612 
FXHW60 PHFO 131339
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
339 AM HST Sat Apr 13 2024

.SYNOPSIS...
A low pressure system northwest of the Hawaii and a surface 
frontal trough moving into the state from the northwest will 
produce southerly winds and periods of heavy showers and 
thunderstorms for the western islands. The initial threat for 
flash flooding with this system is highest for the islands of 
Kauai and Niihau from later tonight through Sunday night. These 
showers will then decrease in intensity as the front weakens and
moves across the eastern half of the state from Monday through 
Tuesday. Improving weather trends will develop after the frontal 
trough passes through each island. Light to moderate trade winds 
will fill in across the Hawaii region from Wednesday onward with 
drying trends forecast for most areas.

&&

.DISCUSSION...
This mornings satellite imagery shows a low pressure center 
lingering 700 miles northwest of Kauai and an associated surface 
frontal trough roughly 100 miles northwest of Barking Sands on 
Kauai. The surface front and a passing upper level trough will 
produce periods of significant rainfall over the western half of 
the state over the next few days. Satellite imagery this morning 
also shows a subtropical jet stream over the island chain 
producing some high elevation cirrus clouds over Maui and the Big
Island. Snow showers over the summits of the Big Island have faded
and the Winter Weather Advisory was cancelled this morning.

Little has changed in the upcoming weather pattern over the past
12 hours. Threats for heavy rain, thunderstorms and flash 
flooding will continue mainly over the western half of the state. 
The latest forecast guidance continues to show southerly winds 
developing ahead of the front with good chances for heavy rainfall
and thunderstorms from late tonight through Sunday over Kauai and
Niihau. A Flood Watch was issued for Kauai and Niihau this 
morning starting late tonight and lasting through early Monday 
morning. Heavy rainfall and thunderstorms will then spread 
eastward along the surface front to Oahu from Sunday afternoon 
through Monday morning. The Flood Watch may be need to be extended
in coverage to include the island of Oahu at some point as the 
forecast time period grows closer. The surface front will continue
to drift eastward and weaken spreading rainfall to all islands in
Maui County from Sunday evening through Monday afternoon. Wet 
weather conditions spread to the Big Island from Monday afternoon 
into Tuesday.

Rainfall amounts will be highest over the western half of the
state from Kauai to Molokai, with decreasing rainfall totals over
Maui and the Big Island as the upper level trough weakens as it
passes the eastern islands. This means the highest threat for
heavy rain with thunderstorms will focus initially on Kauai later
tonight, and then spread to Oahu by Sunday afternoon. Our storm
total rainfall forecast ranges from 3 to 6 inches on Kauai, 2 to 
5 inches on Oahu, 2 to 4 inches on Molokai, with lesser amounts 
forecast for Lanai, Maui and the Big Island. Understand that these
rainfall totals will highly depend on less predictable smaller 
scale heavy rain band and thunderstorm formation areas, which may
increase these island by island storm total rainfall estimates. 

Wet weather conditions will quickly improve from west to east 
next week as the surface trough passes through each island and
diminishes near the Big Island. High pressure will begin to 
filter in north of the state as the low moves northeast away from 
the island chain. Expect a return to trade winds in the light to
moderate range from late Wednesday onward.

&&

.AVIATION...
Light to moderate south to southeast flow will allow for daytime
sea breezes in overnight land breezes in many areas. A few showers
will be possible over south and southeast facing slopes and 
coasts this morning, with a few showers then developing over
interior and windward areas in the afternoon. Showers will likely
pick up over the western islands tonight as a front approaches
from the west. 

AIRMET Sierra remains in effect for mountain over south and
southeast facing sections of Kauai. Conditions should improve here
later this morning. 

AIRMET Tango remains in effect for moderate upper level turbulence
between FL180 and FL380. These conditions will likely hold through
much of the day. 

&&

.MARINE...
Light to moderate southerly winds will prevail over the western 
islands today, while moderate to fresh southeast winds hold over 
the eastern end of the state. A deep low pressure system to the
distant north, will track southeast with the attendant cold front
approaching the western end of the state tonight into Sunday, 
ramping up the southeast to south winds into the fresh to strong 
range. The front will slowly push southeast through the islands 
late Sunday through Monday, before stalling out and slowly 
dissipating near the Big Island through Wednesday. Moderate to 
fresh south winds will prevail through Monday ahead of the front, 
with light to moderate west to northwest winds filling in behind 
it. Winds will become rather light by Monday night and continue 
through next Wednesday, with moderate trades potentially returning 
for the latter part of next week.

North shore surf will remain small through the weekend. Surf will
trend up Sunday night and Monday as a small long-period north- 
northwest swell coincides with a more significant pulse of medium-
period northwest swell. Surf could reach low end advisory levels 
Monday night and Tuesday, with the swell then on the decline 
Wednesday through late next week.

Surf along east facing shores will decline over the weekend, with
only small very small surf expected through the upcoming work 
week.

A small, long-period south-southwest swell will hold through the 
weekend. A new small, but slightly larger, south swell will give 
south shore surf a nice boost Tuesday and Wednesday, with another 
southwest swell giving surf another bump for late next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch from late tonight through late Sunday night for 
Niihau-Kauai-

&&

$$

DISCUSSION...Bohlin
AVIATION...Jelsema
MARINE...Jelsema