Small Craft Advisory 57, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
233 PM AKST Mon Jan 22 2018

.SHORT TERM...Satellite imagery this afternoon showing a surface
low in the western gulf that developed through the day and
associated occluded front moving over the panhandle from south to
north. A second front will rotate around the low but will be
oriented more to the north and south and remain along the
panhandle coast this evening then over the northern panhandle
while shearing apart late tonight. The surface low will weaken
through early Tuesday as another tracks up from the N Pacific
moving to the west of Haida Gwaii by Tuesday afternoon. The
leading front from this low will slowly move over the southern
panhandle into Wednesday then crossing over the central panhandle,
while shearing apart, by Wednesday evening.

The first front progressed northward producing showers over the
the southern panhandle starting early this morning then reaching
the central regions by the afternoon. Typical of showers the
intensity has been highly variable with visibility reductions
swinging from less than 1 mile to 10 miles over an hour.
Precipitation transitioned from snow to rain over the south as a
warmer swath of mid level air moved in. Central and northern areas
expected to stay all snow as 850 mb temps remain at or below -2C.
Even with the impressive snow fall rates (out the window it
finally looks like winter) the speed of the advancing front
keeping snow accumulation in the 1 to 2 inch range today with
additional 1 maybe 2 inches tonight. Model soundings indicate a
good convective environment but not a deep saturated layer or an
impressive dendritic growth zone. Higher snow amounts possible if
showers become oriented along a similar track and begin training.
By Tuesday as the Haida Gwaii moves in producing off shore flow
over the central and northern regions precip will diminish and
some clearing skies are possible. Expect precip for the Wednesday
system to also remain as all snow, but once again no significant
QPF values, temperatures in the upper 20's to low 30's keeping
snow ratios low, thus only a few more inches expected.

Winds showing similar variably to precipitation tonight. As the
front moves north wind directions will flip from north to south
briefly before gradient between the Haida Gwaii low and high
pressure over Canada flips winds back north. Increasing gradient
means increasing winds speeds with gales for the northern inner
channels by Tuesday evening and rather strong outflow winds from
Cross Sound and Taku inlet. Components for mountain wave in
Juneau looking favorable so have gusts upwards of 40 to 45 mph in
the downtown and Douglas areas. As the low begin to pull to the
west, pressure gradient will weaken and winds will diminish
through Wednesday.

NAM/GFS were showing good initialization with the precip
coverage. Not much change through Tue for pressure and winds. On
Wed new models runs showing the low center further southeast and
deeper. Blended in more NAM for pop early on. Tricky part of the
forecast was snow totals. Nudged some QPF values down but with the
convective nature kept toward the 1 to 2 inch range for snow,
even if amounts may be closer to 1 inch. Forecast confidence for
the synoptic pattern is above average but lower confidence on
remaining snow totals.

.LONG TERM.../Wednesday through Monday/...Long range period
begins with an upper low south and east of the Gulf of Alaska
moving east towards Washington. Another upper low located over
the northern interior will be tracking south and be over the
northwest Gulf by Thursday morning. Upstream, a building upper
ridge will be situated over the Bering Sea. The upper low will
slowly track south and dominate the gulf by Friday morning. By
this time, the upstream ridge will have strengthened into an
Omega block, with another upper low over the Sea of Okhotsk. By
the end of next weekend, the Omega block remains strong and in
place while the upper low will have continued a slow track south
of the Gulf of Alaska.

The combined effect of the upper low over the gulf and the
building upper ridge to the west will be to advect in colder low
level air over the panhandle. Some modification of this airmass
will occur as it crosses a considerable distance over the open
ocean, but the ridge will prevent any warmer air from the west
from mixing in. 850H temperatures favoring rain on Wednesday and
Thursday during the day south of Frederick Sound, but then become
more supportive of snow all the way south to the Dixon Entrance
overnight both nights and then through at least Saturday.
1000-500 and 1000-850 thicknesses showing a similar pattern.
Daytime highs over the southern half of the panhandle from
Frederick Sound south indicating that there is a good chance that
at least some of the precipitation for this coming week will be in
the form of rain. Otherwise, what precip we do see may be mostly
in the form of snow. No well defined fronts currently being
depicted in medium to long range models, so convective precip
(showers) is anticipated through at least Wednesday night. 12
hour snow totals currently indicating possibly advisory level
snowfall Wednesday for central zones and Wednesday night for
northern zones south of Berners Bay. Convective nature of this
precipitation will cause the snow totals to be a constantly moving
target even as the event unfolds, so stay tuned for updates as the
week progresses.

No significant changes to pressure or winds. PoP and QPF primarily
from NAM Wednesday and Wednesday night with little change for the
remainder of the long range forecast. Daytime highs and overnight
lows generally nudged down slightly for the second half of the
week based on current model spectrum plots. Overall forecast
confidence is average, but below average for snow totals.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Gale Warning for PKZ022-041.
     Small Craft Advisory for PKZ011>013-021-031-033-035-036-042-043-
     051-052.

&&

PRB/Fritsch

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