Area forecast discussion
National Weather Service Honolulu HI
845 PM HST Friday may 17 2013
Synopsis...
trade winds will weaken tonight and Saturday...as low pressure
develops north of the islands. The weak low will turn winds to the
north on Kauai and Oahu by Saturday night...with light south winds
near The Big Island. With light winds...clouds and showers will
favor afternoon hours...and the interior and upslope areas. The
presence of the low will help to intensify any showers that form
over the islands for the next several days.
&&
Discussion...
currently...water vapor imagery shows a closed low aloft centered
near Oahu...moving slowly toward the east-southeast at about 5 miles per hour. At the
surface...moderate trade winds are being supported by a surface
high centered about 1300 miles NE of the islands. The presence
of the low aloft has destabilized the island airmass...and is
enhancing both the incoming trade wind showers...and the showers
lingering over the Kona slopes that formed in response to afternoon
seabreezes and upslope flow. Meanwhile...water vapor and infrared
imagery show a developing area of low pressure about 600 miles north
of the islands...near 30n 158w...which will become an important
player in island weather over the next several days.
The forecast calls for the current upper low to move east-southeast while
slowly weakening through Saturday...while a new trough aloft digs
aggressively southeast over waters northwest and north of the islands. This strongly
digging trough will help to develop a closed low aloft that will
meander in a position about 550 miles NE of the islands from late
Saturday into the middle of next week. The proximity of the closed
low will maintain a moderately unstable island airmass...and
enhance any showers that develop. The low will linger north of the
islands until the middle of next week before finally dissipating.
A weak surface low is forecast to develop in response to the
developing low aloft to the north of the islands...with a surface trough
extending S over the islands. This will interrupt the trade flow
over the next 24 hours... and although there is some uncertainty as
to where the trough axis will lie...latest guidance indicates that
light to moderate winds will turn to the north over Kauai and Oahu by
Sunday...with S winds near The Big Island. Away from the moderate north
flow on Kauai...land and sea breeze circulations will dominate wind
flow over the islands. This means afternoon clouds and showers over
the islands both Saturday and Sunday...with partial clearing at
night. The trough axis will meander in connection with the
meandering low aloft to the north as we head into next week...and
details on the low level wind direction are uncertain...but
confidence is high that winds will remain light over the
islands...and a continuation of the diurnal convective weather
pattern is expected to continue into next week. There are some minor
differences among the models about how unstable it will be after
Sunday however.
It appears right now as though the middle levels of the atmosphere
will stay a little too warm to support thundershowers...though if
these were to occur the best chance would be over the leeward
slopes and interior portions of The Big Island each afternoon this
weekend. In other areas...localized downpours are possible...
particularly in the afternoons.
&&
Aviation...
clouds and rain showers over windward areas will bring tempo MVFR and isolate
IFR conditions...and isolate MVFR over leeward waters. Trade winds will
decrease through tonight and tomorrow with sea breezes expected over
leeward locations in the afternoon. Clouds and rain showers will increase
over interior sections Saturday afternoon.
&&
Marine...
with trade winds diminishing...the Small Craft Advisory /sca/ for
windier marine zones around Maui and The Big Island was allowed to
expire a short while ago. Winds and seas are expected to remain
below Small Craft Advisory criteria until at least the middle of next week.
Biggest marine highlight is the high surf along S facing shores of
all islands...with a high surf advisory in effect through the
weekend. However...a series of S swells will keep elevated surf
along S facing shores through most of the upcoming week. A small northwest
swell is expected this weekend...with a moderate northwest swell building
Tuesday and peaking Wednesday.
&&
Hfo watches/warnings/advisories...
high surf advisory until 6 PM HST Sunday for south facing shores of
Niihau Kauai Oahu Molokai Maui and The Big Island.
&&
$$
Discussion/marine...birchard
aviation...Morrison
