Heavy Rain in Hawaii, May 3, 2018

Area Forecast Discussion...Corrected
National Weather Service Honolulu HI
946 AM HST Thu May 3 2018

CORRECTED TO ADD END CREDITS

.SYNOPSIS...
A humid airmass over the state will bring showery weather to many
areas over the next few days. An upper level trough transiting
near or over Kauai on Friday and Friday night will bring the
biggest threat of heavy rainfall there, with a somewhat lesser
threat to Oahu. A drier and more stable trade wind pattern will
spread in during the weekend and next week.

&&

.DISCUSSION...
A couple of updates this morning. North to northwest winds have
spread down the island chain from north to south behind a weak
boundary which currently stretches across Maui County. A few
heavier showers developed across windward Oahu behind this
boundary, but it looks like this activity is staying pretty
isolated thus far. Expect showers will concentrate across north
shores and mountain terrain through the day, rain rates do not
look impressive at this time. This boundary will wash tonight
allowing weak trades to move back in across the state by Friday
morning.

Have extended the Flash Flood Watch to cover through Friday night.
The expectation is that the current bands of showers from about
Oahu westward will continue to break apart through the rest of
today and tonight as a short wave ridge builds ahead of the next
upper level trough. This upper trough currently located far
southwest of Kauai near 170W, is expected to lift northeastward
and result in an increase in shower coverage and intensity, along
with the possibility for thunderstorms on Friday through Friday
night from about the Kauai Channel west. There is still a lesser
possibility that heavier shower activity reaches Oahu later
Friday, so the watch is continued for both Kauai and Oahu through
Friday night.

Over the weekend and early next week, a strong high pressure
system builds across the Central Pacific. Moderate trade winds
associated with this high will push a remnant frontal cloud band
across the islands on Monday. Shower activity with this shallow
cloud band will be minimal. Overall we should see drier air moving
into the western half of the state with enough moisture lingering
over Maui and Hawaii Counties for enhanced windward showers into
the first half of next week.

&&

.AVIATION...
Thick layered clouds and areas of stratiform rain will continue
to stream over much of the western half of the state today.
Surface observations have shown mainly light stratiform rain and
VFR conditions prevailing underneath these clouds so far, but
areas of intermittent MVFR ceilings have recently occurred over
Kauai, so Airmet SIERRA for mountain obscuration is in effect for
that island.

The potential remains for locally heavy showers to develop later
today, mainly from Oahu westward, as instability increases with
the approaching upper trough. This could produce additional
periods of MVFR, and possibly even brief IFR, conditions. Oahu may
need to be added to Airmet SIERRA later today if the heavier
showers and associated MVFR conditions extend far enough eastward.

Light rime icing has been reported by several aircraft within the
layered cloud band over the past 24 hours, and this will likely
continue today, mainly between 150-FL280.

&&

.MARINE...
The current small south swell will lower gradually today through
Saturday. Otherwise there will be a series of small, mainly
background southerly swells for most of next week. A new north
northwest swell is expected to arrive Sunday night, peak on
Monday, then lower gradually Monday night. A slightly larger and
longer period north swell is expected to arrive Monday night, peak
early Tuesday, then lower gradually Tuesday night and Wednesday.
Based on latest Wavewatch III model guidance, advisory level surf
is a possibility during the peak of the swell. Strengthening
trade winds early next week will cause an increase in short period
choppy surf along east facing shores.

Winds and seas will remain below Small Craft Advisory levels
through Sunday. Winds and seas are expected to rise Sunday night
and Monday likely requiring a Small Craft Advisory over exposed
zones early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through late Friday night for Niihau-Kauai-
Oahu.

&&

Discussion...Brenchley
Aviation...TS
Marine...Burke