Flash Flood in Hawaii, July 27, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
324 PM HST Tue Jul 27 2021

.SYNOPSIS...

Breezy trades will focus clouds and showers over windward and
mauka areas through the forecast period. Trades will trend drier
from Thursday through the end of the week.

&&

.DISCUSSION...

The eastern flank of a mid-level low is still supporting deep
showers and a few thunderstorms well northwest of the islands
this afternoon. It is the cyclonic flow around the eastern
periphery of this low that is generating the transparent layer of
cirrus that has been streaming across area skies today. For now,
local conditions remain modulated by the continued breezy trades.
Afternoon soundings indicate a gently sloped inversion ranging
from around 5kft at Lihue to around 7kft at Hilo. Although wetter
trades will continue funneling showers into windward sections of
Big Island and Maui, the trend of increasing stability over the
eastern end of the state will continue into tonight and will limit
shower intensity. The exception will be Leeward Big Island where
the destabilization within the reservoir of deeper mid-level
moisture has given rise to a wide coverage of moderate to locally
heavy showers. An isolated rumble of thunder can't be ruled out
there, but potential is too low to warrant including in the
forecast at this time.

A narrow ribbon of relatively dry air has maintained suppressed
cloud and shower activity over windward sections of Kauai through
Molokai, the driest island today being Oahu where only one gauge
has measured any rainfall in the last 12 hours (0.02" at Manoa
Lyon Arboretum). As the airmass currently affecting Big Island
and Maui is ushered north and westward by the breezy trades
tonight, the aforementioned mid-level low will pivot across the
northern end of the state. Forecast soundings indicate a
subsequent loss or, at least, a considerable weakening of the
trade inversion by early Wednesday morning. Increasingly favorable
dynamic support during this time will favor an increased
probability for isolated deep convection or even a few
thunderstorms (confined to marine areas) as far south as the Kauai
Channel during Wednesday morning. The primary adjustment to the
near term forcast was therefore to add isolated thunder to the
waters around Kauai mainly in the 15-21z / 5am-11am period
tomorrow morning.

Trades will briefly weaken over the northern end of the state
tonight in response to passing upper trough and subsequent weakening
or loss of the inversion, but are otherwise forecast to remain
breezy through the forecast period. A noteworthy drying trend is
anticipated from Thursday onward as mid-level heights rebound in
the wake of the departing upper trough. Increasing subsidence
during this time will further limit the intensity of any shower
activity. The extended forecast has accordingly been trended
away from climatology and toward the drier model consensus.

&&

.AVIATION...

Moderate to occasionally breezy trade winds will persist through
24 hours and beyond. This, combined with a weak inversion aloft
around 08 kft, will yield low level turbulence downstream of higher
terrain. AIRMET Tango remains in effect for all islands south
trough west of mountains.

Multiple bands of showers can be seen via satellite embedded in
the trade wind flow. Periods of MVFR ceilings and visibility are
associated with this activity. These showers will impact mainly
north and east facing slopes and coasts, but isolated spillover
showers are to be expected across leeward areas as well. AIRMET
Sierra remains in effect for windward portions of the Big Island,
but may be expanded overnight to include Kauai.

Slight chance of thunderstorms over Interior Big Island through
early evening. Overnight clearing expected across south and west
facing slopes and coasts, but any improvement will be temporary.
Daytime cumulus-- including possible cumulonimbus---will return
to Interior and Leeward Big Island Wednesday afternoon.

&&

.MARINE...

A high pressure system passing by north of the region will keep
fresh to locally strong trade winds in the forecast through
Wednesday, followed by a small speed increase Thursday into the
weekend. A Small Craft Advisory (SCA) remains in effect for the
typically windier waters and channels near Maui County and the Big
Island through Wednesday, and may expand in coverage on Thursday
and Friday.

Moderate choppy surf along east facing shores will slowly trend a
bit lower through Wednesday due to a surface ridge decreasing the
fetch length of the trades upstream of the islands. A slight
increase in the trade wind swell and surf along east facing shores
will develop on Thursday and Friday as a high pressure system
builds in north to northeast of the region.

A mix of minor south and southeast swells will keep small surf
along exposed south shores into the weekend.

A small west-northwest swell, from tropical cyclone activity in
the NW Pacific, may boost surf a bit along some exposed west and
north facing shores starting on Thursday or Friday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Wednesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

DISCUSSION...JVC
AVIATION...Bedal
MARINE...TS