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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 190633

Area Forecast Discussion
National Weather Service Honolulu HI
833 PM HST Mon Nov 18 2019

An unsettled weather pattern will persist across the island chain
through Tuesday, with locally heavy rainfall and thunderstorms
possible statewide. The airmass will stabilize by Wednesday, but
a showery wet trade wind pattern is expected to hold through the
weekend. Breezy to windy conditions are expected Thursday through


An upper low just north of Kauai continues to trigger unsettled
weather across local waters as a surface high far north northeast
of Hawaii drives gentle to moderate trade winds across the area
this evening. Satellite and radar show heavy showers and
thunderstorms have diminished over land within the past few hours,
but persist across local waters mainly northeast and southwest of
Oahu and the islands of Maui County. Conditions will remain ripe
for showers and thunderstorms into tonight. Most of the activity
will remain over coastal waters as light wind flow gives way to
land breezes later on. However, since we can't completely rule out
the possibility of heavy rainfall, The Flash Flood Watch remains
in effect for all islands through tonight.

The upper low will begin filling in and shifting eastward on
Tuesday while low level moisture lingers over the state. Thus, we
will still have a slight chance for thunderstorms and heavy rain
in the forecast for Tuesday. By Tuesday night, the upper low will
move northeast of the state, stabilizing our airmass into
Wednesday. Trade winds will strengthen on Wednesday as high
pressure north of the state builds in. Trade winds will become
strong by Friday into the weekend as the high moves northeast of
Hawaii and strengthens. Clouds and showers will favor windward
and mauka areas through the end of the week, with leeward areas of
the smaller islands getting wet from time to time as well.


Thunderstorm activity continues offshore mostly in coastal waters
south of Oahu and north of Molokai. The chance for some heavy rain
and thunderstorms will continue as the upper level low remains
over the island chain into Tuesday. Low ceilings still remain over
portions of the Big island and should continue through the evening
hours after which we may be able to cancel AIRMET Sierra for
mountain obscuration with some clearing. Other areas may
experience mountain obscuration with rain development and we will
issue an AIRMET accordingly. Winds will be relatively light
across Hawaii and most areas, outside of where rain and possible
thunderstorms occur, will experience nighttime land breezes
tonight and afternoon sea breezes Tuesday afternoon. Inland
showers and possibly thunderstorms are possible Tuesday afternoon.

AIRMET Sierra for mountain obscuration for the Big Island is
currently in effect.


Moderate trade winds are being supported by a surface high
passing well N of the islands, but a well-developed closed low
aloft centered near Kauai is marginally disrupting the flow.
Additionally, erratic and locally gusty winds associated with
thunderstorms/heavy showers forming in response to the low's
instability are disrupting the trades in those areas. As this low
aloft remains nearly stationary overnight, a weak low-level
trough is expected to sharpen slightly along the longitude of
Kauai (160W), leading to a reduced ESE flow beginning tonight,
while continuing to bring the potential for thunderstorms. Lastly,
the Small Craft Advisory (SCA) for locally strong trade winds was
cancelled earlier in the afternoon.

Strengthening trade winds are expected by the middle of the week
as the trough dissipates, and a new high pressure cell builds N of
the islands. The low aloft will also weaken, and the potential
for thunderstorms will diminish after Tuesday. Trade winds may
become locally strong, potentially reaching gale force in some of
the waters around Maui and the Big Island by the end of the week.
Combined seas are expected to exceed 10 feet in many zones by
Wednesday night due to rising NW swell, prompting the issuance of
a SCA that will likely extend to all zones as seas and winds
remain elevated into the weekend.

Surf along all shores will remain below High Surf Advisory (HSA)
levels through Tuesday, but a new long-period NW swell arriving
Wednesday will require a HSA for exposed shores of most islands
through Thursday. If peak swell heights are larger than WaveWatch
guidance (which is pretty much the norm recently), then surf could
approach the 25 foot threshold for a High Surf Warning. Another
NW swell arriving this weekend may also require an HSA. In the
meantime, a new NW swell will likely peak tonight and lower
Tuesday, with peak surf heights remaining below advisory levels.
Strong trade winds will likely drive a steady increase in short-
period wind waves from Wednesday into the weekend, eventually
leading to high surf along E facing shores. No other significant
swells are expected. See the updated Oahu Surf Discussion (SRDHFO)
for increased details as to the sources of swells arriving in
island waters.


Flash Flood Watch through late tonight for all Hawaiian islands.




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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