat may arrive for Fri into Sat as another
low and frontal system move in, but how far N the main low tracks
is a big question. Later in the weekend, both the ECMWF and
Canadian suggest a drier and cooler period, while GFS brings
another system across the N gulf with more precip for the area.
Kept in low POPs for now due to the differences.

Temps were adjusted mainly by blend with NBM. After the Tue system
moves out, not looking like temps will get too far from average
at this point, although trend toward slightly cooler than average
for the end of the week looked reasonable. Still, a lot of issues
with upper air pattern to resolve before forecast confidence rises
above average by then.

&&

.AVIATION...Areas of SC this morning have caused a few of the TAFs
to forecast MVFR, but some locations have great flying wx and
performance. No more nice later tonight and Monday as the
preciptiation moves in with starting as snow especially north.
Look for increasing LLWS tomorrow after 21Z due to the front.

&&

.MARINE...Typical S to SE flow ahead of the front tomorrow then
a real nasty fetch of SW behind it going to all westerlies by Mon
evening on the outside. There is already a swell on its way from
the system on the other side of the Gulf which will be a big lump
under some gnarly wind waves with the westerlies. Tuesday will
have less wind, but expect the outside swell to not lay down for
a while.

&&

.HYDROLOGY...By Tuesday we expect to see some pretty good rises in
streams and rivers due to rainfall and snow melt. At this time we
are NOT seeing any hydro hazards due to the low freezing levels at
the start and the very low winter water levels on the streams. A
few ice chunks will move down the northern streams.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-013-022-031>034-041>043-051-052.
     Small Craft Advisory due to Seas for PKZ053.

&&

JCC/RWT

Visit us at http://www.weather.gov/Juneau