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RH39 %
WindW 21 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

560
FXHW60 PHFO 250200
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 PM HST Mon Jun 24 2019

.SYNOPSIS...
A low, at the surface and aloft, will persist northwest of the
islands through Thursday. The low will dissipate over the second
half of the week as a high builds northeast of the area.

&&

.DISCUSSION...
Satellite loops show north to south bands of overcast, showery
low clouds nearly stationary over Kauai County and Oahu. Radars
show numerous moderate showers moving over Kauai County and Oahu
from the south. There area partly sunny skies over Maui County and
the Big Island with mostly dry weather.

The weather pattern in the north central Pacific remains very
unusual for late June. A 1008 mb surface low is nearly stationary
700 miles northwest of Kauai near 31N168W and a front associated
with the low runs from 30N159W to 135 miles northwest of Kauai. A
1024 mb surface high is far northeast of the islands near 36N138W
with a ridge extending from the high over Maui County. South of
the ridge, moderate to locally fresh easterly trade winds are
continuing to blow near the Big Island. The surface winds veer out
of the southeast around the smaller islands. Southeast flow over
the islands will be partly blocked by the Big Island, so winds
will be gentle and variable over the smaller islands with daytime
sea breezes and nighttime land breezes.

Moisture and instability associated with the low and front west
of the islands is expected to spread east starting tonight. The
threat of heavy rain and thunderstorms will be greatest over Kauai
County tonight, but the computer models show this threat spreading
over Oahu tomorrow. A Flash Flood Watch remains in effect for
Kauai County and Oahu. We may need to expand the watch to include
Maui County. The atmosphere over the Big Island will be drier and
more stable.

Moisture and instability over the islands will subside starting
Wednesday night as the low weakens and high pressure builds
northeast of the state. Trade winds will spread back over the
islands starting Thursday evening bringing drier air and ending
the threat of heavy showers.

&&

.AVIATION...
Moisture and instability over Kauai will produce showers, which
could be heavy at times with the possibility of thunderstorms for
the remainder of Monday. Western portions of Oahu may be clipped
by a few showers as well. Where we have the showers we are
expecting lower level ceilings and possible mountain obscurations,
mainly for Kauai and Oahu Monday evening. The rest of the state
should see passing MVFR conditions with afternoon sea breezes and
overnight land breezes with showers along the windward slopes
possibly spilling over to a few interior sections this afternoon.

AIRMET Sierra is currently in effect for Oahu for mountain
obscurations above 2,000ft. Will monitor Kauai and issue an AIRMET
for mountain obscuration if warranted.

&&

.MARINE...
A potent disturbance aloft will approach the islands from the NW
through Tuesday, supporting the development of a closed low aloft
several hundred miles NW of the islands. The low is expected to
linger for a couple of days, supporting the development of one or
more slow-moving convergence bands over portions of the area.

The threat of unsettled weather will be greatest over waters
around Kauai and Oahu through Thursday, potentially spreading E to
include Maui county waters later Tuesday into Wednesday. Threats
to mariners include locally erratic wind and reduced visibility
in areas of heavy showers/thunderstorms, as well as the potential
for cloud-to-surface lightning strikes and waterspouts. This system
is not expected to bring impactful weather to Big Island waters.

Outside areas of convection, expect moderate to locally fresh E
to SE winds over Big Island waters, while the remainder of the
waters will see light to moderate SE to S winds that may occasionally
shift SW. If a weak surface low develops in response to the low
aloft, S winds may briefly become fresh to strong near Kauai later
in the week. As the low departs to the NW, and high pressure
builds to the NE, E trade winds will build over the area Friday
and Saturday. Until that time, winds and seas are expected to be
below Small Craft Advisory criteria.

Surf along all shores is expected to be below High Surf Advisory
levels through the week, but a long-period S swell arriving next
weekend is expected to bring advisory-level surf to S facing
shores. A small short-period NE swell will linger for another day
or so before diminishing, while building trade winds next weekend
will bring increasingly choppy surf to E facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 6 PM this evening through Wednesday
afternoon for Niihau, Kauai and Oahu.

&&

$$
DISCUSSION...Donaldson
AVIATION...Chevalier
MARINE...Birchard

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