Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
313 PM akst Sat Feb 28 2015
Short term...upper ridge over the eastern Gulf and Southeast Alaska will be
flattened by a shortwave trough moving across the area tonight and
Sun morning...then the ridge will sharpen over the area Sun
afternoon. A weak surface low developing east of Kodiak will move east
across the Gulf tonight...then southeast to haida gwaii by late Sun
afternoon. High pressure will build into the Yukon late tonight
and sun. Models were generally close on the main features...although
the European model (ecmwf) was a bit faster moving the system through the area sun than
the other models.
Main forecast concerns will be timing of precipitation coming in tonight
then exiting the area sun...ptype...and winds. Pretty much
followed the GFS/NAM/Gem consensus on timing of the precipitation moving
in tonight. Looks like it will reach the pael/pasi areas around
06z tonight...then the paoh/pagn/pags areas around 09z. It will
likely get into the pajn/papg/pakw areas by 12z tonight...then
reach the far southeastern areas by 15z sun. Currently think the far northern
areas (paya/pahn/pagy) will be a close call on whether precipitation gets
to them. Keeping maximum probability of precipitation in those areas at chance levels for now.
Think precipitation will last around 4-6 hours over the northern side of the
precipitation shield...and 6-8 hours over the southern side. This ends precipitation
over the northern end around or just after daybreak sun...then by early
afternoon over the southern areas. Looks like clouds will decrease
fairly rapidly once the precipitation ends also...so the northern areas will
get back into sunshine by middle-late morning...with the far southern
areas breaking into sunshine around late afternoon.
As for the ptype...about the only areas with a threat for snow are
over the far northern fringe of the precipitation shield. A tricky aspect of
the forecast though is whether it ends up as a mix or become all
snow. The fairly light winds around pags and pajn and slightly
cooler low level airmass will make for a better chance that snow
melt cooling would occur and allow mainly snow to reach sea level.
This will depend on precipitation rates...and models differ some on this
over the pajn/pags areas. At this time...will limit any snow accums to 1
inch or less. Where airmass will be cool enough for entire event
over the far north...precipitation chances will be lower...so no snow amount
mention will be needed.
As far as winds go...westerly middle-level flow coming in this afternoon
will likely enhance southerly gradient over the far north...so expect Lynn
Canal S winds to pick up late this afternoon to 15-20 knots. These
winds will slowly diminish this evening and into the overnight
hours...then become northerly by daybreak sun as high starts to build to
the north and surface low turns southeast. Further S...winds will generally be 15
knots or less through Sun morning...except around 20 knots over the
southern Gulf waters. Then as low moves off to the southeast and high
builds to the north ... winds should pick up to 15-20 knots during
the late morning and afternoon hours over the remaining Inner
Channels...while the northerly winds over the northern Lynn Canal area should
pick up to Small Craft Advisory levels by late morning.
Long term...high amplitude upper ridge over the Gulf and Panhandle
will keep the Panhandle mostly dry for the first half of the long
range. The only exception to that is a short wave trough that is
traveling up the west side of the trough Monday night into Tuesday. This
will not be enough to get rid of the ridge, but the next one will
be. Another stronger trough will be digging into the western side
of the ridge Wednesday into Thursday and will be pushing the ridge off to the
east. Following this a third trough digs into the Gulf again for
late week and eventually develops into a cut off low over the
northeast Pacific by Sat leaving more east/west flow aloft for the
Panhandle over the weekend.
At the surface, Monday into Monday night will be dry as a 1040 mb
high in the interior of Canada keeps surface flow offshore.
Pressure gradients along the Coast Mountains will be sufficient
for high small craft winds for Lynn Canal and gusty winds for many
of the other outflow regions along the Panhandle. Don't expect
really high winds or really cold temperatures however as most of the
cold air associated with this surface high will be kept bottled in
Canada by just enough southwesterly flow aloft to keep it on course
for the great planes. The Panhandle will just see the fringes of
the cold air which will cool temperatures a few degrees and dry things
out a bit more.
For Tue, the trough that will be moving through will not produce
much more than cloud cover for most of the Panhandle as it will
be mostly overwhelmed with its task of moving through the ridge.
However the further west you are the more likely you will see any
precipitation out of this system. Yakutat will likely see categorical
precipitation on Tuesday with chance probability of precipitation over the rest of the northern
Panhandle. Yakutat may also be cold enough on Tuesday morning to see
some snow but it will likely change back over to rain by
Into Wed, a surface ridge begins to build across the eastern Gulf
and Panhandle behind the first trough. Northerly winds across the
Panhandle will start to switch to a southerly direction. By Wednesday
afternoon the next trough will be moving through but how its
surface reflection will be represented is still up for debate as
the upper ridge breaks down. GFS is easily the strongest with the
surface reflection, tracking a 1005 mb low across the Gulf. The
Gem is a bit weaker and slower, and the European model (ecmwf) is weaker still. Wpc
data went in the direction of the GFS ensemble which had the
strength of the operational GFS but was more like the Gem as far
as timing. Decided on staying close to the previous forecast with
the wpc for the middle week period.
Farther out model agreement breaks down as the models struggle to
resolve what the new pattern will be after the upper ridge breaks
down middle week. General trend out here is for a wetter scenario as
upper flow maintains a more west or SW direction into next weekend
with at least two or three trough moving through by the end of
next weekend. Main storm track out of the Bering Sea and across
the northern Gulf also appears to be generally agreed upon by most
models. It is just timing, structure and strength of the
individual systems that is in dispute. Decided on mainly relying on
wpc data for its use of ensemble means in trying to even out the
differences between models.
Marine...Small Craft Advisory for pkz012.
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