538
FXHW60 PHFO 081300
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
300 AM HST Wed May 8 2024

.SYNOPSIS...
Moderate trades will hold over the eastern islands through
Thursday, while lighter trades and leeward land and sea breezes
prevail over the western islands. Showers will favor windward
slopes and coasts at night and interior and leeward areas each
afternoon. More unsettled weather with thunderstorms and localized
downpours appear in store as a disturbance aloft moves
southeastward across the islands Friday and Saturday. Conditions
should improve Sunday through early next week, as the most
unsettled weather pushes east of the state.

&&

.DISCUSSION...
Currently at the surface, a cold front is located around 825
miles northwest of Kauai, while a 1032 mb high is centered 2200
miles northeast of Honolulu. The trade winds have eased to
moderate levels overnight as the gradient has relaxed a bit.
Infrared satellite imagery shows mostly cloudy conditions in most
windward areas and partly cloudy skies in most leeward locales.
Radar imagery shows scattered showers moving into windward slopes
and coasts, with mostly dry conditions in leeward areas. Main
forecast concern continues to revolve around the potential for
some active and unsettled weather Friday and Saturday.

The cold front to the distant northwest will shift southeastward
and closer to the islands during the next couple of days,
eventually weakening into a broad trough over the islands on
Friday. As a result, the trades will ease during the next few
days. Moderate trade winds will hold over the eastern islands
today and Thursday, while lighter trades over the western end of
the state allow for land and sea breeze development in leeward
areas. The trades ease further on Friday, allowing for sea breeze
development statewide. The broad trough then appears to gradually
dampen out over the weekend, which could allow some light to
moderate trade winds to build back over the islands. By early
next week, a new approaching cold front will begin to shift the
low level boundary layer flow around to the southeast, maintaining
moderate easterly trades over the eastern islands, while land and
sea breezes become more dominant over the western end of the
state.

As for the remaining weather details, a hybrid trade wind pattern
should prevail through Thursday. Showers will favor windward and
mauka areas overnight and during the morning, then transition over
to interior and leeward areas each afternoon into the early
evening. As the boundary layer flow becomes light and variable
Thursday night, most of the shower activity should remain offshore
or close to the islands coasts.

The weather pattern becomes much more interesting Friday through
Saturday as an upper level low swings southeastward across the
island chain, with broad surface troughing holding over the state.
Precipitable water values don`t appear all that high, generally
holding at or below 1.5 inches over the smaller islands, while
reaching as high as 1.7 inches over southeastern sections of the
Big Island. As a result, a widespread heavy rain threat doesn`t
appear all that high, although some downpours will be possible
that could bring some minor flooding impacts. We will continue to
monitor the latest model trends as the need for a Flood Watch is
still not out of the question, particularly over Maui County and
the Big Island.

The more concerning threat revolves around the potential for
severe thunderstorms. Mid-level 700-500 mb lapse rates reach 7 to
7.5 C/KM as the upper level low pivots southeastward through the
islands, while surface based CAPE values increase to between 1000
and 1500 J/KG, and 0-1 KM MUCAPE values reach as high as 2000
J/KG. This in combination with 0-6 KM Bulk Shear values of 25 to
40 knots could lead to some stronger thunderstorms capable of
producing hail and strong gusty winds. The one element that seems
to be holding back the potential for well organized convection is
the lack of a significant surface trigger aside from sea breezes.
Nevertheless, we will need to keep a close eye on how this
pattern evolves during the next couple days.

The most unstable airmass slides east of the state Saturday night,
which should bring a return of a hybrid trade wind pattern
featuring windward and mauka showers at night and interior and
leeward showers during the afternoon and early evening hours. This
pattern appears to generally hold through early next week.

&&

.AVIATION...
Light to moderate trades are expected to persist through
tomorrow. Bands of light showers will continue to favor windward
and mauka areas, and be most active during the overnight hours.
With the lighter flow, sea breezes will develop across wind-
protected leeward areas, bringing an increase in interior clouds
and isolated showers by the afternoon. Land breezes may also help
to clear out cloud cover across leeward areas overnight. Brief
MVFR conditions are to be expected in passing showers, otherwise
VFR conditions will prevail statewide.

No AIRMETs are in effect at this time, nor are any expected to be
needed today.

&&

.MARINE...
Moderate to fresh trade winds will weaken over the next several
days as high pressure northeast of the state moves further away
and a front approaches the islands from the northwest. The front
will weaken into a trough over the islands Friday as wind speeds
look to ease and become light to gentle Friday and Saturday with
southeast flow over the eastern half and northeast to east flow
over the western half of the state. Gentle to locally moderate
east to southeast flow looks to trend into the early half of next
week across the coastal waters. An upper level low will move
across the island chain Friday through Saturday which will
increase the chance for heavy showers and potential thunderstorms.

The current northwest swell will continue to decrease to tiny
conditions through Thursday followed by a reinforcing short-
period north northwest swell Friday into the weekend. Surf along
south facing shores will remain steady through today. Forerunners
from a larger and longer lived south- southwest swell are subject
to arrive later this afternoon. Surf is trending below High Surf
Advisory (HSA) criteria but if nearshore buoy observations come
in above guidance, surf could reach HSA criteria when the swell
peaks Thursday into Friday before declining into the weekend. Surf
along east facing shores should remain small as winds remain
light through the rest of the week.

&&

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

&&

$$

DISCUSSION...Jelsema
AVIATION...Farris
MARINE...Shigesato