Heavy Rain in Hawaii, June 26, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
343 PM HST Sat Jun 26 2021

.SYNOPSIS...
Interior and leeward clouds will clear out as localized land
breezes develop again tonight. Trade winds will pick up from east
to west on Sunday, then gradually strengthen to locally breezy
levels for the second half of the coming week. Stable conditions
will lead to mainly brief showers across windward sections.

&&

.DISCUSSION...
Water vapor imagery shows an upper-level ridge just north of the
state, between lows far to the northeast and west-southwest. At
the surface, a ridge extends just north of Kauai from high
pressure far to the northeast, with a nearly stationary front
farther northwest. 00Z soundings from Hilo and Lihue show strong
inversions around 6kft and precipitable waters of 1.0/1.1 inches
respectively. This is in line with the MIMIC Total Precipitable
Water, which also shows slightly higher values up to 1.3 inches
across the central part of the state.

Clouds filled in across interior and leeward areas today, in
conjunction with onshore sea breezes that developed across the
area under the weak trade flow. With stable conditions, only a
few locations received more than a tenth of an inch of rain over
the past 12 hours, and those were on Maui and Big Island where the
area of greater moisture was located earlier. As we transition to
leeward land breezes tonight, interior clouds will diminish and
windward cloud could fill back in under the light trade flow.

The high to the northeast will strengthen and pull the ridge
northward, leading to a gradual increase in trades, initially as
a surge from east to west. GFS and ECMWF both show an increase in
moisture with this speed convergence spreading from the Big
Island late tonight through Kauai Sunday night, enhancing windward
clouds and showers.

Trade winds will strengthen through mid-week as a larger high
reestablishes itself across the northeastern Pacific. Moderate to
locally fresh winds are expected through the second half of the
week. With the upper-level ridge remaining in place near the
state, forecast soundings show the subsidence inversion holding
fast with a solid cap on trade shower depth.

&&

.AVIATION...
This afternoon, weak easterly trade wind flow continues across the
state as a surface ridge remains stalled north of Kauai. As a
result of the weak flow, sea breezes have supported clouds and an
uptick in showers for interior and leeward sections of the islands
this afternoon. This is especially true for the Big Island
slopes, where interior clouds and showers are the most widespread.
This pattern will continue for the rest of the afternoon and
early this evening before land breezes begin to disperse clouds
and nudge any remaining showers off the islands into the adjacent
waters. VFR conditions are expected to prevail throughout the
next 24 hours with only isolated MVFR ceilings and visibility in
any showers that do occur.

There are currently no AIRMETs in effect.

&&

.MARINE...
A ridge of high pressure will gradually lift north over the next
few days and will bring the return of moderate trade winds across
the areas. Trade winds are expected to increase further towards
the second half of next week as the high far northeast of the
state strengthens. This will likely bring the return of strong
trade winds across the typical windier waters around Maui County
and the Big Island.

The reinforcing northwest swell continues to fill in this
afternoon and will likely peak early tonight and gradually
decrease over the next few days. Another small northwest
swell is possible towards the end of next week. A series of small
south and southeast swells is expected throughout the forecast
period, with a slightly larger south swell possible late Tuesday
and Wednesday. Due to the lack of significant trade winds over and
upwind of the area, surf will remain small along east facing
shores, with increasing surf possible during the second half of
next week as the trades strengthen.

&&

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

&&

DISCUSSION...Bravender
AVIATION...Vaughan
MARINE...Kino