Flash Flood Warning 18, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
958 AM HST Sun Apr 2 2023

.SYNOPSIS...
Instability due to an upper-level disturbance developing over the
area combined with abundant moisture will likely cause increasing
showers, along with a chance of thunderstorms across much of the
state this afternoon through tonight. Trades will become established
across the island chain tonight, with locally breezy trades
expected from Monday into mid-week. A relatively wet trade wind
weather pattern will likely develop from Monday through Tuesday,
with a slight chance of thunderstorms over portions of the Big
Island each afternoon.

&&

.DISCUSSION...

Convergent southeasterly flow in advance of building trades will be
a focus for shower and thunderstorm activity today. Already this
morning, convergent island plumes have been active downwind of the
Big Island and Lanai. Expect this to be a theme today as today as
large scale forcing associated with an upper trough sweeps across
the area. The back edge of high clouds over Kauai has sharpened
within the aforementioned trough and will make for a nice proxy to
track the progress of the best upper level support. Showers, some
locally heavy, and isolated thunderstorms will expand in coverage
this afternoon. Leeward Oahu will be of particular interest and will
be worth monitoring given its propensity to be on the receiving end
of island-enhanced convection originating over Molokai and/or Lanai.
Further east, ESE trades are focusing a band of moisture over
Windward Big Island and, to a lesser extent, Windward Maui. These
showers, too, will increase in coverage and intensity this afternoon
coincident with the advancing upper wave.

The forecast for tonight onward remains in good shape. building
trades will gradually strengthen, with locally breezy conditions
from Monday into mid-week. During this time, the low aloft in the
vicinity of the islands is expected to migrate southwest of the
area. As this occurs, conditions will be favorable for increased mid-
level instability to wrap around the low, which could provide
increased fuel for deeper convection and thunderstorms over, and in
the vicinity of the Big Island on Monday and Tuesday, especially
during the afternoon. At this time, it appears that high cloud cover
will be the decline as mid-level ridging begins building over the
eastern end of the state. The lack of background forcing is
therefore likely to be offset by ample diurnal destabilization both
Monday & Tuesday afternoon's leading to an increased probability of
t-storms over interior and upslope Big Island, especially leeward.
Building upper ridging will nudge the trades toward a more typical
stable character during the latter half of the week
&&

.AVIATION...

Mostly cloudy conditions with localized heavy showers and a few
thunderstorms will remain in the forecast today as an upper
disturbance and surface trough linger nearby. The low-level flow
will remain out of the east-southeast this morning, then gradually
shift out of the east and increase later today through tonight.
Shower activity will favor the adjacent coastal waters and coasts
through the rest of the morning with the land breezes in place,
then shift to windward locations as the winds become easterly. By
Monday, breezy trade winds will return with showers favoring north
through east sections of the islands. The exception will be the
Big Island where leeward areas will see an increase in clouds and
showers as well Monday afternoon, and an isolated thunderstorm
cannot be ruled out over the island's interior.

AIRMET Sierra remains in effect for mountain obscuration for Kauai
and Oahu this morning with showers and low clouds lingering
nearby. However, latest satellite trends show clearing just west
of Kauai that is quickly making its way east, so this AIRMET may
be dropped for Kauai later today.

AIRMET Tango remains in effect for tempo moderate upper-level
turbulence across the islands. This may be needed through today,
but guidance suggests this threat will decrease by this evening.

Finally, a mention of light icing has been included to account
for considerable mid- to high-cloud cover streaming over a large
portion of the state.

&&

.MARINE...

Trade winds will build over the western portions of the state
later this afternoon and strengthen Monday into Tuesday as the
surface trough west of the state moves further away and
dissipates. Trade winds will strengthen to fresh to strong speeds
by Tuesday then gradually weaken during the second half of the
week as the ridge shifts further south and closer to the state.
Small Craft Advisories (SCA) are posted for most waters around
Maui County and the Big Island and will likely need to be
expanded as trades strengthen.

Meanwhile, an upper level trough to the northwest of the islands
will continue to provide instability to spark some thunderstorm
activity. Isolated thunderstorms will be possible throughout the
Hawaiian waters through tonight. The offshore wasters will have a
chance for thunderstorms into the middle of the week with the
main focus today being over the northwestern half of the offshore
waters. That area will shift to the western and then southern
waters over the next couple of days.

No significant swells expected through the week. Background
energy from the northwest and trade wind wrap is expected along
north facing shore for most of this week. A small north- northwest
(340 degrees) swell is possible towards next weekend. Trade winds
strengthening during the next few days should bring a small boost
to surf heights and an increase of choppy conditions along east
facing shores by Tuesday. Small background energy from a variety
of southerly directions will maintain small surf along south
facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 AM HST Monday for Big Island
Summits.

Small Craft Advisory until 6 PM HST this evening for Maui County
Windward Waters-Big Island Windward Waters.

Small Craft Advisory until 6 AM HST Tuesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

DISCUSSION...JVC
AVIATION...Vaughan
MARINE...Kino