Heavy Rain in Hawaii, March 24, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Fri Mar 24 2017

.SYNOPSIS...
Trade wind weather will prevail through next week with passing
showers favoring the windward and mountain areas, especially during
the night and morning hours. The strength of the trades will
increase to moderate to locally strong for the next few days.

&&

.DISCUSSION...
The line of low clouds that affected the southern islands windward
areas has pretty much stayed south and continues to affect windward
Big Island at this time. Meanwhile, more low clouds have developed
over the coastal waters just east of the islands. These clouds will
bring a little more showers to the northern islands, especially
windward Oahu this morning. Early morning Lihue sounding exhibited
little changes from Thursday afternoon run, though Hilo sounding has
become quite a bit more moist, which is expected due to the present
of the low clouds and showers. Both soundings do indicate airmass in
the island vicinity is still somewhat unstable. Therefore, these low
clouds are capable of bringing measurable precipitation, even
locally heavier showers. Forecast solutions still depict a drier
airmass will spread from the east later today. Hence, expect a
little more clouds and showers over the windward areas today, with
improving conditions late today. Lee areas should stay mostly dry,
except for the Big Island lee areas in the afternoon and evening due
to daytime convection.

A boost in the trade winds is expected today, as the high pressure
northeast strengthens through the next few days. Expect winds to
become locally brisk through Sunday. Winds are then expected to ease
slightly next week as the high pressure northeast moves further away
from the islands. Regardless, trade wind weather is here to stay
through next week, as indicated by forecast models. Moisture
embedded in the trade wind flow will reach the area at times,
bringing some showers mainly to windward and mountain areas. Airmass
is expected to become somewhat stable and so rainfall amounts will
not be a cause for concern.

Long range models still hint the trades may weaken further toward
the later part of next week, as a stronger front may approach the
area from the west. There is disagreement upon how close this front
will get to the islands, though both solutions indicate at least
trade winds will hold.

&&

.AVIATION...
Low clouds from an old frontal boundary are embedded within
prevailing easterly flow. Showers are being enhanced near the Big
Island this morning by an upper trough. These clouds will produce
isolated to scattered showers with brief periods of MVFR ceilings
along eastern slopes of all islands during the overnight period.
Drier VFR conditions expected elsewhere.

An AIRMET for TEMPO MTN OBSC in effect for eastern half of Big
Island from Upolu Point to PHTO to South Cape.

&&

.MARINE...
Winds are expected to begin to increase today, with the typical
windier locations reaching Small Craft Advisory (SCA) levels
today. An SCA is being issued with the morning package for those
areas, however winds are expected to increase over other areas
through the weekend as a new high strengthens to the north of the
islands.

A very large west-northwest swell is expected to build
across the island chain Saturday and Saturday night, with high
end advisory/low end warning level surf expected for north and
west facing shores. Surf will be the largest on Kauai and Niihau,
and slightly lower over the rest of the islands due to some island
shadowing. Rough and choppy conditions will persist along east
facing shores due to the strengthening trade winds this weekend.
The incoming swell will also raise seas to SCA levels this
weekend for almost all coastal waters, with the SCA seas reaching
the waters northwest of Kauai Saturday morning.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

Aviation...Bohlin
Marine...MBallard
Rest...Hui