Drought in Hawaii, August 6 to 31, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
852 PM HST Mon Aug 5 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 anticipated with light to
moderate trade winds.

&&

.DISCUSSION...
Recent satellite imagery showed Post-Tropical Cyclone Flossie
skirting the Maui windward waters on a west-northwest track around
12 MPH and a trailing plume of deep tropical moisture extending
southward just east of 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. Some heavy showers
have already begun to develop and move over Oahu this evening where
1 to 1.5 inch accumulations have been observed over the past few
hours (peak at the Palisades and Waiawa C. F. gages through 7 PM
HST). Elsewhere across the state, isolated to scattered showers have
been observed with recent totals ranging from a quarter to half of
an inch.

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 overnight into
Tuesday, then over the central islands late Tuesday through
Wednesday. Winds will become 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 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 remaining 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 tonight
and into Tuesday morning as the remnants of Flossie pass just to
the north of the Islands. Scattered to numerous showers will exist
across the entire area with greatest coverage along the windward
slopes and coasts. MVFR conditions are expected at times due to
the thick low clouds and showers. Also, instability from a weak
trough west of the area may generate isolated thunderstorms.

AIRMET Sierra remains in effect for northeast sections of Oahu,
Molokai, and Maui, and Lanai due to tempo mountain obscuration
above 2000 feet.

&&

.MARINE..
Flossie is now post-tropical and located about 80nm north of the
Big Island. As Flossie tracks northwest tonight, north to
northeast winds are expected to shift to the southeast over the
Big Island and Maui County waters while winds over the western
half of the state weaken and shift to northerly. As Flossie
dissipates, high pressure will build northeast of the state,
allowing moderate to fresh east-southeasterly trades to return
from east to west across the islands Wednesday through Thursday.

The latest observations from the PacIOOS buoy at Hilo showed the
seas dropping below the 10-foot Small Craft Advisory (SCA) level,
but still elevated enough to justify keeping the advisory going
through tonight for exposed waters near Big Island and Maui. Will
likely drop it with the morning forecast package.

Surf height observations are still high along east facing shores
of the Big Island this evening and will likely remain so through
Tuesday. Thus, the High Surf Advisory (HSA) has been extended for
all exposed east facing shores through Tuesday afternoon. This
swell should quickly trend down later Tuesday as Flossie weakens
and tracks north. 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...
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.

Flash Flood Watch through late tonight for Maui-Big Island.

Small Craft Advisory until 6 AM HST Tuesday for Maui County
Windward Waters-Alenuihaha Channel-Big Island Windward Waters.

&&

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