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Hawaiian Islands Synoptic Discussion and Guidance

454
FXHW60 PHFO 191339
AFDHFO

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

.SYNOPSIS...
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, but a
showery wet trade wind pattern is expected to hold through the
weekend. Breezy to windy conditions are expected Thursday through
Saturday.

&&

.DISCUSSION...
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 morning. After an active evening, satellite and radar show
heavy showers and thunderstorms have diminished over land within
the past few hours, but persist across local waters mainly south
of Oahu and within a thin band across and around Kauai. Conditions
will remain ripe for showers and thunderstorms through this
afternoon. Most of the activity will remain over coastal waters
this morning as land breezes persist. Once daytime heating sets
in, the risk for heavy rainfall over land increases. Since
thunderstorms persist across local waters this morning and the
upper low seems to be nearly stationary, we can't rule out the
possibility of heavy rainfall over land today. Therefore, the
Flash Flood Watch has been extended for all Hawaiian Islands
through this afternoon.

The upper low will begin filling in and shifting eastward later
today while low level moisture lingers over the state. Thus, we
will still have a slight chance for thunderstorms and heavy rain
in the forecast through this afternoon. By tonight, 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.

&&

.AVIATION...
Widespread rain showers continue across the state from Kauai to
Maui with a few embedded heavy rain. The chance for some heavy
rain and thunderstorms will continue as the upper level low
remains over the island chain through the day. Low ceilings still
remain over portions of the Oahu although most of the showers have
dissipated. Another round of heavy rain is moving onto the
southern shore of Oahu so will leave the AIRMET for mountain
obscuration in place for now. 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 Oahu is currently in
effect.

&&

.MARINE...
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, a weak low-level trough is
expected to sharpen slightly along the longitude of Kauai (160W),
leading to a reduced ESE flow, while continuing to bring the
potential for thunderstorms through the day.

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 by later today. 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, which will prompt the
issuance of a Small Craft Advisory 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 tonight, but 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 (which is pretty much the norm recently), then surf could
approach the 25-foot threshold for a High Surf Warning for north
facing shores. Another NW swell arriving this weekend may also
require an HSA. In the meantime, the current moderate NW swell
will lower today, 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.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through this afternoon for all Hawaiian
islands.

&&

$$

DISCUSSION...Powell
AVIATION...Chevalier
MARINE...Kinel

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



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