Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
406 PM akst Tuesday Dec 17 2013
Short term...weather pattern is much quieter over the Panhandle
today than it was yesterday. Last night's low that went through
the southern Panhandle is now well into Canada. It dumped up to
4 inches of snow around the Clarence Strait area and up to
7 inches around Hyder last night. Currently we are left with
lingering showers of snow, and in some southern areas, rain that
are gradually getting pushed out by northerly outflow winds.
Speaking of the outflow, Lynn Canal, Skagway, and Haines have
seen windy conditions since late last night. Eldred Rock has been
gusting as high as 60 knots and Skagway has seen gusts to 33 knots so
far. Temperatures have been on a downward trend as well with
Haines, Skagway, and Yakutat in the teens right now.
The northerly outflow will continue for the next 24 hours but
will gradually be decreasing in intensity. This is in response to
the pressure gradient becoming more and more parallel to the
channels rather then perpendicular as the Yukon high slides into
northern British Columbia by Wednesday. In Lynn Canal, the gales will
gradually give way to small craft winds by late tonight and then
to around 10 knots by Wednesday afternoon. Skagway and Haines should lose
their strong wind headlines by late this evening.
As for clouds and showers, those should be gradually diminishing
from north to south this afternoon and evening. Temperatures will be
cold enough that most places will still see those showers as snow
but Ketchikan, Hydaburg and points south should be just warm
enough for some rain as well. Any additional snow accumulations
are expected to be less then 2 inches. Clouds will also be
diminishing, promising a nice but cold night for the northern
Panhandle at least. Overnight lows are expected to get into the
teens to single digits above zero tonight in the northern half of
the Panhandle especially in the wind sheltered areas. Southern
Panhandle will still have a few clouds around to keep temperatures in
the 20s. The clearer skies will continue into tomorrow for the
Panhandle but the Gulf and Yakutat will start seeing more clouds
come in ahead of the next front tomorrow afternoon.
Short range models were in good agreement today with most having
a decent handle on the pressure gradient in the northern
Panhandle this morning. Changes from the previous forecast were
minor. Generally preferred the NAM for overall changes.
.Long term...Gale force front associated with a 980 mb over the
western tip of the Aleutians will track eastward into the central
Gulf Wednesday evening with a triple point low developing along
the front near Prince William Sound. The front will cross over
the Gulf and Alaska Panhandle Wednesday night into Thursday. The
front and triple point low will weaken and dissipate as high
pressure builds over the Gulf Friday. By the weekend the original
parent low over the Aleutians will track eastward into the
western Gulf with another front approaching the Alaska Panhandle.
With cold air in place the warm moist air mass associated with
the front should set up for an over running event over the
central Panhandle. Have issued a Winter Storm Watch with storm
total accumulations in the 4 to 12 inch range possible. Main
issue for snow totals will be the quantitative precipitation forecast amounts from the front and
timing of warm air mixing down to the surface. Not seeing as well
developed sub tropical connection with this system as seen from
previous storms which is reflected in new model runs showing
lower 6 hour quantitative precipitation forecast totals. As the front advances temperatures will
rise through Thursday and continue to rise into Thursday night
with precipitation beginning to transition from snow to rain.
Think this will mainly be over the southern and coastal areas but
with the warmer temperatures snow should become wet and thus additional
accumulations and snow ratios will be lower. Higher snow totals
looks to keep out of the far northern Panhandle and with strong
winds kept watch out of Skagway for now.
Winds over the coastal waters will pick up to maximum gales with the
advancing front and over the Inner Channels as the front moves
over the Panhandle. 850 mb low level jet sweeps over the southern and
central Panhandle with strong winds developing. Winds will
diminishing overnight Thursday then begin to shift from north to
south as the high builds in for the end of the week.
Models were in good agreement with the system but fall apart
dealing with the remnants. European model (ecmwf) drops the low quickly to the
south while NAM/GFS/Gem keep a low in the northern Gulf with a
easterly barrier jet longer then shifts winds to the north by
Friday. Used a blend of NAM/Gem through day 4 then used wpc for
an ensemble approach. Confidence for the Wednesday and Thursday
system is above average however with questions on precipitation
totals. Confidence drops with the large model spread for the
remainder of the long range as the low exits the region.
public...Winter Storm Watch from late Wednesday night through Thursday
afternoon for akz017-019>021-024>026.
Strong wind until midnight akst tonight for akz018-019.
Marine...heavy freezing spray warning for pkz012.
Gale Warning for pkz012-013.
Small Craft Advisory for pkz011-022-031-032-034-041.
Small Craft Advisory due to seas for pkz042.