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FXAK67 PAJK 192245
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
245 PM AKDT Tue Sep 19 2017

.SYNOPSIS...An upper low west of Haida Gwaii will continue to
track southeast Tuesday night. A surface ridge will shift east
across the gulf through Wednesday night and displace a trough of
low pressure south and east.

&&

.SHORT TERM.../Tuesday night through Wednesday night/...Little
change in the short term forecast, as all is progressing according
to the previous forecast. Easterly flow aloft is visible in
satellite imagery this afternoon, but cloud cover is relatively
thin. There are, however, a few showers occurring over the
southern panhandle south of Admiralty Island. Over the gulf,
evidence of an inverted trough exists in the form of developing
convection that is rotating about a trough axis that appears to
extend northwest from south of Haida Gwaii. Showers will diminish
and end tonight with decreasing cloud cover across the panhandle.
Increase in radiational cooling overnight is expected to result in
areas of fog across the panhandle tonight and this has been
retained in the afternoon forecast. High pressure over the western
gulf will shift east during the short term forecast period,
displacing the inverted trough to the east as well. This will set
up a solid north or northwest wind regime over the eastern gulf
and inner channels. Winds will remain on the light side except
for coastal marine zone 41. There, winds will pick up to small
craft level of 25 kts very late tonight or by early tomorrow
morning. Otherwise, winds 20 kts or less on the outside and over
Lynn Canal with 15 kts or less elsewhere.

Ensemble approach was used to update pressures with GFS as a
foundation for winds over the gulf with little change to the
previous forecast. Updated PoP and QPF with NAM and SREF - again
with little change. Temps adjusted using bias correct MOS. Overall
forecast confidence is above average.

.LONG TERM.../Thursday through Monday as of 8 pm Monday/ The
extended forecast begins dry; however, it will quickly shift to a
wet pattern as a front approaches from the west. Increasing clouds
and rising PoP values are the trend for Thursday as the front
approaches. Yakutat remains the area that will see the earliest
onset of precipitation with rain likely by Thursday afternoon. The
rest of the panhandle may stay dry until Thursday evening.
Nonetheless, once precipitation begins, a wet period continues
through the long term. As the effects of this first front begin to
diminish, a low pressure system and its associated weather front
will begin to affect the panhandle Saturday. This will act to
increase rainfall across the panhandle once again. Little day to
day temperature variation is expected in the long term with highs
in the 50s across the panhandle through the extended.

A few significant changes were made in the long term. The front
approaching Thursday has trended slower, so its arrival was slowed
down. Also, there is enhanced model agreement that a surface low
develops over the pacific and tracks northeast into the gulf this
weekend. Operational models are in good agreement with this system
but diverge some from the WPC solution so some detail surrounding
the low was added with winds being increased to 20 kts over the
gulf; however, this may be trended upward as the event draws
closer. Precipitation associated with the low may be another
concern as models show tropical influence in the low`s
development. A blend of the NAM and ECMWF was used for Thursday
and more input from the ECWMF on Friday. The weekend low was
handled with a blend of the WPC, ECMWF, and GFS solutions;
however, before this timeframe the GFS is an outlier.

&&

.AVIATION...Main concern will be fog tonight. Expect most cloud
cover to dissipate and with light flow in lower levels, a good
setup for fog is developing. Have the usual fog spots (PAYA, PAGS,
PAJN, PAPG, PAWG, PAKW) lowering to IFR conditions late tonight.
Other locations appear to have a bit too much low level flow to
allow fog to develop in any substantive way, although will need to
watch for a stratus layer that could form below 1000 feet. The fog
should lift to IFR stratus around 18z, then mix out by 20z.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ041.

&&

$$

Fritsch/RWT/Byrd

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