Heavy Rain in Hawaii, February 24, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
959 AM HST Sun Feb 24 2019

.SYNOPSIS...
Gentle to moderate northeasterly winds will prevail into Tuesday
as surface high pressure passes north of the islands. A surface
trough just east of the state will keep shower chances higher over
the Big Island today, with the rest of the state experiencing
minimal showers over windward and north facing slopes. A band of
low level moisture will produce an enhancement of rainfall as it
drifts southward down the island chain beginning this afternoon
on Kauai. On Tuesday and Wednesday, a passing upper level
disturbance may trigger heavy showers along the band as it moves
over the eastern half of the state. During this time, a developing
surface low well northeast of the islands will generate cool and
locally breezy north winds. Winds will ease on Friday.

&&

.DISCUSSION...
The islands remain under a gentle to locally moderate northeast
winds, and an eastward-drifting surface trough just east of the
state is helping to maintain greater instability over the Big
Island. Morning satellite imagery shows precipitable water (PW)
values in excess of 1.5 inches along the trough, with an area of
showers beginning to push onshore along the Hamakua coast of the
Big Island. Also, an upper level disturbance passing overhead is
triggering a few heavy showers just off the Kona coast. To the
west, PW values drop rapidly to around an inch, with clouds and a
few light showers banked along the northern and windward slopes
from Maui to Kauai and mostly clear conditions leeward. Just
north of Kauai, the tail end of a dissipating front is marked by
a diffuse, 100 mile-wide band of low level moisture that is
drifting southward.

Little change is due through the day. The trough and its
associated moisture will linger near the Big Island, as the axis
of the upper level disturbance moves overhead. The resulting
instability will likely produce another round of heavy showers
and isolated thunderstorms over the Big Island, along with the
possibility of a snow shower on the high summits where a Winter
Weather Advisory remains in effect. Weak mid level ridging, which
was evident on the overnight Lihue sounding, should provide a
strong enough cap to inhibit heavy showers elsewhere. As a result,
expect only modest rainfall along windward and north facing
slopes elsewhere. The exception will be on Kauai, where the
approaching band of low level moisture will provide a boost in
shower activity late in the day.

The upper level disturbance will move east of the state overnight,
and the surface trough will be pushed eastward by an advancing
surface high far northwest of Kauai. Some convection will likely
linger over and around the Big Island during the evening, but
otherwise expect somewhat stable conditions and gentle to
moderate northeast winds to focus showers over windward and
northeast slopes. The bulk of the shower activity should be along
the above mentioned moisture band, which will be draped over
Kauai. The band will drift over Oahu late Monday and Monday night.

An increase winds and the potential for unsettled weather over
portions of the state is due on Tuesday and Wednesday. The band of
moisture will be pushed southeastward across Maui County on
Tuesday and the Big Island on Wednesday as strong high pressure
settles far northwest of the state and surface low pressure forms
far to the northeast. A rather potent upper level trough will
swing overhead during this time, which may lead to heavy showers
along the band for the eastern half of the island chain. Locally
breezy north winds will ramp up first for the western half of the
state on Tuesday, then state-wide on Wednesday. This will usher in
a cool air mass, with dew points falling from near the current
near normal levels down into the lower 50s.

Winds will drop off on Thursday and Friday, with modest showers
moving along the northerly flow.

&&

.AVIATION...
A weakening surface trough near the Big Island will move slowly
east today with drying conditions and light northeast trades
filling behind the feature. Thus, isolated showers will mainly be
limited to the windward sides of the islands. Tonight, light
northeast flow will continue, once again bringing isolated to
scattered showers to mainly the windward slopes and coasts. Brief
MVFR conditions can not be ruled out due to lowered visibility and
ceilings produced by some of the heavier showers.

AIRMET SIERRA remains in effect for north through southeast
sections of the Big Island due to mountain obscuration above 2500
ft.

&&

.MARINE...
High pressure far northwest of the state will maintain moderate to
fresh northeast winds through Tuesday across most areas. The one
exception will be for Maalaea Bay, Alenuihaha Channel, and
Big Island Leeward areas where a surge of strong northeast winds
is expected late this afternoon through midnight. A Small Craft
Advisory (SCA) is in effect through midnight tonight for the surge
of strong winds. Starting Wednesday, a front will move down the
island chain and will produce widespread fresh to strong northerly
winds Wednesday and Wednesday night and possibly into Thursday.

Small surf is expected on all shores today and tonight, with a
small east swell fading and tiny northwest swell holding well
below the winter time average. A pulse of moderate northwest swell
is expected on Monday, followed by a large, long period northwest
swell that will build on Tuesday, peak Tuesday night through
Wednesday night, then slowly fade through late next week. This
large swell will produce advisory level surf for most north and
west facing shores, with some areas possibly reaching warning
levels. The combination of this large northwest swell, large wind
waves associated with fresh to potentially strong north winds,
along with a short period north swell, will lead to rough
conditions that will likely require SCAs for most waters and
possibly a High Surf Advisory (HSA) for east facing shores as
well. Winds and seas should decrease below SCA levels by late
Friday or Friday night, with HSA level surf possibly holding along
east facing shores exposed to the declining north swell into
early Friday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM HST this evening for Big
Island Summits.

Small Craft Advisory until midnight HST tonight for Maalaea Bay-
Alenuihaha Channel-Big Island Leeward Waters.

&&

DISCUSSION...Wroe
AVIATION...JT
MARINE...Kino