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Hawaiian Islands Synoptic Discussion and Guidance

683
FXHW60 PHFO 190639
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
839 PM HST Thu Jul 18 2019

.SYNOPSIS...
Gusty trade winds will continue through Friday, then slightly trend
down into the moderate to breezy category over the upcoming weekend
and early next week. Clouds and showers will favor windward and
mountain locations, especially through the overnight and early
morning periods when pockets of higher moisture move through. The
exception will be over the leeward sections of the Big Island,
where afternoon and evening scattered showers associated with the
sea breeze develop.

&&

.DISCUSSION...
Satellite imagery showed the leading edge of a broken deck of low
clouds associated with a large area of higher moisture (1.6-1.8 inch
PWs) quickly advancing westward across the state. The afternoon
soundings reflected this increase in moisture from west to east with
1.18 to 1.40 inch PWs shown from Lihue to Hilo. Despite the
abundance of dry mid-level air above a strong subsidence inversion
positioned around 7000 ft acting as a cap to the low clouds moving
in, an upward trend in windward and mountain shower coverage is
anticipated through the overnight period tonight. Observations at
Hilo are already reflecting this with showers moving through
periodically reducing visibilities early this evening. Although
most of this coverage will focus over windward locales, a few
showers will manage to spill over into leeward sections of the
smaller islands later tonight through Friday morning with the
gusty trades in place.

Guidance remains in decent agreement and shows the back edge of the
aforementioned area of higher moisture shifting west of the state
late Friday through Friday night. PWs are forecast to quickly taper
off from east to west through this time with a return to near normal
(drier; around 1.3") by Saturday morning. This downward trend in
moisture will act to reduce the windward shower coverage, which
should continue through the rest of the weekend. The exception
will be due to pockets of higher moisture moving through from east
to west, which will modulate shower coverage.

Trades will remain gusty through Friday due to the tight gradient
associated with strong high pressure to the north. Some of the
windier locations (Maui Central Valley, Lanai and Kohala on the Big
Island) may near advisory criteria once again due to terrain
accelerations during the afternoon period. Guidance suggests the
trades will ease into the moderate to locally breezy category over
the weekend in response to the area of high pressure slightly
weakening to the north.

For the extended period, moderate to breezy trades are forecast to
continue through early next week, then potentially increase by
midweek as the gradient tightens between the subtropical ridge to
the north and a weak trough passing to the south. Showers will
continue to favor windward areas, leaving leeward sections mostly
dry (exception being over leeward Big Island during the
afternoons/evenings). The GFS depicts an area of 1.5 to 2 inch PWs
(above average moisture) clipping this islands Monday night through
Tuesday as it moves through from east to west. The ECMWF, however,
shows this area remaining south of the islands. As a result of
these differences, no significant changes in the forecast are
anticipated through this time until better agreement is shown.

&&

.AVIATION...
The high situated north of the islands is still providing
moderate to breezy wind conditions. Some of these winds will
weaken overnight but pick back up through Friday as we continue to
have persistent windy conditions. As a result of the these winds
across the state, we will leave AIRMET Tango active for moderate
turbulence below 080.

The difference in the weather through this period is a batch of
low level clouds that are moving in across Hawaii. These clouds
may have enough moisture to produce showers, mainly on the
windward side of the islands. The winds in some areas may push a
few showers over interior sections of the islands but this rain
will be rather light. These clouds will continue to produce low
ceilings and visibilities, especially on the northeast sections of
the islands. AIRMET Sierra for mountain obscuration is in effect
for northeast sections of the Big Island, Maui, Molokai, Oahu, and
Kauai.

&&

.MARINE...
No significant changes to the forecast are anticipated this
evening, with high pressure continuing to dominate the forecast.
The high north of the islands will continue to bring fresh to
strong trade winds to the region through Friday, with some
weakening expected over the weekend. Further weakening of the
winds are expected early next week.

The current Small Craft Advisory (SCA) continues for all coastal
waters through Friday afternoon for a combination of winds and
seas exceeding criteria. The SCA then is trimmed back to the
typical windier areas around Maui and the Big Island through the
weekend.

East facing shores will continue to see rough and choppy surf just
below advisory levels in response to the trade wind swell through
Friday. As the winds ease this weekend, the surf will drop
slightly. Some small background south and southwest swells are
expected through the weekend, with an increase in surf along
south facing shores possible early next week as a couple small
southwest swells arrive in the islands.

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 Sunday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory until 6 PM HST Friday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Big Island
Windward Waters.

&&

$$

DISCUSSION...Gibbs
AVIATION...Chevalier
MARINE...M Ballard

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



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