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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 151942

Area Forecast Discussion
National Weather Service Honolulu HI
942 AM HST Tue Oct 15 2019

A front approaching from the north, combined with a developing
upper trough, will support increased rainfall coverage across the
western half of the island chain beginning Wednesday.
Thunderstorms are possible Wednesday and Wednesday night across
the western half of the state. Drier trade wind weather will
return Friday.


Ridging far north of the main Hawaiian Islands continues to drive
moderate trade winds across local waters in spite of the presence
of a frontal boundary along 30N this morning. Satellite and radar
show scattered low clouds, along with isolated showers, moving in
on the trades and affecting mainly windward shores and slopes.
Overnight soundings show a capping inversion between 5600 and 7000
feet, likely due to ridging aloft, acting to limit convection.

Models show a shift in this pattern over the next few days as the
front to our north moves toward the islands and the upper ridge is
replaced by an upper trough developing from the northeast. PW
will likely increase from current values near 1.5 inches to just
over 2.0 inches. This increased moisture, combined with cooler
temperatures aloft, will act to increase shower coverage and
intensity Wednesday into early Thursday, mainly across the
western half of the island chain. A slight chance of thunderstorms
remains in the forecast for Kauai and Oahu, as well as for
adjacent waters, for Wednesday and Wednesday night. Bouts of heavy
rainfall are possible as well and we are considering issuing a
Flash Flood Watch for Kauai and Oahu through this period.

The upper trough will pinch off into a weak upper low which will
drift west of the main islands through the day Thursday. The
frontal boundary will stall near Kauai and weaken during this
time, allowing stronger trade flow and drier conditions to build
in from the east. Typical trade wind weather will then dominate
into early next week.


Moderate east-northeasterly trade wind flow will slowly trend down
across the state through tonight ahead of a front approaching from
the northwest. Passing light showers will continue to focus on
windward and mauka areas, but VFR conditions will prevail with no
AIRMETs expected through the forecast period.


A ridge of high pressure north of the state will maintain moderate
to fresh northeast trades through tonight. A trough approaching
the area from the north on Wednesday will cause the winds to veer
to the east and will bring an increase in showers especially
across the western waters, where thunderstorms remain a
possibility Wednesday and Wednesday night. As the trough exits
the coastal waters Thursday, a ridge of high pressure will
strengthen north of the state and will bring the return of fresh
to locally strong trades Thursday night through Saturday. Winds
may slightly weaken on Sunday as a front passing northwest of the
state weakens the ridge.

Surf is expected to remain below advisory levels along all shores
into the weekend. A series of south and south-southwest swells
will continue to move through the islands. A small long-period
west swell generated by former Tropical Cyclone Hagibis is
expected to peak today then drop off Wednesday. Some of the swell
energy could wrap into some northern exposures.

As Hagibis recurved off of Japan, it tracked northeast towards the
Aleutian Islands. A small compact area of strong gales were seen
on ASCAT near 40N 155E with seas growing to 25 feet on 10/13. This
should bring a small long-period west-northwest starting Wednesday
night. Several smaller pulses should follow Thursday night
through the weekend out of the north-northwest.

Rough and choppy surf along east facing shores will begin trending
down today through Thursday as the trades ease. East shore surf
will then trend up toward the end of the week and over the weekend
as the trades increase.






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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