Area Forecast Discussion
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018
FXAK67 PAJK 171807 CCA
AFDAJK

Southeast Alaska Forecast Discussion...CORRECTED
National Weather Service Juneau AK
907 AM AKST Tue Jan 17 2017

.SHORT TERM...A colder airmass has arrived over southeast Alaska
from the west and Yakutat has been reporting accumulating snow
all night long. Farther south and east, Gustavus began reporting
snow at 4 am. Many northern locations will see daytime highs early
in the day. As temps cool, a transition from rain showers to snow
showers will occur, but precip will be diminishing at the same
time. End result is that, apart from Yakutat and possibly
Gustavus, no significant accumulations expected. Farther south, a
developing gale force low is currently west of Vancouver Island
and moving north. Associated front will spread rain into the
southern zones later today. Temperatures are cooler here too, but
not cool enough for snow. Exception is Hyder in Misty Fjords,
where colder temps will result in around 2 inches of snow today
and another 4 inches tonight. In the middle, between Icy Strait
and Sumner Strait, look for a rain-snow mix later today as
temperatures cool down. As the gale force low continues moving
north, low level warm air advection will cause temperatures to
remain steady or rise overnight for all locations but the far
northern inner channels with a transition back to rain by
Wednesday morning except for Haines and Skagway.

Blizzard warning for the Klondike Highway remained in effect as
of forecast issuance. Have some concerns about actual conditions
at White Pass based on winds at Summit Creek - 60 kts with 70 kt
gusts. No observations at White Pass, however, and night webcam
images from Fraser Camp do not appear to be indicating reduced
visibility. Decision to expire or extend will be made prior to 6
am.

Employed ensemble approach to pressure updates with no significant
changes from previous forecast. Updated winds over the gulf with
latest NAM. Winds over public zones and inner channels largely
unchanged. Latest models in good agreement regarding approaching
frontal precipitation from the south, so PoP and QPF updated using
NAMDNG5 and SREF. Daytime highs weighted heavily toward current
conditions. Overnight lows from GFS MOS with the model curve being
used rather than a diurnal sine wave. Snow level from GFS as well.
Overall forecast confidence is average.

.LONG TERM...Main focus of the long range remains the compact low
that will be moving north along the outer coast Wednesday into
Wednesday night. Track guidance is rather poor at the moment with
the Nam showing the best consistency on position and timing over
the last few runs (track heading toward Cape Edgecombe by Wed
evening). The Gfs has trended slower, but still takes it in near
Cape Edgecombe. Meanwhile, the Ecmwf has similar timing to the
Nam but sends the low toward Cape Spencer instead. The Canadian is
much faster, tracks it toward Yakutat, and has terrible
consistency between its individual model runs. In light of the
guidance disagreements decided to stick with the more consistent
Nam forecast.

As for what to expect out of this system. Wind gusts to 40 or 45
mph are likely Wed morning for the southern panhandle as 850 mb
winds reach 35 to 45 kt on the east side of the system. This will
be followed by some gusty winds traveling north along the outer
coast Wed and Wed night as stronger SW winds in the wake of the
low move in. Not expecting any strong wind gusts out of the
southwesterlies at this time. As for precip, the northern inner
channels should start out as snow Wed morning with light
accumulations as easterly flow ahead of the low pulls cooler air
out of Canada, but that should change back to rain by afternoon as
flow turns more southerly. Otherwise, precip is expected to
remain rain for other areas.

For the rest of the extended period, forecast is mainly
characterized as cooler and drier as the main Pacific storm track
is forced south into the Pacific Northwest and cold air in the
Alaskan Interior tries to move south and east. Here in Southeast
we will mainly be stuck in a low flow area between a stalled upper
low SW of Vancouver Island late week and the very cold air mass
trying to move in from the NW. Current thinking has enough of the
cold air moving in to cool temperatures down enough that any
light precip that does develop will mostly be in the form of snow
but with low level flow turning mostly offshore or along shore into
next weekend precip will be limited if it occurs at all. Extended
forecast indicates that the Pacific storm track will turn back to
the north early next week with more warm wet southerly flow being
pumped into the area.

&&

.AVIATION...Marginal VMC will prevail with areas of IMC due
primarily to low CIGS. Heavy icing reported yesterday in PIREPS.
Suspect that will be the case again today, so watch those wings
for ice formation. LLWS and turbulence becomes an issue again
tonight for the southern panhandle as the gale force low
approaches the southern zones. Turbulence below ridge tops also
expected over Lynn Canal and near Skagway and Haines this morning.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Blizzard Warning until 6 AM AKST early this morning for AKZ018.
     Strong Wind late tonight for AKZ028.
MARINE...Small Craft Advisory for PKZ012-036-041>043-051-052.
     Small Craft Advisory due to Seas for PKZ022-053.
&&

$$

Fritsch/EAL

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