Heavy Rain in Hawaii, March 22 to 23, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
409 PM HST Mon Mar 22 2021

.SYNOPSIS...
The trade winds will weaken and veer southeast tonight due to an
approaching cold front. Nighttime land breezes and daytime sea
breezes will replace the trades after midnight for many areas.
A pre-frontal convergence band will set up across the smaller
islands Tuesday night and bring widespread showers that will
continue through Thursday over these areas. Trade winds will push
the boundary westward to Kauai on Friday with drier weather by
the weekend.

&&

.DISCUSSION...
A 1035 mb surface high is centered around 1500 miles northeast of
Hawaii driving moderate trade winds this afternoon. A cold front
is located about 500 miles northwest of Kauai moving toward the
islands at about 25 mph. Winds are expected weaken and veer
southeasterly tonight before becoming light and variable on
Tuesday. Nighttime land breezes will set in for some areas around
the state after midnight with sea breezes expected for all islands
Tuesday. The southeast flow is also moving clouds and showers over
the Big Island this afternoon.

An upper-level trough east of the Big Island will slowly weaken
tonight. Some instability will remain in the vicinity of the Big
Island into Tuesday. Given the moisture around the Big Island at
this time, a slight chance of thunderstorms is forecast for
Tuesday afternoon. The smaller islands will likely get afternoon
cloud build ups and a few interior showers as it is more stable.

As the front moves closer, a pre-frontal convergent band will set
up over the smaller islands beginning Tuesday night. The rain
coverage could cover anywhere from Kauai to Maui, but looks like
the central to western portion of the state will be the main
focus. The upper level support for this front is expected to stay
too far north for any significant rainfall/flash flooding, but
with the southerly flow and the band moving very slowly, light to
moderate rain could linger over the same islands until Thursday.

Remnant moisture will hang around Kauai until Friday as the trade
winds build in from the east and push the boundary back to the
west. Expect drier trade wind weather for the weekend.

&&

.AVIATION...
Moderate trade winds will continue to ease through tonight, becoming
light and variable. The overall wind pattern will become more
southeast then south Tuesday as a cold front approaches from the
northwest. Showers will continue to favor windward and mauka areas
through tonight, with mostly dry conditions persisting over
leeward sections of the smaller islands. Brief MVFR conditions are
possible, though VFR conditions should prevail. Windward Big
Island remains quite showery this afternoon with tempo MVFR
cigs/vis, but should dry out more tonight. Thunderstorms remain
possible over interior Big Island through this afternoon, which
will clear out this evening as land breezes develop.

AIRMET Sierra remains in effect for mountain obscuration over
windward sections of the Big Island and will likely continue
through much of evening.

&&

.MARINE...
Satellite shows a line of clouds associated with a cold front
approaching the offshore waters from the northwest. This front is
already having impacts on the weather over the coastal and
offshore waters, with winds on the decline. The morning ASCAT pass
showed winds were, as expected, weaker than last night. The
morning pass did indicate a couple of points in the typical
windier locations were near Small Craft Advisory (SCA) levels.
However with weaker winds expected, and overall being experienced
across the area today, suspect that the next pass will show all
areas below SCA. Seas have also subsided below SCA levels, so the
the SCA has been cancelled with the afternoon package.

The cold front approaching from the northwest is expected to
enter the offshore waters Tuesday morning, and will shift winds
out ahead of the front to a southerly direction. The front is
expected to reach the northwest coastal waters Wednesday, and
stall over the central waters later in the week.

The high pressure system far to the northeast has continued to
boost surf along east facing shores with the long trade wind
swell. However as the high is moving away and the trades
weakening, surf along east facing shores is on the decline. Surf
heights have fallen below advisory levels, and so the High Surf
Advisory (HSA) has been cancelled with the afternoon package. A
new northwest swell is expected to arrive tomorrow night, and will
boost surf along north and west facing shore, with surf nearing
advisory levels during the peak of the event Wednesday night
through Thursday. This swell is then expected to decline Friday
and Saturday.

Some small long period south swells will continue to boost surf
along south facing shores through the week.

&&

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

&&

DISCUSSION...Foster
AVIATION...TS
MARINE...M Ballard