950
FXHW60 PHFO 142015
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1015 AM HST Tue May 14 2024

.SYNOPSIS...
A lingering upper level low northeast of the islands is keeping
the local airmass relatively moist and unstable. Spotty, hit-or-
miss heavy downpours will continue to be possible over the next
couple of days. A late season kona low will develop several
hundred miles north of the islands Tuesday night, and then meander
around well northwest of the islands the rest of the week. The
kona low will turn our winds southerly as early as Tuesday, 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.

&&

.DISCUSSION...
A kona low developing north of the state tonight will bring a
multi-day heavy rain event starting as early as tonight. 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 will
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.

Wednesday and Wednesday night...
Showers increasing through the day especially over the western
half of the state. By Wed 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 the
band of heavy rain will shift late Wednesday into Wednesday night.

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 and Sunday...
As the band of convergence shifts towards Kauai over the weekend,
even more copious moisture (TPW near or possibly even in excess
of 2 inches) will be pulled northward toward Kauai and possibly
Oahu. A shortwave aloft rotating around the kona low and favorable
jet difluence along may help to enhance rainfall even more. This
scenario is less confident, however, since the band may slow to a
crawl and intensify mostly west of the Garden Isle over the
weekend. Also, there is the possibility for one or more
thunderstorm complexes to develop SW of the islands that might rob
us of some moisture and instability. Nevertheless, we want to
stress that this is the type of pattern that can lead to some
pretty serious flooding. 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. SHRA is possible off and on through the
forecast period. MVFR conds are also possible in those heavier
SHRA. Light to moderate southeasterlies will veer more southerly
through the day.

No AIRMETs currently in effect. However, this will be watched and
an AIRMET for mtn obsc may be needed at least for the smaller
islands.

&&

.MARINE...
Light to moderate southeast to south winds will persist today
into Wednesday as an area of low pressure develops 450 nm north of
the state. A cold front extending from the low will pass over the
western islands early evening Wednesday then stall near or over
the central islands 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 begin to
fill in this afternoon, keeping surf along south facing shores
near to just below average in the short term. A second set of
southerly swells will fill in late Wednesday and could bring above
average surf Thursday into Friday. Surf should remain elevated
through the weekend.

A new small medium-period northwest swell will arrive later today
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