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Hawaiian Islands Synoptic Discussion and Guidance

765
FXHW60 PHFO 181333
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
333 AM HST Thu Jul 18 2019

.SYNOPSIS...
A broad area of high pressure centered far north of the islands
will continue to feed the islands with a typical summertime trade
wind pattern through early next week. Showers riding in with the
trades will favor the windward and mountain areas, with the
highest frequency of these showers occurring during the nights and
mornings. Most lee areas of the smaller islands will remain dry,
while the lee side of the Big Island will have its usual clouds
and a few showers every afternoon and evening.

&&

.DISCUSSION...
A 1032 mb surface high is centered about 1100 miles north of the
Big Island this morning, providing the islands with a nice
tradewind breeze. The high will meander around that area well into
next week. But the high will cycle through some weakening and
strengthening phases during this stretch. The high will weaken to
1029 mb about Friday, causing the trades to ease off slightly.
The high will then get reinforced around Monday night to 1031 mb,
resulting in a slight boost in the trades.

The 500 mb low is now a deep trough just west of Kauai. It hasn't
really done much. Low level moisture appears limited, but there is
line of clouds and showers heading towards Oahu that will the
island a couple of hours of clouds and some passing showers. It
should be past of Oahu by mid morning. There are some smaller
patches of low clouds upwind of Kauai and Maui, and these islands
too, will get a few morning showers. The large cloud mass upwind
of the Big Island continues to close in. As of 230 am HST, the
leading edge is 140 miles east of Cape Kumukahi, moving west at 15
mph. This leading edge consist of stable stratocumulus, and not
much showers are expected initially. The more showery pocket lies
further east, and is not expected to reach eastern Big Island
until 12 noon. The mass will spread westward with the trades,
reaching Oahu at around sunset, then Kauai towards midnight
tonight. The mass should clear all but Kauai County by mid Friday
morning.

The upper low just west of Kauai will shift north and deepen some
400 miles north-northwest of Kauai by Friday afternoon. This will
bring a southerly flow aloft over the islands. This setup will
probably draw some high clouds over the islands from the south.
Further more, the GFS is showing the depth of the marine layer
rising from 6 to 7k feet to 10-12k feet on Saturday afternoon
over the smaller islands. We may not see an increase in trade
showers, but the showers will be a little bit more robust or
heavier with this profile. The Big Island could be just outside
the range of this upper low, meaning the threat of thunderstorms
over the interior, higher slopes, is nill at this time. This air
mass will hold into the second half of next week.


&&

.AVIATION...
Relatively strong high pressure still sits well to the north of
Hawaii. This will continue the recent trend of moderate to breezy
northeasterly trade winds across the island chain. AIRMET Tango
remains in place for moderate turbulence below 7,000ft and this
AIRMET will most likely continue into Thursday. Expect lower
clouds and showers on the windward slopes therefore passing
periods of MVFR conditions are possible. Otherwise, VFR for the
rest of the state.

&&

.MARINE...
Good model agreement continues and as a result, little change to
the forecast with the morning package. High pressure north of the
state will maintain breezy to strong trade winds through Friday.
The high is expected to weaken some this afternoon, which will
help the winds ease slightly.

The Small Craft Advisory (SCA) continues through Friday afternoon for
all waters in response to the trade winds. The forecast calls for
the winds to start to weaken Friday night, which will drop the SCA
back to the typical windier areas around Maui and the Big Island.
This SCA will likely be extended through the remainder of the
weekend.

Expect choppy surf along the east facing shores with the breezy to
strong trade winds through Friday. Will continue to monitor the
surf heights, but at this time expecting surf to remain below
advisory levels. East facing shores will see surf slowly lower
over the weekend as trades ease.

Some small background south and southwest swells are expected
through the weekend. An increase in surf along south facing shores
is possible early next week as a couple 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 AM HST Saturday for Maalaea Bay,
Pailolo and Alenuihaha channels, and waters south of the
Big Island.

Small Craft Advisory until 6 PM HST Friday for all Hawaiian
Waters.

&&

$$


H Lau/Chevalier/M Ballard

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