Flash Flooding and Heavy Rain in Hawaii, April 2 to 5, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
402 AM HST Mon Apr 2 2018

.SYNOPSIS...
A wet weather pattern will develop this week across the Hawaiian
Islands as several low pressure systems track across the state.
Humid and Voggy south to southwesterly winds will continue through
Thursday. The first trough will move into the islands from the
west this morning producing showers over Kauai and Oahu, this
system will spread clouds and showers down the island chain to
Maui County later this afternoon. A second trough brings showers
across the islands from west to east starting on Wednesday
morning. Deep moisture will continue to hang over the islands
through the week with a weak cold front sweeping a third round of
showers through the state on Thursday and Friday. Trade winds
return on Saturday in a continued humid and cloudy weather
pattern.

&&

.DISCUSSION...
The Water Vapor satellite imagery this morning shows an extensive
band of mid and high clouds from a strong jet stream spreading
across the islands from west to east. An upper level trough is
approaching Hawaii from the northwest. Local doppler radar shows
showers already developing just west of Niihau and Kauai, with
some showers moving into the Waianae Coast of Oahu. Shower
activity will increase and spread eastward through the morning
hours as the upper level trough moves over Kauai.

In the big picture, the East Pacific high remains locked in place
off the California coast as a broad area of low pressure moves
through the Central Pacific. A slow moving cold front lingers
between these two air masses dragging a nearly stationary remnant
frontal boundary just northwest of the Hawaiian Islands. Meanwhile
tropical moisture with voggy kona winds persists over the entire state.
A complicated wet weather pattern will develop through the work week
with several upper level short wave troughs passing west to east
through the region. These upper troughs will cause weak lows to
develop along the remnant front, lifting clouds and showers
across each island as they pass.

Today and Tuesday...The first in a series of troughs moves into
Kauai from the northwest this morning with upper level forcing
spreading clouds and showers eastward over Kauai and Oahu. The
trough continues to push showers eastward into Maui County by
Monday afternoon with enhanced showers reaching the Big Island on
Tuesday. Upper level wind speeds will increase this morning over
the Big Island summits of Maunaloa and Maunakea as the trough
approaches from the west. A wind advisory was issued this morning
for both summits.

Wednesday through Friday...Meanwhile far to the west of the
islands, a second stronger trough moves into the picture passing
through Kauai and Oahu on Wednesday morning. On Thursday, a high
pressure system moving across the Central Pacific will help push
a band of clouds and showers across Maui and Hawaii Counties
through Friday.

The long range outlook for next weekend still appears humid and
cloudy as a surface trough will linger over the state. Weather
model guidance continues to show kona winds replaced by trade
winds on Saturday. These winds will be strong enough to blow all
the Volcanic fOG (VOG) away from the smaller islands. Another
round of showers is possible from Kauai to Maui this weekend.

The timing on precipitation impacts for each island remains
challenging in a complicated weather pattern. The American (GFS)
global weather model and the European (ECMWF) model solutions are
improving with the track and timing of precipitation impacts from
these upper troughs. Overall we expect periods of moderate to
locally heavy rainfall through the upcoming work week. Significant
adjustments to precipitation chances were made with the morning
forecast grid package. Short term precipitation forecasts were
created from a blend of larger scale and smaller mesoscale model
rainfall solutions. Stay tuned for forecast updates this week as
the weather pattern evolves over time.

&&

.AVIATION...
Moisture associated with a weak frontal boundary just northwest
of Kauai will advance to the western islands today. Southerly
winds are expected to increase a bit across the islands as well,
bringing more moisture from the south. Thus, more widespread MVFR
ceiling showers conditions as well as isolated IFR conditions
will affect Kauai early today, then spreading east to Oahu later.
This may prompt the issuance of AIRMET mountain obscuration for
those islands. Rest of the islands will see mainly VFR conditions
early today, though increasing moisture from the south coupled
with daytime heating may bring some MVFR ceiling conditions to
those islands.

An AIRMET for upper level TEMPO moderate turbulence has been
canceled. The non-standard AIRMET for TEMPO light icing remains
in effect across the area between 160 and FL250 due to the present
of upper level moisture.

&&

.MARINE...
Light to moderate southerly winds are forecast to shift out of
the south-southwest Tuesday through midweek and increase into the
fresh to strong category as low pressure develops and lifts
northeast, potentially dragging a cold front into the area by
late Thursday. The best chance for heavy showers will remain over
the northwest waters today due to a lingering frontal boundary
positioned northwest of the state. More of a widespread rainfall
threat will become a possibility Tuesday through the second half
of the week as the front begins to advance down the island chain.

The anomalous pattern across the northern Pacific that has
suppressed our more typical swell energy from the northwest
direction since the second half of February has begun to shift.
The ridge over the northwest Pacific has broken down, which has
allowed the storm track to shift eastward. The latest model
guidance reflects this and depicts a series of gales setting up
over the northwest Pacific through the week that will bring
consistent surf out of the northwest (310-330 deg) to our north
and west facing shores through the upcoming weekend and next week.

The current west-northwest (300 deg) swell that brought advisory-
level surf to exposed north and west facing shores early Saturday
will continue to slowly ease through Tuesday. A small
reinforcement out of the same direction associated with a gale
tracking eastward north of Midway around 30 to 35N this morning
is forecast to fill in locally Wednesday through Thursday.

Another gale is forecast to develop between 35N and 40N just east
of the date line Wednesday as it tracks southeastward aiming seas
at the local area out of 330 deg. Surf will respond from this
source Friday and persist through the weekend.

Choppy and short period surf along south facing shores will
become a possibility Tuesday through midweek as fresh to strong
south-southwest setup over the state.

For the long term (Sunday night into next week), guidance remains
persistent and depicts a large area of gale- to storm-force
westerly winds developing over the far northwest Pacific near the
western Aleutians south of 967 mb low pressure Thursday through
Friday. ECMWF-Wave and WAVEWATCH III solutions are similar and
show a large NNW (330-340 deg) swell hitting our NW offshore
buoys Sunday night. If this scenario evolves as predicted, warning
level surf will become a possibility along north and west facing
shores early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 11 AM this morning to 5 AM HST Tuesday for
Big Island Summits.

&&

DISCUSSION...Bohlin
AVIATION...Hui
MARINE...Lau