Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
400 am HST Friday Aug 28 2015
humid...moist...and somewhat unstable conditions will persist today
as weak trade winds gradually rebuild. Although showers will favor
windward and Mauka areas...afternoon clouds and spotty showers can
be expected over leeward areas with flash flooding remaining a
possibility over the smaller islands. Trade winds will strengthen
during the weekend...though humid conditions will continue...
especially across the northern islands. Hurricane Ignacio will
approach the state late in the weekend and will affect island
weather next week.
trade winds are slowly building...but sea breezes will trigger
spotty heavy showers and possibly thunderstorms this afternoon. A
surface trough about 150 miles north of the islands will continue to
slowly fill today...causing the local pressure gradient to tighten
and usher back in light trade winds. However...these winds will not
be strong enough to hold back afternoon sea breezes across leeward
areas. The atmosphere remains quite moist with precipitable water
values running near 2 inches across the smaller islands and 1.6 on
The Big Island...and an upper level trough just north of the islands
will maintain instability. With the moist and somewhat unstable
conditions remaining in place...sea breezes should trigger spotty
heavy showers and a few thunderstorms over interior and leeward
sections today...mainly on the smaller islands...and with grounds
still nearly saturated from recent rainfall...a Flash Flood Watch
remains in place from Kauai to Maui.
Although afternoon high temperatures will be around seasonable normal...dew
points in the middle 70s will maintain humid conditions with heat index
values rising to the middle to upper 90/S. Winds will also be light...
making the humid conditions feel even worse.
Trade winds will gradually strengthen on Saturday. Higher moisture
will linger over the western end of the state...as drier air slowly
filling in from the east limits shower activity on The Big Island.
Expect showers to favor windward slopes...but sea breezes will
continue to produce afternoon clouds and a few showers over leeward
terrain. The upper level trough will remain close enough to the
state to pose the threat of isolated heavy showers for the smaller
islands...but confidence is too low to mention in the forecast at
Stronger trade winds are due on Sunday. Hurricane Ignacio will be
approaching the region from the east...and moisture along the outer
fringe of the system could lead to an increase in trade wind showers
across The Big Island.
Forecast uncertainty increases significantly on Monday and
Tuesday...hinging on the eventual track of Hurricane Ignacio.
Forecast track errors at that time range are roughly 150 to 200
miles...so it is still too early to tell what the direct impacts
will be. Everyone in the state should continue to monitor the
progress of Ignacio during the next several days...and the National
Weather Service urges individuals to have an action plan in case
Ignacio...or any hurricane...threatens.
VFR conditions are expected to prevail through much of the morning.
Daytime heating could bring clouds and showers to the islands which
could bring MVFR and IFR. No airmets at this time...however
depending on the amount of development...mountain obscuration could
be possible. Expect any clouds and showers to clear again tonight.
all mariners should monitor the progress of Hurricane Ignacio during
the next several days.
Winds and seas should remain below Small Craft Advisory levels into
Saturday...with elevated seas becoming a concern by Sunday for
portions of the state.
In the short term...a west to west-northwest swell will be the main
concern. Buoy 51001 and the Hanalei buoy showed an increase in long
period energy overnight. While the impacts of this swell will be
modest on most islands...buoy data and wavewatch guidance support
the issuance of a high surf advisory along west facing shores of The
Big Island...where the advisory threshold is lower than the other
islands. Surf around the advisory level is expected on The Big
Island through Saturday.
Uncertainty is very high regarding the potential impacts from
Hurricane Ignacio early next week...but we are confident that an
east-southeast swell will arrive on Saturday. The bulk of this swell
energy will initially impact The Big Island and Maui...while the
other islands will be largely shadowed. Surf along east facing
shores on The Big Island and east Maui is expected to rise to
advisory levels as early as Saturday...then possibly climb to
warning levels on Sunday. This large surf will likely spread to east
facing shores of the rest of the state early next week.
Flash Flood Watch through 6 PM HST today for Niihau-Kauai-Oahu-
High surf advisory through 6 PM HST Saturday for west facing shores
of The Big Island.