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Temp8.5 C
RH29 %
WindNNE 16 mph
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Hawaiian Islands Synoptic Discussion and Guidance

327
FXHW60 PHFO 180218
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
418 PM HST Sat Aug 17 2019

.SYNOPSIS...
The high pressure ridge north of the Hawaiian Islands will weaken
today with lighter trade winds forecast on Sunday and more
variable winds expected on Monday. Trade wind speeds will trend
higher by the middle of next week as the high pressure ridge
builds north of the state. Additional moisture will arrive mainly
affecting the windward slopes of the southeastern islands with
elevated humidity levels and enhanced shower activity as two low
level troughs pass south of the state. Moderate to locally breezy
trade winds will continue into next weekend.

&&

.DISCUSSION...
Only subtle changes were introduced to the forecast since the
mornings grids were issued. The trade wind speeds were increased
slightly for tomorrow due to a slightly stronger ridge.
Precipitation chances for next week were decreased, mainly over
the islands just west of Maui as the tropical disturbance appears
to track further south than previous guidance.

A mid-latitude trough passing through the Central Pacific will
weaken the high pressure ridge north of the islands decreasing
trade wind speeds each day from Sunday to Monday. Onshore sea
breezes caused by day time surface heating will develop over each
island with offshore land breezes each night, especially along the
more sheltered west facing slopes. Drier trends are forecast for
the next few days under the subsidence from a strong upper level
ridge. The trade wind subsidence inversion heights will range from
5000 to 7000 feet through Monday afternoon.

By Monday evening the weather pattern begins to change as
additional low level troughs move into the Hawaii region. The
first of these trough arrives from Monday night through Tuesday
morning with some enhanced showers developing mainly along
windward slopes of each island. A second tropical disturbance is
now forecast to pass south of the Big Island on Tuesday night with
potentially less impacts on rainfall and humidity levels across
the islands by Wednesday. Trade wind speeds will increase starting
on Tuesday, as the ridge builds north of the islands and this
trough passes to the south of the state. Moderate to locally
breezy trades are forecast from Wednesday onward. Higher chances
for elevated moisture levels and showers will linger over the Big
Island from Tuesday to Thursday with lower precipitation impacts
for the rest of the state. We will probably trend precipitation
chances downward, in the afternoon forecast grids, across the
western islands for the Wednesday through Thursday time periods.

Moderate to locally breezy trades are forecast to continue on Friday
and next Saturday as the high pressure ridge builds in north of
the islands. An upper level cut off low drops in from the north
likely enhancing shower regional shower activity especially in the
overnight and early morning hours.

&&

.AVIATION...
A 1028 mb surface high far to the northeast of the state will
maintain light to moderate easterly trade winds through this
evening. This flow will help generate isolated showers along the
windward slopes and coastal areas, which are then forecast to
become scattered after sun down. Brief MVFR conditions will remain
possible tonight due to reduced visibility and mountain
obscuration from low clouds and showers.

No AIRMETs are currently in effect.

&&

.MARINE...
A surface high pressure system far northeast of the state will
gradually weaken tonight, which will cause the trade winds to
diminish slightly through early next week. A new surface high
pressure system building far north of the state by the middle of
next week is expected to cause the trade winds to gradually
strengthen starting next Wednesday. We may see the return of Small
Craft Advisory winds for the typical windy areas around Maui and
the Big Island during the second half of next week.

Currently, we have mainly background energy from the south and
southeast along south facing shores. A small bump is expected
Sunday as a small southwest swell fills in. A large, long-period
south-southwest swell arriving Monday is expected to peak
Tuesday, then gradually lower through the rest of next week.
Latest observations from the Pago Pago buoy is indicating the
swell running higher than the WaveWatch 3 guidance with energy
similar to the July 14th high surf event. This suggests surf
heights may near High Surf Warning levels during its peak along
south facing shores on Tuesday.

The current west swell produced by tropical cyclones in the
Northwest Pacific will continue to slowly decline through early
next week. Surf heights produced by this lingering swell will be
largest along shorelines with a westerly exposure. A small
northwest swell will be possible from next Thursday through
Friday. Modest surf will persist along east facing shores today,
then will slowly subside from Sunday through early next week as
the trade winds weaken.

For additional details on swells and surf, please see the latest
Collaborative Nearshore Swell and Wind Forecast for Oahu (SRDHFO)
product.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Bohlin
AVIATION...JT
MARINE...Kino

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