FXHW60 PHFO 171408
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
408 AM HST Wed Jul 17 2024

.SYNOPSIS...
Trade winds will gradually relax just a bit over the next few
days, as high pressure far to our north moves slowly northeast.
Dry and stable air over the state should limit windward showers,
which will favor the overnight and early morning periods. Isolated
afternoon showers can be expected to form on the sea breeze over
the lower slopes of leeward Big Island. A weak trough aloft moving
over the state from east to west this weekend could bring a minor
increase in showers for portions of the state. Stronger trades
are expected to return again early next week.

&&

.DISCUSSION...
Surface high 1030 mb is located about 1400 mi NW of Kauai. The
high is moving toward the ENE very slowly. A ridge extends SE from
the high, helping to maintain the pressure gradient over the state
for breezy trades. Over the next few days the high will move very
slowly NE and the ridge will weaken, while a weak, stationary
surface trough develops 600 mi NE of the islands and lingers
through Sat. This should allow the trades to gradually relax just
a bit. Starting Sunday, the trough will begin moving W and weaken,
allowing the now-stronger high to build back toward the S, far N
of the islands. This will allow locally windy trades to return
about Mon.

The 12z RAOBs showed a strong inversion remaining over the islands
from about 5500 ft at Lihue to 6500 ft at Hilo. This is due in
part to weak mid-level ridging over the islands, accompanied by
warm temperatures aloft. GOES-derived TPW shows a wedge of drier
than normal air for this time of year over the islands as well.
All of this spells generally fair weather this predawn, with just
a few mainly light windward showers.

This general cycle is expected to continue for the next few days.
Models show a weak trough at 700 mb moving in from the E over the
weekend. This will be enough to raise the inversion a bit, and
bring in some additional moisture (a small pocket of 1.5-1.8 inch
precipitable water), allowing for an increase in mainly windward
showers starting Friday night, especially for Maui and the Big
Island. There are some differences in the model guidance by Sunday
as to how much moisture will get entangled with the smaller
islands, but the multi-model ensemble mean PW does seem to suggest
some increase in showers lingering for the smaller islands into
Sunday. Not a washout weekend by any means, but additional
showers to contend with, mainly windward, and mainly during the
nights and mornings. The leeward slopes of the Big Island should
see an increase in sea breeze showers each afternoon this weekend
as well as moisture wraps around that island and gets trapped in
the leeside eddy.

Models and ensembles are in good agreement that drier and more
stable air will return briefly for Mon, then suggest another
influx of moisture is possible about Mon night or Tue. This
moisture would be associated with a surface trough, the remnant of
whatever is left of a current tropical disturbance near 17N 125W,
which is unlikely to develop further, per NHC.

&&

.AVIATION...

Breezy trades will gradually ease through the end of the week.
SHRA and low cigs will favor windward and mauka areas and could
bring brief periods of MVFR. Otherwise, VFR should prevail.

AIRMET Sierra is in effect for mtn obsc for Oahu and Molokai.
Conds should improve by mid-morning.

AIRMET Tango for mod low-level turb lee of island terrain remains
in effect and will continue through today.

&&

.MARINE...
Overnight ASCAT pass showed mostly moderate tradewinds across the
state with borderline Small Craft Advisory (SCA) winds around the
typical windier waters of Maui County and the Big Island. The
forecast remains on track with the pressure gradient weakening
around the state today and thus wind speeds easing below SCA
criteria. Thus the SCA has been cancelled. SCA conditions may
return over the weekend and into early next week as the trough
dissipated and a strong high builds north of the state.

Surf along south facing shores will remain elevated today with a
gradual downtrend through Thursday. Background south swell energy
is then expected Friday through the rest of the forecast period.

Rough, choppy surf along east facing shores begin to trend down
today through the rest of the week as the tradewind swell eases
and the small northeast swell fades.

Tiny surf along north facing shores will linger through today
from the combination of a small, medium period northwest swell and
the small northeast swell mentioned above but should return to
nearly flat summertime conditions Thursday.

&&

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

&&

$$

DISCUSSION...R Ballard
AVIATION...Walsh
MARINE...Almanza