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Area Forecast Discussion (AFD)

Issued by Juneau, AK (AJK)

FXAK67 PAJK 212307

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
307 PM AKDT Sun Oct 21 2018

.SHORT TERM...A stalled front oriented north-south over the 
eastern gulf has been producing heavy rain and strong winds over 
Yakutat for much of the day. The front is forecast to weaken later
tonight as a developing wave moves north along the frontal 
boundary. Early afternoon observations indicate that this is 
already happening. As the front wavers east and west, winds will 
diminish significantly, as will the rain rates. The Situk River is
currently at bankfull and is still rising. Current projections 
are for the Situk to continue to rise to near minor flood stage 
later tonight. As the developing wave tracks north along the 
frontal boundary, gale force southeasterlies over the eastern gulf
will expand south, resulting in strong wind gusts along the 
southern outer coast from late tonight through Monday afternoon. 
Additionally, heavy rain will move over the Panhandle from the 
south overnight even as the heavy rain ends over the northeast 
gulf coast. Following close behind the developing wave is a 
developing storm force low that is forecast to enter the eastern 
gulf from the south on Monday night. This will result in more 
strong winds along the outer coast. These winds will spread north 
along the outer coast through Tuesday before diminishing Tuesday 
night. High wind warnings are not out of the question at this 
point, as minor variations in the track of the low with respect to
the coast could result in significant changes to wind speeds. 

Lightning continues to be an issue. The airmass to the west of the present
stalled front is clearly convective as evidenced by satellite
imagery, and lightning has been detected all day in Prince 
William Sound and the Kenai Peninsula. As the front associated 
with the storm force low tracks north, the atmosphere east of 140W
will destabilize as well. In addition to aiding stronger winds 
aloft to mix down to the surface, large scale ascent in the low to
mid levels and strong vorticity in the mid levels will likely 
trigger significant thunderstorm activity along the eastern gulf 
and outer coast. This potential has been included in the forecast 
for Tuesday afternoon and evening.

Used a blend of NAM and the GFS with a little help from the ECMWF
for updates to pressure. Used GFS as a foundation for wind grids
with the net effect over the gulf being an increase in wind speed.
Changes to wind on the inside mostly local effects with some minor
increases to wind speeds. Overnight lows and daytime highs raised
by a few degrees. Temperatures will be a struggle due to increased
mixing, including during the overnight hours, and locations that
do see increased winds will see warmer temps as well. PoP from a 
blend of GFS and NAM with some local edits based on MOS. QPF from 
NAM with local edits. Overall forecast confidence is average.

.LONG TERM.../ Wednesday to Saturday as of 10 pm Saturday / A 
broad complex pattern of upper level troughs are rotating about 
general low aloft over the far western Gulf of Alaska, the end of 
the week the complex shifts south to the far southwestern gulf.

At the surface the active weather pattern persists. A moderate to
strong low in the lower 970 mb range moves across the central
gulf. Model solution still having some minor disagreements to
strength and position, we made minimal changes at the present,
though it may get fined tune over the next day or so. Look for
gale force winds with the front and wet conditions for the coastal
area. The front is weakening slightly as it moves through the
inner channels so heavier amounts will linger more over the 
outer coastal region. 

A brief break as the following showers band impact eastern gulf
and coastal areas. The next low develops over eastern pacific and
then curves north on a fairly similar storm track through the 
gulf to the north-northwest. At present, the various models 
solution are still agreeing on a significant storm feature with a
depth of sub 960 mb for Wednesday night. They are also agreeing 
on placement better than yesterday as well. So we have deepened 
the storm just a little more, and increased the winds in the gulf 
around the low and frontal band to storm force, the seas with the 
frontal band area could be impactful as well with a fair section 
of 25+ foot wave heights. The low track appears to be far enough 
west at the present to make this more of a marine issue and not 
slam the coastal communities as hard as if it was closer to the 
panhandle. They may see strong wind headlines as the date of the 
event is closer.

The pattern after the low and front from late Thursday to the 
weekend is confused with no strong signal from any of the
operational solution and WPC solution just hangs onto a general
area of low pressure over the central gulf. Which will mean a
continued stream of short waves and frontal band into the
panhandle for the end of the week an the weekend. Long range
forecast confidence is average to moderate. 


.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind from 1 AM AKDT Monday through Monday afternoon for 
     Strong Wind until 1 AM AKDT Monday for AKZ017.
MARINE...Gale Warning for PKZ036-041>043-051. 
     Small Craft Advisory for PKZ011-021-022-033>035-052. 
     Small Craft Advisory due to Seas for PKZ053. 




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