612
FXHW60 PHFO 141412
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
412 AM HST Tue May 14 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
couple of days. 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
kona low will turn our winds southerly as early as Tuesday, and
eventually 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 continues to show an upper level low located
approximately 350 mi NE of Honolulu. This feature, along with a
relatively large area of relatively moist air (TPW 1.6-1.7 in the
predawn balloon soundings) is helping to enhance showers around
the islands. Winds at the surface are mostly light land breezes
early this morning but become S-SE in the lower levels just off
the surface, which is pulling showers from the downwind Big
Island convergence zone northward Oahu and Maui county.

The various convection-allowing-models all have their own ideas
about the details of the next couple of days, but the overall
theme seems to be that the general unsettled weather pattern will
continue through Wed, with clouds and showers being drawn up
northward over the smaller islands, as well as the possibility for
some afternoon convective development over the interiors and
north shores (where enough morning sunshine might allow).

The very unusual weather pattern for mid-May continues for latter
half of the week.  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. 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 focus some of this moisture northward along a developing
convergence band over Oahu and Maui county 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.

Although the model QPFs are not incredibly impressive, this
general pattern, with a relatively slow moving band of moisture
convergence over the islands could lead to locally excessive
rainfall. Flash flooding will be a concern, especially given the
wet antecedent conditions from recent heavy rainfall. Have nudged
QPF toward the highest NBM percentile values for Wed night into
Thu, where confidence is highest for heavy rain timing and
placement in the near term.

As the band of convergence shifts westward toward the weekend,
even more copious moisture (TPW near or possibly even in excess of
2 inches) will be pulled northward toward Kauai and Oahu. A
shortwave aloft rotating around the kona low and favorable jet
diffluence along may help to enhance rainfall even more. This
scenario is less confident, however, since the band may slow to a
crawl and intensify mostly west of the Garden Isle over the
weekend. Also, there is the possibility for one or more
thunderstorm complexes to develop SW of the islands that might rob
us of some moisture and instability. Nevertheless, we want to
stress that this is the type of pattern that can lead to some
pretty serious flooding. We will continue to monitor the situation
and expect that details will become more clear as we get closer.

It is worth noting that as all the action shifts west, the Big
Island and perhaps Maui county will be under increasing subsidence
and warming aloft, with some dry air possibly trying to make
inroads from the E, and a mid- level ridge located E of the
islands. So the weather there by the weekend may end up being
drier, with locally breezy E or ESE winds thanks to a nearly
stationary surface high far to the NE of the Big Island.

&&

.AVIATION...

VFR prevails this morning as land breezes have scoured out low
clouds and convective debris from yesterday afternoon and evening
lingers as sct-bkn cigs around 10kft. Showers have largely
refocused offshore where they will remain for the remainder of the
early morning, save for a few locales along the immediate coast,
before refocusing over island interiors again late this morning
into this afternoon. Light to moderate southeasterlies will veer
to southerly during this time.

No AIRMETs in effect.

&&

.MARINE...

Light to moderate east southeast to south winds will persist today
into Wednesday as an area of low pressure develops 450 nm north
of the state. A frontal boundary extending from the low will move
over the western islands early evening Wednesday then stall near
or 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.

A series of overlapping, long-period south swells will begin to
fill in this afternoon, keeping surf along south facing shores
near to just below average. A second set of southerly swells will
fill in late Wednesday and could bring above average surf
Thursday. Surf should remain elevated through the weekend.

A small, medium-period northwest swell will slowly decline today.
A new small, medium period north-northwest swell will fill in
later today, and peak Wednesday. As the low mentioned above
develops near Hawaii, this source may send a moderate, short
period northwest swell Wednesday night, peaking near to just below
advisory levels Thursday into Friday. Model guidance tends to
under-forecast swell heights with sources developing at a close
proximity. Therefore, there is a potential for surf to reach High
Surf Advisory criteria and will continue to monitor all available
guidance.

West facing shores will see a bump 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