Flash Flood and Heavy Rain in Hawaii, September 22 to 25, 2022

Area Forecast Discussion
National Weather Service Honolulu HI
352 PM HST Thu Sep 22 2022

.SYNOPSIS...
An area of low pressure far northwest of the Hawaiian Islands,
will further weaken trade winds, allowing for land and sea breeze
development across the islands into early next week. This pattern
will maintain afternoon and early evening clouds and showers
favoring mountain and island interiors, while land breeze
development each night will limit clouds and showers through the
early morning hours.

&&

.DISCUSSION...
Low pressure well northwest of the islands will quickly drop
southwest through the weekend, while maintaining a track over 500
miles west of Kauai. This system will eventually stall west of the
state by Sunday, while a surface boundary drops in well north and
northeast of the state. Additionally, a strong upper low (also
well northwest of the islands) will follow a similar path of the
southwest drifting surface low through the weekend, while model
solutions continue to diverge by the beginning of next week. This
pattern will weaken the pressure gradient north of the state and
support weakly organized east to southeast trades through the
weekend, which will reinforce the diurnally driven local sea and
land breeze circulations.

Current radar trends show scattered showers lingering
offshore this afternoon, with the areal coverage of showers
expanding rapidly across interior sections of the islands.
Overall expect that showers will continue to expand across
mountain and interior regions through early evening, as weak
background trades maintain windward showers, and sea breeze
circulations support leeward showers and clouds.

Model data shows moisture depths remaining around 10k ft
through tonight, with CIMSS satellite based precipitable water
values indicating PWs near 1.55 inches, while the latest HLI and
HTO soundings show PW values near 1.34 inches which should help
limit widespread heavy rainfall. However, still expect to see some
enhanced rainfall possible across interior sections and mountains
into this evening as sea breeze and upslope winds enhance lift.
Locally enhanced rainfall potential is further supported by dual
pol inst precip rates estimates nearing 0.75 inches per hour on
cells just offshore. However the areal coverage of showers will
quickly diminish this evening due to limited instability and depth
of moisture, combined with winds quickly shifting back toward
land breeze this evening. The weather pattern seen across this
islands today, will repeat again Friday as the overall pattern
remains similar. However, shower and cloud extend will become more
limited this weekend as inversion heights and moisture depths
decrease slightly.

Monday through next week, the weak surface boundary drifting in
far north-northeast of the state to begin the work week, will
maintain lighter winds across the islands. This will tend to favor
the continued development of nighttime land breezes, with clouds
and showers mainly over windward areas. Meanwhile, daytime heating
and afternoon sea breezes will lead to the development of clouds
and showers for interior leeward areas. Guidance continues to
trend towards a moderate trade wind pattern returning late next
week as high pressure builds far northeast of the state.

&&

.AVIATION...
Weak ridging north of the area will sustain light to moderate
winds across the island chain through Friday, generally in an east
to east-southeast direction. With this regime in place, daytime
sea breezes and nighttime land breezes will develop in many
locales. Afternoon cloud buildups, with some embedded showers,
have been developing over interior, mauka, and/or leeward
sections, especially on Maui and the Big Island. Much of the
cloudiness will tend to dissipate in the overnight hours as land
breezes take hold.

No AIRMETs are in effect.

&&

.MARINE...
A ridge to the north of the area will slowly shift to the south,
maintaining a light to moderate east to southeast flow. Winds are
expected to remain below Small Craft Advisory through the weekend.

The Lanai buoy has been reporting 3.6 to 4.0 feet at 17 seconds
over the last several hours. With little change in the buoy
observations, have opted to extend the High Surf Advisory through
midnight tonight. Expect the swell to be on the decline over the
next few hours, and at some point this evening, expecting a drop
in surf heights. The latest runs of the WW3 and NWPS continue to
show swell of 2 feet, falling about 2 feet below the buoy
observation. The forecast reflects the bump in the south swell for
tonight. This swell will slowly decline Friday, and then arrival
of a reinforcing pulse of south swell energy on Saturday, which
will likely maintain surf near the seasonal average along south
facing shores this weekend. Surf will gradually lower along south
facing shores early next week.

Surf will remain below the seasonal average along north facing
shores through early next week. The current small northwest swell
will fade late Friday. A small, short period north-northwest
swell is possible during the weekend, followed by another small
northwest swell next Tuesday. The weak trade winds over and
upstream of the islands will maintain small surf along east
facing shores surf through early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until midnight HST tonight for Niihau-Kauai
Leeward-Waianae Coast-Kahoolawe-Maui Leeward West-Kona-Kauai
South-East Honolulu-Honolulu Metro-Ewa Plain-Molokai Southeast-
Molokai Leeward South-Lanai Leeward-Lanai South-Maui Central
Valley South-South Maui/Upcountry-South Haleakala-Big Island
South-Big Island Southeast.

&&

DISCUSSION...SWR
AVIATION...Kinel
MARINE...M Ballard