Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 251401

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST WED MAY 25 2016

Light trade winds will continue over the state as the pressure
gradient weakens across the area. Showers, some locally heavy, will
remain focused over the windward slopes today. Widespread afternoon
sea breezes will trigger isolated heavy downpours, mainly over
interior and mauka sections, as the atmosphere becomes increasingly
unstable. A more stable but humid, hybrid trade wind and weak
afternoon convective pattern will prevail on Thursday, followed by
light and variable winds with land and sea breeze weather regime
heading into the weekend.


Trade winds have become rather light overnight across the state, as
a surface trough deepens far north of the state. An area of moisture
has reached the eastern islands early this morning, bringing an
increase in shower coverage. Meanwhile, a rather strong upper level
trough moving near Kauai from the northwest early this morning, as
revealed by the water vapor imagery, will continue to advance east
through the day. This trough is bringing cooler air aloft, resulting
in the air mass becoming more unstable as the day progresses. Early
morning soundings from Lihue and Hilo both revealed deeper low
level moisture, reaching over eleven thousand feet while air mass is
unstable. Therefore, showers will be enhanced, increasing the chance
of locally heavy rainfall. Since trade winds are still prevailing,
much of the clouds and showers will affect windward areas this
morning. By the afternoon, developing sea breezes will trigger
convection over the islands, especially over the interior and lee
areas. Given there will be deep moisture available, the threat of
heavy showers remains. Moreover, air mass may become unstable
enough for a slight chance of thunderstorms over the islands to
develop during the afternoon and evening.

As the upper level feature exits east on Thursday, airmass in the
island vicinity will become more stable. Hence, a hybrid trade wind
and convective weather pattern is expected tomorrow. The weak trade
winds will bring some showers to the windward slopes while afternoon
sea breezes will generate clouds and a few afternoon showers over
the leeward and interior sections. There may be some clearing over
the islands overnight as land breezes develop.

Light wind regime over the state will persist into the holiday
weekend, as the aforementioned surface trough stays north of the
islands. Land and sea breezes weather regime will continue over the
islands, featuring afternoon clouds and spotty interior showers then
partial clearing at night. It may feel rather humid as well.
Volcanic emissions from the Big Island may spread to the smaller
islands over the weekend under the light southeasterly winds.


Increased low-level moisture moving over the islands from the E
will combine with a passing trough aloft to create an increasingly
unstable island atmosphere that will be conducive for the
development of locally heavy showers today. While trade winds are
expected to gradually weaken, they remain sufficiently strong
this morning for the best chance of showers to remain over
windward coasts, slopes and adjacent waters. One exception is the
leeward near shore waters of the Big Island, where pooled moisture
is triggering isol +SHRA. Lighter winds will allow clouds and
showers to develop over interior and leeward areas of the islands
from late this morning into this evening.

Isolated areas of IFR VIS/CIG in +SHRA/+TSRA are possible today,
with the potential for fairly widespread MVFR CIG/VIS to develop.
An AIRMET for mountain obscuration is currently posted for
windward portions of Maui and the Big Island, and may need to be
expanded to cover additional areas/islands later today, depending
on the cloud and shower coverage that develops. The passage of the
trough aloft also brings the potential for moderate turbulence
aloft today. As the trough aloft exits to the E tonight, instability
will slowly diminish.


No small craft wind and seas conditions are expected through the
rest of the week as winds will stay below criteria level while only
moderate swells are expected. Light to moderate east to southeast
winds may persist over the windward coastal waters in the next few

A moderate southwest swell may arrive late Wednesday, peak late
Thursday and Friday, then decline during the weekend. This swell
will originate from the Tasman Sea, and given the large amount of
south Pacific islands along the path of the swells from this
direction, surf locally tends to be inconsistent. See the the
Collaborative Nearshore Swell and Wind Forecast (SRDHFO) for more
details. Otherwise rather small surf is expected.





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