Heavy Rain in Hawaii, February 2, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
956 AM HST Sat Feb 2 2019

.SYNOPSIS...
Wet, breezy, and unsettled weather will persist across much of
the state into tonight as a band of low level moisture drifts
south over the islands and an upper disturbance passes overhead.
Rainfall will be focused over windward slopes, though showers will
periodically spill over to leeward sections. A return of a more
typical trade wind pattern is expected Sunday into Tuesday as
drier air moves over the islands. Unsettled weather and disrupted
trade winds are possible during middle to latter half of the
week.

&&

.DISCUSSION...
Wet, breezy, and unsettled conditions are in place from Kauai to
Maui this morning. A southward-drifting band of low level
moisture with precipitable water values of 1.3 to 1.7 inches
covers these islands, while an upper level disturbance passing
overhead is triggering isolated heavy showers. In the past six
hours, generally less than an inch of rainfall has been measured
across windward slopes from Kauai to Maui, though a few standout
gauges have collected in excess of 3 inches during this time. As a
result, water levels in some streams are elevated, and with
additional rainfall and potentially a thunderstorm in the forecast
for the rest of the day, isolated nuisance flooding is a
possibility. High pressure sitting far north of the state is
driving moderate to locally breezy trades, which will easily carry
showers to leeward areas at times. On the Big Island, relatively
dry conditions are in place this morning, but as the band of
moisture drifts southward today, expect shower activity to pick
up, especially for windward areas and the upper leeward slopes.

Wet and unsettled trade wind flow will persist across much of the
state tonight. The band of low level moisture will continue to
drift southward as the upper level disturbance tracks
southeastward. While all islands will continue to experience
rainfall, especially windward, chances for heavy rainfall will
diminish over Kauai and Oahu. The ground may become saturated as
the day progresses, so chances of some flooding impacts will
increase heading into tonight.

A return to typical trade wind weather is expected on Sunday. The
upper level disturbance will move east of the Big Island in the
morning, and ridging aloft will build in from the west, bringing a
return to stable conditions. The band of low level moisture will
break up over the eastern end of the island chain. This may allow
showers to linger across windward slopes of the Big Island and
Maui, while the rest of the state sees a decreasing amount of
mainly windward shower activity.

For Monday and much of Tuesday, expect typical trade wind weather.
Moderate northeasterly trade winds will produce modest showers
over windward slopes. Ridging aloft will maintain stable
conditions, though this ridge will break down late Tuesday.

Trades will likely become disrupted by midweek as another upper
trough moves into the area. Guidance shows a broad surface trough
evolving over the state in response to the upper feature Wednesday
through Thursday. If this pattern sets up as projected, light
winds along with increasing moisture and instability will
translate to higher rainfall chances once again. Additional
modifications will be likely in the forecast through this extended
period as confidence steadily rises over the coming days.

&&

.AVIATION...
A line of enhanced clouds over much of the main Hawaiian islands
will bring periods of rain with MVFR VIS and/or CIG, and even brief
IFR conditions, especially across windward and mountain areas today.
Furthermore, the airmass in the island vicinity is becoming more
unstable as an upper level trough passes over the area today,
resulting in a slight chance of thunderstorms state-wide. The
deteriorated conditions are expected to last through much of
today, so AIRMET SIERRA for mountain obscuration will remain in
effect through at least this evening, and likely overnight as
well.

Moderate to locally breezy northeast to east trade winds will
continue over the next few days. However, wind speeds are expected
to stay just below the threshold for widespread, low-level
mechanical turbulence.

&&

.MARINE...
High pressure north of the state will keep moderate to strong
trades in place through tonight. A weak surface trough currently
passing southwest of Kauai may produce an isolated thunderstorm
later today over the western waters. The trades will diminish to
moderate levels Sunday through Monday, then strengthen briefly
back into the fresh to strong range on Tuesday as the gradient
tightens over the island chain. A surface trough will likely
develop over the state Wednesday, causing unsettled weather and
weakening the trades into the light to moderate range through
Thursday. Fresh to strong northerly winds may then develop by the
end of next week as high pressure sets up northwest of the state
and low pressure develops off to the northeast.

A Small Craft Advisory (SCA) remains in effect for much of the
marine area through tonight, due to winds, seas or a combination
of the two. Seas will remain elevated across portions of the
marine area through early Sunday, due to a moderate west-northwest
swell moving through the area.

Surf along east facing shores will remain elevated from Kauai
eastward to Maui through tonight, and as a result, a High Surf
Advisory (HSA) remains in effect. Surf is expected to drop below
HSA levels by Sunday morning. Several west-northwest swells are
expected through middle of next week and will continue to produce
moderate surf along north and west facing shores. The first in the
series of swells is expected to arrive this afternoon into the
evening hours, producing surf just below the advisory level along
north and west facing shores of Niihau and Kauai this afternoon
and tonight, and near the advisory level along west facing shores
of the Big Island tonight and Sunday. The next west-northwest
swell will likely remain below advisory levels for all shores
Monday through Tuesday. Another west-northwest swell due in
Tuesday night and Wednesday, may flirt with advisory levels for
north and west facing shores of Niihau and Kauai. A much larger
northwest swell is then expected to build late next week,
potentially bringing warning level surf to north and west facing
shores on Friday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Sunday for east facing shores
of Kauai, Oahu, Molokai, and Maui.

Small Craft Advisory until 6 AM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Windward Waters.

Small Craft Advisory until 6 PM HST this evening for Oahu
Leeward Waters.

&&

DISCUSSION...Wroe
AVIATION...TS
MARINE...Kino