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RH91 %
WindSSW 6 mph
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Hawaiian Islands Synoptic Discussion and Guidance

889
FXHW60 PHFO 221353
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
353 AM HST Sun Sep 22 2019

.SYNOPSIS...
Increasing moisture moving over the islands from south to north
and lighter winds will bring increasing showers statewide. An
upper level trough to the west of the islands will maintain some
instability that could cause some of the showers to be heavy, and
could lead to a few thunderstorms. Drier weather is expected by
Tuesday for Maui County and the Big Island, with lingering showers
expected over the western islands.

&&

.DISCUSSION...
No significant changes to the forecast with the morning package.
Have brought the slight chance for thunderstorms a bit further
north over the waters to the northeast and southwest of Oahu based
on some areas of towering cumulus and brief thunderstorms
overnight.

Water vapor shows the upper level low to be about 230 miles
north-northwest of Kauai overnight and continuing to move to the
west. High clouds that had been over the southern end of the
island chain are spreading to the west, and will are expected to
cover all the islands and surrounding waters by this afternoon.

Satellite derived precipitable water (PW) shows 2+ inches reaching
the Big Island this morning, which is also reflected in the
overnight sounding at Hilo which reported 2.12 inches of PW, while
the Lihue sounding had 1.66 inches. 500 mb temperatures from the
soundings remained near -6 at Hilo and -8 at Lihue. Models
continue to show a gradual warming over the next 36 hours or so.

Winds are expected to weaken and turn more to the southeast today,
which will allow for some localized sea breezes. This is in
response to a surface reflection of the upper level trough to the
northwest of the islands. This will contribute to more shower
activity over leeward and interior sections. The combination of
increased PW and the unstable airmass nearby due to the upper
level low means we still could see some heavier showers and/or a
few thunderstorms. The most unstable airmass is to the northeast
of the moisture maximum, but the pieces are close enough to
support some convection.

The upper level low is still expected to lift northward during the
first part of the work week and being absorbed into another upper
level feature to the north-northwest of the islands. This will
leave a trailing convergence area over the western islands, so
expect more showers there than over the eastern islands during the
middle to latter part of the week.

&&

.AVIATION...
High pressure far northeast of the state will maintain east to
northeast flow over the state this morning. Surface winds will
veer to to east southeast this afternoon in reaction to a surface
reflection of the upper level trough to the northwest. Meanwhile,
an upper low centered just west of Kauai will keep the air mass
unstable statewide.

Unstable trade wind flow will yield scattered showers and isolated
thunderstorms this morning, mainly across windward slopes and
coasts. As deeper moisture moves in from the southeast later
today, expect increased chances for MVFR or IFR conditions in
locally heavy showers and thunderstorms. AIRMET Sierra for
mountain obscuration may be needed for east and southeast facing
slopes of Big Island and Maui towards evening.

Meanwhile, a band of layered middle and high clouds over the Big
Island is expected to spread northwestward across the remainder of
the state by early evening. AIRMET Zulu for light to moderate
icing may be needed above fl180.

&&

.MARINE...
A surface reflection of the upper level trough to the northwest of
the islands will move over the state and will cause the winds to
veer to the east-southeast and decrease. As this feature passes
over the state today, winds will drop to light to moderate speeds
over most areas. The channels and windward zones near the Big
Island and Maui County, though, will likely see fresh speeds due
to terrain accelerations. Fresh to locally strong speeds are
expected to return late Monday as the trough moves west of the
state and the persistent high, far north-northeast of the state,
builds back across the area. A brief return to Small Craft
Advisory (SCA) conditions for one or more of the typically windy
zones near the Big Island and Maui is a possibility.

A front developing northwest of the state early in the week should
cause winds to decrease to light to moderate east-southeast winds
by midweek. Beyond Tuesday, winds are expected to remain in an
east to southeast direction and below SCA levels as the front
remains northwest of the state and the high persists north-
northeast of the islands.

A series of small swells from the southwest and south are
expected through the week. A new moderate period south-southwest
swell is due Monday and will be followed by a larger long-period
swell later in the week. This latter swell may reach advisory
levels along south facing shores late Thursday into Friday. A new
northwest swell will continue to fill in across the state and
peak today, and lower through the day Monday. A few other small
northwest swells will be possible towards the end of the week.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$


DISCUSSION...M Ballard
AVIATION...Bedal
MARINE...Kinel

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