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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 181950
Area Forecast Discussion
National Weather Service Honolulu HI
950 AM HST Thu Apr 18 2019
Moderate to locally breezy trade winds will prevail through much
of the week as surface high pressure holds north of the state.
Rather stable conditions will remain in place, leading to a
typical pattern of mainly windward and mauka showers.
A stable and moderate to locally breezy trade wind flow is place
across most of the state. The trades are being driven by a 1027
mb high passing about 1,100 miles north of the state. A mid level
ridge is building over the islands from the west, pushing a weak
mid level trough a couple hundred miles east of the Big Island.
This is causing the inversion to be slightly higher and weaker
over the eastern end of the state, with the overnight soundings
from Hilo and Lihue showing the inversion near 8,500 ft and 6,000
ft, respectively. As a result, a small pocket of moisture is
producing active showers over windward Big Island, while modest
rainfall has been confined to windward terrain elsewhere. Expect
showers to decrease on windward Big Island this afternoon, as
spotty showers develop over the Kona slopes. Elsewhere, little
change is expected today.
Overall, a typical trade wind weather pattern is expected through
the upcoming week. The surface high will settle to the northeast
of the state, while its associated surface stalls about 300 to 500
miles north of the islands. We could see an uptick in trade winds
during the weekend as the ridge briefly strengthens. Stable
conditions will dominate through much of the week as the mid level
ridge will be parked overhead. Generally, precipitable water will
be near normal, suggesting that showers will be modest and
confined to windward slopes. As random pockets of low level
moisture move through on the trade wind flow, expect brief and
localized increases in showers. Aside from afternoon showers on
the Kona slopes of the Big Island, leeward areas will be mostly
Strong high pressure far north of the islands will support a
locally brisk trade wind pattern through twenty-four hours and
beyond. Clouds and showers will continue to favor windward slopes
and coasts of all islands.
The air mass over the eastern third of the state is expected to
become mildly unstable this afternoon. Sea breeze driven cumulus
development across leeward and interior Big Island could produce
brief heavy downpours this afternoon and early evening.
Elsewhere, the air mass will remain mostly dry and mostly stable.
Shower activity across the smaller islands is expected to remain
light and patchy.
Trade winds will continue to strengthen as the day progresses.
AIRMET Tango, already in effect for mechanical turbulence below 8k
feet, will continue until further notice, possibly for several
Fresh to strong easterly trade winds associated with a
strengthening ridge north of the islands will fill in today and
continue through early next week. A Small Craft Advisory is in
effect for the Pailolo and Alenuihaha channels, Maalaea Bay, and
over the waters around South Point of the Big Island through
Sunday where the strong winds are expected.
Surf along north and west facing shores will remain small today,
then rise tonight into Friday as a new north-northwest (320-340
deg) swell builds down the island chain. This source will be from
a gale that was centered over the Aleutians near the Date Line
Sunday that quickly tracked south-southeast and weakened Monday
through Tuesday. After the peak on Friday, the swell will quickly
ease Friday night into the weekend. A small westerly swell from a
compact gale that has developed east of Japan will be possible
early next week.
Surf along south facing shores will remain small through the weekend
with mainly background south-southwest pulses moving through. An
upward trend is anticipated early next week, beginning as early as
Sunday night due to activity south to southeast of New Zealand over
the past few days. Guidance depicts this source peaking at Pago Pago
Saturday, which should correlate to above average south shore surf
continuing into midweek.
Surf along east facing shores will remain small today, then
slightly come up Friday through the weekend due to fresh trades
locally and just upstream. Expect this trend to continue early
Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office