Heavy Rain in Hawaii, September 9, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
404 PM HST Sat Sep 9 2017

.SYNOPSIS...
High pressure northeast of the state will weaken through the rest
of the weekend with land and sea breezes becoming more prominent
by Sunday. Light background easterly winds will still bring
clouds and showers to windward areas at times, with sea breezes
brining clouds and showers to leeward and interior sections each
afternoon. Trade winds are expected to fill back in around Friday.

&&

.DISCUSSION...
High pressure northeast of Hawaii is driving gentle to moderate
trade winds this afternoon, with winds light enough in some areas
for leeward and interior cloud build ups. Radar shows scattered
showers over leeward Kauai, Maui and the Big Island this
afternoon, which will likely dissipate around sunset. Windward
showers were more prevalent early this morning, but have tapered
off through the day.

The reason for the showers is due to the relatively higher moisture
in the atmosphere. Afternoon soundings showed above average
precipitable water (PW) values of 1.48 inches at Lihue and 1.66
inches at Hilo. A weak upper low is centered about about 400
miles west southwest of Kauai. Also, a weak tropical disturbance
is about 500 miles southeast of Hilo tracking westward and is will
bring some additional moisture to the Big Island along it's
northern periphery as it passes south of the state. Showers will
likely increase again this evening into Sunday for all windward
sections and southeast Big Island as the aforementioned moisture
skirts by south of the state, and additional clouds and showers
arrive in the trade wind flow.

A cold front currently far north-northwest of Hawaii is expected
to advance southeast through the next few days, then stall and
gradually weaken through Thursday. Daytime sea breezes and
nighttime land breezes will begin to be the dominant weather
pattern beginning tomorrow. Forecast guidance shows background
synoptic winds will stay easterly enough such that emissions from
the Big Island will remain south of the smaller islands. Expect
muggier sensible weather under the lighter winds through at least
Thursday.

Low clouds and showers embedded in the light easterly synoptic
winds will reach the windward areas at times, while clouds and
showers will develop over the interior sections of the islands
during daytime under the sea breezes. Latest model solutions hint
stronger trade winds may not return to the islands until around
Friday.

&&

.AVIATION...
Clouds and showers are expected to continue to primarily impact
the windward and mountain sides of the islands of the smaller
islands, with predominantly VFR conditions expected. Clouds and
some showers over the Big Island are providing some mountain
obscuration, while the clouds over Kauai have started to thin.
Will likely post AIRMET SIERRA for mountain obscuration for the
Big Island and drop it for Kauai.

Heading into tonight and tomorrow, trades will weaken and some
land and sea breezes will begin to set up. This will lead to at
least some clearing overnight, and cloud build ups Sunday
afternoon.

&&

.MARINE...
Trades are forecast to weaken the rest of this weekend as a cold
front approaches the region from the northwest. This will allow
land and sea breezes to setup by Sunday afternoon and continue
through early next week near the coast. Surf along east facing
shores will trend down over the weekend as the trades relax
locally and upstream of the state.

Surf along north facing shores will remain elevated, but below
advisory level through the weekend due to overlapping north and
northwest swells from recent sources across the northern Pacific.
Another small northerly (340-010 deg) swell should reach the
waters Tuesday, then hold through midweek before slowly trending
down. Surf from this source will remain small along north facing
shores.

Surf along south facing shores will remain small through the weekend
into early next week with a small southeast swell holding through
the next couple of days. A small to moderate southwest (220 deg)
swell that was generated in the Tasman Sea should fill in around
the midweek time frame and lead to a slight increase in surf
through the second half of the upcoming week along south facing
shores.

For the extended, the latest surface analysis across the southern
Pacific showed gales out of the southwest across the Tasman Sea
associated with a 976 mb low over southern New Zealand. This
energy should fill in across the local waters late next week
through the weekend out of 220 deg, resulting in rising surf
along south facing shores.

&&

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

&&

DISCUSSION...Foster
AVIATION...M Ballard
MARINE...TS