542
FXHW60 PHFO 132015
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1015 AM HST Mon May 13 2024

.SYNOPSIS...
Light southeasterly winds, afternoon sea breezes and weak
troughing aloft will produce brief periods of slow moving showers
over most islands through Tuesday. Some of these showers may
become heavy at times especially along the eastern slopes of Oahu
and Maui, and the southeastern slopes of the Big Island. On
Wednesday the weather pattern trends more unstable as southerly
winds develop over the state in response to a kona low approaching
the islands from the north. Deep unstable tropical moisture will
move over all islands from the south, resulting in periods of
heavy rainfall, thunderstorms and strong gusty winds. The threats
from flooding and thunderstorms will be elevated for all islands
from Wednesday into the weekend.

&&

.DISCUSSION...
An upper level low continues to linger north of the state and
continues to produce unstable conditions over the state. A surface
trough over the western end of the state will bring light east to
southeast winds today and light southeast to south winds on
Tuesday. Overall the weather pattern looks fairly similar today
and tomorrow with heavy showers possible each afternoon due to the
sea breeze convergence. The most likely area for heavy rain will
be over interior and mountain areas. With steep low level lapse
rates and ample moisture (precipitable water values of 1.58
inches) as seen on the 12Z sounding from Lihue should allow for
slow moving heavy downpours today. Currently, heavy showers are
already ongoing across windward Oahu, where we have a Flash Flood
Warning and we are seeing heavy showers starting to develop over
other parts of the state. With the heaviest rain expected over
interior and mountain areas, please remember that flash flooding
can occur well downstream from the mountains in areas where it may
not be even raining. Although the threat for heavy rain will
continue to exist the next few days, a more widespread rain event
is expected during the second half of the week.

A strong kona low will develop north of the state and its
associated frontal boundary will heavily impact the state during
the second half of the week. The frontal boundary will move into
the state Tuesday night into Wednesday with increasing chances of
heavy rain as deep tropical moisture moves in from the south. By
Wednesday night, precipitable water values will likely increase
over 1.8 inches across parts of the state and the frontal boundary
is expected to stall somewhere around Oahu and Maui County based
on the latest guidance. Where this moisture boundary stalls (late
Wednesday through Friday), will be the area where there will be
significant potential for flash flooding.

Several threats are likely in this weather pattern from Wednesday
onward including heavy rain with flooding, thunderstorms in the
strong to severe range, and strong gusty winds. The severe
thunderstorm threat will favor the western islands closer to the
approaching surface front/trough, the upper level cold pool and
the strongest wind shear. The forecast guidance over the past few
days suggests these threats will last for several days from
Wednesday through Friday with even deeper moisture moving in from
the south on Saturday. Latest guidance is showing deep tropical
moisture with precipitable water values of over 2 inches moving
into the western half of the state on Saturday. This will increase
the threat of flash flooding, especially over Kauai and Oahu on
Saturday. Long story short, this is a strong kona low event that
will last for many days (starting Wednesday and persisting through
the weekend) and will likely produce record rainfall in many
areas for the month of May and will impact many people. Please
stay tuned to the latest forecast as we fine tune the details.


&&

.AVIATION...
Lingering instability aloft combined with a moist southeasterly
flow will allow for a few SHRA across the state through the
forecast period. Winds will remain light enough to allow for a
land/sea breeze pattern where clouds and SHRA will build up over
island interiors during the afternoon/early evening hours and
clear out overnight. MVFR conds are expected in heavier SHRA,
with brief IFR conds possible.

AIRMET Sierra for mtn obsc is in effect over east facing areas of
Oahu and Kauai. AIRMET Sierra for IFR conds may also be needed
for the southeastern slopes of the Big Island later today as
showers periodically become heavy.

&&

.MARINE...The air mass over the state will remain moist and
unstable, creating a favorable environment for locally heavy
showers and possible thunderstorms. Light to locally moderate
southeasterly flow will hold over the next twenty-four hours.
Winds will shift to the south Tuesday night when a new cold front
will approach the islands from the northwest. The front will move
into the western islands Wednesday night and Thursday, then weaken
into a trough and shift westward Friday. Light to moderate
northwest winds may briefly develop behind the front over the far
western end of the state, otherwise moderate southerly winds
should prevail into the weekend.

Surf along south facing shores will slowly decline today and
tonight. A series of overlapping, long-period south swells will
then move through the islands Tuesday into the weekend. This will
be a long lived south shore event, with multiple overlapping
pulses. Surf may approach High Surf Advisory criteria on
Thursday.

A small, long-period northwest swell peaked early this morning will
slowly decline through tomorrow. A new northwest swell will then
arrive on Wednesday. This will bring moderate sized surf moving
into north facing shores Wednesday night through Friday.

Surf along east facing shores will remain well below normal
levels through the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$


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