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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 250639
Area Forecast Discussion
National Weather Service Honolulu HI
839 PM HST Mon Jul 24 2017
A drier, more comfortable trade wind regime will be building
across the area tonight and Tuesday as high pressure strengthens
across the area. And this regime will persist into the upcoming
weekend. Showers riding in with the trades will favor the
windward and mountain areas. Lee areas of the smaller islands will
be dry. Leeward Big Islands will get their usually afternoon and
evening clouds and scattered showers.
The uncomfortable air mass brought on by the remnants of tropical
cyclone Fernanda continues to exit the main Hawaiian Island this
evening. It is being nudged out by a much more comfortable trade
wind air mass. There is a sign that this air mass is here, by
noting the dew point reading from Kahului airport where it has
lowered from 74 degrees at 4 pm HST to 71 at 7 pm. Also, the the
trades are apparently back, blowing out of the NE at 13 mph with
gust to 22 at Kahului Airport.
The upper level trough that aided with the flare ups of
thunderstorms earlier this morning, is still located 240 miles
west of Kauai this hour. The models are weakening this trough
overnight and into Tuesday. Thus, it will no longer be a factor to
our local weather.
The shower activity over on Oahu will continue to subside in the
next hour. Expect trade wind showers to take over after midnight
on Oahu. The last of the tropical elements should vacate Kauai
by noon Tuesday. Once the trades become established, it will be
here for the rest of the week with precipitable water (PW) values
of around 1.50, 1.65 inches. These values translate to more or
less of a typical trade wind regime with showers likely for the
nights and mornings across the windward and mountain areas, and
drier during the afternoon hours. A light shower or two will leak
over into the lee areas of the smaller islands, particularly
during the nights and morning. Lee side of the Big Island will
maintain their usual afternoon clouds and scattered showers,
lasting well into the night, but shifting from the interior
to along the coast as the evening progresses.
The return of the trades will be that of a nice breeze, at
moderate to locally strong speeds. The trades then ease off
slightly to moderate speeds on Wednesday night. This comes after
the surface high supplying the islands weakens slightly from 1031
mb to 1029 mb some 1300 miles NE of the islands. It remains at
moderate speed to until Friday night when the trades strengthens
to locally strong for the weekend.
A low pressure system, associated with the remnants of Fernanda,
will continue to move north away from the state tonight. Moderate
trade winds will return to the islands by morning bringing drier
and more stable air to the region. Expect mostly VFR conditions
with a few areas of low clouds lingering in the overnight hours.
There are no AIRMET's in effect. However, stronger winds may
produce low level turbulence over and downwind of mountain ranges
by Tuesday afternoon.
The trade winds are gradually spreading across the area from east
to west this evening as the remnants of Fernanda, located just
west of Kauai, moves further away from the islands. The trade
winds have already reached Maui with Kahului airport coming in at
15 mph and gusts to 23. The models are suggesting the trades to be
locally strong once it comes in tonight. Thus, the Small Craft
Advisory (SCA), slated to go into effect at 6 am HST Tuesday, will
go into effect shortly at 9 pm. The SCA will continue through
Wednesday night for the typically windy zones around Maui County
and the Big Island. The high to the northeast is expected to
weaken Thursday through the weekend, resulting in winds easing
during this period.
Moisture associated with the surface trough near Kauai may
interact with a broad upper air feature west of the area. This
interaction could result in a slight chance for thunderstorms over
coastal waters northwest of Kauai.
One final large high tide of the month will occur tonight. This
tide may result in coastal flooding in some areas. A Special
Weather Statement (see SPSHFO product on our website) is posted to
highlight the potential hazards.
No significant swells are expected through the week. Surf will
likely remain below high surf criteria on all beaches.
Small Craft Advisory to 6 AM HST Thursday for Maalaea Bay, the
Pailolo and Alenuihaha Channels, and waters south of the Big
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office