FXAK67 PAJK 252240

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
240 PM AKDT Tue Sep 25 2018

.SHORT TERM...Rain continues throughout the panhandle this
afternoon as our atmospheric river continues to pump in the
moisture, which can be traced on MIMIC imagery all the way back
to Hawaii. A relatively strong jet aloft is helping to channel
this subtropical moisture plume directly towards SE Alaska,
functioning somewhat like a fire hose. In addition, WV satellite
imagery shows an upper level ridge building in over the gulf,
which will act to pivot and, eventually, cut off this flow of
moisture. Currently, the heavier rain is centered around Baranof
Island and includes much of the central panhandle. Both the Sitka
and Juneau ASOS are currently reporting an observation of
moderate rainfall. Sitka has consistently been the pivot point
we've seen with these systems the last few days, with previous
spotter reports indicating they've received anywhere from an inch
and half to two inches of rain these last two days, not including
today's accumulation. Orographic flow enhancement is also
providing additional lift to trigger heavier precipitation in
areas near higher terrain.

As mentioned previously, the central panhandle will remain the
target for higher rain chances this evening before the building
ridge over the gulf pivots this moisture stream south late
tonight. Southern areas, including Klawock, Ketchikan, and
Metlakatla, should see an increase in rain chances and
accumulation tonight and continue through tomorrow. Tomorrow
afternoon and into tomorrow night, this is when we should see the
ridge begin to shutoff the subtropical moisture flow. Rain
chances should then diminish. However, newer model runs have
begun to show some additional moisture getting trapped within the
ridge as it further extends north. Due to how stable the
atmosphere will be, this may be a case of drizzle or maybe even
some light rain, so we've increased chances of rain slightly for
northern areas on Thursday. Regardless, this ridge feature moving
in will likely keep a very moist boundary layer nearer to the
surface, giving us a nice soupy marine layer rather than clearer
skies. Forecast soundings then hint of a return to northwest flow
aloft beginning Thursday night, which will act to start drying
things out for much of the panhandle.

Winds should remain a non-concern through the period with no
major frontal features and a slackening pressure gradient.
For Thursday into Thursday night, daytime high temperatures were
decreased and nighttime temperatures were raised to account for
this marine layer in place, preventing things from heating up
during the day and cooling down at night. It's likely that Yakutat
could see some clearing tonight, allowing some radiational cooling
to occur with possible fog development. Wednesday night may also
yield some fog development for northern and central locations
depending on if any breaks occur, albeit briefly.

.LONG TERM.../Thursday through Tuesday/ As of Monday evening. The
precipitation pendulum is swinging quickly as another stretch of dry
weather is on its way. Thursday will still be damp over the southern
half, while the north will be dry. The rest of the area has to wait
for the rain to slip to the south through the day Friday for the dry
weather. By Saturday, dry weather and sun will abound and
continue through Tuesday.

At present, many stations in SE Alaska are well below normal for
rainfall. Juneau, for one, has the driest September on record so far
this month. With a week to go and another dry spell only a few days
away, we will have to wait to see if we break any monthly records.
With a moderate drought still in place over much the area, this is
one record most are not hoping to see.

The building ridge Thursday will produce another blocking pattern
for the Panhandle. Once this ridge in place it should provide a nice
defensive line against any approaching lows at least through
Tuesday. With warmer temperatures aloft will need to keep an eye on
strong outflow winds. Lack of clouds will give us warmer daytime
temperatures, Look for highs in the low to mid 60s, which is above
normal for this time of the year. The mercury will drop quickly
overnight with all the radiational heat loss. Temperatures will be
in the 40s, with a few locations dipping down into the upper 30s.

Models for a second day continue to be in good agreement. The GFS
and ECMWF were again the favored models from Thursday through the
weekend, trending towards WPC over the weekend. Forecast confidence
is above average.


.AJK Watches/Warnings/Advisories...
MARINE...Small Craft Advisory for PKZ041.
Small Craft Advisory due to Seas for PKZ022-042-043-051-052.




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