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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 230622

Area Forecast Discussion
National Weather Service Honolulu HI
822 PM HST Mon Jul 22 2024

An area of higher moisture will move into the islands tonight
and Tuesday, bringing an increase in windward shower activity.
Moderate to breezy trade winds will continue into Tuesday, then
increase to strong speeds Tuesday night through Friday. Drier air
will settle over the region during the second half of the week.


A 1034 mb high pressure center remains far north of the main
Hawaiian Islands, with a surface trough to its south preventing
the local trades from being as strong as they typically would with
a high that strong. An earlier ASCAT pass and surface obs showed
east-northeasterly trades in the moderate to locally breezy range
statewide. Meanwhile, mid- and upper-level troughing just west of
the islands is enhancing some showers north and west of Kauai,
while sending some thin high clouds over the western end of the
state. Overall though, stable conditions have kept showers and
cloud cover limited over the islands today.

CIMSS MIMIC layer precipitable water (PW) depicts a band of higher
moisture upstream of the islands approaching from the east. This
band of 1.5 to 1.8-inch PW is visible on regional satellite imagery
approaching the eastern end of the state this evening, and will
increase windward rainfall chances tonight through Tuesday as it
moves into the state on the trades. Additionally, the leeward
slopes of the Big Island, and possibly Haleakala on Maui, will
likely be more active Tuesday afternoon. Leeward areas elsewhere
will experience a few passing showers carried over the mountains,
but remain drier overall.

Models remain consistent that drier conditions and stronger trade
winds will develop later Wednesday and Thursday as the surface
trough moves away and the high to the north builds. Expect minimal
windward rainfall and dry conditions over most leeward areas during
the second half of the week, with trades easing this weekend.


Moderate to locally breezy trades will continue and gradually
strengthen into tomorrow. Showers and low clouds will favor windward
and mountain areas. An area of enhanced moisture moving in from
the east is expected to bring an increase in shower coverage tonight
into tomorrow, mostly for windward areas. In addition, clouds and
showers will continue to build up on the Kona slopes of the Big
Island each afternoon. MVFR conditions will be possible with showers.
Otherwise, VFR should prevail.

No AIRMETs are currently in effect. However, AIRMET Sierra may be
needed later this evening as shower activity increases.


Fresh to locally strong trade winds will persist through Tuesday
due to a 1035 mb high far north of the state. A Small Craft
Advisory (SCA) remains in effect for the typical windier waters
around Maui County and the Big Island through Tuesday night. Winds
are expected to increase to strong to near Gales Wednesday through
the the end of the week. This will likely expand the SCA to
additional waters during the second half of the week. The high is
expected to weaken some over the weekend which would allow the
trades to ease back to fresh to locally strong speeds

With the current locally strong trade winds, and the increase
expected over the next few days, surf will build along east
facing shores throughout the work week. Rough and choppy surf
should peak above the summer average (5 ft) Friday into Saturday
then gradually ease early next week. Local windswell may wrap into
select north and south facing shores. The current background
south swell will continue its decline into the middle of the week.
Otherwise, no significant swells are expected this week.

Local tide gauges are still showing water levels running about a
half a foot above coastal flooding guidance at Hilo, Kahului, and
Honolulu. The Coastal Flood Statement for isolated minor flooding
remains in effect through Tuesday afternoon.


Moderate to breezy trade winds will prevail through Tuesday, with
wetter conditions expected as an area of higher moisture moves
through. However, beginning on Wednesday, trade winds will increase
as somewhat dry and stable conditions develop. This could present
some fire weather concerns Thursday and possibly Friday.


Small Craft Advisory until 6 AM HST Wednesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.



MARINE...M Ballard

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office