FXAK67 PAJK 171254
Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
354 AM AKST Sat Nov 17 2018
.SHORT TERM...Main forecast concerns through Sunday night revolve
around ongoing significant heavy, wet snowfall for the far
northern channels as well as heavy rainfall for the remainder of
the northern Panhandle.
Currently, swath of deep subtropical moisture clearly seen on WV
and layered precipitable water analysis early this morning
streaming into the NE Gulf coast and far northern Panhandle.
1 hr precipitation amounts have been as high as 0.4 inches per
hour along the NE Gulf coast with 0.1 to 0.2 per hour over the
remainder of the northern channels. Deep layer SSW flow between 40
and 50kt on the VWP this morning is leading to favorable
orographic enhancement of the precipitation. This stream of
moisture will be augmented by deep forcing for ascent ahead of an
elongated PV anomaly seen on WV imagery this morning over the
eastern Gulf. The heavy precipitation will spread southward
through the day and eventually weaken as the PV anomaly tracks
into Canada. To the west, a second mid level trough will track
eastward on Sunday and will act to slowly bring the front and
associated moisture back northward. The remnant front and the new
system will eventually merge over the eastern Gulf by late Sunday
night. The end result will be a prolonged period of rainfall
especially for the northern half of the Panhandle. At this time,
no flooding is expecting although smaller streams will likely see
some rapid rises during periods of heavier precipitation.
Low level thermal profiles from surface and higher elevation obs
over the far northern inner channels reveal a deep, near
isothermal slightly below freezing layer early this morning.
Strong forcing for ascent and very stable low levels will lead to
the potential for very heavy, wet snowfall. Given the upstream
hourly precip totals, think 1 inch per hour rates are likely for a
few hours early this morning. Thus increased snow total around the
Haines area a bit this morning. Once the precipitation rates let
up later this morning, think the low level warm advection will be
able to win out and precipitation will mix with or change to rain
below 2500 ft. Very strong warm advection noted between 3 and 5
kft which may be just enough for a transient period of freezing
rain around mid morning through the Chilkat valley before the
precip either lets up or changes to rain. After this afternoon,
snow levels look to be high enough that only the top of White Pass
will continue to see snow and even the pass may mix with rain at
times. Decided to leave product expiration times as is since the
heaviest precip rates are expected to sink south of the warning
area by late morning.
.LONG TERM.../Monday through Saturday as of 10 pm Friday/
Main adjustment to longer range period was to slow the movement of
a front across the area Mon-Mon night. Models have trended toward
a more amplified upper pattern which will be the slowing
mechanism. Also, there may end up being at least 1 low pressure
wave that rides the front N through the eastern gulf during this
time, which would initially slow the eastward progress of the
front, until the wave passes into the far N gulf, when it should
push in a little faster. Other than that, the model differences
precluded any changes to the Tue period onward. Models are
hinting at a more progressive pattern from midweek into next
weekend, with periods of wetter weather interspersed with drier
days. Should also end up with some cooler temps as well especially
after upper trofs move inland and a more NW flow aloft can develop
for a bit.
PUBLIC...Winter Storm Warning until 9 AM AKST this morning for AKZ019.
Winter Storm Warning until 3 PM AKST this afternoon for AKZ018.
MARINE...Gale Warning for PKZ022-042-043-051-052.
Small Craft Advisory for PKZ012-013-021-031>036-041-053.
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