581
FXHW60 PHFO 010157
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
357 PM HST Fri May 31 2024

.SYNOPSIS...
Surface high pressure positioned far northeast of Hawaii will
maintain a tight pressure gradient across Central Pacific. This
scenario will generally support breezy to locally windy trade
winds through early next week. Showers moving in on trade flow
will favor windward areas and likely become more prevalent during
the nocturnal hours. An upper trough developing over the islands
this weekend will be the impetus to increased shower frequency and
areal rain coverage.


&&

.DISCUSSION...
The synoptic set up across the Central Pacific has changed very
little these past few days. This being of a near 1030 mb high
centered approximately 1,000 miles from Oahu with a pesky surface
trough about 1,100 miles west of Kauai. The tight pressure gradient
between these two features will hold primarily breezy regional
trade winds in place over the next several days. The surface high
will gradually settle south as the trough continues to weaken and
wash out by early Saturday morning. The high`s subtle shift to
the south will slightly tighten the pressure gradient over and
upstream of the islands enough to provide a minor uptick in weekend
trade wind magnitudes. Recent local soundings have been evidence
to the resident dry air mass. This morning`s precipitable water
values registered in the lower 10% percentile range for late May
climatology. Enhanced stability is also noted in satellite, radar
and afternoon soundings as a 6 to 8k ft foot subsidence inversion
pins down a relatively dry boundary layer. The only other near
term mentionable change will occur with increasing mid level
moisture this weekend. This will likely lead to more frequent
windward showers that will expand across a larger area. Better
organized windward showers will hold on and occasional spill over
into leeward locales.

Heights will lower over the state into the weekend. The extended
NWP guidance is still in disagreement on how to handle these
height falls...whether it be from broad trough passage north of
the state (EC) or take the form of a weak low settling in over
the islands (GFS). The emergence of the subtropical jet over or
just south of the state this weekend may offer more argument to a
wetter commencement of June, but this is very dependent upon its
latitudinal position. While there will be some eastern moisture
advection, the mid levels of the atmosphere will remain fairly dry
until Tuesday and this may weigh forecast philosophy toward more
dry. Regardless of any minor model nuances, there continues to be
moderate confidence that next week`s upper troughing scenario will
increase areal shower coverage and intensity...especially going
into the middle of next week due to the increased lower and mid
layer moisture. Trades may become disrupted late next week in
response to the passage of a surface trough to the north. This
will place the islands in more of a east-southeast flow pattern.
If this does occur, the end of week weather pattern will
transition to weaker winds with higher humidity.



&&

.AVIATION...
Breezy to locally strong easterly trade winds are expected to
increase slightly this weekend, as a surface high moves closer
to the state from the northeast. Clouds and showers will continue
to favor windward slopes with periodic MVFR conditions possible,
especially during the overnight to early morning hours when an
uptick in shower activity is expected. Otherwise and elsewhere,
VFR conditions will prevail.

Winds of 20 to 30 kts are expected to persist below an inversion
between 5000 and 7000 feet. These conditions justify continuance
of AIRMET Tango for tempo moderate low-level turbulence south
through west of terrain. In addition, strong jet stream winds
passing over Hawaii today through Friday warrant AIRMET Tango for
tempo moderate turbulence aloft between FL300-FL400.

AIRMET Sierra may be needed for mountain obscuration later tonight
as shower activity picks up.


&&

.MARINE...
A 1030 mb surface high centered 1100 nm NE of the islands will
change little as it sags slowly S over the next couple of days,
resulting in a slight increase in ENE trade wind speeds. Fresh to
strong trade winds will prevail into early next week, and an
existing Small Craft Advisory (SCA) for the windier zones around
Maui and the Big Island has been expanded to include most zones.
Long-range guidance indicates a trend toward diminishing and
veering winds around the middle of next week as the ridge to the N
is weakened by a passing front.

There will be an extended run of elevated surf along S facing
shores over the next week (and likely longer), with the largest
of the swells likely to produce surf near High Surf Advisory
(HSA) levels of 10 feet. The first of several pulses of long-
period SSW swell will gradually build tonight, and peak near HSA
heights over the weekend. Latest buoy observations indicate
continued modest energy increases in the longer periods,
resulting in inconsistent and small sets at the moment.
Additional pulses of swell arriving early next week will persist
through the middle of the week, with the potential for a larger
swell by the following weekend.

A strong, slow-moving low in the distant NW Pacific has generated
a small NW swell that will arrive Monday and Tuesday, with the
potential for a small follow-up NW swell generated by the extra-
tropical version of former west Pacific Typhoon Ewiniar. Trade
winds will continue to generate short-period wind waves that will
support choppy surf along exposed E facing shores.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Saturday for Kauai Northwest
Waters-Kauai Leeward Waters-Kauai Channel-Oahu Windward Waters-
Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters.

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


&&

$$


DISCUSSION...Blood
AVIATION...TS
MARINE...Birchard