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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 240204

Area Forecast Discussion
National Weather Service Honolulu HI
404 PM HST Thu Mar 23 2023

Light winds and humid conditions will prevail through Friday as a
trough stalls and gradually weakens near Kauai and Oahu.
Thunderstorms and locally heavy showers will remain possible
through the evening and into tonight for the central islands. A
drier and more settled trade wind weather pattern is expected as
the weekend begins, with limited showers expected into early next
week as trade winds gradually turn to the southeast and diminish
Sunday and Monday. Light winds and increased moisture are
anticipated again during the middle part of next week.


This afternoon, deep low pressure is located roughly 600 miles
due north of Kauai. This low will be moving eastward over the next
several days, eventually lifting well away from the islands. A
cold front extends southward into the offshore waters but
transitions to a surface trough that currently extends through the
eastern coast of Kauai. Throughout the day, thunderstorms with
frequent lightning have been occurring along and ahead of the
front in the offshore waters. Closer to the islands, a convergent
band stretching across Kauai and extending southwest has sparked
a line of heavy showers and thunderstorms this afternoon. This
convection was likely enhanced by an outflow boundary that
resulted from storms that occurred earlier this morning south of
Oahu, which could be seen on satellite and radar imagery. As it
moved into Kauai from the west and tracked across the island,
rainfall rates of 1.5 to 2.0 inches were observed at the Kalaheo
gauge, but radar data suggested that even heavier rates likely
occurred just offshore. Elsewhere, convection also continues this
afternoon about 50 miles south of Oahu, and afternoon sea breeze-
induced thunderstorms have also been observed at times over the
saddle area of the Big Island.

Light winds are expected to persist into Friday. Therefore, the
threat for sea-breeze thunderstorms over island interiors will
continue for the rest of this afternoon. Additionally, instability
will linger across the islands into this evening with plentiful
moisture and bulk shear hovering around 30kts. Based on the
latest hires guidance, areal coverage of thunderstorm chances has
been increased for the evening hours across the central islands
and their adjacent waters. Although Oahu has not received all that
much rain today, there still exists the possibility for locally
heavy rain this evening and tonight as the weakening trough moves
over the islands. To err on the side of caution, the Flood Watch
for Oahu has been extended in time until 6am Friday.

The trough is forecast to weaken and dissipate as is moves over
the islands. On Friday, drier conditions are expected in general
as the low far north of the state moves further away. With the
light winds, afternoon sea breezes will be the best trigger for
shower development over island interiors. Although there will be
enough instability for a terrain-induced afternoon thunderstorm or
two on Maui and the Big Island, the threat for more widespread
thunderstorms will end by early Friday.

Light to moderate trade winds are expected for the the weekend as
the high to the NE resumes control over island weather, with a
stable regime leading to just a few windward showers. By early
next week, a regime similar to the current one appears to unfold
once again. Low pressure currently to the distant NW will pass N
of the islands, leading to a stalling low- level trough that is
likely to bring light winds and increased moisture from Tuesday
through Thursday.


Troughing across the islands will trigger isolated to scattered
thunderstorms around Kauai and Oahu through tonight. Light
southeast to south winds with local land/sea breezes will
continue as well. Expect periods of MVFR conditions embedded
within showers, with isolated IFR conditions in heavier

As the trough moves across the island chain, clouds and showers
will push in from the south. Chances for showers and thunderstorms
will increase as afternoon convection sets in, then decrease at
night. An upper trough may enhance instability this evening,
increasing the chances for heavy rain and thunderstorm across all
the islands. AIRMET Sierra remains in effect for mountain
obscuration over Kauai and Oahu due to frequent clouds and
showers. This will likely continue overnight and may need to be
expanded to other islands.


A surface low pressure system north- northwest of Kauai will
continue to move toward the northeast. A surface trough associated
with the low is located near Kauai. Clouds and showers with
embedded isolated thunderstorms are along this trough, which is
moving slowly eastward. This feature is expected to move across
the western end of the island chain through tonight, and
eventually dissipate by Friday. Elsewhere, a surface high pressure
system is centered far northeast of Hilo. These weather features
will keep the background winds gentle across the western half of
the state, while moderate east to southeast winds will persist
over the eastern half of the state today. High pressure building
north of the region from Friday night through Saturday will cause
the trade winds to strengthen over the eastern end of the state.
This may result in the issuance of a Small Craft Advisory (SCA)
for the Alenuihaha Channel and waters south of the Big Island
starting Saturday. Another potential surface low pressure may move
to a position far northwest of the region from Sunday through
Monday. This scenario will likely cause the winds to gradually
weaken early next week.

Surf along east facing shores will gradually lower from this
evening through Friday. The current short-period north swell (020
degrees) will steadily decline tonight. A mix of small northwest
and north- northwest swells (310-330 degrees) will maintain small
surf along most exposed north and west facing shorelines later
tonight into this weekend. The current small, medium- period south
swell (190 degrees) will slowly fade through Friday, with only
small background south swell energy expected this weekend.


Flood Watch through late tonight for Oahu-




Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office