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FXAK67 PAJK 231529
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
629 AM AKST Thu Nov 23 2017

.SHORT TERM...The start of the holiday will see periods of
rain/snow during the day, a quick moving gale force front tonight
followed by showers for Friday.

A low pressure system will develop as it moves into the NW gulf
today. A pre-frontal trough is already spreading precip across the
panhandle this morning. Expect periods of rain or snow to
continue through the day. As is typically the case with SE
winters, the rain/snow line will be a challenge and somewhat
variable. Most locations but the far southern and outer coast
are cold enough for snow this morning but have a mix as far north
as Hoonah this afternoon. Some areas, such as Juneau, will see
1-3 inches during the day.

The main front assoc with the low pressure system will move across
the eastern gulf this afternoon and into the panhandle this
evening. This is a quick moving front with winds expected to
increase to gale force out of the SE as it moves onshore. Have
many locations along the outer coast with a mention of gusts to
40mph and other inland locations can expect wind gust of around
30-35mph for a short period tonight. Greatest precip rates along
the front will accompany the wind. Have issued a Winter Weather
Advisory for Gustavus and Juneau for 3-6in expected to fall over
a 12hr period, majority during the early morning hours between
midnight and 6am. Other areas nearby such as Hoonah and Haines
could see up to 4 inches with this event, but have not included
them in the advisory at this time since rates should be lower for
Haines and some mixing could limit accumulations in Hoonah. The
snowfall amounts for this event will likely be difficult to
measure since there will be periods of snow during the day before
the front and scattered to numerous snow showers behind it, making
the start/end time hard to determine.

The low will meander around the south central gulf through most
of Friday with bands of vorticity rotating around it and causing
showers across most of the panhandle. The low will then start
shifting south heading into Friday night. This will cause winds to
shift back to the north with increasing outflow winds.

Made adjustments today using an ensemble approach of all available
models with most emphasis on the GFS. Forecast confidence is
average for wind and slightly lower for the rain/snow line.
Biggest concern is the winds with the front shifting to the S-SE
in more locations across the north and warming the surface enough
for a change over to rain.

.LONG TERM.../Saturday through Thursday/ Active weather pattern
continues through the long range. Uncertainty is still the word
for the weekend forecast. While there is some agreement at mid and
upper level patterns the details with surface features are
causing large variability. What is expected after the weekend is a
shift from the cold arctic air in place over the central and
northern panhandle to a warming trend. Needless to say forecast
confidence is low.

High amplitude pattern at mid levels on Saturday with upper level
trough over the eastern AK gulf dropping almost down to S
California with a sharp ridge to the west extending from the N
Pacific and all the way into the Chukchi Sea. The trough/ridge
will start to shift to the east and flatten through Sunday as
another low drops from the Bering Sea over the Aleutians and over
the western gulf Monday. As this low moves east will have a
repeating pattern of ridge following the low, followed by another
low tracking across the Aleutians and into the western gulf. The
weekend surface feature is causing the most difficulty. Low
pressure over the gulf on Friday was originally shown to move
quickly to the east but now indications that this feature will
become quasi stationary into Sunday before weakening and moving to
the south. If the GFS/NAM are correct the more southerly
positions of the low pushes more warm air over the panhandle,
raising temps and snow levels but would be drier. The ECMWF keeps
the low further north, keeps precip in longer but keeps northerly
flow and thus lower temps and snow levels. ECMWF has shown more
run to run variability for the weekend system but is more
consistent for the systems next week. The Monday low looks like a
typical SE AK system moving a low center into the western gulf
with frontal boundary moving across the eastern gulf then over the
panhandle Monday night into Tuesday. While models differ on the
position and strength of the low center, they are in general
agreement on southerly flow which would once again raise temps and
snow levels. Began the process of increasing temps and for now
have more mix of rain/snow for areas north of Frederick Sound, but
could be switching this to all rain at lower elevations. In
addition to the precip and warming temps each low will bring in
another round of stronger gulf winds, with gales likely but
potential for stronger winds by Tuesday over the gulf. Not seeing
any real change to this warmer/wetter pattern for the rest of the
week.

Initially models for Saturday had enough spread that changes
seemed undesired from the previous ensemble type forecast. Latest
check now shows, at least until 12z Sunday, more agreement on
rotating the low center CCW over the southcentral AK gulf before
dissipating. Best agreement between the newest GFS/NAM but held
off on using that blend until more detail for the Friday system
could be established. For the rest of the time frame utilized
latest WPC, which was pretty much in line with previous WPC
resulting in some minor adjustments to wind speeds.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind this evening for AKZ023-027.
     Winter Weather Advisory from 9 PM this evening to 9 AM AKST
     Friday for AKZ020-025.
     Strong Wind from this evening through late tonight for AKZ022-
     028.
     Strong Wind from this evening through late tonight for AKZ018.
     Strong Wind late tonight for AKZ017.
MARINE...Gale Warning for PKZ012-022-036-043-051.
     Small Craft Advisory for PKZ011-013-021-031>035-041-042-052-053.

&&

$$

Ferrin/PRB

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