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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 200632

Area Forecast Discussion
National Weather Service Honolulu HI
832 PM HST Tue Nov 19 2019

An unsettled weather pattern will persist across the island chain
through tonight, with locally heavy rainfall and thunderstorms
possible. The airmass will stabilize Wednesday with trade winds
building back in. Trades will strengthen and become windy Thursday
through the weekend.


An upper low remains centered just north of the Big Island this
evening, providing enough instability to trigger areas of heavy
rainfall and thunderstorms. That said, this low actually weakened
somewhat over the past 24 hours and is expected to slowly move
towards the east and off the island chain Wednesday. While shower
activity is less than it was this afternoon, abundant moisture
combined with the instability from the upper low may fuel
additonal heavy rainfall and thunderstorms through tonight. The
Flash Flood Watch remains in effect for all of the main Hawaiian
Islands through 6 am HST Wednesday.

A 1029 mb high is located far northeast of the state, with a
nearly stationary surface trough noted near Kauai. The resulting
light and variable winds dominate the western end of the state,
while light to moderate east southeast winds persist across Maui
County and the Big Island.

On Wednesday, the upper low will move northeast of the state and
will be replaced by more stable zonal flow aloft. Trade winds will
also strengthen as the surface trough weakens and a new surface
high builds in north of the area. Winds will focus incoming
clouds and showers over windward and mauka areas. Trade winds
will become strong by Thursday into the weekend as the surface
high moves northeast of Hawaii and strengthens. Windward and mauka
showers will continue, with leeward areas expected to see passing
showers at times due to the breezy winds.


Showers are still numerous across the state with some embedded
heavy rain showers and thunderstorms, especially on the Big island
this evening. Will issue AIRMET Sierra for mountain obscuration
for portions of the Hamakua coast and the southern shore of the
Big Island. We may also need to add portions of other islands
tonight, depending on where heavy rains pop up. Conditions are
expected to improve as we progress through the night thus we are
anticipating eliminating AIRMETs early Wednesday. A moderate
trade wind pattern is expected to build back in for Wednesday.


The low aloft has been moving slowly east and weakening, while a
weak surface trough near the islands has been nearly stationary
and will gradually dissipate. That means the atmosphere is
becoming more stable, though there is still a possibility of heavy
downpours and/or thunderstorms through tonight.

Trade winds will gradually strengthen tonight and Wednesday, and
then become breezy to strong beginning Wednesday night into the
weekend. This will be in response to strong high pressure far
north and northeast of the area as it becomes the dominant
feature affecting the islands. Also, combined seas are expected
to exceed 10 feet in most coastal zones by Wednesday night or
Thursday due to a building northwest swell. Small Craft
Advisories will be posted as warranted, with a possibility of
Gale Warnings in some channels toward the end of the work week.

Surf along all shores will remain below High Surf Advisory (HSA)
levels through tonight as the current northwest swell lowers.
However, as alluded to above, a new long-period northwest swell
arriving Wednesday afternoon and evening will require an HSA for
exposed shores of most islands through Thursday. If peak swell
heights are larger than WaveWatch guidance, as they have been
recently, surf could approach the 25-foot threshold for High Surf
Warning along north facing shores. Another northwest swell
arriving this weekend may also require an HSA. Strong trade winds
will likely drive a steady increase in short-period wind waves
Wednesday into the weekend, eventually leading to an HSA for east
facing shores. No other significant swells are expected. See the
latest Oahu Surf Discussion (SRDHFO) for details on swell sources.


Flash Flood Watch until 6 AM HST Wednesday for all Hawaiian




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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