FXAK67 PAJK 252054 CCA
Southeast Alaska Forecast Discussion...CORRECTED
National Weather Service Juneau AK
1154 AM AKST Sun Feb 25 2018
.SHORT TERM...Some lingering snow showers persist as one low
weakens and exits later today. Our attention now shifts to a
second storm force low tracking east across the northern Gulf and
reaching Yakutat by early Monday.
Coupled with strong upper-level dynamics, some of the main
concerns associated with this next system are heavy snowfall,
strong winds, 25+ foot seas in the Gulf, higher CAPE values over
the Gulf (meaning some coastal locations could see lightning), and
local areas of blowing snow. Blowing snow will begin in Yakutat
early tomorrow morning and then spread to the northern inner
channels, before temperatures warm up.
Models are in decent agreement, with a blend of NAM/GFS used per
changes for Monday. The biggest change to winds was upgrading to
storm force over northern gulf and Cross sounds along with
increasing southerlies and southwesterlies over the northern
inner channels with the low track. This system will rapidly
strengthen and develop as it moves onshore, with strong west winds
following behind and to the south of the low Sunday night into
Monday. Gales will spread over the area tomorrow in the northern
inner channels to small crafts in Stephen's Passage and southern
inner channels, as the ridge moves eastward. Winds were increased
to storms in Cross Sound from the west as the low moves inland.
A ridge of high pressure moving through the Gulf this afternoon,
will keep clouds at bay for today and sunshine will persist
throughout by the afternoon. However, as this ridge exits later
this afternoon, clouds will be introduced earlier than before.
POP's were increased to categorical over Yakutat beginning late
tonight into early tomorrow. POP was blended with the SREF-
keeping precipitation around a little longer (into Monday
evening). Previous model guidance had the precipitation exiting
too quickly in the presence of onshore flow. Snowfall will begin
early Monday morning, with most areas across the panhandle seeing
3 to 5 inches of new snow accumulation over a 12 hour period.
Snow showers will persist through Monday night with strong upper-level
energy. There is a potential for some of them to be strong and
train over areas to produce good snow amounts along the northern
coast and inner channels. Stay tuned for updates on this.
Maximum temperatures were decreased for today and increase for
tomorrow based on MOS Guidance and pockets of CAA and WAA,
respectively. Minimum temperatures were decreased for tonight
based on cloud cover and tomorrow night into Tuesday as the shot
of colder air pushes through.
Confidence remains about average with the timing of precipitation
and snowfall amounts across the panhandle.
.LONG TERM.../Tuesday through Sunday/ As of 11 pm Saturday: At mid
levels 500 mb trough extending from a low over the arctic digs
into the central AK gulf with amplified ridging building over the
western gulf Tuesday. The trough will dig towards Haida Gwaii as
the sharpening ridge axis builds toward the Chukchi Sea setting up
an omega block for the rest of the week. At the surface the
remnants of the Monday system in the NE gulf will push some
remaining shower activity over the panhandle then transition to a
dry period. As the energy from this system drops south under the
digging mid level trough a low will reform over Haida Gwaii,
resulting in increasing pressure gradient between it and high
pressure over the Yukon. Main impact from this developing pattern,
besides dry and clear, will be an increase in outflow and
northerly winds, at least gale force over the inner channels and
out of interior passes with gusts of 40 to 50 mph. Also expect
much colder air to stream in from western Canada with temps
dropping into the teens to possible single digits over night.
Models were in fair agreement initially with the timing of the
building ridge. Used a blend of 00z NAM/GFS which over all still
in line with previous forecast. Used more NBM/WPC for Thursday to
increase outflow winds over the panhandle and lower temps. could
still drop temps a few more degrees. Typical with any blocking
pattern models are showing large spread in determining how/when
the ridge will break down with a transition to more zonal flow.
GFS breaks the ridge down the fastest, so held off on using it in
later grids keeping with WPC. Forecast confidence is average
initially then drops by the weekend.
MARINE...Gale Warning for PKZ012-013-041-042.
Small Craft Advisory for PKZ031>034-036-043-051-052.
Small Craft Advisory due to Seas for PKZ022-053.
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