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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 190151

Area Forecast Discussion
National Weather Service Honolulu HI
351 PM HST Thu Aug 18 2022

Trade winds will gradually strengthen tonight and Friday, and
increased stability will prevail Friday through the weekend,
focusing modest rainfall over windward areas as well as across
the Kona slopes of the Big Island each afternoon. Trade winds may
ease early next week as another disturbance potentially brings an
increase in shower activity.


A building surface ridge located about 400 miles north of Kauai is
driving a gradual increase in trade winds this afternoon. Daytime
sea breezes were able to overpower the building trades over many
leeward areas, and an upper level trough just west of the state
has provided enough instability to trigger a few thunderstorms on
the middle slopes of Mauna Loa and Mauna Kea on the Big Island.
That said, afternoon shower activity over the rest of the state
was subdued compared to the past couple of days. Leeward
convection will abate by sundown, and the focus for rainfall will
shift back to windward slopes. A small pocket of low level
moisture will move across Maui County and Oahu this evening then
pass over Kauai late tonight, providing typical windward showers.

Trade winds will be at moderate strength Friday through the
weekend as the surface ridge holds to the north, and a ridge aloft
building in from the east will bring increased stability. No
significant sources of moisture are noted in the trade wind flow,
pointing toward modest rainfall over windward areas and a few
showers over leeward Big Island slopes each afternoon. Other
leeward areas will be mainly dry.

Trade winds will ease early next week as the surface ridge to the
north is eroded. An increase in showers will be possible late
Monday through Wednesday as a mid level trough and some associated
moisture moves in from the east. We could see another period of
increased humidity and daytime sea breezes triggering afternoon
clouds and a few showers over leeward areas. At this time, heavy
rainfall appears unlikely.


Through this evening, weak trade winds will help generate
isolated showers along the windward coasts and slopes. Weak
instability will allow for showers and isolated thunderstorms
over the Big Island interior. However, this activity is expected
to dwindle with the loss of diurnal heating.

By tonight, trade winds will strengthen to moderate and help
generate scattered showers along windward areas. This activity
may bring brief periods of MVFR ceilings and visibility, though
not anticipating anything widespread.


A surface ridge north of the area is forecast to drift further
northward over the next couple of days resulting in trade winds
increasing. A Small Craft Advisory is currently posted for the
typically windier waters around Maui County and the Big Island.
Trade winds may become a bit lighter Monday and Tuesday as a
frontal system far northwest of the area nudges the ridge axis
closer to the state.

The current small south swell will linger through tonight. A new
long period south swell will begin to fill in late Friday and
Friday night. This swell is expected to be rather long lived
peaking around Sunday with surf heights possibly reaching low end
advisory levels. This swell will slowly diminish early next week.
Short period choppy surf will increase into the weekend with the
expected uptick in trade wind speeds over and upwind of the state.
North facing shores will remain just above summertime flat levels
through the weekend. A frontal system far northwest of the state
may send a small northwest pulse during the first half of next


Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.




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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