Heavy Rain in Hawaii, May 8, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
959 AM HST Mon May 8 2017

.SYNOPSIS...
A cold front lies 300 miles northwest of Kauai while a 1030 mb high
is 1400 miles northeast of the state this morning. A trough aloft
will bring some instability to the islands but affecting mainly
Kauai and Oahu with a chance of heavy afternoon showers. The trade
winds returns on Tuesday, but the lingering trough aloft will
maintain a wet trade wind pattern for the rest of the week.

&&

.DISCUSSION...
A cold front lies 300 miles northwest of Kauai while a 1030 mb high
is 1400 miles northeast of the state this morning. Moderate southeast
winds prevail as a result of these two features, however winds
are weaker across most islands due the blocking from the Big
Island. An upper trough extending from an upper low far NW of
Kauai will settle over the islands today, bringing along with it a
pool of cooler air. The air mass aloft will cool off a couple of
degrees lowering from a -8 C at 500 mb to minus -10 C over the
western end of the state by this afternoon, meaning more unstable
atmospheric conditions. Accentuated by afternoon heating, locally
heavy showers are possible for Kauai and Oahu this afternoon.
The rest of the island chain will also see clouds and showers
this afternoon, but it is not expected to be quite as heavy.

The trade winds are expected to start returning late tonight over
the eastern end of the state, and gradually strengthen over the
islands on Tuesday and Tuesday night. Parts of Kauai and Oahu may
still experience some afternoon convective type showers Tuesday
afternoon since the deeper and stronger trades will not reach
these two islands until Tuesday night. Once the trades build back
in, they should last through the weekend and into next week.

The upper trough will linger west of the state through the rest
of the week and will continue to produce a wetter-than-normal
trade wind weather pattern. Model soundings keep the inversion
elevated and weak, meaning showers may periodically become heavy
at times.

&&

.AVIATION...
High pressure northeast of the state and a trough of low pressure
to the west, will keep a east-southeasterly trade wind flow in
place through tonight. Sheltering effects from the island terrain
can be expected as a result of this wind direction, with daytime
sea breezes and overnight land breezes expected in many areas.
Showers will favor the coastal waters and windward areas under an
east-southeasterly wind direction through the morning hours, and
the interior of the islands during the afternoon and evening
hours.

No AIRMETs are currently in effect. AIRMET Sierra may be needed
this afternoon across portions of the island chain as shower
coverage and intensity increases.

&&

.MARINE...
The moderate, out-of-season, northwest swell filled in overnight
and appears to be peaking this morning. Despite yesterday
afternoon's obs from Buoy 51101 and 51001 peaking around a foot
above wave model guidance, surf is struggling to reach advisory
levels on north and west facing shores so far. However, the swell
is just now peaking, so will monitor the obs through the day to
help decide the fate of the High Surf Advisory this afternoon. As
it stands, the Advisory runs through 6 pm today.

Surf along south-facing shores will remain small through the
weekend and early next week due to mainly small background energy over
the next several days. Surf along east-facing shores will lower slightly
today due to relaxed trade winds.

Breezier trades should return by Tuesday night, however, with
Small Craft Advisories (SCA) favoring the typically windier marine
zones around Maui County and south of the Big Island again. Surf
will also rebound along the east-facing shores as a result.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for north and west
facing shores of Niihau and Kauai, and north facing shores of
Oahu, Molokai, and Maui.

&&

DISCUSSION...Foster
AVIATION...Jelsema
MARINE...TS