Heavy Rain in Hawaii, August 6 to 10, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
349 AM HST Tue Aug 6 2019

.SYNOPSIS...
Wet and humid conditions are anticipated across the state through
midweek due to a combination of light winds and deep tropical
moisture pooling northward in the wake of Post-Tropical Cyclone
Flossie passing to the north. In addition to the wet pattern, a few
thunderstorms will remain possible due to an upper trough over the
region. Later in the week and into the upcoming weekend, a return of
a more stable and drier pattern is expected with light to
moderate trade winds.

&&

.DISCUSSION...
Recent satellite imagery showed the remnant low associated with
Flossie beginning to lift northward away from the islands and a
trailing plume of deep tropical moisture extending southward
around the Big Island. Satellite-derived precipitable water (PW)
imagery reflects this and depicts a large area of near to above 2
inches over the state. Most of the overnight shower coverage has
focused over the southeast slopes of the Big Island where
accumulations have ranged from a quarter to a little over half of
an inch in the past six hours (through 2 AM HST). Elsewhere,
mostly quiet conditions have been in place due to the early
morning land breeze.

The short-term guidance remains in good agreement and keeps the
wet and humid pattern in place through midweek. As Flossie lifts
north and tracks away from the area, the aforementioned trailing
plume of moisture will setup over the eastern end of the state
this morning, then over the central islands later today through
Wednesday. Winds will remain light and variable, which will allow
for a land and sea breeze regime to become established. Dewpoints
will hover in the low to mid 70s. A combination of this moisture
pooling northward over the state trailing Flossie and an upper
trough positioned west of Kauai will keep the threat for heavy
rainfall and flash flooding in the forecast. Although the
potential for heavy showers will continue each day through
Wednesday, the best coverage of heavy showers will favor interior
sections of the islands through the afternoon and evening periods
due to the sea breeze and light south-southeast low-level
background flow. Additionally, a few thunderstorms can't be ruled
out today due to the proximity of the upper trough and added
instability.

Later this week and into the upcoming weekend, guidance remains in
decent agreement and depicts a return of a drier and more stable
pattern as upper heights rise over the region. Trades are forecast
to return, but could remain in the light to moderate category due to
a weakness in the subtropical ridge north-northwest of the
islands. Showers will favor windward areas, except where sea
breezes setup over the interior and leeward locations through the
afternoon and evening periods.

&&

.AVIATION...
Light and variable winds will ensue for the remainder of this morning
as the remnants of Flossie pass just to the north of the Islands.
This light flow regime will lead to modified sea breeze
development this afternoon with showers and isolated thunderstorms
forming over the islands interiors. Expect MVFR conditions at
times due lowered visibility and mountain obscuration from thick
low clouds and showers.

No AIRMETs are currently in effect.

&&

.MARINE..
As the remnants of Flossie track northward through the day, winds
are expected to shift to the southeast over the Big Island and
Maui County waters while weakening and shifting northerly over the
western end of the state. High pressure will then build northeast
of the state, allowing moderate to fresh east trades to return
from east to west across the islands Wednesday through Thursday.

The latest observations from the PacIOOS buoy at Hilo seas around
6 to 7 feet and will continue to lower through the day, so the
Small Craft Advisory has been dropped for all remaining waters.

Surf height and near-shore buoy observations are still elevated
enough along east facing shores of the islands this morning, so
will let the High Surf Advisory (HSA) continue through this
afternoon for all east facing shores. Will continue to monitor in
case the HSA can be dropped sooner as the swell eases through the
day. Surf along south facing shores will rise through the second
half of the week as overlapping south-southwest swells move
through.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through this afternoon for Maui-Big Island.

High Surf Advisory until 6 PM HST this evening for Kauai
Windward-Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-
Windward Haleakala-South Big Island-Big Island North and East.

&&

DISCUSSION...Gibbs
AVIATION...Thomas
MARINE...Stall