681
FXHW60 PHFO 152016
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1016 AM HST Wed May 15 2024

.SYNOPSIS...
A kona low developing about 600 miles north of Kauai today will
intensify and meander around well northwest of the islands into
next week. The low will turn our local winds southerly, pulling
copious moisture from the deep tropics over the islands. A slow
moving band of downpours and a few thunderstorms is expected to
develop near Oahu and possibly Maui county tonight into Thursday,
then slowly shift westward toward Kauai by Friday. Excessive
rainfall and flash flooding will be a significant concern.
Significant flooding will be possible over Oahu over the next few
days and the threat will shift towards Kauai on Friday. Other
areas away from the main band could see spotty downpours and
localized flooding problems as well. The weather over the Aloha
State this weekend and beyond will depend highly on the later
evolution and movement of the kona low far to our northwest.

&&

.DISCUSSION...
A kona low has developed north of Kauai and will bring a multi-
day heavy rain event mainly starting this afternoon over the
western half of the state and really ramping up over Oahu tonight.
Not everywhere across the state will see the heavy rain and
flooding impacts, but in the areas that do see heavy rain,
significant flooding will be possible. 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 latest guidance is
coming into better agreement that the fire hose will be focused
mainly over Oahu.

For Oahu, numerous showers will be moving in throughout the day
today from the south with heavy rain showers and embedded
thunderstorms ramping up tonight. Confidence is increasing for
the fire hose of rain to focus over Oahu tonight throughout the
day Thursday and persisting through Thursday night. Initial
rainfall estimates for Oahu is a widespread 4 to 6 inches of rain
with isolated pockets of 8 to 10 inches through Thursday night.
The most likely area for the heaviest rainfall totals will be over
leeward and mountain areas. The time period for the heaviest
rainfall rates should be starting around midnight tonight and
persisting through Thursday with steady 1 inch per hour rates and
pockets of 2 inch per hour rates. Heavier rates will be possible
with any thunderstorms. Due to the widespread rain event, large
rises should be expected over streams. Weather conditions should
gradually improve Friday, but will still remain quite showery.
For the weekend, there is still quite a bit of uncertainty with
the rain coverage on Oahu.

For Maui County, although the current forecast is for the fire
house of rain to be focused over Oahu, a slight shift toward the
east could bring steady periods of heavy rain over parts of Maui
County. For now, we are expecting showers to continue to move in
from the south through the rest of the day today with an increase
of moderate to locally heavy showers tonight through Thursday
night. The most likely area for the heavier showers will likely be
from west Maui westward including Lanai and especially Molokai
where they will be closer to the heavy bands of rain. Breezy east
to east-southeast trades will fill in on Friday, which will bring
stable conditions and drier conditions over leeward areas. Over
on east Maui, passing showers will likely increase on Friday.

For Big Island, confidence is fairly high that the main moisture
band will remain east of the Big Island. Some spotty afternoon
showers will continue to remain possible, especially over the
slopes, but with 500 mb temperatures warming (-7C), the atmosphere
should be too stable to produce heavy rain rates. Over the Kau and
Puna district steady south winds will likely bring showery weather
to the area but intensities should remain light to moderate. A
Flood Watch still remains in effect, but we are currently looking
into possibly dropping it with the afternoon package. As far as
winds, we do expect breezy southerly winds to pick up today and
become strong and gusty on Thursday and possibly Friday. Wind
gusts of up to 40 mph will be possible across select localized
areas downwind of terrain over Puna and South Hilo districts and
areas just north of the Kona airport in the north Kona district.

For Kauai, showers will increase throughout the day today and
tonight, especially over leeward areas. As the frontal boundary
moves through this evening, shower coverage should decrease for
most areas tonight, but increase over north Kauai where periods of
moderate rain will be possible on Thursday. Although it will be
wet and showery at times through Thursday, the main heavy rain
event will likely begin on Friday and likely persist through the
weekend. There is still quite a bit of uncertainty regarding this
weekend as a vigorous shortwave trough swings through the area.
Stay tuned for more details, but there is potential for a
significant flooding event over Kauai starting from Friday through
the weekend.




&&

.AVIATION...
An active weather pattern will continue through the rest of the
week, especially as a kona low develops about 600 nm to the
NNW of Kauai. SHRA and a few TS will bring the potential for
periodic IFR/MVFR conds on and off throughout the period. The
main focus for SHRA/TS will likely be around Oahu and Kauai today
and tonight as the kona low taps into some tropical moisture to
the south and pulls it up through the western half of the state.
Moderate southerly winds are expected today with higher gusts
possible within/near SHRA/TS, then winds will become light once
again overnight.

AIRMET Sierra is in effect for all of the smaller islands and
windward Big Island for mtn obsc. Sierra may be needed for IFR
conds later this afternoon.

AIRMET Tango will go into effect at 21Z for the entire area for
mod turb btn FL300 and FL400. Tango may also been needed for turb
blw 080 as the kona low gets closer and conditions deteriorate.


&&

.MARINE...
A low developing north of the main Hawaiian Islands will push a
front toward the state from the northwest today which will advance
eastward until stalling over Oahu late Thursday and weaken. The
remnant will then drift west through the remainder of the week.
Heavy showers and isolated thunderstorms are expected along and
ahead of the front.

A series of overlapping, long period south swells will maintain
near to below average surf along south facing shores through the
near term. Renewed energy from the south will fill in late
tomorrow night, generating above average surf by Thursday. Surf
will then remain elevated through the weekend. A small medium
period northwest swell will peak today. By tonight, a short period
northwest swell associated with the developing low will spread
across the area. This swell will peak near to just below High Surf
Advisory threshold Thursday into Friday. West facing shores will
see a bump in surf through the week due to overlapping northwest
and south swells wrapping into exposed areas. Surf along east
facing shores will remain well below normal through the weekend.
&&

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


&&

$$


DISCUSSION...Kino
AVIATION...Walsh
MARINE...Powell