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Southeast Alaska forecast discussion 
National Weather Service Juneau Alaska
600 am akst Sat Dec 27 2014

Short term...weak low over southeastern Panhandle is exiting to
the east and leaving only small craft advisories in northwesterly
flow over marine zones 41-43 and Clarence Strait. Buoy 46084 never
reached gale level and we downgraded winds there from gale several
hours ago. Showers have exited and brief minor fog episodes have
characterized several locations over the central/northern
Panhandle over the last several hours as precipitation and cloud cover
begins to decrease. Overall the trend is less fog than
anticipated. Some central/northern locations are reporting clear
skies already...as the cooling trend begins at low-levels.

Meanwhile winds are starting to increase over northern Lynn as
anticyclonic conditions begin to take over at all levels and this
trend will continue over the next 12 hours. Eldred Rock now is
gusting to 30 and Small Craft Advisory begins at 15z and then spreads to Stephens
Passage and southern Lynn over the next 12-15 hours or so as
northerly gradients increase in those areas. Central High pressure
just to the northeast over southern Yukon surpasses 1050mb by
Sunday morning and will cause cold outflow winds in the favored
passes. Only minor tweaks to existing grids were required
otherwise as dry, partly cloudy to sunny conditions take over and
fog risk is minimized by the influx of drier low- level air. Do
not feel in this scenario that fog will be a threat tonight.
Adjustments to wind fields included aligning some of The Channel
winds per the northerly gradient and some tweaking of
direction/speed in Yakutat Bay. The cross barrier flow increases
to 20kt+ by 15z Sunday for the Juneau vicinity. The NAM forecasts
further increases and winds become more favorably oriented after
this period...this will raise the threat of some gustiness on
Sunday downtown/Douglas. Current guidance suggests potential
mountain wave acceleration after 15z Sunday.

Utilized primarily ec/NAM models for the minor updates.

Long term...little change in thinking from previous few days in
the extended range. High amplitude ridge will flatten and shift to
the south and east as a front moves into the eastern Gulf early
next week. This weakening front will push across the Panhandle
Tuesday and Wednesday. Models continue to struggle with pattern
evolution after middle week but general idea is the ridge will begin
to reestablish itself over the interior and southward into the
eastern Gulf next weekend.

High pressure will dominate weather over the Panhandle through
Monday. Outflow winds will be on the diminishing trend over the
Inner Channels on Monday as the surface high slides southeast into
British Columbia. This will set up a weak wind regime over the
Inner Channels Monday and Tuesday. The airmass associated with
this high is colder than what we have experienced recently, but
nothing extreme for this time of year. The upper level ridge
begins to flatten out on Tuesday in response to an upper level
trough riding over top of it. This will push a weather front into
the eastern Gulf, reaching the Panhandle late Tuesday. The front
is weakening as it reaches the area so am expecting less than one
half inch of liquid precipitation in most areas. Due to the cooler
airmass in place (although it will be modified by this point) and
likelihood that their will be some drier air in place for
evaporative cooling have continued with snow chances at the onset
of precipitation Tuesday night over the northern Panhandle.
However, have reduced snow chances to only Tuesday night as
significant warm air advection changes precipitation to all rain
on Wednesday expect in the Haines and Skagway areas. 850 mb
temperatures warm to -2 to 0 c with 1000-850 mb thicknesses rising
above 1300 M. While the Haines and Skagway areas could see a
little bit more, am not expecting much accumulation at sea level
for the remainder of the northern Panhandle.

The front pushes through early Thursday with model disagreement
reigning supreme thereafter. The 00z European model (ecmwf) is a change from its
previous run with a shortwave moving through a weakness in the
ridge. The associated surface low tracks across the southern Gulf
spreading more precipitation into the area on Thursday night and
Friday. This would be an interesting setup as the Arctic front
slips to the south and interaction with this system could produce
snowfall over a portion of the area. The GFS is depicting a weaker
and farther south solution. Have kept with idea of a drier end of
the week, but this will need to be monitored as the European model (ecmwf) solution
would have the potential to produce accumulating snowfall over a
portion of the region. Plenty of time to watch this. After this
system pushes to the east, models are indicating the upper high
may try to reestablish itself over the Gulf next weekend.

Continued with NAM/European model (ecmwf) blend through day 4 then kept much of
the remainder of the extended forecast as is. Main changes were
to limit snow potential for middle next week and weaken winds over
the Inner Channels early next week. Forecaster confidence is above
average through middle next week then decreases as models struggle
with pattern evolution.

&&

Ajk watches/warnings/advisories...
public...none.
Marine...Small Craft Advisory for pkz012-013-022-031-036-041>043-051-052.

&&

$$

Wesley/tps

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