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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 211326
Area Forecast Discussion
National Weather Service Honolulu HI
326 AM HST Mon Aug 21 2017
Increasing moisture combined with an upper disturbance over the
region will support heavy showers and a few thunderstorms through
Tuesday. Drier and windier conditions will return Wednesday through
the second half of the week as the ridge strengthens to our north
and the upper disturbance shifts away from the region.
Recently animated water vapor loop showed plenty of middle- to upper-
level moisture streaming northward over the islands due to a broad
upper low drifting southwestward just north of Kauai. This upper low
combined with a sufficient amount of instability and moisture will
keep the chance of heavy rain and thunderstorms in the forecast
through Tuesday. The early morning upper air sounding at Lihue
reflected this instability and depicted a relatively steep mid-level
lapse rate with -9C at 500 mb. The latest satellite-derived
precipitable water (PW) imagery showed values ranging from 1.5-2"
over and around the islands, which is above normal for this time of
year. As a result, localized flooding will remain a concern
through Tuesday, mainly where the heavy showers become focused.
Some of this activity will become focused over the leeward areas
through the afternoon and early evening periods where sea breezes
For the extended period, the GFS and ECMWF both depict breezy trades
and drier conditions returning across the state beginning Wednesday
as the upper low shifts away from the region and high pressure
builds north of the state. Model PWs support this and dip back down
to below average for this time of year (near 1"). Showers will
remain focused over windward and mauka areas each day.
Surface high pressure to the north will maintain moderate east
northeast surface winds across the area. These winds will carry
very moist air over the area. A low aloft 150 nm north northwest of
Kauai is forecast to move south southwest over the next 24 hours,
then move off to the northwest. The low aloft will make the atmosphere
prevail over southwest sections of all the islands this morning, but
TEMPO MVFR and ISOL IFR conditions are expected over northeast sections
of Maui, Molokai and Oahu. AIRMET SIERRA is currently in effect for
MTN OBSC over the northeast section of Maui. MTN OBSC is expected to
spread to the northeast sections of Molokai and Oahu before sunrise.
Thunderstorms may develop again over land as the islands heat up today.
Winds are expected to remain in the moderate to fresh range
through tonight, then strengthen into the fresh to locally strong
category Tuesday through the remainder of the week. Increasing
shower coverage will remain in the forecast through Tuesday as an
upper trough drifts southwestward over the region. This upper
trough will also support a slight chance of thunderstorms through
Tuesday. Drier conditions will return Wednesday through the second
half of the week.
Surf will remain generally small all around. Small background
long-period pulses from the south-southwest (210-220 deg) and
south- southeast (130-160 deg) will keep the surf from being flat
along south facing shores this week. A slightly larger south-
southwest (220 deg) swell will be possible this coming weekend due
to a recent gale over the Tasman Sea.
Surf along east facing shores will remain generally small through
tonight, then build Tuesday through the rest of the week as
stronger trades return. Additionally, a long-period swell
associated with tropical activity across the eastern Pacific will
be possible Wednesday through Friday, especially along the exposed
beaches of the Big Island.
Expect higher than normal water levels due to a combination of an
oceanic eddy moving through from east to west and the regular
King Tide cycles. Water levels will slowly lower each day through
the week. Please refer to the Special Weather Statement for more
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office