034
FXHW60 PHFO 131353
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
353 AM HST Mon May 13 2024

.SYNOPSIS...
Light southeasterly winds, afternoon sea breezes and weak
troughing aloft will produce brief periods of slow moving showers
over some islands through Tuesday. Some of these showers may
become heavy at times especially along the eastern slopes of Oahu
and Maui, and the southeastern slopes of the Big Island. On
Wednesday the weather pattern trends more unstable as southerly
winds develop over the state in response to a kona low approaching
the islands from the north. Deep unstable tropical moisture will
move over all islands from the south, resulting in periods of
heavy rainfall, thunderstorms and strong gusty winds. The threats
from flooding and thunderstorms will be elevated for all islands
from Wednesday into the weekend.

&&

.DISCUSSION...
Local radar imagery this morning shows moderate showers anchored
over the Koolau Mountains on Oahu from a Molokai convergence plume
caught up in the light southeasterly winds. Showers over the
eastern mountain slopes of Oahu will continue at least through the
morning hours. Satellite imagery using the water vapor channel
continues to show the weak upper low drifting northeastward away
from the Hawaiian Islands today. Upper air balloon soundings from
Hilo and Lihue show very weak to non-existent subsidence
inversion. This means periods of wet weather should be expected
today statewide. The forecast grids were updated significantly
this morning in the medium to long range forecast to include
higher chances for showers and thunderstorms from Wednesday on
into the weekend.

The weather pattern for the next few days keeps the Hawaiian
Islands in a light southeasterly wind pattern with land and sea
breezes forming over each island through Tuesday. Forecast
guidance and satellite water vapor imagery continues to show weak
upper level troughing that will enhance some shower activity
across the state. Southeasterly winds tend to produce small scale
convergence plumes in the lee of island mountains and when
combined with upper level instability tend to create enhanced
showers over eastern Oahu. Southeasterly flow will also combine
with sea breezes to drive clouds and low moving moderate to heavy
showers up the mountain slopes of east Maui and the southeast
slopes of the Big Island over the Kau and Puna districts.
Localized slow moving heavy rain may also develop and Flood
Advisories may be needed mainly in the afternoon to early evening
hours through Tuesday.

Wednesday through Friday...Another unstable weather pattern
develops across the Hawaiian Islands as a unseasonably deep kona
low approaches the islands from the north. A cold front riding in
ahead of this system will help to produce southerly kona winds
over the islands starting early Wednesday morning. These southerly
winds will draw up a deep and unstable tropical moisture plume ahead
of the approaching cold front. The cold front will weaken into a
trough on Thursday as it merges with the tropical moisture plume
right over the islands. Forecast models have been remarkably
consistent in showing both features will impact all islands with
heavy rain and thunderstorm threats during this time period.

Several threats are likely in this weather pattern from Wednesday
onward including heavy rain with flooding, thunderstorms in the
strong to severe range, and strong gusty winds. The severe
thunderstorm threat will favor the western islands closer to the
approaching surface front/trough, the upper level cold pool and
the strongest wind shear. The forecast guidance over the past few
days suggests these threats will last for several days from
Wednesday through Friday and possibly linger into next weekend. A
very wet and unstable pattern for this time of year. Stay tuned.

&&

.AVIATION...
Lingering instability aloft combined with a moist southeasterly
flow will allow for a few heavy showers across the state through
the forecast period, with the slight possibility remaining for
isolated thunderstorms over the Big Island today. The heaviest
showers will mostly be focusing over southeast facing slopes of
each island. Winds will remain light enough to allow for a
land/sea breeze pattern where clouds and showers will build up
over island interiors during the afternoon/early evening hours and
clear out overnight. MVFR conditions are expected in any heavier
showers, with brief IFR conditions possible.

AIRMET Sierra for tempo mountain obscuration is in effect over the
the southeast facing areas of the Big Island and east facing areas
of Oahu. AIRMET Sierra for IFR conditions may also be needed for
the southeastern slopes of the Big Island later today as showers
periodically become heavy.

&&

.MARINE...
Moderate trade winds will gradually ease today and tonight, then
become more southerly on Tuesday as troughing amplifies to the
west of the state. Moderate southerly winds will develop in
advance of a front Wednesday. The front will move into the western
islands Wednesday night and Thursday, then weaken into a trough
and shift westward Friday. Light to moderate northwest winds may
briefly develop behind the front over the far western end of the
state, otherwise moderate southerly winds should prevail into the
weekend.

Surf along south facing shores will slowly decline today and
tonight. A series of overlapping, long-period south swells will
then move through the islands Tuesday into the weekend. This will
be a long lived south shore event, with surf holding well above
the summertime average Wednesday through the weekend.

A small, long-period northwest swell will peak today, then slowly
decline Tuesday, giving north shore surf a slight boost. A series
of reinforcing northwest swells will keep some small to moderate
sized surf moving into north facing shores Wednesday through the
weekend.

Surf along east facing shores will remain well below normal
levels through the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

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