666
FXHW60 PHFO 080632
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
832 PM HST Tue May 7 2024

.SYNOPSIS...
The trades will ease tonight, then hold at moderate levels 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 900
miles northwest of Kauai, while a 1032 mb high is centered 2000
miles northeast of Honolulu. Moderate to locally breezy trade
winds prevail across the state, strongest over the eastern
islands. Infrared satellite imagery shows partly to mostly cloudy
conditions, with cloud coverage the greatest over windward areas
and leeward sections of the Big Island. Radar imagery shows
scattered showers moving into windward slopes and coasts, with
mostly dry conditions in leeward areas, except for leeward Big
Island where scattered daytime heating showers persist. 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
Wednesday and Thursday, while lighter trades allow for land and
sea breeze development in leeward areas. The trades lower even
more on Friday, allowing for sea breeze development statewide. The
broad trough to appears to gradually dampen out over the weekend,
which could allow some light to moderate trade winds to build back
over the islands, but there are conflicting signals on this. 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 beginning Friday
and continuing 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 the eastern end of the state.

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...
Moderate trade winds will gradually ease to become light to
moderate tomorrow. Bands of light showers will continue to favor
windward and mauka areas, and be most active during the overnight
hours. 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...unless low clouds and showers increase in coverage across
windward areas more than expected tonight into tomorrow morning.

&&

.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 late Friday through Saturday which will
increase the chance for heavy showers and potential thunderstorms.

The current northwest swell will decrease through Wednesday
followed by a reinforcing short-period north northwest swell
Friday into the weekend. Surf along south facing shores will
remain steady through tonight. Forerunners from a larger and
longer lived south- southwest swell will arrive Wednesday, as surf
is expected to be near, or at, High Surf Advisory 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...
High Wind Warning until 6 AM HST Wednesday for Big Island
Summits.

&&

$$

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