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Hawaiian Islands Synoptic Discussion and Guidance

987
FXHW60 PHFO 190651
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
851 PM HST Sat Aug 18 2018

.SYNOPSIS...
Humid and unsettled conditions associated with a trough of low
pressure moving through the state from east to west will keep the
rainfall and thunderstorm chances in the forecast through Sunday.
Although the bulk of this activity will favor windward and mauka
locations with moderate trades in place, some showers will manage to
develop across leeward areas periodically. Breezy and drier trade
wind conditions are expected to fill in Sunday night through the
early portion of next week. Although it remains too early to
determine how the islands will be impacted, Hurricane Lane is
forecast to track through the region near the islands through the
middle part of next week.

&&

.DISCUSSION...
Short-term (through Sunday night) guidance remains in good agreement
and is initializing well with the current pattern and trends.
Consensus supports humid (mid 70 dewpoints) and unsettled weather
conditions continuing through Sunday as a surface trough tracks
westward across the state. The latest surface analysis and satellite
imagery showed this feature approaching Kauai this evening. Plenty
of deep tropical moisture associated with it combined with modest
mid-level lapse rates led to an active day today for the eastern end
of the state. Heavy showers and thunderstorms developed over the
windward and southeast slopes of the Big Island through the
afternoon hours with peak rainfall totals near the 5" mark at
Papaikou Well (4.9" in 12 hrs). Elsewhere, 12-hour totals ranged
from 2-4" from Hilo to Glenwood.

Although the convective activity over the Big Island from earlier
has diminished this evening, the potential remains in place
overnight through Sunday, mainly for the western end of the state
closer to the aforementioned trough. The latest hi-res model
reflectivities support this potential and depict heavy showers
developing over Oahu/Kauai beginning later tonight into Sunday.
Localized flooding will remain a possibility if and where any heavy
activity develops and becomes focused for any given duration. Over
the eastern end of state, a return of moderate to breezy trades
along with more stable conditions are expected to gradually fill
in from east to west.

The extended (Monday through Friday) guidance remains in
decent agreement and supports a drier and more stable trade wind
pattern filling in from east to west through Tuesday. Trades will
likely climb into the breezy category due to a strengthening
pressure gradient between high pressure to the north and Hurricane
Lane approaching the region from the east-southeast. Forecast
uncertainty ramps up heading into the middle/latter half of next
week as Hurricane Lane passes near or just south of the islands. It
remains too early to forecast the exact location of Lane and whether
it will directly impact portions the state. Interests are encouraged
to monitor the latest advisories for Lane from the Central Pacific
Hurricane Center.

&&

.AVIATION...
High pressure far northeast of the state will keep light to
moderate trade winds in place through Sunday. A weak trough of low
pressure will move from west to east through the island chain
tonight through early Sunday, bringing some enhanced showers and
perhaps an isolated thunderstorm along with MVFR cigs/vsbys
mainly to windward areas. Drier conditions will overspread the
entire state Sunday afternoon, although an isolated thunderstorm
or two can't be ruled out over leeward sections of the Big Island.

AIRMET Sierra is in effect for mountain obscuration across
windward sections of Kauai, Oahu, Molokai, Maui, and the Big
Island. These conditions will likely remain in place through much
of the night.

&&

.MARINE...
Light to moderate trade winds are expected to continue tonight
and remain below Small Craft Advisory (SCA) levels. A weak trough
will continue to move through the islands tonight, for the
potential of some heavier showers and thunderstorms. High
pressure will then build well northeast of the state through the
middle of next week, while Hurricane Lane is forecast to pass by
to the south of the Big Island Tuesday night through Wednesday. As
a result, trade winds will increase again with SCA conditions
returning first to the typically windy waters around Maui and the
Big Island Sunday night, then possibly expanding to additional
zones as moderate to strong trade winds continue through the
middle of next week. It is still too early to determine the exact
impacts that Hurricane Lane may have on the coastal waters, so
mariners are urged to keep up to date with the latest forecasts.

No significant swells are expected through the weekend, so surf
will remain small on all shores. A small, north-northwest swell
is expected to arrive on Monday and continue through Tuesday. A
larger north swell is expected to arrive late Tuesday, peak
Tuesday night and early Wednesday, then slowly fade through
Thursday. We will continue to monitor the track and intensity of
Hurricane Lane as it moves closer to the islands, as an elevated
easterly swell is expected to move into the coastal waters Sunday
night through the middle of next week. This swell will likely
result in large surf for east and southeast facing shores of the
Big Island and potentially large surf for east facing shores of
Maui as well.

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

&&

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

&&

$$

DISCUSSION...Gibbs
AVIATION...Jelsema
MARINE...Kino

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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