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Hawaiian Islands Synoptic Discussion and Guidance

957
FXHW60 PHFO 180630
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Tue May 17 2022

.SYNOPSIS...
Southerly winds will focus clouds and showers across leeward
areas for the next few days. Heavy rainfall will bring the
potential for flooding to Kauai late tonight through Wednesday
night. A few thunderstorms are also possible. For the remaining
islands, coverage and duration of heavier showers is expected to
be limited.

&&

.DISCUSSION...
While cloud cover increased this afternoon across mauka areas of
the islands, this should briefly decrease with the onset of
nighttime cooling. Low level flow is from the south near Kauai to
southeast near the Big Island, thanks to flow deformation ahead of
an approaching front far northwest of the main Hawaiian Islands.
The associated low near 37N167W is lifting out to the northeast
but, as the supporting upper trough digs southward and swings
eastward toward Kauai, the probability for adverse weather along
and ahead of the front is increasing.

Models show southerly flow across the western islands becoming
more moist and unstable as the night wears on. Shower bands will
form and intensify within this convergent flow, mostly over and
near Kauai. Islands farther to the east may see some of this
activity as well, but with less total rainfall. Southerly wind
speeds will also increase a bit statewide. The probability for
heavy showers on Kauai will steadily increase through the night.

On Wednesday, especially by midday, the digging upper trough will
begin to pass over Kauai. This, combined with strengthening moist
southerly flow, will significantly boost the probability for
flooding on Kauai as heavy showers become more abundant and
thunderstorms become possible. Although convection will tend to
focus over south-facing slopes, the flood threat will encompass
the entire island. A Flood Watch remains in effect for Kauai and
Niihau from 6 am HST Wednesday through Wednesday night. In
addition, there is a very low, but non-zero, probability that
there will be enough vertical shear to generate waterspouts in
some of the strongest convection Wednesday afternoon and evening.
Temperatures aloft will probably be too warm and conditions too
stable for hail development, but small hail is not entirely out of
the question either.

The upper trough will lift northeast and moisture will spread
east over Oahu Wednesday night. However, diminishing convective
forcing will likely keep shower intensity in check. A few locally
heavy showers may still occur on Oahu, but with less abundance
than on Kauai. For Kauai, the departure of the upper trough will
mark a transition to orographic forcing. Enhanced moisture will
remain in place and heavy rainfall may continue there through
Wednesday night. If this happens, the Flood Watch may need to be
extended into the first half of Thursday.

Models show southerly flow will weaken considerably Thursday into
Friday as the distant surface low lifts northeastward. With the
remnant moisture band still lingering over the state, any
afternoon showers over island interiors would have the potential
to produce heavy rainfall. Therefore, heavy rainfall may occur
Friday and Saturday as afternoon sea breezes develop.

&&

.AVIATION...
Moisture will increase across western and portions of the central
islands through Wednesday, in advance of a slowly approaching
frontal system. VFR conds currently across Kauai are expected to
quickly deteriorate to MVFR, and possibly IFR, cigs early
Wednesday before expanding eastward toward Oahu. Also, vsby
restrictions are expected to develop across PHLI through much of
the TAF period, resulting from potentially heavy rainfall. Further
east, limited moisture and forcing will reduce showers and clouds
to more favored southeast trade wind and sea breeze locations.
Thunderstorms are also a possibility across Kauai to Oahu into
Wednesday. Otherwise, winds will increase from the south over the
west end of the island chain, while remaining weaker from the
southeast elsewhere.

No AIRMETs are currently in effect. Mountain obscuration is
possible at times especially for south facing slopes of Kauai and
Oahu. AIRMET Tango for low-level mechanical turbulence is
possible later tonight as well as southerly winds increase
around Kauai. A period of light ice could potentially develop in a
layer between 140/FL240 early Wednesday for mainly western and
central islands, as a band of mid and upper lvl moisture continues
to stream up toward these islands.

&&

.MARINE...
Southerly winds are strengthening over the Oahu and Kauai waters
this evening and this trend will continue overnight through
Wednesday as a cold front moves into the area and stalls near or
just west of the Kauai waters. Winds are forecast to reach Small
Craft Advisory levels early Wednesday morning, then continue into
Thursday before easing. Seas will respond and quickly rise over
waters that are normally protected during trade wind conditions.
Expect locally higher winds and seas in and around heavy showers
and thunderstorms that develop as the front draws closer (best
chance late tonight through Wednesday night). A return of easterly
trade winds is expected over the upcoming weekend as the ridge
lifts northward.

For the eastern end of the state, fresh to locally strong southeast
winds are expected late tonight through Wednesday night - strongest
being around the typical coastal areas that experience terrain-
induced accelerations with this wind direction (Hamakua coast and
waters around South Point of the Big Island).

Surf along south facing shores will remain up into the second half
of the week as the south swell that arrived early today continues
to move through and becomes reinforced through the day Thursday.
Offshore buoys to the south and the nearshore PacIOOS buoy at
Barbers reflect this with the peak of the energy registering in
the 15 and 17 second bands. In addition to the long-period energy
moving through that is typical for this time of year, southerly
winds picking up overnight through midweek will add to the mix
with short period choppy/rough conditions expected (roughest
Kauai where locally strong south winds are possible).

Looking out into the long range (late next week/last weekend of
May), lots of activity brewing down south within Hawaii's swell
window around New Zealand. GFS and ECMWF both depicting around 960
mb low pressure passing through/near this area later this weekend
and into this coming weekend. First phase of this for Hawaii will
potentially be out of the southwest direction from the initial
phase over the Tasman Sea around next Thursday (the 26th of May)
followed by a upward trend through the end of the month. Will
monitor the progression of this system later this week/weekend
which will lead to more confidence in what to expect locally
toward the end of the month.

Surf along north facing shores will remain small through midweek,
then trend up Thursday as a small, medium-period northwest swell
arrives. Satellite data reflected this potential over the past 24
hours, which showed a decent sized batch of strong northwest
winds focused at the islands within the 300 to 320 degree
directional band. A return of summer-like conditions is expected
over the weekend as this source moves out.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch from 6 AM HST Wednesday through late Wednesday night
for Niihau-Kauai-

Small Craft Advisory until 6 AM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters.

&&

$$

DISCUSSION...Powell
AVIATION...SWR
MARINE...Gibbs

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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