Current Conditions
Temp-0.5 C
RH74 %
WindNE 0 mph
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 190214

Area Forecast Discussion
National Weather Service Honolulu HI
414 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


Water vapor imagery shows a stationary upper low centered about 80
miles northeast of Lihue this afternoon. 00Z soundings showed 500
mb temperature showed -13 C at Lihue and -10 C at Hilo with
precipitable water (PW) near normal at 1.3 to 1.4 inches. Meanwhile,
at the surface, a 1029 mb high centered around 1500 miles north
northeast of Hawaii is driving gentle to moderate trade winds
across the island chain while a broad surface trough is located
near Kauai and is somewhat disrupting the trade wind flow. Visible
satellite and radar heavy showers and thunderstorms firing over
Big Island, Maui County and to a lesser extent, Kauai.

Conditions will remain ripe for showers and thunderstorms into
tonight. Thinking that most of the activity will remain over
coastal waters as the light wind flow will give way to land
breezes later on, however, can't rule out some heavy rain/thunderstorms
making their way onshore resulting in flash flooding. The Flash
Flood Watch has been extended for all islands through tonight.

The upper low will begin filling in and shifting eastward on
Tuesday while the low level moisture hangs around 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 with more stability
then building into Wednesday. Trade winds will also strengthen on
Wednesday as the earlier mentioned surface trough moves west of
the state and a new high pressure system north of the state builds
in. The trade winds will become strong by Friday into the weekend
as the high moves northeast of Hawaii and strengthens. The trades will
keep windward and mauka showers in the forecast through the end
of the week with leeward sides of the smaller islands getting
passing trade wind showers at times due to the breezy winds.


The potential for heavy showers and isolated thunderstorms will
remain in the forecast through Tuesday due to an upper low lingering
over the area. Although clouds and showers will be most active over
north and northeast facing slopes and coastal waters with gentle to
breezy trades in place, some will manage to spill over into
leeward areas periodically, or develop in afternoon sea breezes
over sheltered leeward areas. Trades will gradually trend down
tonight and Tuesday to light and variable for Oahu and Kauai,
which may help to keep some of the SHRA/TSRA offshore. MVFR
conditions are likely in and around SHRA/TSRA. AIRMET Sierra for
mountain obscuration is now in effect over north and east sections
of Maui, but may also be needed elsewhere as conditions evolve
through the night.


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 aloft is disrupting the flow somewhat.
Additionally, erratic and locally gusty winds associated with
thunderstorms/heavy showers forming in response to the low's
instability are disrupting the trades. 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. The Small Craft Advisory (SCA)
for locally strong trade winds has been cancelled.

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 next weekend may also require a 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


Flash Flood Watch through late tonight for all Hawaii islands.




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