Heavy Rain in Hawaii, June 16 to 17, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
819 PM HST Sat Jun 16 2018

.SYNOPSIS...
A weak ridge north of the islands will keep a light trade wind
pattern with land and sea breezes in the forecast along south and
west slopes of all islands through Tuesday. Clouds and showers
will develop over mountains and island interior sections each day
with partial clearing at night. Deep northeasterly winds are
developing over the islands this evening, these winds should be
strong enough to push the VOG plume towards the southwest
overnight and keeping it away from the smaller islands by Sunday
morning.

&&

.DISCUSSION...
The high pressure ridge north of the island will remain weak for
several days as frontal systems transit across the North Pacific.
Winds aloft however have switched to northeasterlies through a
deep layer, these winds should be strong enough to push the VOG
(Volcanic smOG) plume towards the southwest and away from the
smaller island by Sunday morning. This weak ridge north of the
state will keep light trades in the forecast through Wednesday,
with daytime sea breezes and overnight land breezes along south
and west slopes of all islands.

In the short range forecast, expect clouds and a few showers
developing each day over mountains and island interior sections
through Tuesday as daytime heating and weak trade wind interaction
causes convection. From Tuesday night into Wednesday northeasterly
flow will bring remnant clouds from an old frontal boundary into
the Hawaiian Islands increasing cloud cover across the state. Wind
directions on Wednesday may briefly switch back to southeasterlies
for a long enough time for another short round of VOG over the
smaller islands.

In the longer forecast range, the high pressure ridge in the
northeast Pacific strengthens by the end of the week with more
moderate strength trade winds returning to the islands lasting
through next weekend. Clouds and showers will favor windward and
mountain areas during this time period.

&&

.AVIATION...
Pressure gradient remains slack as a high pressure ridge is near
the state. Thus winds will stay light with day-time sea breezes
and night-time land breezes over the islands. Anticipate VFR
conditions for much of the state through early Sunday morning,
though the on-going volcanic emissions on the Big Island may
bring areas of MVFR VIS HZ to that island.

&&

.MARINE...
A weak ridge of high pressure will remain nearly stationary just N
of Kauai through the weekend, supporting light to locally moderate
E trade winds. A small "bubble" high is expected to develop
within the ridge to the NW of Kauai over the next 24 hours, with
low level winds remaining light, but backing toward the NE over
some of the area. A dissipating front will drift toward the
islands from the N late Monday into Tuesday, maintaining light
winds that may become more variable in direction through
Wednesday. High pressure will consolidate far NE of the area
Thursday and Friday, supporting seasonable moderate to strong
trade winds.

A long-period WNW swell and a long-period S swell will continue to
provide moderate surf to most island shores this weekend, with
surf along E facing shores smaller than normal due to the lighter-
than- normal trade winds. These two swells are combining to
produce surf up to 8 feet along the W facing shores of the Big
Island, which warrants a High Surf Advisory (HSA), which is
currently in effect through early Sunday. The WNW swell is
expected to change little through Sunday before only slowly
diminishing through Wednesday, and the HSA may need to be
extended. The S swell is expected to gradually diminish through
Monday. Little other swell is expected through much of next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Sunday for Kona-Kohala.

&&

DISCUSSION...Bohlin
AVIATION...Hui
MARINE...Birchard