490
FXHW60 PHFO 150159
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
359 PM HST Tue May 14 2024

.SYNOPSIS...
A late season kona low will develop several hundred miles north
of the islands tonight, and then meander around well northwest of
the islands the rest of the week. The kona low will bring an
increase of southerly winds starting tonight, and eventually tap
into deep tropical moisture lurking south of the islands. This
will cause a slow-moving band of showers, some heavy, and a few
embedded thunderstorms over some parts of the state for the second
half of the week. Excessive rainfall and flash flooding will
continue to be a concern especially over Oahu, Maui County and
Kauai.

&&

.DISCUSSION...
An upper level low northeast of the state continues to produce
unstable conditions across the state this afternoon. Convection
developed across many interior and mountain areas this afternoon
with showers nearly stationary or drifting northward for most
areas. These showers should decrease this evening. For tonight, we
could see some showers move in from the south, especially around
Oahu, where a plume of moisture is seen south of the islands on
satellite.

A kona low developing north of the state tonight will bring a
multi-day heavy rain event starting Wednesday. Unlike this past
week where we had scattered heavy showers with intense rainfall
rates, this week we have less instability, but a lot more moisture
and the focus of the heavy rain will be over leeward areas rather
than windward areas. The best way to describe this event, is like
a fire hose shooting water. A steady stream of rain will be
concentrated along a band and steadily move east through Wednesday
night then stall somewhere around Oahu or Maui County then shift
back westward towards Kauai on Friday. The most likely area for
the heaviest rain would be in areas under the fire hose,
especially leeward areas. A Flood Watch is in effect for the
entire state starting Wednesday, but for the Wednesday and
Thursday time frame, the greatest chance for flooding impacts will
be over Oahu and Maui County with Big Island seeing the least
chance for flooding impacts.

Wednesday and Wednesday night...
Showers increasing through the day especially over the western
half of the state. By Wednesday afternoon, a frontal boundary
approaching Kauai will enhance the lift and instability ahead of
the front. Models are showing CAPE values of around 1400 j/kg
with precipitable water values increasing to around 1.80 inches.
This will allow for heavy bands of rain to develop with embedded
thunderstorms along the convergent bands starting around Wednesday
afternoon over Oahu and expanding eastward towards Maui County
late afternoon through the evening. Confidence for heavy rain is
highest over Oahu with some uncertainty on how far eastward into
Maui County the band of heavy rain will shift Wednesday afternoon.
By Wednesday night, the band of heavy rain should make its way
over to Maui County. Wednesday night into Thursday looks to the
time period for the potential for flash flooding is the highest.
Due to the widespread rain event, large rises should be expected
over streams.

Thursday and Thursday night...
Moderate to locally breezy southerly winds will continue to pump
up moisture form the south with a steady stream of rain focused
over Oahu and Maui county. Instability does decrease slightly,
which could help decrease the rain rates a touch. But nonetheless,
a steady stream of moisture with prolonged periods of moderate to
locally heavy rain will persist around Oahu and Maui County
throughout the day.

Friday and Friday night...
As the kona low shifts westward, the associated convergent bands
will begin to shift westward towards Kauai. Winds will pick up and
become breezy out of the southeast direction over the eastern end
of the state with drier air approaching the eastern half of the
state. With the drier air moving in from the east, this will
decrease the chance for any flooding impacts, but showers will
still be possible. Over on the western half of the state,
especially Kauai, moisture is expected to ramp up and a steady
stream of rain will be moving in from the south impacting many
leeward areas. For Oahu, with the steady stream of rain focused
mainly over Kauai, the rain will likely not be as widespread, but
showery weather should persist for most of the day. By Friday
night, the main concern will be over Kauai.

Saturday through Sunday...
As the band of convergence shifts towards Kauai over the weekend,
even more moisture (precipitable water values near or possibly
even in excess of 2 inches) will be pulled northward toward Kauai
and possibly Oahu. The GFS and ECMWF models continue to show
differences in how much moisture will be over Oahu during this
time this weekend, but confidence remains high for the potential
for significant flooding impacts over Kauai this weekend. We will
continue to monitor the situation and expect that details will
become more clear as we get closer.

&&

.AVIATION...
An unstable and moist airmass remains in place due to a upper
level low northeast of the islands. In the meantime a low is
developing northwest and will move closer to the islands and
become a kona low tomorrow. SHRA is possible off and on through
the forecast period becoming more widespread in coverage and
intensity with embd TS. IFR/MVFR conds are possible in heavier
SHRA/TS. Southerly winds will become light overnight before
picking up tomorrow. Moderate winds are expected with gusts
possible within/near heavy SHRA/TS.

AIRMET Sierra is in effect for all the smaller islands and
windward Big Island for mtn obsc. Sierra may be needed for IFR
conds and Tango for turb as the kona low gets closer and conds
deteriorate.

&&

.MARINE...
A new subtropical low will develop about 450 nm north of Kauai
tonight. A cold front extending from the low will approach the
state from the northwest this evening, pass over Kauai tomorrow
night, and stall over Oahu late Thursday. The front will then
rapidly weaken into a trough and shift westward Friday. Scattered
heavy showers are possible through the week with the threat of
thunderstorms developing Wednesday evening and persisting into the
weekend.

A series of overlapping, long-period south swells will keep surf
along south facing shores near to just below average in the short
term. A second set of southerly swells will fill in late tomorrow
night and could bring above average surf Thursday. Surf should
remain elevated through the weekend.

A new small medium-period northwest swell will fill in this
evening and peak Wednesday. Then, on Wednesday night, a new
short- period northwest swell, associated with the developing low
described above, is expected to build in. This swell will likely
peak near to just below advisory levels Thursday into Friday.
Model guidance tends to under-forecast swell heights with sources
developing at a close proximity. Therefore, there is a potential
for surf to reach High Surf Advisory criteria, and we will
continue to monitor all available guidance.

West facing shores will see a bump in surf through the week due
to the overlapping northwest and south swells wrapping into some
exposed areas. Surf along east facing shores will remain well
below normal levels through the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch from Wednesday morning through Friday evening for
all Hawaii islands-


&&

$$


DISCUSSION...Kino
AVIATION...Walsh
MARINE...Bedal/Birchard