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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 130619

Area Forecast Discussion
National Weather Service Honolulu HI
819 PM HST Sun Jul 12 2020

A high pressure system north of the islands will keep moderate to
locally breezy trade winds in the forecast through Monday. An
upper level disturbance will line up with a low level trough
passing from east to west across the state. Expect increasing
shower activity tonight through Monday as both system move across
the state. Trade winds will decrease in strength starting on
Tuesday as the high pressure center drifts further north away from
the Hawaiian Islands. These lighter trades will allow light sea
breezes to develop over shelters leeward areas each day lasting
through the end of next week. Stronger trade winds are forecast to
return by Saturday.


This evenings satellite imagery shows an upper level closed low
roughly 600 miles north of the Big Island drifting slowly westward
along the axis of an upper level trough. The subtropical jet
stream continues to spread cirrus clouds from west to east across
the islands. In the lower levels clouds continue to move in an
east to west direction with the moderate to breezy trade winds.
Several bands of unstable open cell cumulus clouds are shown along
a low level 700 mb trough axis moving into the islands from the
east. Low level convergence near this trough will link up with
upper level lift from an upper level trough over the islands,
increasing shower activity from east to west through Monday.
Periods of wet trade wind showers remain in the forecast for the
next 24 hours.

In the big picture, a high pressure system north of the Hawaiian
Islands will keep moderate to locally breezy trade winds in the
forecast through Monday. By Tuesday this high pressure system
drifts northeast away from the islands, decreasing local wind
speeds, and allowing thermally driven land/sea breezes to develop
over leeward areas. This light wind speed pattern will continue
through Friday. High pressure builds back in by Saturday with
another increase in showers just in time for the weekend.

The forecast grids this evening look good, no evening updates are


An elongated ridge of high pressure north of the state will
maintain moderate to occasionally breezy trades across the islands
tonight. Scattered to numerous showers will mainly affect
windward locations and bring periods of MVFR conditions due to
lowered ceilings and visibility.

AIRMET Tango remains in effect below 8000 feet for south thru west
of mountains due to tempo moderate turbulence.


High pressure north-northeast of the islands will keep fresh to
strong trades in place through Monday. As a result, a Small Craft
Advisory (SCA) remains posted for the typically windy waters
around Maui County and the Big Island. A couple troughs of low
pressure will pass by to the north of the state Tuesday through
Friday, easing the trades into the light to moderate range in most
areas, with fresh trades holding in the typically windier channel
waters. Trades should then ramp back up to locally strong levels
next weekend as high pressure strengthens far to the north of the
island chain.

There will be a series of small southerly swells over the next
several days that will keep south shore surf near of just above
background levels. A slightly larger long period south swell may
fill in Friday and Saturday. East facing shores will continue to
experience short period choppy surf produced by the trade winds. A
small but slightly longer period east swell produced from
former tropical cyclone Cristina, may provide a slight uptick in
surf heights during the late Tuesday through Thursday time
frame. No other significant swells are expected.


Small Craft Advisory until 6 PM HST Monday 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