FXAK67 PAJK 211354
Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
554 AM AKDT Mon Aug 21 2017
.SYNOPSIS...A wide swath of tropical moisture associated with
former typhoon Banyan spread across Southeast Alaska overnight
resulting in bands of moderate to heavy rainfall across the
Panhandle, but a developing wave approaching the southeast gulf
will re-focus the heaviest rainfall across the southern and
south-central Panhandle today. Another front developing tonight
in the gulf will extend the heavy rainfall's duration into Tuesday
.SHORT TERM...Monday through Tuesday night will be wet but a
drying trend begins late Tuesday as a low begins to cross the gulf
southwest of Southeast Alaska. Forecast temperatures were raised
Monday through Tuesday, even lows Tuesday morning, by 1 or 2
degrees. Winds were increased a little for the south/central Inner
Channels to coincide with expected frontal passages. Changes were
made to QPF, detailed in the hydrology section.
Instability will increase as a parent upper low moves into the
central and southeast gulf Monday night through Tuesday night. We
considered a slight risk of thunderstorms for the southern gulf
waters, but given our best verification seems to occur with
onshore systems and interaction with terrain, we chose to leave
out at this time.
NAM/GFS/ECMWF all helped guide changes.
.LONG TERM.../Wednesday through Sunday/ Upper level low in the
southeast gulf will weaken and move to the SE followed by a ridge
building over the gulf and then panhandle. By Thursday another
upper level low tracking E from Kamchatka will drop over the
western AK gulf. Estimate this low center will eventually swing
up to the northeast gulf coast by Saturday with a broad area of
low pressure still covering most of the gulf. Through the weekend
expect a series of waves to rotate around the main low and then
over the panhandle. At the surface a low in the SE gulf on
Wednesday will have potential to produce small craft winds over
the coastal waters before it weakens and moves inland.
Precipitation likely to diminish from N to S as this low moves
into southern British Columbia with some lingering shower
activity. Any dry period will be short lived as the next surface
low moves over the AK peninsula and its associated front crosses
the gulf and then over the panhandle late on Friday. Current
projections have a wet weekend as short waves at the surface
continue to move in bands of rain. In addition to the rain, there
is potential for stronger winds with this low and front, at least
getting up to small craft levels over the gulf coast. Once again
this system showing low fetch of moisture and while GFS IVT
analysis is aiming the heaviest amounts just south of the
panhandle still potential for periods of heavy rain.
Models still in fair agreement for the Wednesday system but did
use a blend of GFS with some NAM in nudge winds up. Later in the
period operational GFS and ECMWF still in good agreement and with
more detail than WPC were used to refresh grids to Friday. For
the rest of the time kept with WPC guidance. Forecast confidence
is bit above average due to model agreement.
.AVIATION...Given such deep moisture, it is no surprise that TAFs
depict lower ceilings and visibilities within moderate to heavier
rain bands: generally MVMC with some temporary IMC conditions. Low
level wind shear also largely corresponds to frontal passages
through the Panhandle.
.MARINE...Frontal activity across the Gulf of Alaska will keep
southeast winds up to 25-30 kt across the outside waters and
Clarence Strait today. Despite waning winds tonight over the gulf,
high seas to 8 feet will necessitate small craft advisories into
tonight. Winds will be more stubborn through Clarence Strait.
Elsewhere, southerlies will periodically rise to 15 to 20 kt as
fronts weaken onshore. Winds in the far northern Inner Channels
should remain light.
.HYDROLOGY...Some changes were made to QPF, and most importantly
the track and duration of heavy rain bands today due to an
increasingly apparent wave forming in the southeast gulf overnight.
After initial moderate to heavy rain bands slide through
Ketchikan and the southern Panhandle this morning they will slowly
move westward and reach Wrangell/Petersburg before moving back
toward Ketchikan this evening. This will likely temporarily halt
or slow any rises in creeks over the far south. However, Prince of
Wales Island looks to be persistently very wet through tonight.
Juneau and a few points north and northwest along the mountains
may see amounts increase this afternoon as the swath of moisture
pivots farther west today, but the main focus continues to be
southeast of the capital city.
Another front curves into Southeast Alaska round from the
southwest on Tuesday, this one also drawing moisture ahead to the
northwest along the Coast Mountains. However, the front's forward
progress will push the heaviest precipitation eastward into
Canada by Tuesday afternoon.
Southern Panhandle streams rose overnight but have begun to level
off or decrease as rain rates have eased. These should begin to
ascend again this morning when heavier bands begin to pass to the
northwest. Northern Panhandle streams likewise have fallen or
plateaued as rates fell off around midnight. These will likely
rise again today as rain returns. While, Staney Creek and the
Stikine as well as flashier streams like Montana Creek and
Government Creek will be monitored, we continue to not foresee any
flooding. The breaks in heavier precipitation will be important in
keeping levels manageable even as watersheds are primed. However,
it must be said, rainfall rates have appeared to max around one-
quarter inch per hour in lowland locations and have not been as
consistent as we feared. Rainfall Monday through Tuesday
afternoon will amount to between 0.5 and 2.5 inches over the north
and 2 to 5 inches for areas Petersburg south. Higher elevations
could see as 4 to 9 inches, especially across the south.
MARINE...Small Craft Advisory for PKZ036-041-051-052.
Small Craft Advisory due to Seas for PKZ042-043.
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