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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 180140

Area Forecast Discussion
National Weather Service Honolulu HI
340 PM HST Wed Jul 17 2019

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.


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 Friday, with breezy ENE trade winds
easing somewhat for the weekend. A low aloft will linger in the
island vicinity, 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 clouds and showers every
afternoon and evening due to upslope flow driven by daytime heating.

High-resolution GOES-17 visible imagery shows fewer low clouds
immediately upstream of the islands as compared to this morning,
with a smattering of randomly distributed low clouds dotting skies
between the islands and 150W. With PWAT expected to drop to a near
normal ~1.2" in the short term, a near normal trade shower
distribution is expected overnight into Thursday.

Between 150W and 140W at the latitude of the islands, a broken to
overcast mix of stable and showery low clouds is noted, associated
with an increase in PWAT to near ~1.7". Guidance is consistent in
depicting the leading edge of this increased moisture arriving
Thursday morning on the Big Island, spreading westward through the
day, and arriving on Oahu in the afternoon. This will likely bring
an increase in windward showers, and keep skies a little cloudier
than normal, with the bulk of this moisture moving W of all islands
by Friday morning. Thereafter, models indicate low level moisture
streaming in from the E that will likely keep a seasonal windward
shower regime intact.

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 over the islands. Models do
not depict significant destabilization of the lower portions island
atmosphere, but incoming cumuli will be able to grow sufficiently
tall to drop showers as the subsidence inversion is weakened.
Additionally, periods of high clouds are possible from Thursday
through Saturday due to instability aloft, with the high clouds
potentially becoming thick over parts of the state at times. Outside
chance of high-based convection developing over the Big Island upper
slopes the next couple of days, but since it didn't happen today, it
won't be included in the forecast for now.


High pressure north of the Aloha State continues to drive moderate
to breezy trades across the area. PHOG was gusting to 40kts earlier
this afternoon, but winds should be continuing to trend down into
the evening and overnight hours. Clouds and showers that have
favored mainly windward coasts and slopes today will also be on the
decline overnight as an upper level disturbance moves away from the
island chain. VFR conditions are expected to prevail for most
locations, with brief periods of MVFR conditions possible for
windward areas this evening.

AIRMET Tango remains in effect below 7000 ft for areas south through
west of mountain ranges due to tempo moderate turbulence.


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. The Small
Craft Advisory (SCA) remains in effect and has been expanded to
include all coastal zones. Winds are expected to trend down over the
weekend, when the SCA may be scaled back to just the typically windy
areas around Maui County and the Big Island.

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, with
a boost through Friday as the upstream fetch of trade winds has been
increasing (although surf is expected to remain just below advisory
levels). Surf will slowly lower over the weekend on east shores as
trades ease.


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 Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu
Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County
Windward Waters-Maui County Leeward Waters-Big Island Windward




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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