Heavy Rain in Hawaii, April 18, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
344 PM HST Tue Apr 18 2017

.SYNOPSIS...
Lingering clouds and showers will persist over the southern half
of the Big Island into Wednesday. Trade winds will decline tonight
and begin to veer southeasterly as the subtropical ridge is
pushed toward the islands by an advancing front. Humid kona winds
and spotty showers will develop over the smaller islands on
Wednesday as the late season front nears the state. The front will
reach Kauai on Thursday, then stall over the smaller islands on
Friday and Saturday as trade winds rebuild.

&&

.DISCUSSION...
Have made some minor tweaks to the forecast over the Big Island
tonight, but otherwise the forecast seems to be on track.

The 1023 mb high was analyzed about 450 miles northeast of the
islands this morning with the ridge sinking over the islands.
Winds have been on the decrease today as a result, and we expect
further weakening tonight. The high will be moving to the east,
with a front approaching from the northwest. This should result in
winds becoming more southeasterly tonight and a bit more southerly
Wednesday.

The band of clouds and showers over the Big Island has slowly
shifted to the southern end of the island, primarily impacting the
Kau and Puna districts late this afternoon. Expect these areas to
remain the focus of these showers into Wednesday. Rainfall totals
have been dropping, but will continue to monitor the amount of
rain in the area.

The afternoon soundings had the precipitable water (PW) at Lihue
increasing to 1.01 inches, and Hilo at 1.48, which is near the
overnight value. Rainfall should remain limited over the smaller
islands overnight.

The GFS and ECMWF remain in very good agreement regarding the
the frontal system approaching from the northwest. As such the
forecast for that feature remains unchanged. As the winds shift to
the southeast tonight and south on Wednesday, expect an increase
in humidity and volcanic haze, particularly over the smaller
islands. The models are suggesting the portion of the stalled
band of moisture currently sitting west of the Big Island will be
drawn northward. A broad mid to upper level trough will dig toward
the state during this time, weakening and possibly eliminating
the inversion. This could trigger spotty heavy showers over the
western half of the island chain on Wednesday.

Both models show this feature reaching Kauai on Thursday, then
stalling over the central islands on Friday. Moisture along the
front will likely be shallow, not too unlike the band that passed
over the smaller islands a few days ago and is still impacting the
Big Island, but the mid to upper level trough could produce
enough instability to generate an isolated heavy shower or two.
A building ridge to the north of the islands will help to usher
back the trade winds Friday, and strengthen them this weekend as
the front dissipates, leaving some enhanced moisture to produce
rainfall over windward slopes.

&&

.AVIATION...
AIRMET SIERRA for mountain obscuration remains posted for the
windward and mountain areas of the Big Island. Low level moisture
surrounding the Big Island is helping to produce persistent low
clouds and showers (MVFR to isol IFR conditions), especially for
the Hilo, Puna, and Kau areas. We should start to see some
improvement Wednesday as the winds veer out of the southeast and
south tonight. Elsewhere, predominately VFR conditions prevail,
with scattered to broken clouds and isolated showers favoring the
windward areas.

A front northwest of the state will continue to advance towards
Kauai, pushing the weak surface high to the east, producing
southeast to south winds over the state starting as early as
tonight. The southerly flow will allow for sea breezes to form,
shifting clouds and showers to the leeward and interior areas for
Wednesday.

&&

.MARINE...
High pressure north of the area continues to weaken as a cold
front approaches the area. This will result in the winds shifting
out of the south-southwest Wednesday and Wednesday night,
especially for the smaller islands. Guidance remains persistent
and shows this front moving down the island chain late Wednesday
night through Thursday night with moderate to fresh northerly
winds filling in behind it. Trade winds are forecast to return
Friday through the weekend as high pressure builds north of the
area in the wake of the front.

A new long-period northwest (320 deg) swell has begun to fill in
today, which will continue to impact the exposed waters and
beaches into Wednesday. Surf associated with this source will
remain well below advisory levels along north and west facing
shores through Wednesday night. A much larger northwest (330 deg)
swell associated with gale currently digging southeast across the
central north Pacific near the Date Line will fill in late
Thursday, peak Thursday night into Friday, then slowly ease into
the upcoming weekend. Surf may near warning levels through the
peak of this event along exposed north and west facing shores.

Another small south (180 deg) swell, similar to this past weekend,
will gradually fill in Wednesday and lead to a slight increase in
surf along south facing shores through Friday before trending down
into the weekend. Surf associated with this source will remain below
advisory levels.

A small northerly swell associated with an upstream fetch well north
of the state has affected the north and east facing shores today
and will continue into Wednesday. This small northerly swell will
then ease into the second half of the week. Small surf will result
along exposed north and east facing shores.

For more detailed information on the local surf and outlook for
Oahu, see the latest Collaborative Nearshore Swell and Wind Forecast
for Oahu that was issued Monday afternoon.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

DISCUSSION...M Ballard
AVIATION...Eaton
MARINE...Evans