Flash Flooding and Heavy Rain in Hawaii, November 8 to 10, 2018

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
425 PM HST Thu Nov 8 2018

.SYNOPSIS...
A cold front to the northwest of the islands will strengthen as it
moves towards the islands. The front is expected to reach Kauai by
Friday morning, and move down the island chain, pushing south of
the Big Island on Sunday. Showers will accompany the front, with
cool, dry air moving in behind the front. A new high will build
in behind the front, returning trade winds next week.

&&

.DISCUSSION...
Have made some modifications to the PoP and related fields over
the first couple of days in the forecast to better handle the
approaching front. Otherwise the forecast remains largely intact.

Satellite shows the cloud band associated with the front is about
80 miles to the northwest of Kauai this afternoon. Early this
morning there were a couple thunderstorms along the line, however
nothing has been observed today. Models show the front
strengthening as it moves towards and over the islands over the
next 24 hours.

Afternoon soundings from Lihue and Hilo show precipitable water
(PW) values of 1.40 and 1.52 respectively, both near normal for
November. This matches well with satellite derived PW, which shows
PWs to the northwest of Kauai reaching near 1.7, which would be
above normal for Kauai, and at the high end of normal for Hilo.
Satellite shows PW dropping off about 200 miles northwest of
Kauai. Models show some 500 mb temperatures on the cold end of
normal moving in with the front.

Have opted to not put mention of thunderstorms in the forecast,
but will continue to monitor the front's development to re-examine
the threat.

Behind the front, cool, dry, northerly air is expected to move in,
lowering dewpoints in to the low 60s or even upper 50s in some
locations. Kauai could see the cooler dewpoints as early as
Saturday afternoon, while they wouldn't reach Oahu until Saturday
night/Sunday morning.

High pressure behind the front will help re-establish trade winds
on Monday, with the high moving to the northeast of the islands by
midweek, with a ridge remaining north of the islands.

&&

.AVIATION...
The cold front approaching from the northwest is within 100nm of
Kauai. Precipitation from the front could reach Kauai Friday
morning, and move down the chain to Oahu as the day wears on. The
loose pressure gradient will keep winds relatively light so
expect daytime sea breezes and nighttime land breezes across the
island chain. As the front approaches, there will also be
temporary wind shifts to the south-southwest and then to the
north after frontal passage.

There are no AIRMETs currently in effect.

&&

.MARINE...
A front is currently located about 70 nm NW of Kauai's northwest
coast. The front is slowly moving SE, toward the islands, and is
expected to reach the western coastal waters west of Kauai Friday
morning, Friday evening for Oahu, early Saturday morning for Maui
County, and by Saturday evening for the Big Island. The front
should be moving east of the Big Island on Sunday morning. There
is a degree of uncertainty with this time line, so the forecast is
subject to change.

Before the actual front arrives, light northeast winds will
spread over the main Hawaiian Islands tonight. We will know when
the front is here as moderate to strong NE winds moves in right
behind the front. This will warrant the issuance of a Small Craft
Advisory (SCA) for most of the near-shore waters through early
next week.

There will be a series of N to NNW swells impacting the islands
through next week. The current NNW swell is expected to hold
through Friday. A larger short to mid period N swell is expected
to move in Friday and may bring near advisory surf to north
facing shores over the weekend. As this swell slowly fades in the
ensuing days, a small NW swell with a 15 second period will reach
our shores on Monday. A larger NNW swell will be possible Tuesday
that may warrant another high surf advisory for at least the north
facing shores.

A small south-southwest swell is providing a slight increase in
surf across south facing shores today. A larger and very long
period southwest swell is expected to fill in late Friday, peak
Saturday at heights around the summer average and gradually
decline early next week.

&&

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

&&

DISCUSSION...M Ballard
AVIATION...Chevalier
MARINE...Kino