Heavy Rain in Hawaii, July 30, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
344 AM HST Mon Jul 30 2018

.SYNOPSIS...
High pressure north of the state will keep moderate trade winds
in place through tonight, with the trades then strengthening to
breezy levels on Tuesday and continuing through the end of the
work week. A wetter than normal trade wind shower pattern is
expected through Tuesday as a weak disturbance moves through the
island chain. A more typical trade wind pattern is expected to
return Tuesday night through Friday, although wetter than normal
conditions may continue across the Big Island through Wednesday.
Showers will favor windward and mauka areas through the week, with
a few showers affecting leeward communities from time to time as
well.

&&

.DISCUSSION...
Currently at the surface, a 1032 mb high centered around 1725
miles north-northeast of Honolulu, is driving moderate trade
winds across the island chain early this morning. Meanwhile, a
weak trough of low pressure is producing some disorganized
thunderstorm activity around 550 miles south-southeast of the Big
Island, and the post tropical remnant low of Gilma is located
around 800 miles east-southeast of Hilo. Infrared satellite
imagery shows partly to mostly cloudy skies in place across the
state, due to a combination of low clouds embedded within the
trades and scattered to broken jet stream cirrus. Radar imagery
shows scattered to numerous showers moving into windward areas,
with a few showers affecting leeward areas as well. Main short
term concerns revolve around rain chances and fluctuating trade
wind speeds over the next couple days.

High pressure to the north-northeast of the state will shift
slowly southward over the next couple of days while the post
tropical remnant low of Gilma tracks westward and opens up into a
trough. The trough is expected to pass by approximately 250 miles
south of the Big Island on Wednesday. As a result, the pressure
gradient across the island chain will strengthen through the first
half of the work week, with moderate trade winds today and
tonight, increasing to breezy levels Tuesday through mid week.
The high pressure ridge axis will then settle southward through
the end of the work week, while another high strengthens well
north-northwest of the state. This will maintain breezy trade
winds through Friday with a slight easing of the trades expected
over the weekend.

As for sensible weather details, we should see a wetter than
normal trade wind shower pattern continue through Tuesday due to
the convergent low level flow in advance of the strengthening
trade winds, as well as above normal precipitable water values
ranging from 1.6 to 1.9 inches. As a result, shower coverage and
intensity will likely be a bit higher than normal statewide.
Additionally, with the moist and unstable airmass over the Big
Island along with sea breezes resulting from the blocking effects
of the terrain, an isolated thunderstorm or two will be possible
during the afternoon hours both today and Tuesday.

Precipitable water values will trend down slightly Tuesday night
through Friday, particularly over the smaller islands, so we
should see more of a typical trade wind shower pattern return.
Across the Big Island however, precipitable water values will
remain elevated due to the proximity of the remnants of Gilma
passing by to the south, so shower coverage and intensity will
likely remain a bit elevated here Tuesday night and Wednesday,
before trending closer to climatology Wednesday night through
Friday.

Confidence in the long range forecast Friday night through the
upcoming weekend remains below normal, as the model solutions
continue to show differences in the amplitude and positioning of
an upper level trough developing in the vicinity of the islands.
The GFS paints a much more pessimistic forecast than the ECMWF,
showing a deeper trough and more deep tropical moisture streaming
through the islands. Given the uncertainties this far out, will
utilize a blended approach for forecast details, and show a wetter
than normal trade wind pattern statewide Friday night through the
weekend.

&&

.AVIATION...
Low clouds are pushing across north through east portions of the
islands this morning. While VFR conditions continue to dominate
at the terminals, brief MVFR conditions may occur across windward
slopes today and tonight. Brief MVFR conditions may also occur at
PHNY. AIRMET Sierra may be needed later this morning if mountain
obscuration becomes widespread. Only light turbulence is expected
over and immediately south and west of mountain ranges as low
level winds are generally running in the 10 to 20 kt range.

&&

.MARINE...
High pressure north of the islands will maintain a breezy trade
wind flow over the region through the week. An uptick in winds is
expected by midweek as the high strengthens. The current Small
Craft Advisory (SCA) remains posted for the typically windier
locations near Maui County and the Big Island through Tuesday
afternoon. With winds expected to remain elevated beyond that,
expect the SCA to be extended. As the trades respond to the
strengthening high, additional areas may need to be added to the
SCA.

No significant swells are expected. With the breezy trade winds,
small, choppy breakers will continue along east facing shores
today. As the trade winds strengthen, the wind swell along east
facing shores will also increase. Small southeast and south swells
will maintain small background surf along south facing shores
through the week. A small and short-lived north-northwest swell is
expected to arrive on Wednesday along north facing shores.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

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

&&

DISCUSSION...Jelsema
AVIATION...Powell
MARINE...M Ballard