Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 211948

Area Forecast Discussion
National Weather Service Honolulu HI
948 AM HST Mon Aug 21 2017

Trade winds will prevail through the week as a ridge remains to
our north. Tropical moisture overrunning the islands from the east
will maintain muggy conditions with enhanced showers through
Tuesday. Instability aloft will introduce a slight chance of
thunderstorms. Drier and windier conditions will return Wednesday
as the ridge strengthens to our north.


Morning satellite, lightning, and radar data show moderate trade
showers moving into windward areas, along with and area of thunderstorms
about 100 miles northwest through north of Kauai. The thunderstorms
are forming under an upper low that is moving slowly southward,
bringing cooler temperature aloft and instability to the
atmosphere. Showers are expected to increase statewide this
afternoon with land heating lending some extra oomph at the
surface to combine with the instability of the upper low. The
combination of the nearby upper low, afternoon land heating, and
enhanced low level moisture will bring the chance of thunderstorms
and locally heavy showers both this afternoon and possibly again
Tuesday afternoon. Overnight, showers will focus mainly over
windward slopes and mountain areas with passing showers over
leeward locations. The elevated low level moisture has brought
uncomfortable conditions with dew points in the low to mid 70`s.
Muggy conditions will continue through Tuesday.

The upper level low will push southwest near Kauai tonight and
Tuesday maintaining instability over the western islands. By
Wednesday, the low is expected to move farther west of Kauai with
ridging building over the state from the east. A slight chance of
thunderstorms is possible mainly near Kauai and Oahu through
Tuesday. The possibility of afternoon upslope thunderstorms over
the Big Island will remain through Tuesday due to the elevated
terrain, enhanced low level moisture, and slight instability.

Outside of the thunderstorm threat, the general weather pattern
will consist of windward clouds and showers merging with
afternoon and evening leeward clouds and showers. By Wednesday,
the upper low will be far west of the islands, and the ridge to
the north is expected to rebound and drive stronger trade winds
across local waters. More stable conditions will prevail as
ridging aloft builds over the state.


A ridge of high pressure north of the state will keep moderate
trade winds in place through tonight. Clouds and showers will
favor windward and mauka areas, but daytime heating will result in
showers across leeward areas as well. An unstable airmass over
the islands may also result in thunderstorms across portions of
the island chain this afternoon into early this evening. MVFR and
lower conditions will become more common as shower coverage and
intensity increases across the state this afternoon into this
evening. Otherwise, MVFR cigs/vsbys should remain primarily
confined to windward and mauka locales.

AIRMET Sierra remains in effect for mountain obscuration on the
island of Oahu. Conditions should improve here later this
morning. This afternoon, mountain obscuration will likely become
an issue once again across portions of the State, and will issue
AIRMETs as conditions develop.


A cold front far to the north of the Hawaiian Islands has weakened
the subtropical ridge of high pressure resulting in moderate trade
winds across the region. As the front moves eastward, high
pressure north of the state will strengthen and increase the trade
winds. The trades are expected to reach Small Craft Advisory
criteria on Tuesday for the normally windier channels between the
islands and the coastal waters south of the Big Island.

The above-mentioned upper level low will continue to produce a
slight chance for thunderstorms over coastal and offshore waters
through Tuesday. The highest risk is for the waters in the west
half of the forecast area. More stable and drier conditions are
expected on Wednesday and through the second half of the week.

Surf is expected to remain small through the week. East-facing
shores should see a slight increase in surf heights as the trade
winds rebuild on Tuesday. A longer period east swell may also
arrive during the second half of the week as a result of tropical
cyclone activity in the eastern Pacific. South-facing shores may
see a slight increase in surf this weekend from a swell train that
originated from a Tasman Sea gale. A small background southeast
swell is also expected through the forecast period.





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