Small Craft Advisory 571, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
348 PM AKST Wed Nov 15 2017

.SYNOPSIS...An area of high pressure in the Yukon will maintain
strong outflow winds through Wednesday night. A low pressure
system will develop over north central gulf on Thursday then track
to the southeast through Friday night.

&&

.SHORT TERM...Transition period beginning Wednesday evening as
the high pressure over the Yukon shifts to the east and the low
near Haida Gwaii continues to track to the south. The strong
northerly outflow winds have started to diminish and by Thursday
inner channel winds will have flipped to the south as pressure
gradient becomes parallel. Another cold night in store due to
clear skies, radiational cooling, and with the diminishing winds
no downslope warming to temper these lows. The quiet day on
Thursday will be short lived as another low moves into the
Northeast gulf starting Thursday evening. At first models have
been in good agreement with the timing and position of the low
Thursday evening. Did move the low center further to the
northwest which would delay the onset of precip in Yakutat a few
hours. From there expect the low to take a southeasterly track as
the mid level trough over the eastern gulf drops to the south. At
this point, Friday, models show more spread. While all still had
the low center further north than previous runs there was a split
between the ECMWF/NAM to the east with deeper GFS/Canadian to the
west. Results of this more northerly low center were to increase
temperatures with southwesterly flow over the far southern
panhandle. As the low moves to south, northerly pressure gradient
will increase once again with gale force winds for northern inner
channels.

Main issue from this system will be the precipitation. Good
agreement of swath of PoPs spreading from west to east through
Friday, then dropping to the south. The forecast problem is where
the rain-snow line will occur and QPF amounts. Current thinking
is coastal areas, Yakutat, Pelican, Sitka, Angoon will start as
snow but transition to rain and thus limit significant snow
accumulations. Keeping this regions in a 1 to 5 inch range, where
cold air damming would allow more snow. For the far southern
panhandle the transition from snow to rain expected to be faster
with only an inch or two of accumulation. For the central and
northern panhandle good set up for an overrunning event with
possible 4 to 9 inches. Everyone note that 9 inches is on the
extreme end of snow totals. Cold air in place at the surface with
easterly flow, isothermal or saturated layer in dendritic growth
zone, and later inversion with elevated lift. Limiting factor for
the snow will be final QPF values. It has already been a harsh
winter for SE AK and it is only November. GFS/ECMWF were on the
high side but while NAM values were more reasonable for amounts
the position of the NAM moved this further south.

Overall little change to the forecast grids initially due to
better model agreement. Temps were warmed with the next system to
the south with the new position of the low using a NAM/ECMWF/GFS
blend. Increase confidence in a snow event occurring but due to
the low confidence and model spread of the low position variations
to the forecast are likely.

.LONG TERM.../Friday through Wednesday/...Confidence is
increasing for a potential winter storm event for heavy snow
later this week which may last into the weekend. Models continue
to be in good agreement on a short wave that will drop southeast
out of the interior Thursday evening. Rapid cyclogenesis will
occur over the far north-central gulf Thursday evening, followed
by frontogenesis early Thursday night. The low itself is forecast
to remain out over the gulf, but precipitation in the form of snow
will reach the northeast gulf coast and Yakutat area just prior
to the beginning of this long range forecast period. By Friday
morning, snow will stretch along the the entire length of the
outer coast and will have penetrated into some of the inner
channels. By mid morning, a transition to a rain/snow mix is
likely for the outer coast from Sitka south, and for the southern
inner channels by Friday afternoon. Meanwhile, snow rates will
increase over the inner channels from north to south with the
heaviest forecast snow falling between a line from Cape
Fairweather to Berners Bay, thence south to Frederick Sound
between Friday morning and Saturday morning. By Saturday morning,
the heaviest precipitation will have shifted south as the low
tracks off to the southeast but snow is not expected to end
entirely until Sunday afternoon, if at all. A follow-on low and
associated front approaching from the west on Sunday may provide
for continued snowfall for many locations. Confidence on this
second system is not very good at the present time and PoP grids
for late this weekend and early next week reflect this with a very
broad area of PoP and little in the way of details. If models
continue to remain in good agreement, a Winter Storm Watch may
become necessary within the next day or two.

Even as all of this is happening, strong northerly outflow winds
continue, with gales over the northern inner channels, coast
marine zones between Cape Edgecumbe and Icy Bay, and out of
Yakutat Bay and the Dangerous River. By Saturday night, outflow
winds will diminish a bit, but will still represent a significant
marine hazard for anything less than open-ocean type vessels.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind until 9 PM AKST this evening for AKZ025.
     Winter Storm Watch from late Thursday night through late Friday
     night for AKZ019>021-025.
MARINE...Gale Warning for PKZ012-013-031.
     Small Craft Advisory for PKZ011-021-022-032>034-042-043-051.

&&

PRB/Fritsch

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