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Hawaiian Islands Synoptic Discussion and Guidance

931
FXHW60 PHFO 230632
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
832 PM HST Tue Oct 22 2019

.SYNOPSIS...
High pressure northeast of the state will keep moderate to
locally breezy trade winds in place through Wednesday. The trades
will ease Thursday through Saturday as a series of fronts
approach from the northwest. The trades are then expected to
strengthen Sunday through early next week. Fairly typical trade
wind weather will prevail, with showers favoring windward and
mauka areas and a stray shower reaching leeward locales from time
to time.

&&

.DISCUSSION...
Currently at the surface, a 1032 mb high is centered around 1750
miles northeast of Honolulu, while a cold front is located around
725 miles northwest of Kauai. Meanwhile, Invest 93C is located
around 1000 miles southeast of Hilo. The local pressure gradient
is producing moderate to locally breezy trade winds across the
island chain this evening. Infrared satellite imagery shows
partly cloudy skies in most areas, although mostly cloudy
conditions are present across much of windward Maui and most of
the Big Island. Radar imagery shows isolated to scattered showers
moving into windward areas, with shower coverage the greatest
across windward Big Island. Mainly rain free conditions prevail in
leeward locales. Main short term concerns over the next several
days revolve around trade wind trends and rainfall chances.

High pressure northeast of the state will keep moderate to
locally breezy trade winds in place through Wednesday. A
weakening cold front will approach from the northwest Wednesday
night and Thursday, then stall out a few hundred miles north of
Kauai through the weekend. A secondary cold front will approach
from the northwest over the weekend and stall out in roughly the
same position, a few hundred miles north of Kauai early next week.
These fronts should ease the trades into the light to moderate
range Thursday through Saturday, with localized land and sea
breezes possible in the more sheltered locations. Moderate trade
winds will try to make a return Sunday through early next week as
the gradient across the island chain tightens up due to high
pressure strengthening north of the state.

As for forecast details, fairly typical trade wind weather should
prevail during the next 7 days, with showers favoring windward
and mauka areas, particularly at night through mid morning each
day. A stray shower will also reach leeward areas from time to
time. There could be a slight increase in leeward shower activity
Thursday through Saturday as the trade winds ease, allowing for
some sea breeze driven shower development. Overall, rainfall
amounts are expected to remain on the light side through early
next week.

&&

.AVIATION...
Moderate to breezy trade winds will continue through Wednesday,
then shift out of the east-southeast and weaken Wednesday night.
Clouds and showers will favor windward and mountain locations,
with the best coverage expected through the overnight and early
morning hours as pockets of moisture move through. Brief periods
with MVFR ceilings and visibilities will be possible in and
around passing showers. Elsewhere, mostly dry VFR conditions will
prevail.

AIRMET Tango remains in effect for areas south and west of the
mountains of all islands for low-level turbulence below 8,000
feet. This will be dropped later tonight or Wednesday as the
trades begin to weaken.

&&

.MARINE...
A ridge of high pressure several hundred miles N of Kauai continues
to support locally strong trade winds, and a Small Craft Advisory
(SCA) remains posted for several marine zones. An approaching front
will force the ridge S and weaken it over the next couple of days,
leading to weakening winds that will veer to the SE. Like last
weekend, this effect will be most pronounced on the W end of the
island chain, but light E to SE winds will prevail over most areas.

The front will stall and dissipate well N of the area Thursday
and Friday, but the ridge will remain suppressed southward near
the islands into Saturday as a second front approaches. High
pressure building NW of this second front will start to tighten
the local pressure gradient by Sunday, leading to a slight
increase in trade wind speeds heading into next week. As this
high passes N of the area early next week, it looks to support
light to moderate trade winds.

Surf along N and W facing shores will be increasing to
climatological levels over the next several days, with increasing
potential for High Surf Advisory (HSA) level surf late this
weekend into early next week. A mid-period NW swell arriving
Thursday will back to the N on Friday while peaking below advisory
levels, then diminish Saturday. A powerful low (with hurricane-
force winds) will develop in the NW Pacific the next couple of
days, generating a long-period NW swell that will arrive early
Sunday and persist through Tuesday. Peak surf heights will likely
warrant a HSA for several days. Short-period wind waves will
gradually diminish along E facing shores as trade winds ease the
next couple of days. A slight increase in long-period S swell is
expected Wednesday, holding into Thursday before fading Friday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Wednesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory until 6 AM HST Wednesday for Oahu Leeward
Waters-Kaiwi Channel-Maui County Windward Waters.

&&

$$

DISCUSSION...Jelsema
AVIATION...Gibbs
MARINE...Birchard

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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