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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 121933

Area Forecast Discussion
National Weather Service Honolulu HI
933 AM HST Wed Aug 12 2020

A nearby upper low to the northwest of the state will bring a
slight chance of thunderstorms for areas around Kauai through
tonight. Otherwise, trade wind weather is expected into early next
week with a slight weakening of the wind speeds Thursday and
Friday. Clouds and passing showers will tend to favor windward and
mountain areas, but localized sea breezes in protected areas may
bring a few afternoon clouds as the background winds weaken.


No changes to the forecast this morning, with the forecast
seemingly on track.

Satellite and lightning networks continue to show thunderstorm
activity to the northwest of Kauai over the last several hours.
Most of the activity has remained about 130 miles west-northwest
of Kauai in the offshore waters. However there were a couple
thunderstorms in the far western coastal waters, west of Niihau
just before 8 am.

The thunderstorms are the result of an upper level low to the west
of the islands. Precipitable water (PW) values in the models are
around 1.5 inches, with 500 mb temperatures around -9/-10 celsius.
While the PW values are near normal for August, the 500 mb
temperatures are colder than normal, an indicator of the unstable
airmass making its way over the waters to the northwest of Kauai.
This low is expected to linger, and possibly edge closer to the
state over the next 36 hours or so, before moving to the west. We
have the chance for thunderstorms near Kauai today and tonight,
and will be monitoring the situation with the latest model runs to
determine if that threat will continue beyond that.

Otherwise, high pressure far north-northeast of the islands will
remain in place through much of the week. This will maintain a
moderate trade wind pattern, with clouds and showers focused over
the windward and mountains of the islands. Some areas could see
lighter winds at times, which could lead to some localized sea
breezes during the afternoon hours. If that happens, those areas
could see a few more afternoon clouds and showers.


A weak upper level low northwest of Kauai, with associated cold
and unstable air aloft, will allow for increased shower and
isolated thunderstorm activity over Kauai and surrounding waters.
Elsewhere, persistent high pressure north of the state will bring
moderate trades to the islands. Thus, expect isolated to scattered
showers to mainly affect the windward coasts and slopes through
this afternoon. Brief MVFR conditions may occur with this
activity, but not expecting anything widespread.

No AIRMETs are currently in effect.


A large and fairly strong surface high far north of the area will
move east slowly and weaken through Friday. A new high will build
northwest of the area over the weekend then move southeast through
the middle of next week. These highs will maintain moderate to
fresh northeast to east trade winds over Hawaiian waters through
at least early next week. Winds over the coastal waters are expected
to remain below Small Craft Advisory (SCA) speeds. A low aloft
will bring a slight chance of thunderstorms to waters around Kauai
through tonight, and some of the trade wind showers there may be
heavier than normal.

Surf will remain below advisory levels along all shores into next
week. Surf will remain quit small along north facing shores, but a
relatively short-period NE swell expected Thursday and Friday may
reach some locations. Small surf along south facing shores will
build starting over the weekend. A small long- period E swell from
Hurricane Elida is possible over the weekend.






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