Flash Flood Warning 16, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
946 AM HST Wed Mar 29 2023

.SYNOPSIS...
Showers and thunderstorms are likely over the western end of the
state today. Heavy rainfall is possible across Niihau, Kauai, and
Oahu, where a Flood Watch remains in effect through the afternoon.
Thunderstorms may be particularly strong this morning. Further
east, lighter showers will favor southeast- facing slopes.
Thunderstorms are possible over interior Big Island slopes this
afternoon and Thursday afternoon. Winds will remain southeasterly
through the weekend, then return to a more easterly direction
early next week.

&&

.DISCUSSION...
A surface low northwest of the islands is supported by a deep
upper low that is producing a broad area of cyclonic flow aloft. A
stationary front lies a couple hundred miles northwest of Kauai.
Satellite loop shows deep convection and overcast layered clouds
across the state, with the coldest cloud tops west of Molokai at
this hour. Southeast flow near the surface contrasts with
southwest to northeast flow aloft. Within southeast surface flow,
a convergence band continues to trigger thunderstorms over and
around Kauai and Niihau. The area of thunderstorms is drifting
eastward as an upper trough passes by. Elsewhere, showers embedded
with southeast flow continue to favor southeast-facing shores and
slopes.

Abundant moisture and instability will keep the threat of heavy
rainfall and thunderstorms in the forecast for the western islands
through today, with an additional threat of thunderstorms for Big
Island interior areas each afternoon through tomorrow. Some
thunderstorms may be particularly strong this morning. A Flood
Watch remains in effect for Niihau, Kauai, and Oahu through this
afternoon.

Models show southeast surface flow will continue into the
weekend, with locally breezy wind speeds indicated around Maui
County and the Big Island and light to moderate wind speeds
indicated elsewhere. Increased stability will introduce drier
conditions, though lingering moisture around Kauai will contribute
to increased shower coverage there. There is some uncertainty in
the forecast for early next week as models show an upper low or
trough approaching the islands from the northwest. This pattern,
if it were to pan out, would drive an increase in shower activity
during the first part of the work week.

&&

.AVIATION...
A low pressure system far northwest of the state will continue to
produce unstable southerly winds over Kauai and Oahu through tonight.
Bands of showers and embedded thunderstorms will continue to
develop in this wet, unstable flow and move northward over Kauai
and Oahu, with periods of MVFR and brief IFR conditions. Shower
coverage remains significantly less across the eastern half of the
state, with only brief periods of MVFR conditions in showers from
Molokai to the Big Island. However, thunderstorms are possible
again over the Big Island interior this afternoon, due to
instability, abundant moisture and surface heating above the trade
wind inversion. Overall shower activity is forecast to decrease
gradually through Thursday as the aforementioned low moves away
to the northwest.

AIRMET Sierra remains in effect for mountain obscurations above
020 for the islands of Kauai and Oahu this morning. This AIRMET
will likely continue into this evening.

Tempo light rime icing will also be present in the layered higher
clouds between 130-FL180.

&&

.MARINE...
Thunderstorms remain active around the Kauai waters this morning
especially Kauai Leeward waters. For rest of the day today, we
will continue to see scattered thunderstorms around the waters of
Kauai with the main line of thunderstorms gradually shifting
eastward. We should also see an increase of thunderstorms around
Oahu waters, especially Oahu Windward waters. The threat for
thunderstorms will begin to diminish tonight, but isolated
thunderstorms will still remain possible around Kauai waters.

A broad low pressure system west of the state will keep moderate
to fresh southerly winds over the western islands and fresh to
strong southeasterly winds over the eastern islands through
tonight. Trade winds will begin to slowly build back in from east
to west across the islands Thursday through the weekend, with the
strongest winds holding over the eastern waters. A Small Craft
Advisory is in effect for most waters surrounding the Big Island
through 6 PM Thursday.

No significant swells are expected through the middle of next
week. A small pulse from the northwest (320 degrees) will build
late this afternoon and tonight, peak Thursday and Thursday
night, then slowly decline through Saturday. North shore surf will
be nearly flat Sunday through the middle of next week.

Fresh trades upstream of the state will continue to produce near
seasonal surf along east facing shores during the next 7 days.
Small background energy from the southwest, south and southeast
will continue to produce small surf along south facing shores for
the foreseeable future.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through this afternoon for Niihau-Kauai Leeward-
Kauai Mountains-Waianae Coast-Oahu North Shore-Olomana-Central
Oahu-Waianae Mountains-Kauai North-Kauai East-Kauai South-East
Honolulu-Honolulu Metro-Ewa Plain-Koolau Windward-Koolau
Leeward.

Small Craft Advisory until 6 PM HST Thursday for Big Island
Windward Waters-Big Island Leeward Waters-Big Island Southeast
Waters.

&&

DISCUSSION...Powell
AVIATION...TS
MARINE...Kino

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