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Hawaiian Islands Synoptic Discussion and Guidance

733
FXHW60 PHFO 241957
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
957 AM HST Mon Jun 24 2019

.SYNOPSIS...
A low, at the surface and aloft, will persist northwest of the
islands through Thursday. The low will dissipate over the second
half of the week as a high builds northeast of the area.

&&

.DISCUSSION...
The heavy showers and thunderstorms that affected the main
Hawaiian Islands yesterday have dissipated. Satellite loops show
mostly cloudy skies over Oahu and Maui County and partly sunny
skies over the Big Island and Kauai. The weather is mostly dry
over the Big Island, Maui County and Kauai with scattered light
to moderate showers moving over Oahu and Niihau.

The weather pattern in the north central Pacific remains very
unusual for late June. A 1010 mb surface low is nearly stationary
915 miles northwest of Kauai near 31N171W and a front associated
with the low runs from 30N161W to 200 miles northwest of Kauai to
21N164W. A 1026 mb surface high is far northwest of the islands
near 37N139W with a ridge extending from the high over Maui
County. South of the ridge, moderate to locally fresh easterly
trade winds are blowing near the Big Island. The surface winds
veer out of the southeast around the smaller islands. Southeast
flow over the islands will be partly blocked by the Big Island, so
winds will be gentle and variable over the smaller islands with
daytime sea breezes and nighttime land breezes.

Moisture and instability associated with the low and front west
of the islands is expected to spread east starting tonight. The
threat of heavy rain and thunderstorms will be greatest over Kauai
County tonight, but the computer models show this threat spreading
over Oahu on Tuesday and possibly reaching Maui County. A Flash
Flood Watch remains in effect for Kauai County and Oahu. We may
need to expand the watch to include Maui County. The atmosphere
over the Big Island will be drier and more stable, but afternoon
heating could produce spotty heavy showers or even isolated
thunderstorms over the Big Island slopes today.

Moisture and instability over the islands will subside starting
Wednesday night as the low weakens and high pressure builds
northeast of the state. Trade winds will spread back over the
islands starting Thursday evening bringing drier air and ending
the threat of heavy showers.

&&

.AVIATION...
There is enough moisture and instability over Kauai that there
should be rain, which could be heavy at times with the possibility
of thunderstorms. Western portions of Oahu may be clipped by a
few showers as well. Where we have the showers we are expecting
lower level ceilings and possible mountain obscurations, mainly
for Kauai and Oahu today. The rest of the state should see passing
MVFR conditions with afternoon sea breezes and overnight land
breezes with showers along the windward slopes possibly spilling
over to a few interior sections this afternoon.

AIRMET Sierra is currently in effect for Oahu for mountain
obscurations above 2,000ft. Will monitor Kauai and issue an AIRMET
for mountain obscuration if warranted.

&&

.MARINE...
Heavy showers and thunderstorms are increasingly likely from
tonight Tuesday through Thursday, as a potent disturbance aloft
approaches from the NW and supports the development of a slow-
moving convergence band over portions of the area. Threats to
mariners include locally gusty wind and reduced visibility in
areas of heavy showers/thunderstorms, as well as dangerous cloud-
to-surface lightning strikes, and the potential for waterspouts.

The threat of unsettled weather will initially be greatest over
waters around Kauai and Oahu from tonight into Tuesday, potentially
spreading E to include Maui county waters later Tuesday into
Wednesday. Unsettled weather is expected to continue over (at
least) waters around Kauai and Oahu through Thursday, before the
convergence band gradually moves away to the W Friday into
Saturday.

Outside areas of convection, expect moderate to locally fresh E
to SE winds over Big Island waters, while the remainder of the
waters can expect light to moderate SE to S winds that may
occasionally shift SW. Seasonably strong trade winds will build
over the area from the E Friday and Saturday. Until that time,
winds and seas are expected to be below Small Craft Advisory
criteria.

Surf along all shores is expected to be below High Surf Advisory
levels through the week, but a S swell arriving next weekend is
expected to bring advisory-level surf to S facing shores. A small
short-period NE swell will linger for another day or so before
diminishing, while building trade winds next weekend will bring
increasingly choppy surf to E facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 6 PM HST this evening through Wednesday
afternoon for Niihau, Kauai and Oahu.

&&

$$
DISCUSSION...Donaldson
AVIATION...Chevalier
MARINE...Birchard

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office