Flash Flood and Heavy Rain in Hawaii, June 23 to 28, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
835 PM HST Sat Jun 22 2019

.SYNOPSIS...
Light east to southeast winds will continue through the first half
of next week due to an area of low pressure positioned northwest of
the state. Warm and humid conditions with overnight land breezes and
daytime sea breezes will result. Daytime heating combined with the
afternoon sea breezes will translate to the best cloud and shower
coverage through the afternoon periods, mostly over interior and
leeward portions of the islands. Shower coverage may increase early
next week as an upper trough moves into the area, especially over
the western end of the state. A return of a more typical trade wind
pattern is not anticipated until the latter half of next week.

&&

.DISCUSSION...
The latest surface analysis showed a weak front extending
southward from an area low pressure a few hundred miles west of
Kauai. Plenty of deep moisture is pooling northward over the
region from the deep tropics due to a combination of this surface
feature and an anomalous upper low centered around a 1000 miles to
the northwest. Recent satellite-derived precipitable water (PW)
imagery reflected this and showed a broad area of above average
moisture (2.25" PW values) being drawn northward toward the state.
The low-level flow remains light out of the southeast over the
islands, which has translated to warm and humid conditions, with
dewpoints now in the lower 70s this evening. The afternoon high
temperature at the Kahului airport on Maui managed to reach 93
degrees today, which was the highest recorded across the state.
Despite the minimal shower coverage this afternoon over the
islands, a thunderstorm developed over the central or interior
section of the Big Island near or over Saddle Road and produced a
brief period of heavy rain.

The latest short-term guidance remains in decent agreement through
Monday and depicts no significant synoptic pattern changes due to
the aforementioned area of low pressure and front lurking to the
west. Warm and humid conditions with overnight land breezes and
daytime sea breezes will prevail each day. Dewpoints will hover in
the upper 60s to low 70s with PW values climbing to around the 2"
mark (highest moisture western end of the state). Daytime heating
combined with the afternoon sea breezes will translate to the
best potential for cloud and shower coverage through the afternoon
periods, mostly over interior and leeward portions of the
islands. Simulated model reflectivities show the best afternoon
coverage setting up over Kauai each day.

Despite some notable differences between model solutions, a wet
pattern with above average moisture and increasing instability due
to lowering upper heights is becoming more likely Monday night
through Wednesday. The better source of deeper moisture will favor
the western end of the state where PWs are forecast to exceed the 2"
mark. This deep tropical moisture combined with the added
instability will be enough to support heavy showers and even a few
thunderstorms. Although it remains too early for any details, if
this scenario materializes, localized flooding can't be ruled out
through this time, if and where heavy showers become focused for
any duration.

All of the model guidance begins to favor a return of a more
typical summer trade wind pattern spreading over the entire state
late next week.

&&

.AVIATION...
High pressure northeast of the state and an area of low pressure
a few hundred miles northwest of Kauai, will keep light to
moderate east-southeast winds in place through Sunday. This will
allow for overnight land breezes and daytime sea breezes to
develop in many leeward areas. Showers will favor areas near the
coast during the overnight and morning hours, and interior and
mountain areas during the afternoon and evening. Brief MVFR
cigs/vsbys will be possible as showers move through, but
predominantly VFR conditions are expected during the next 24
hours.

No AIRMETs are currently in effect.

&&

.MARINE...
High pressure will remain far northeast of the state and a front
will continue to linger west of the islands through the weekend.
This will cause moderate east to southeast winds over the coastal
waters, with localized fresh breezes expected in the windy areas
around Maui and the Big Island over the next few days. Compared to
today, wind direction will be slightly more east-southeasterly
rather than southeasterly for Sunday. Near the coasts, expect
daytime sea breezes to develop, followed by nighttime land
breezes. Looking ahead to next week, an upper trough will approach
the area from the west and northwest, and will likely bring some
unsettled weather across the area waters. Although the details are
still unclear, there will likely be numerous showers over the
western half of the coastal waters Tuesday into Wednesday and
possibly some thunderstorms. Winds, for the most part, will be
light to moderate, and out of the southeast or south through the
first half of next week.

Surf along south facing shores will remain small through early
next week with mainly a combination of short-period southeast
trade wind energy and overlapping background southerly swells
moving through. Long range guidance is showing the next larger
bump in south shore surf next weekend, from a developing storm-force
low passing south of New Zealand this weekend. Recent altimeter
pass revealed a large area of seas greater than 40 feet south-
southwest of New Zealand with ASCAT showing storm-force winds.
Depending on the storm intensity and track over the next 24 to 36
hours, we may see advisory level south shore surf next weekend.

A mid-period NE swell transiting through the area was generated
by a steady stream of fresh to strong trade winds upstream from
the state. Surf heights, however, are expected to remain below
advisory level for east facing shores through the peak this
weekend.

&&

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

&&

DISCUSSION...Gibbs
AVIATION...Jelsema
MARINE...Kino