621
FXHW60 PHFO 072004
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1004 AM HST Tue May 7 2024

.SYNOPSIS...
A low pressure system moving into the Central Pacific basin north
of Hawaii will weaken the subtropical ridge and keep light to
moderate trade winds in the forecast this week. An unstable
environment will bring the threat for flash flooding and isolated
thunderstorms late this week and possibly into the weekend as an
upper level low moves through the state.

&&

.DISCUSSION...
The trade winds have weakened a notch from yesterday and for rest
of the day today, we should see moderate trades with some
scattered showers over windward areas and the kona slopes of the
Big Island this afternoon. The trades may be light enough for
select leeward areas, where we could possibly see some isolated
showers this afternoon due to the sea breezes. On the summits of
the Big Island, we do have a High Wind Warning in effect through
this evening and wind observations are showing wind gusts of over
70 mph. The winds should taper off on Wednesday, but the High Wind
Warning will probably need to be extended through tonight.

Starting Wednesday, the trades will weaken further as a surface
trough develops north of the state. Light to moderate trades are
expected across the state with wide spread sea breezes developing
during the day time, which will allow for scattered showers over
interior and leeward areas during the afternoon.

Starting Thursday, the atmosphere will begin to change as an upper
level trough approaches the state. The surface winds will continue
to remain light with daytime sea breezes developing. Temperatures
aloft will begin to cool, which will bring instability over the
state. With the added instability and some low level moisture,
showers should increase during the afternoon on Thursday. Some of
the showers can be locally heavy and the most likely area for some
downpours looks to be over windward and interior areas.

A deep upper level trough will continue to approach the state
Thursday night with a cold cut off low developing over or very
close to the state by Friday afternoon. 500 mb temperatures on the
ECMWF is showing around -14C for Lihue, which is very unusual for
this time of year. This will produce a very unstable weather
pattern for Friday and will bring the threat for flash flooding
and isolated thunderstorms to the state. It is too early to tell
exactly where we could see the heavier showers and thunderstorms
and it will highly depend on the position of the upper level low.
Latest GFS and ECMWF has the upper level low centered around the
western half of the state Friday afternoon. This will cause the
most favorable area for thunderstorms and heavy rain to be over
the eastern half of the state. But if the upper level low was to
shift slightly more west, Oahu and Kauai could also be under the
same threat. But for now, we are looking at the possibility of
intense rain showers and isolated thunderstorms starting as early
as Friday and most likely between Friday afternoon through the
evening for the entire state. A Flood Watch will likely be issued
sometime 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 prevail across the island chain today,
then ease up a bit tonight especially over the western islands.
Low clouds and showers will continue to favor windward and mauka
areas, and be most active during the overnight hours. Brief MVFR
CIG and VIS are likely in passing showers, otherwise VFR
conditions are expected to prevail through at least early evening.

AIRMET Sierra for mountain obscuration has been canceled, but
conditions could redevelop along windward slopes late tonight or
early Wednesday.

&&

.MARINE...
A Small Craft Advisory (SCA) remains in effect for the typically
windy waters around the Big Island and Maui County through this
afternoon due to fresh to strong winds. The winds will slowly
ease today as high pressure northeast of the state moves further
away.

A surface trough will form north of the state Wednesday. As this
feature drifts southward near or over the islands Thursday into
Saturday, the trade winds will weaken further, and chances for
heavy showers will increase through the weekend. An upper low will
move across the island chain Friday through Sunday and bring a
slight chance of thunderstorms.

Near shore buoys are showing signs of fading energy in the 12 to
14 sec range suggesting the current south swell is slowly
beginning to fade today into Wednesday. 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
gradually decline through the weekend.

A small northwest swell of around 3 feet will peak today, then
decline Wednesday. Wind driven waves will gradually decline to
around May average today, then drop to below average for the rest
of the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 6 PM HST this evening for Big Island
Summits.

Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.


&&

$$


DISCUSSION...Kino
AVIATION...Bedal
MARINE...Foster