Flash Flood Warning 24, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
317 AM HST Fri May 19 2023

.SYNOPSIS...
Humid and wet conditions with light southeast winds will continue
into the weekend as an upper disturbance drifts eastward over the
state. Some showers may be heavy at times, which could lead to
localized flooding concerns today. Southeast winds will linger
into the weekend, with a transition back to easterly trade winds
and drier conditions late Saturday through early next week.

&&

.DISCUSSION...
Short-term guidance remains in good agreement through the weekend
and shows the upper low centered near Kauai drifting eastward
over the state through tonight, then opening up and shifting east
of the area over the weekend. Southerly flow ahead of it will
continue to draw deep tropical moisture northward over the
islands. This influx of moisture, coupled with enhanced lift and
added instability associated with upper height falls, will keep
the chances up for localized heavy showers and a few
thunderstorms. Early morning radar trends support this and show
plenty of moderate to heavy showers developing near the upper low
over the leeward/southern Kauai waters, with another burst of
convection located around 100 miles south of Oahu/Maui County.

The timing and spatial extent of this activity will be mostly
influenced by the progression of the aforementioned upper low
moving through. Based on the current high-resolution guidance,
the heavy showers developing over and around Niihau will continue
and gradually shift eastward to Kauai by or around daybreak, then
to Oahu and the eastern end of the state later today through
tonight. The initial concern will remain over Kauai and whether or
not the heaviest activity stays off the coast to the south or
fills in over the island this morning. If the latter scenario
evolves, the potential for localized flooding will increase,
especially since some areas have already received 2-4 inches over
the past 12 to 24 hours. A similar threat will be in place over
Oahu and Maui County through the day, then the Big Island this
evening through early Saturday. In addition to the rainfall
potential, the threat of a wintry mix remains in place over the
Big Island Summits through early Saturday as the cold pool aloft
associated with the upper low moves through.

Improving conditions are expected over the weekend through early
next week as the weakening upper low departs and upper heights
rise. This will shift the deep moisture in the area westward with
surface high pressure building to northeast. This pattern shift
will result in a more typical easterly trade wind set up with
showers returning to windward and mauka locations each day.

&&

.AVIATION...
Early this morning, light showers and layered clouds are
impacting much of the state as a low aloft moves over the islands
and tracks east through tonight. Reduced visibilities and MVFR
ceilings can be expected in and around showers tracking from the
southwest. Deeper convection is observed on radar imagery in the
waters southwest of the islands, and this activity will have the
potential to spark isolated thunderstorms and occasional heavy
rain through the morning.

Scattered shower activity and occasional MVFR conditions are
expected to continue throughout the day, particularly for the
western islands, before diminishing by tonight as the upper low
opens up and shifts east of the state. Easterly trade winds will
fill back into the region this weekend.

AIRMET Sierra remains in effect for tempo mountain obscuration
for Kauai this morning. Additionally, with thick layered clouds
overhead, light icing remains possible from roughly 13,000 through
23,000 feet statewide.

&&

.MARINE...
A surface trough will linger over western waters through today
and will produce moderate southeast to south winds across most
areas with locally fresh southeast winds across select areas of
Maui and the Big Island. The threat for isolated thunderstorms
over western waters remain and moderate rainfall will be possible
at times especially around the waters of Kauai and Oahu. The
surface trough will dissipate on Saturday, leading to a return of
easterly trade winds and Small Craft Advisory conditions possible
over typical windy waters around the Big Island and Maui county.
Trades will likely strengthen to strong speeds early next week as
high pressure strengthens far northeast of the state.

A small, short-period north-northeast swell will steadily decline
tonight through the day. Some of this swell will wrap into
exposed eastern shores, but otherwise, east shore surf will be
below May average as the nearby trade wind flow will be weaker
than normal into Saturday. A small northwest swell is possible
early next week, while returning strong trade winds will increase
east shore surf.

A mix of small south-southwest and south-southeast swell should
produce inconsistent surf near the seasonal average through
the weekend. Expect south shore surf to be at summertime
background heights from Monday through the middle of next week.

&&

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

&&

DISCUSSION...Gibbs
AVIATION...Vaughan
MARINE...Shigesato