917
FXHW60 PHFO 140736
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
936 PM HST Mon May 13 2024

.SYNOPSIS...
A lingering upper level low northeast of the islands is keeping
the local airmass relatively moist and unstable. Spotty, hit-or-
miss heavy downpours will continue to be possible over the next
day or so. A late season kona low will develop several hundred
miles north of the islands Tuesday night, and then meander around
well northwest of the islands the rest of the week. The developing
kona low will turn our winds southerly on Tuesday, and start to
tap into deep tropical moisture lurking south of the islands.
This will cause a slow-moving band of showers, some heavy, and a
few embedded thunderstorms over some parts of the state for the
second half of the week. Excessive rainfall and flash flooding
will continue to be a concern.

&&

.DISCUSSION...
Water vapor imagery shows the old upper low that`s been giving us
all the active weather the last few days continues to sit and spin
about 300 miles NE of Honolulu, with a weak trough extending to
the west. Both CIMSS MIMIC TPW and GOES derived TPW show a large
area of relatively moist air with values around 1.5 inches moving
across the islands. Although the mid-level temperatures are
nowhere near as cold as they were last weekend, the additional
moisture and the presence of the nearby upper low is enough for
about 700 J/kg effective layer CAPE, and the weak SE flow is
helping to keep spotty heavy showers slow moving and, at times,
anchored to terrain. This general pattern should continue into
Tue.

The model guidance for this week looks anything typical of mid-
May for Hawaii, to say the least. Another strong shortwave digs
into the mean longwave trough position near the islands to
generate a kona low about 500 mi N of Kauai (near 30N 160W) Tue
night. This low is then forecast to strengthen and meander around
several hundred miles NW of the islands for the rest of this week.
Our low level ESE flow currently will to veer around to southerly
on Tue, and remain that way through at least Fri. A large reservoir
of deep, moist, tropical air with TPW of greater than 2 inches is
lurking about 400 mi or so SW of the islands, and the models bring
some of this moisture northward along a developing convergence
band over the middle main islands Wed night. After that, the
models slowly take the very moist convergence band slowly
westward, with another surge of very high TPW air riding up the
boundary as we approach the weekend. The GFS has this band mostly
west of Kauai by Sat, while the latest ECMWF is a little slower.
The eastern end of the state should be not nearly as active,
epecially once the band starts moving westward.

I`m a little surprised that deterministic model QPFs are not
higher with this band, and a bit perplexed as to why that would
be. This seems like the type of pattern that would be very
favorable for heavy and possibly excessive rainfall. Nevertheless,
the probabilistic QPFs from the NBM show some very impressive
rainfall rates which does indeed suggest that flash flooding is ia
major concern for the latter half of the week, especially given
the already wet antecedent conditions. It is possible that by the
weekend, one or more active thunderstorm complexes could develop
south of the islands, which might impede some of the incoming
moisture and help to locally stabilize the airmass over us, but
don`t want to give anyone false hope about what appears to be a
low-chance possibility.

&&

.AVIATION...
Low VFR to MVFR prevail as showers remain active over the area.
The light gradient will allow land breezes to develop overnight
which will bring clearing to most areas. However, showers have
developed south of Molokai through Kauai and are advancing slowly
toward those islands this evening. Winds will veer to southerly as
the night progresses, and that may provide enough of a push to
bring periods of heavy showers to those islands, including to
PHNL. Showers will redevelop again late tomorrow morning into
tomorrow afternoon.

AIRMET Sierra remains in effect for all islands.

&&

.MARINE...
Light to moderate east southeast winds will begin to veer out the
south Tuesday as an area of low pressure develops 450 nm north of
the state. A frontal boundary will develop and move over the
western islands early evening Wednesday then stall over the
central islands Thursday. The front will then weaken into a trough
and shift westward Friday. Scattered heavy showers are possible
through the week with the threat of thunderstorms developing
Wednesday and persisting into the weekend.

Surf along south facing shores will remain small overnight. A
series of overlapping, long-period south swells will begin to fill
in Tuesday afternoon and persist through the weekend. The first
set of southerly swells will begin to fill in Tuesday peaking near
average Wednesday. A second set of will fill in late Wednesday and
could bring surf up to above average Thursday. Surf should remain
elevated through the weekend.

A small, long-period northwest swell will slowly decline through
Tuesday. A new small, medium period north-northwest swell will
fill in late Tuesday, peaking Wednesday. The the low mentioned
above develops and strengthens as it nears Hawaii, this will
send a moderate, medium period northwest swell Wednesday night,
peaking Thursday. This could bring moderate sized surf along
north facing shores Wednesday night through Friday. The swell
height may need to be adjusted up later due to model guidance
consistently under representing swell heights from sources at
close proximity to the islands. We will continue to monitor all
available guidance to assess the potential for surf to reach High
Surf Advisory criteria Thursday and early Friday if the swell
height does come in higher.

West facing shores will see an uptick in surf through the week due
to the overlapping northwest and south swells wrapping into some
exposed areas. Surf along east facing shores will remain well
below normal levels through the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch from Wednesday morning through Friday evening for
all Hawaii islands-

&&

$$

DISCUSSION...R Ballard
AVIATION...JVC
MARINE...Almanza