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Hawaiian Islands Synoptic Discussion and Guidance

243
FXHW60 PHFO 171948
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
947 AM HST Wed Jul 17 2019

.SYNOPSIS...
Typical summertime weather is expected through the weekend, with
locally breezy trade winds delivering clouds and passing showers to
windward areas, while most leeward areas remain dry. On the Big
Island, leeward areas will see clouds and showers every afternoon
and evening.

&&

.DISCUSSION...
A typical summertime weather regime is in place, and is expected to
remain so into next week. Surface high pressure centered to the
distant N will weaken slightly over the weekend, with breezy trade
winds perhaps becoming a little less gusty. A low aloft will linger
in the island vicinity into next week, allowing low clouds arriving
on the trade wind flow to drop a few showers, mainly over windward
areas - although a few will drift over to leeward areas on the
smaller islands. Meanwhile, the Kona side of the Big Island will see
afternoon clouds and showers every afternoon and evening due to
upslope flow driven by daytime heating. With elevated instability
noted in the overnight sounding, a repeat of yesterday's isolated
thunderstorm over the slopes cannot be ruled out.

High-resolution GOES-17 visible imagery shows a loosely organized NW-
SE oriented band of stable and showery low clouds approaching the
islands from the E. This band will fuel a few windward showers in
the short term before diminishing and moving W of the area later
this afternoon, with standard fare trade showers anticipated
overnight as PWAT drops to ~1.2". Models depict PWAT increasing (to
~1.6") from E to W Thursday and Thursday night, with the leading
edge of this area now along 145W. This will likely fuel an increase
in windward showers, with other randomly distributed clusters of
showery low clouds continuing to do so through the foreseeable
future thereafter.

While the nearby low aloft will drift N over the next day or so, it
will move back toward the islands on Friday, with the associated E-W
oriented trough becoming N-S oriented. Models do not depict
significant destabilization of the island atmosphere due to the
proximity of this feature, but its presence will allow incoming
cumuli to grow sufficiently tall to drop showers as the subsidence
inversion is weakened. Additionally, periods of high clouds are
possible from Thursday through Saturday, potentially becoming thick
over parts of the state on Friday.

&&

.AVIATION...
High pressure north of the Aloha State continues to drive moderate
to breezy trades across the area. Cloud and shower activity has
increased from yesterday morning to this morning in response to an
upper level disturbance near the island chain. This may lead to the
development of MVFR conditions, particularly along windward coasts
and slopes. Isolated leeward showers will also be possible this
afternoon, with a slight chance of thunderstorms for leeward Big
Island.

AIRMET Tango remains in effect below 8000 ft for areas south through
west of mountain ranges due to tempo moderate turbulence.

&&

.MARINE...
High pressure north of the state will keep breezy to strong trades
in place through Friday. The trades are expected to ease slightly
over the weekend, as high pressure to the north weakens. A Small
Craft Advisory (SCA) remains in effect for most waters between Oahu
and the Big Island. Breezy trades are expected for the remaining
waters not in the SCA, with winds likely flirting with SCA levels at
times. We will expand the SCA to additional marine zones if
warranted.

The current south swell will continue to slowly subside through
tonight, with only small background south and southwest swells
expected Thursday through the weekend. A modest increase in south
shore surf is possible early next week as a pair of overlapping
southwest swells move through. Locally strong trade winds will
produce choppy waves along east facing shores into the weekend. East
shore surf will get a boost Thursday and Friday as the upstream
fetch of trade winds increases, although surf is expected to remain
below advisory levels. Surf will slowly lower over the weekend on
east shores as trades ease.

&&

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

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

&&

$$

DISCUSSION...Birchard
AVIATION...Dye
MARINE...Kinel

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