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000
FXHW60 PHFO 130656
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
856 PM HST Fri Apr 12 2024

.SYNOPSIS...
A low pressure system northwest of the Hawaiian Islands and a
surface frontal trough will continue to produce periods of wet
weather for the western islands later this weekend and then
spreading across the state into early next week. Heavy rain and
thunderstorms are possible with this system elevating the threat
for flash flooding over some islands. Improving weather trends
will spread rapidly from west to east from Tuesday into Wednesday
as this low pressure system swiftly moves northeast away from the
island chain. Light to moderate trade winds will fill in across
the Hawaii region from Wednesday onward with drying trends for
most areas.

&&

.DISCUSSION...
A strong low pressure center currently 700 miles northwest of
Kauai and an associated surface trough 100 miles northwest of
Barking Sands on Kauai will continue to produce periods of
significant rainfall over the western half of the state over the
next few days. Satellite imagery this evening also shows a
subtropical jet stream over the island chain producing some high
elevation snow showers over the summits of Mauna Loa and Mauna
Kea on the Big Island. A Winter Weather Advisory remains in effect
through early Saturday morning for the highest summits of both
mountains. Snow showers at the summits will likely diminish in the
early morning hours.

An upper level trough and a surface front associated with the low
pressure system northwest of the islands will combine forces over
the next few days and increase the threats for heavy rain,
thunderstorms and flash flooding over the western half of the
state. The latest forecast guidance continues to show good chances
for heavy rainfall and thunderstorms from late Saturday night
through Sunday over Kauai and Niihau. Heavy rainfall and
thunderstorms will spread eastward along the surface front to
Oahu from Sunday afternoon through Monday morning. The surface
front will continue to drift eastward spreading heavy rainfall to
the 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 by
Saturday night and then spread to Oahu by Sunday afternoon.
Our forecast rainfall estimates range 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 amounts will highly depend on smaller scale heavy rain
band and thunderstorm formation which can change these island by
island storm total rainfall estimates. Given these rainfall
amounts and potential for thunderstorms a Flood Watch issuance is
likely to start over Kauai by Saturday night and then possibly
extend over Oahu on Sunday.

Wet weather conditions will quickly improve from west to east
next week as the surface trough passes through each island. High
pressure will begin to filter in north of the state as the low
moves northeast away from the island chain. Expect light to
moderate trade wind returning from late Wednesday onward.

&&

.AVIATION...
S to SE winds will gradually trend down over the next couple of
days. Low cigs and SHRA will continue to be carried into E
through SE facing areas of the islands through Saturday, with
brief periods of MVFR conds amid prevailing VFR. A slight chance
of TS remains through Saturday for the far western edge of the
Kauai coastal waters.

AIRMET Sierra remains in effect for mtn obsc over E thru SE
facing areas of the Big Island, as low cigs/vsbys and SHRA
continue.

AIRMET Tango remains in effect for tempo mod turb between 080 and
FL180.

AIRMET Zulu remains in effect for light icing from Oahu through
Big Island, as layered mid- and upper-level clouds continue to
stream up from the southwest.

&&

.MARINE...
Moderate to fresh southeast to south winds will ease overnight as
a deep surface low to the distant northwest drifts northeastward
and further away from the island chain. Light to moderate
southerly winds will prevail over the western islands Saturday,
while moderate to fresh southeast winds hold over the eastern end
of the state. The deep low will track southeast with the
attendant cold front approaching the western end of the state
Saturday night 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 Monday night through next Wednesday, with moderate trades
potentially return 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 arriving tonight 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...
Winter Weather Advisory until 6 AM HST Saturday for Big Island
Summits.

&&

$$

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