924
FXHW60 PHFO 080206
AFDHFO

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

.SYNOPSIS...
Trade winds will continue to weaken tonight through Thursday as a
trough approaches the state. An unusually strong upper level low
for this time of year will produce an unstable weather pattern
late Thursday through around Saturday. Even though a wide spread
heavy rain event is currently not expected, showers that do
develop will bring the threat of flash flooding and isolated
thunderstorms especially Friday afternoon through the evening.
Weather should gradually improve from west to east this weekend,
but some unsettled weather will likely linger over the eastern
half of the state on Saturday.

&&

.DISCUSSION...
In the short term, the trade winds will continue to weaken further
tonight into Wednesday with sea breezes expected over leeward
areas by Wednesday afternoon. This will allow for scattered
afternoon showers to develop tomorrow over interior and leeward
areas. On the summits of the Big Island, wind observations
continue to show wind gusts over 70 mph, thus the High Wind
Warning has been extended through tonight. Winds should taper off
by Wednesday but still may remain breezy at the summits throughout
the day.

Starting Thursday, the atmosphere will begin to change
as an upper level low approaches the state. The surface winds
will likely weaken further as a surface trough begins to develop
over the state. Temperatures aloft will begin to cool, which will
bring instability over the state. With the added instability,
some low level moisture, and sea breeze convergence, showers
should increase during the afternoon on Thursday over interior and
mountain areas. Due to light winds in the lower half of the
atmosphere, any showers that develop will be slow moving and will
be capable of producing some downpours in the afternoon.

Although some locally heavy showers will be possible Thursday
afternoon, the main concerning period will be Friday through
Saturday. An unusually cold upper level low will approach the
state Thursday night and is expected to be over or very close to
the state by Friday afternoon. 500 mb temperatures on the ECMWF
and GFS is showing around -14C to -15C for Lihue, which is very
unusual for this time of year. This will produce a very unstable
weather pattern for Friday and will allow for any showers that do
develop to become a heavy rain threat. Also, with temperatures
that cold aloft, we will likely see isolated thunderstorms with
some of the stronger storms capable of producing small hail. At
this time, it is too early to tell exactly where the heavier
showers and thunderstorms will develop, and it will highly depend
on the position of the upper level low. Latest GFS and ECMWF has
the upper level low centered just north of Kauai Friday
afternoon. This will cause the most favorable area for
thunderstorms and heavy rain to be from Oahu eastward. But if the
upper level low was to shift slightly more west, the entire state
could see the threat of heavy rain and isolated thunderstorms.
Although the details are still uncertain, the main takeaway is
that we are not expecting a widespread rain event. But, any
showers that do develop on Friday will be capable of producing
intense rainfall rates. With the potential for intense rainfall
rates and the slow movement of any showers or thunderstorms, a
Flood Watch could be issued in the coming days.

For the weekend, there is still quite a bit of uncertainty on how
quickly the upper level low will move through the state (from
west to east) with the latest global models trending slower.
Although the models have trended slower, there is fairly good
agreement with the upper level low being east of Kauai by
Saturday. The threat for flash flooding and isolated thunderstorms
should decrease from west to east as the upper level low slowly
progresses eastward. At this time, the eastern half of the state,
sees the greatest chance for some unsettled weather this weekend.


&&

.AVIATION...
Moderate trade winds will gradually ease over the next twenty-four
to thirty-six hours. Bands of light showers off the Pacific will
continue to favor windward and mauka areas, and be most active
during the overnight hours. Isolated moderate showers are likely over
island interiors this evening. Brief MVFR CIG and VIS are to be
expected in passing showers, otherwise VFR conditions will prevail
statewide.

&&

.MARINE...
Trade winds will weaken over the next several days as high
pressure northeast of the state moves further away and a surface
trough builds over the island chain. Chances for heavy showers
will increase Thursday through the weekend. An upper low will move
across the island chain Friday through Sunday and also bring a
slight chance of thunderstorms.

The current northwest swell will decrease tonight and Wednesday.
followed by a reinforcing a short-period north northwest swell
Friday into the weekend. Forerunners from a larger and longer
lived south-southwest swell will arrive Wednesday, and surf is
expected rise near, or to, the High Surf Advisory level during the
peak of the swell Thursday and Friday. This swell will decline
through 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...Kino
AVIATION...Bedal
MARINE...Foster