Small Craft Advisory 696, NWS Juneau, AK

Southeast Alaska Forecast Discussion...CORRECTED
National Weather Service Juneau AK
921 AM AKST Fri Nov 11 2016

.SHORT TERM...The Pharaoh of Egypt caved after ten plagues. The
forecasters of Southeast Alaska fortunately only deal with four
this morning: thunderstorms...heavy rain showers...heavy
snow...and strong winds. But we do have concerns that two of these
plagues will mushroom.

A vigorous gale-force low churns northward across the central
Gulf on its way to Prince William Sound this evening. This parent
low of the front that wrought high winds of 60 to 100 mph to the
southeast gulf coast yesterday carries its own black magic. A
highly unstable atmosphere with midlevel lapse rates of 7.5 C/km
and CAPE values in excess of 500 J/kg has spread across the gulf
and into Southeast. With such atmospheric volatility, we have been
dealing with a thunderstorm outbreak since yesterday afternoon. A
marine weather statement was issued yesterday for the outside
waters and will continue through the morning. Hail was reported to
have covered roads near Petersburg yesterday on social media, and
judging by the Annette sounding and some of the radar signatures
possessing rotation in the low to mid-levels, we may be dealing
with the strongest cells this area can possibly offer. Isolated
thunderstorms may indeed make it as far inland as the Juneau area
today.

Winds aloft have had no trouble reaching the surface tonight, and
as a low level jet of 35 to 45 kt at 925 mb passes overhead of
the Panhandle this morning, we will see strong wind gusts of 40 to
50 mph across Southeast and gales throughout many of the Inner
Channels today. While winds across the south should begin
to diminish later this morning, winds over the north will remain
strong well into mid-day and afternoon. As the low moves onshore
this evening, forcing from aloft skips inland into Canada
tonight. Therefore, we expect gradients and winds to drop off
gradually tonight. We do have a slight fear that some place may
see an isolated gust around 60 mph with such a strong system but
our confidence is not high enough to post a warning for high wind.
Gradients will increase this afternoon through Lynn Canal and
Skagway's winds will begin gusting strongly into the evening.

Showers, as one might expect in a thunderstorm environment, have
been heavy at times with this system, and we expect that to
continue today before dynamics wane tonight. Showers are moving
northward generally so quickly that 1-hour totals have not been
super impressive, but rates have over much shorter time intervals.
We look for the heaviest rates to be concentrated across the north
today as the low and its associated vorticity passes across the
eastern gulf and northern and central Panhandle today.

Finally, the atmosphere has cooled aloft and temperatures at
White Pass dropped below freezing overnight. Web cams confirm
heavy snow showers at White Pass. With steering flow at 700 mb
directing a deep moist layer northward up Lynn Canal, we have
issued a winter weather advisory for up to 8 inches of snow
through this evening above 2500 feet on the Klondike Highway. It
may be possible that snow amounts will be higher but warmer
temperatures and wetter snow early may keep accumulations down
initially. Haines Highway drivers will be saved today with warmer
temperatures. However, snow levels drop to the highway tonight.
Right now, we do not think this will be an accumulating snow, but
we will study its potential for significance.

NAM was used for guidance with the most attention given to today's
weather since it will have the greatest impacts.

.LONG TERM...Active weather continues through the long range as an
upper low/trough remains fixed over the eastern Aleutians
Islands/Bering Sea. The resulting Pacific storm track remains
aimed at the Gulf of Alaska with a few gale force fronts expected
to slam the panhandle through early to mid next week.

The strongest of the incoming systems will be during the weekend as
first one gale force front pushes in on Sat followed by a nearly
equal strength system on Sun. Current model runs are doing better
at representing this time period with better consistency between
models. Main difference compared to yesterday is that there is a
more clear distinction on where the first front ends and the
second begins with better agreement on timing of the features.

Wide spread small craft advisory and gale force winds are expected
across the outside and inside waters as these fronts move through.
Land areas can expect strong wind gusts especially along the
southern inner channels and outer coast through the weekend.
Warm southerly flow accompanying these systems will keep precip
rain at most locations except for the higher elevations areas
along the Haines and Klondike highways.

Forecast starts drying out and possibly cooling down mid to late
next week as the Pacific storm track get pushed south by more
zonal flow aloft. Temperatures will be getting a little colder by
this point as well but we should be drying out enough that most
precip will be over before it start getting cold enough to snow.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...STRONG WIND until noon AKST today for AKZ018-020-027.
 STRONG WIND until 3 PM AKST this afternoon for AKZ022-023-025.
 STRONG WIND until 9 AM AKST this morning for AKZ017-028.
 Winter Weather Advisory until 9 PM AKST this 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.

&&

JWA/EAL

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