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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 191951

Area Forecast Discussion
National Weather Service Honolulu HI
951 AM HST Tue Nov 19 2019

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


Water vapor loop shows the upper low that brought the unstable
weather across the islands the past couple of days remains near
Kauai this morning. The low has weakened somewhat and broadened in
coverage areas over the past 12 hours and is expected to slowly
move towards the east and off the island chain by tonight. For
now, the chance for heavy rain and thunderstorms will continue and
a Flash Flood Watch remains in effect through 6 pm HST this
evening. Visible satellite and radar shows most of the showers and
thunderstorms focusing over the coastal waters of Kauai and Oahu
late this morning. The islands have seen an overall decrease in
rain over the past couple of hours with some portions of the state
seeing a bit of sunshine.

At the surface, a 1028 mb high is located far northeast of the
Hawaii with a nearly stationary broad trough across Kauai. These
features are producing light and variable winds across the western
end of the state and gentle to locally moderate winds from the
east southeast across Maui County and the Big Island. The light
wind flow and will allow sea breezes to set up over most islands
this afternoon with warm and muggy conditions expected. Clouds will
likely fill in over the island interiors around midday. The
abundant moisture around, combined with the lift and convergence
provided by the islands and the instability from the upper low
will bring the chance that heavy showers and thunderstorms may
occur again this afternoon into early evening.

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


Another round of heavy showers and thunderstorms are possible
across the island chain today. As of mid morning, the bulk of the
TSRA activity appears to be focused over the western half of the
state as well as south of the Big Island. AIRMET Sierra for
mountain obscuration may be needed later today. Winds are very
light across the area, which will lead to sea breeze development
by this afternoon and an uptick in interior showers. TEMPO MVFR
conditions are likely with the heavier showers and storms.
Overnight, we'll see conditions improve as the upper low finally
moves away from the state and trades begin to make a comeback for


Moderate ESE winds are supported by a surface high passing well N
of the islands, but a well-developed closed low aloft and weak
surface trough near Kauai are marginally disrupting the flow.
Additionally, thunderstorms/heavy showers forming in response to
the low's instability are causing erratic and locally gusty winds.
Thunderstorm chances will continue through today, as this low
remains nearly stationary, but diminish as the low weakens and
shifts E of the state tonight.

Winds are expected to strengthen and shift to a more ENE direction
by the middle of the week as the trough near Kauai dissipates and
high pressure builds N of the islands. Trade winds will 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 the 10-foot Small craft Advisory (SCA) threshold
for most zones by Wednesday night due to rising NW swell. Increasing
winds will likely extend the SCA to all zones through much of the
weekend as seas and winds remain elevated.

Surf along all shores will remain below High Surf Advisory (HSA)
levels through tonight as the current NW swell lowers. However, a
new, long- period NW swell arriving Wednesday 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 NW 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, likely leading to an HSA for E 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 PM HST this evening for all Hawaii




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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