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Hawaiian Islands Synoptic Discussion and Guidance

329
FXHW60 PHFO 190625
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
825 PM HST Fri Oct 18 2019

.SYNOPSIS...
Trade winds will continue through the weekend and much of next
week, delivering clouds and showers to windward areas, with a
stray shower reaching leeward locales from time to time. High
clouds of varying thicknesses will occasionally move over the
islands through early next week.

&&

.DISCUSSION...
Currently at the surface, a 1028 mb high is centered around 1000
miles northeast of Honolulu, while a weak trough of low pressure
is located around 450 miles south-southeast of the Big Island. The
resulting gradient is producing moderate trade winds across the
island chain this evening. Infrared satellite imagery shows low
clouds moving into windward areas with the trades, with high
clouds associated with a jet stream aloft spreading from southwest
to northeast across the state. Radar imagery shows scattered
showers moving into windward areas, with a stray shower drifting
leeward from time to time. Main short term concern revolves around
rainfall chances and trade wind trends over the next several days.

Trade winds will prevail for the next 7 days, with only some minor
fluctuations in speed expected. Moderate to locally breezy trades
will hold in place though Sunday, then ease slightly late Sunday
through Monday as a weakening cold front approaches from the
northwest. The trades are expected to edge up late Monday through
Wednesday as high pressure northeast of the state strengthens,
before trending back down late next week as a new cold front
approaches from the northwest.

Fairly typical trade wind weather is expected to continue during
the next 7 days, with showers favoring windward and mauka areas
along with the occasional leeward spillover. Showers are expected
to be most common between 6 PM and 10 AM each day. There could be
a bit of a downward trend in shower coverage through the weekend
and possibly into Monday, as the airmass remains fairly dry and
stable. Slightly higher precipitable water values move in late
Monday and remain in place through the end of the work week. This
should increase shower coverage to more typical levels for this
time of year.

Periods of high cloud cover associated with a jet stream aloft
will continue to move overhead through the weekend and into early
next week, resulting in partly to mostly cloudy conditions. The
high clouds should thin out Tuesday through late next week.

&&

.AVIATION...
Breezy trades associated with high pressure northeast of the
state will continue through Saturday. Clouds and showers will
favor windward and mountain locations, with the best coverage
likely occurring through the overnight and early morning hours.

No AIRMETs are in effect or anticipated at this time. Brief MVFR
ceilings and visibilities are possible in and around passing
showers. Otherwise, VFR conditions will prevail for all areas.

&&

.MARINE...
High pressure northeast of the state will maintain the fresh to
strong easterly trade winds through Saturday afternoon, so the
Small Craft Advisory (SCA) continues for the typical windier
waters near Maui County and the Big Island.

As a front moves closer to the state from the northwest late
Saturday night, winds are expected to shift to the east-southeast.
The front is expected to stay north of the islands, and will
likely weaken the winds over the western half of the coastal
waters. Winds are expected to shift back to a more easterly
direction Monday.

Surf is expected to remain below advisory levels through the
forecast period. The current combo north and northwest swell will
continue to gradually decline through the weekend. A new small
long-period north-northwest swell expected Saturday will help
maintain small surf along exposed shores.

A gale low is expected to develop near the Kurils Saturday night
into Sunday, and then track to the east toward the Date Line
Sunday into Monday. This would generate a northwest swell
that could reach the islands late next week.

Last week, a storm force low far southeast of New Zealand tracked
east with a captured fetch aimed east of the state. Due to angular
spreading, we should see an increase in surf tonight through the
weekend along south facing shores. Near-shore buoys are showing
some long-period forerunners from the south this evening. Surf
should increase to around the summer average on Saturday and hold
through the weekend before dropping off early next week.

The fresh to locally strong trade winds will maintain rough surf
along east facing shores through Saturday. As the winds weaken
Sunday, surf heights should subside and conditions should improve.

&&

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

&&

$$

DISCUSSION...Jelsema
AVIATION...Gibbs
MARINE...Kino

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