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Hawaiian Islands Synoptic Discussion and Guidance

787
FXHW60 PHFO 220153
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
353 PM HST Thu Feb 21 2019

.SYNOPSIS...
Generally light winds are expected across much of the area for the
next couple days. Fairly dry conditions will also remain over the
western half of the state. A surface trough lingering near the
Big Island will enhance clouds and showers over Maui and the Big
Island into the weekend. High pressure systems moving in from the
northwest may strengthen the winds early next week.

&&

.DISCUSSION...
Water vapor imagery shows an upper level ridge just east of the
state with a shortwave trough passing by to our north. At the
surface, a trough is located just east of the Big Island and a
weak low is a few hundred miles west-northwest of Kauai, with a
very weak pressure gradient between them across the islands. The
00z sounding from Lihue shows a solid inversion just below 6kft
and a precipitable water of 0.80 inches (which is about a
standard deviation below normal for this time of year). The Hilo
sounding showed a weaker low-level inversion and a more
pronounced one near 13kft, and a PW of 1.46 inches (which is about
a standard deviation above normal). MIMIC total precipitable
water imagery captured this gradient across the islands, with
higher values above 2 inches just offshore south and east of the
Big Island that have made only slight progress westward during
the day.

With the light gradient in place across the state, onshore sea
breezes were pretty widespread today. Cloud buildups have mainly
affected interior and northeast sections of the islands, with
little in the way of rainfall from them. The afternoon clouds are
most noticeable across Kauai/Oahu away from the more widespread
mid-level clouds farther east. The sea breezes will give way to
offshore land breezes this evening, leading to some clearing
overnight, at least across the western islands. Skies will remain
at least partly cloudy with some showers in the east, where there
is greater moisture closer to the surface trough.

The surface trough near the Big Island will drift westward slowly
through Friday, spreading additional low-level moisture across
the Big Island and eventually Maui County as well. Rainfall
chances are greatest on Friday, then start to taper off Saturday
into Sunday. With the wind shifting more southeast near the Big
Island, highest rainfall amounts will be focused across windward
and southeast sections. An upper level trough moving through the
area may enhance lift, and with the marginally unstable airmass
and abundant moisture we could see embedded heavy rain showers.
However, the upper trough and surface feature will be weakening
with time and don't appear to provide a strong focus for
convection. At this point we're not expecting widespread flash
flooding and therefore do not anticipate a flash flood watch,
although localized heavy rain is possible.

Over the weekend, the surface trough will weaken and shift
eastward as a new high builds in from the northwest of the state.
Deeper moisture and some showers will linger longest across the
Big Island, but conditions there should start to stabilize by
Sunday night/Monday. This first high will be somewhat brief,
dissipating early next week as a front passes by far north of the
state. Another high will build in behind the weakening front,
bringing an increase to north/northeast winds and cooler/drier
conditions by the middle of next week.

&&

.AVIATION...
A surface trough near the Big Island will slowly track westward
tonight resulting in an increase in showers over the Big Island
tonight and Maui on Friday. Mountain obscurations due to these
clouds and showers may become more extensive tonight and require
an AIRMET later on. No AIRMETs are currently in effect at this
time.

A mix of mid and high clouds continue to stream across the central
and eastern portion of the state. Low clouds over the islands are
due to the afternoon sea breezes will provide brief mountain
obscurations late this afternoon, which should begin to dissipate
after sunset as land breezes take over.

&&

.MARINE...
A trough near the Big Island of Hawaii will maintain light to
gentle variable winds over most Hawaiian waters and gentle to
moderate southeast winds over waters east of the Big Island. Winds
are forecast to remain below Small Craft Advisory (SCA) speeds
through at least early next week. The trough will slowly weaken
and shift east as high pressure builds far northwest of the main
Hawaiian Islands. Light to moderate north winds will develop
starting Saturday night. Winds will strengthen and shift out of
the northeast by the middle of next week. Winds could reach SCA
speeds over some areas at that time.

Surf will remain below the advisory threshold along all shores
through the weekend. A new northwest swell is expected to produce
surf near advisory levels Monday night. A very large northwest
swell may produce warning level surf by the middle of next week.
This swell is also expected to boost wave heights above the 10
foot SCA threshold.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until midnight HST tonight for Big Island Summits.

&&

$$


DISCUSSION...Bravender
AVIATION...Foster
MARINE...Donaldson

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