792
FXHW60 PHFO 171950
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
950 AM HST Fri May 17 2024

.SYNOPSIS...
A kona low north of the state will continue drawing unstable
tropical moisture northward over the western islands into
Saturday. The focus for the heaviest rain will be Oahu initially
today, then shift westward to Kauai this evening and Saturday.
The kona low will begin slowly drift away from the state this
weekend with improving trends spreading from east to west, though
enhanced rainfall may linger over Kauai and Niihau into Sunday. By
Monday all islands will see an improvement in weather conditions
as easterly trade winds return.

&&

.DISCUSSION...
Latest satellite imagery shows a kona low located roughly 730 nm
northwest of Kauai. A decaying stationary front is currently
located over Oahu, which is serving as a focus for moderate to
locally heavy showers this morning. Radar estimates and rain
gauges are showing that rainfall rates are currently in the 1-2
inch per hour range, which has resulted in some nuisance flooding
and elevated streams across Oahu. 24 hour rainfall totals across
Oahu have exceeded 10 inches in some locations, and the ground
remains highly saturated and susceptible to runoff and flooding
should heavy rain continue to move over the island. A Flood Watch
remains in effect for Niihau, Kauai, and Oahu today, but changes
to the watch area are likely by this afternoon.

Latest guidance and radar trends suggest that this main
band of rain will slowly move west throughout the day, focusing
mainly within the Kauai Channel this afternoon and eventually the
island of Kauai by this evening. The main change to the previous
forecast has been to slow this westward progression a bit, but
still think that conditions should improve across Oahu by early
afternoon. Of note, some of the latest hi-res guidance has been
largely discounted, namely the ARW and FV3, which do not appear to
have initialized correctly.

Focus will then shift to Kauai this evening, where this rain band
will provide widespread moderate but pockets of heavy rain to the
Garden Isle through at least Saturday. The main consideration for
the island of Kauai will be how long the band lingers over the
island before moving off to the northwest as the kona low exits
the region. While consensus is that conditions will begin to
improve by early Sunday, the flooding concerns could linger a bit
longer should this be a slower transition. Models also are hinting
that one last moisture surge back towards Oahu is possible on
Saturday night, but confidence is low in this occurring at this
time.

High pressure will slowly build into the region this weekend as
the kona low exits the pattern to the north. Expect more
stabilizing trends with decreasing showers as east to southeast
winds spread across the state. Likely by Sunday, the moderate to
heavy rainfall threat will diminish over Kauai as more stable
conditions fill in across the state.

More typical easterly trade winds will return across the state from
Monday onward with only brief passing showers in the forecast
favoring the typical windward and mountain areas through the
middle of next week.

&&

.AVIATION...
A moist and unstable cloud band associated with a kona low far to
our northwest is currently over Oahu and will slowly move west
and affect Kauai later today. MVFR with embedded IFR conditions
will continue affecting portions of Oahu this morning but then the
focus will shift to Kauai as the cloud band moves west. Winds
should remain out of the south-southeast through today. Lingering
moisture could still bring short periods of MVFR conditions to any
of the islands today and primarily focus over the southeast
facing slopes.

AIRMET Sierra is in effect for Kauai, Oahu, and windward Big
Island. IFR conds over Oahu will likely improve towards noon, but
mountain obscuration will likely remain an issue for Oahu and
Kauai through twenty-four hours.

AIRMET Tango is in effect for low level mechanical turbulence
north through east of the Big Island and East Maui. Also upper
level turbulence could be a possibility tomorrow morning.

Occasional light rime icing is expected from Kauai to Oahu in
layer 140-FL250.

&&

.MARINE...
Fresh to locally strong south to southeast winds associated with
a kona low north-northwest of the state will persist through
tonight. A shift out of the east to southeast direction is
expected over the weekend through early next week as it lifts
northward and away from the area. A Small Craft Advisory remains
in effect for the Windward and Leeward Big Island waters (Hamakua
Coast and waters near South Point) through tonight. In addition to
the winds, expect locally rough seas in heavy showers or storms
that develop, with the best chances being over the waters west of
Maui County. Seas should remain just below the advisory level over
the exposed Kauai waters as a short-period northwest swell moves
through (downward trend expected this weekend).

Surf along south facing shores will remain up through early next
week due to overlapping, long-period south-southwest swells. The
first swell has filled in and will peak today, likely holding at
the advisory level into Saturday before briefly lowering.
Observations at offshore buoys 51003 and 51004 (51002 remains
down) reflect this with the long-period peak energy now in the
15-17 second bands. Expect rough conditions for most southerly
exposures today due to a combination of the onshore winds, swell,
and periodic heavy showers moving through. As a result, a High
Surf Advisory remains in effect through Saturday. A fresh, long-
period south-southwest swell will fill in through the day Sunday,
then peak at a similar level early next week before easing
through midweek.

Surf along north and west facing shores will remain up into
Saturday due to a short-period northwest swell generated from the
kona low. This swell will lower over the weekend.

Surf along east facing shores will remain well below average
through the weekend. An upward trend is possible next week as the
trades return locally and far upstream over the eastern Pacific.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 8 PM HST this evening for Niihau, Kauai, and
Oahu.

Wind Advisory until 6 PM HST this evening for Big Island summits.

Small Craft Advisory until 6 AM HST Saturday for Big Island
Windward Waters-Big Island Leeward Waters.

High Surf Advisory until 6 PM HST Saturday for south facing shores
of all islands.

&&

$$

DISCUSSION...Vaughan
AVIATION...Tsamous
MARINE...JVC