Heavy Rain in Hawaii, August 23, 2022

Area Forecast Discussion
National Weather Service Honolulu HI
346 PM HST Tue Aug 23 2022

.SYNOPSIS...
Weak high pressure ridge north of the islands will maintain light
trade wind flow into Thursday, while afternoon sea breeze showers
develop across some sheltered interior leeward areas. A surface
trough slowly drifting across the state from east to west, will
enhance rainfall coverage through Wednesday. A transition toward
more typical trade wind weather is expected to begin Thursday as
the trough exits west of the area. This will allow the ridge to
restrengthen north of the state Friday through the weekend.

&&

.DISCUSSION...
The current satellite loop shows an area of clouds maneuvering
northwestward along the northern edge of the islands, with the
greatest cloud cover remaining anchored along the central smaller
islands and extending toward Kauai. Radar data shows the areal
coverage of showers has continued to diminish across the state
through the afternoon, and mainly focused along windward locations
in weakened trade wind flow. CIMSS satellite data continues to
show increased layered precipitable water streaming in from the
east, with sfc-925mb southeast streamlines likely maintaining a
bulk of the showers in a narrow band right along the coastal
areas of windward locations into this evening. Expect the current
radar trends, which show showers along and just offshore of
windward locations, will continue through tonight as low level
wind flow remains orientated largely from the southeast. However
the areal coverage of the showers will likely increase again
overnight in the typical diurnal pattern.

Otherwise, the weak surface trough slowly drifting westward
across the region through Wednesday will continue to enhance low
level moisture and chances for showers across the state. The
lingering weak trade wind flow will help focus a majority of the
showers across windward locations, with greatest coverage
occurring overnight through the early morning hours. The overall
weak background winds will also promote daily sea breeze clouds
and showers over interior leeward areas into Thursday. Any
interior showers and clouds will then slowly diminish in the
evening as the flow transitions to land breeze.

Thursday will begin the transition back to a more dominant trade
wind weather pattern, as the surface trough exits west, allowing
the ridge to restrengthen north of the islands. Trade winds will
continue to strengthen through Friday and are expected to become
moderate to breezy trades this weekend which will linger through
the first half of next week. Showers will favor typical windward
and mountain areas Friday through early next week.

&&

.AVIATION...
Moisture associated with a low-level trough continues to
sluggishly lift through the forecast area this afternoon. Diurnal
heating has brought improving conditions to windward zones with
showers and MVFR decreasing in frequency. The remainder of the
afternoon will feature a tendency for showers to refocus inland
before clearing out with land breeze development overnight. Low
clouds will increase over Windward Kauai this afternoon as the
lead edge of the trough approaches from the west. Light southeast
flow will continue into Thursday.

AIRMET Sierra for mountain obscuration is in effect for windward
Big Island through Oahu.

&&

.MARINE...
A ridge of high pressure just north of the state along with a
weak low level trough over the west end of the island chain will
maintain light to moderate trade winds through Friday. Trade winds
will gradually increase over the weekend as the ridge lifts northward
and strengthens.

The current long period south swell will continue to lower gradually
tonight through Wednesday. A much smaller south swell is expected
during the Thursday through Sunday time period. The current small
mid period northwest swell will lower gradually Wednesday through
Friday. Short period choppy surf along east facing shores will
remain rather small through Friday with a gradual uptick expected
over the weekend as trade winds gradually strengthen.

&&

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

&&

DISCUSSION...SWR
AVIATION...JVC
MARINE...Burke