Flash Flooding and Lightning Strike in Hawaii, August 20 to 21, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
1018 AM HST Sun Aug 20 2017

.SYNOPSIS...
Light to moderate trade winds will prevail through early next
week as a weak ridge remains to our north. Tropical moisture from
the east will increase humidity and showers today, and keep muggy
conditions with enhanced showers around through Tuesday. Instability
aloft will introduce a slight chance of thunderstorms through
Tuesday. Drier and windier conditions will return Wednesday as the
ridge strengthens to our north.

&&

.DISCUSSION...
Moderate trade winds this morning are pushing clouds and showers
into windward and mountain areas with a few showers reaching
select leeward locations. Mid morning radar imagery shows a patch
of showers heading into Puna District of the Big Island, so expect
clouds and wet weather to increase over that area today. Otherwise,
windward areas will continue to see mostly cloudy skies with
passing showers through the afternoon. Showers are expected to
increase tonight as enhanced low level moisture moves in the from
the east. This moisture will also increase our dew points into the
low to mid 70's by tonight, with humid conditions remaining
through Tuesday.

Along with the low level moisture, an upper level trough will push
south over the state tonight and Monday bringing slight instability
over the islands. The main axis of the trough looks to remain
near Kauai through Tuesday before pushing west of the state. A
slight chance of thunderstorms is possible mainly near Kauai and
Oahu tonight through Tuesday. There is also the possibility of
upslope thunderstorms over the Big Island due to the elevated
terrain, enhanced low level moisture, and slight instability.

Outside of the thunderstorm threat, the general weather pattern
today through Tuesday will consist of windward clouds and showers
merging with afternoon and evening leeward clouds and showers,
with a hybrid light trade winds with leeward sea breeze setup. By
Wednesday, the upper trough will be far west of the islands, and
the ridge to the north is expected to rebound and drive stronger
trade winds across local waters. More stable conditions will
prevail as ridging aloft builds over the state.

&&

.AVIATION...
A moist and unstable airmass will remain in place over the state
through tonight. Clouds and showers will focus primarily over
windward and mauka areas, but sea breezes will also result in
leeward shower development this afternoon into this evening. An
isolated thunderstorm may also affect interior and leeward
sections of the Big Island this afternoon into early this
evening. MVFR cigs/vsbys will be possible in windward areas
through the next 24 hours, with these conditions being most common
during the overnight and early morning hours. PHLI, PHTO and PHNY
are the most likely places to see reduced cigs/vsbys. Elsewhere,
predominantly VFR conditions are expected. Light to moderate
trades will prevail, thanks to a ridge of high pressure to the
north of the islands.

No AIRMETs are currently in effect. An AIRMET for mountain
obscuration may be needed later today or tonight for portions of
the island chain as showers increase in coverage and intensity.

&&

.MARINE...
Winds and seas are expected to remain in the light to moderate
range through Tuesday, then strengthen to moderate to locally
strong Wednesday through the remainder of the week. Increasing
shower coverage will become a possibility through the first half
of the work week as an upper trough drifts southwestward to a
location just west of Kauai tonight and Monday. This upper trough
will also support a slight chance of thunderstorms over the
offshore waters mainly north through west through south of Kauai
this afternoon through Tuesday.

Surf along south facing shores will continue to lower today as a
south-southeast swell continues to fade. Looking ahead, mainly
small background long- period reinforcing pulses out of the south-
southwest (210-220 deg) and south- southeast (130-160 deg) will
keep the surf from going flat along south facing shores this week.
A slightly larger south- southwest (220 deg) swell will be
possible next weekend due to a gale over the Tasman Sea. For more
detailed surf information, see the latest Collaborative Nearshore
Swell and Wind Forecast at
www.prh.noaa.gov/hnl/pages/SRF.php#Discussion.

Surf along east facing shore will remain small through the first
half of the week, then build through the second half as stronger
trades return. Additionally, a long-period swell associated with
tropical activity across the eastern Pacific will be possible
Wednesday through Friday, especially along the exposed beaches of
the Big Island.

Water levels Saturday afternoon ended up coming in and exceeding
the record water level measured at Honolulu back at the end of
April and May (the Honolulu station recorded a preliminary water
level of 3.284 ft relative to MLLW). These water levels are in
response to a combination of an oceanic eddy moving through from
east to west and the regular King Tide cycles. Water levels will
begin to slowly lower each day through the week. Please refer to
the Special Weather Statement for more details.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

Morrison...Discussion
Jelsema...Aviation
Lau...Marine