912
FXHW60 PHFO 150808
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1008 PM HST Tue May 14 2024

.SYNOPSIS...
A kona low will develop about 600 miles north of Kauai overnight
into Wednesday, then intensify and meander around well northwest
of the islands into next week. The kona low will turn our local
winds southerly, pulling copious moisture from the deep tropics
over the islands. A slow moving band of downpours and a few
thunderstorms is expected to develop near Oahu and Maui county
Wednesday night into Thursday, then slowly shift westward toward
Kauai by Friday. Excessive rainfall and flash flooding will
certainly be possible. Other areas away from the main band could
see spotty downpours and localized flooding problems as well. The
weather over the Aloha State this weekend and beyond will depend
highly on the later evolution and movement of the kona low far to
our northwest.

&&

.DISCUSSION...
A strong digging shortwave trough is quite clear on water vapor
imagery this evening well to the NNW of the islands, and this
feature will be the one to initiate cyclogenesis approximately 600
mi N of Kauai. Background low level southerly flow has already
started creeping our dewpoints upward, approaching or exceeding
70 at some lower-elevation stations. Both the GOES-derived and
CIMSS-MIMIC total precipitable water fields are showing a large
wedge of relatively moist air pushing northward across the area
surrounding Hawaii, with even deeper ITCZ moisture well to our S
beginning to creep N under the influence of the sharply digging
trough aloft.

In the short term, numerous disorganized showers and the odd,
brief thundershower or two are pushing northward across Oahu and
Maui county, with the Big Island on the periphery of this area and
Kauai mostly quiet so far this evening. The convection-allowing
models (CAMs) and HREF suggest that this will continue to be the
case overnight, and also showing the need to keep an eye on the
ESE flow into the Kau district of the Big Island.

As the kona low develops, the convergence band over the islands
will sharpen, and the focus for heavier spotty showers will shift
westward for a time on Wed. Although the CAMs have minor
differences in the details, there is good agreement about a fairly
solid band of heavy showers and embedded thunderstorms developing
somewhere near or over Oahu Wed night and lingering into Thu. The
HREF shows increasing probabilities for excessive rainfall rates
focusing on Oahu as an area of favorable upper level jet stream
diffluence spreads over the low level convergent band. There are
still some probs for intense rainfall on Maui county, but a subtle
westward shift in the guidance with time has been noted over the
last few HREF runs.

One other thing to note is the risk of some gusty downslope winds
on the normal-windward side of the islands. The models don`t seem
to be super-stoked about the possiblity for damaging downslope
winds, but some gusts over 30 knots appear possible on Wed into
Wed night.

The global models and CAMs show that the band may break up or
weaken a bit by Thu night as the best diffluence and stronger
shortwave energy shifts NE of the islands. Heavy showers will
still be possible but they may become more spotty and less
organized for a time before reintensification starting Friday.
One of the big questions that remains will be how far W will the
band be when it reintesifies under stronger upper level diffluence
Fri into the weekend. The NBM thunderstorm probabilities show
values increasing again near Kauai on Friday before shifting
westward to the coastal and offshore waters W of Kauai Fri night.
Global model rain rates increase quite a bit within the band this
weekend, but by then hopefully the heaviest rain is falling out
over the water. Still, the models can change on these details and
we`ll need to keep an eye on this.

The global models show that the middle and eastern islands this
weekend will have an odd combination of very moist air, with
precipitable water near 2 inches...but also a low level subsidence
inversion that may try to keep somewhat of a lid the moisture.
Nevertheless, with ESE or SE flow continuing, there should be some
fairly prolific afternoon showers in any areas shadowed by the
background flow. Kauai remains a big question mark during this
period.

There`s considerable uncertainty after the weekend, with lingering
moisture gradually diminishing and the kona low far to the north
likely weakening, leaving a surface trough behind W of the islands.
There should be a continuing trend toward more settled weather
statewide, with weak mid-level ridging eventually trying to build
in. In the meantime, there will be a daily chance of spotty heavy
showers, mainly in the afternoons and evenings.

&&

.AVIATION...

The aviation weather forecast will continue to be active through
the rest of the week, especially as a kona low develops to the
northwest of the island chain. Showers and thunderstorms will
bring the potential for periodic MVFR or lower conditions on and
off throughout the period. Southerly winds will be light overnight
before picking up tomorrow. Moderate winds are expected with
higher gusts possible within/near heavy showers or thunderstorms.

AIRMET Sierra is in effect for all the smaller islands and
windward Big Island for mtn obsc. Sierra may be needed for IFR
conds and Tango for turb as the kona low gets closer and conds
deteriorate.

&&

.MARINE...
Low pressure developing north of Kauai will usher a cold front
toward the state from the northwest this evening. The front will
advance eastward until stalling over Oahu late Thursday and
weakening. The remnant boundary will then drift west during the
remainder of the week. Heavy showers and isolated thunderstorms
are expected along and ahead of the front.

A series of overlapping, long period S swells will maintain near
to below average surf along S facing shores through the near
term. Renewed energy out of the southerly quadrant will then fill
in late tomorrow night bringing above average surf by Thursday.
Surf remains elevated in through the weekend.

A small medium period NW swell is filling in this evening and will
peak on Wednesday. By Wednesday night, a short period fresh NW
swell associated with the developing low will spread into the
area. This swell is forecast to peak near to just below the High
Surf Advisory threshold Thursday into Friday.

West facing shores see a bump in surf through the week due to
overlapping NW swell and S swells wrapping into exposed areas.
Surf along E facing shores remains well below normal through the
weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch from 6 AM HST Wednesday through Friday evening for
all Hawaii islands-

&&

$$

DISCUSSION...R Ballard
AVIATION...Farris
MARINE...JVC