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RH69 %
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 171316

Area Forecast Discussion
National Weather Service Honolulu HI
316 AM HST Tue Jul 17 2018

High pressure north of the state will keep a breezy trade wind
flow in place through the next few days. Clouds and showers will
favor windward and mauka areas, with a few showers spilling
leeward from time to time. Little change is expected into the
upcoming weekend.


Breezy trade wind weather prevails on this early Tuesday morning.
Satellite and radar shows areas of clouds and scattered showers
over the windward waters, with some showers moving across windward
land areas. Rain gauges report only very light rainfall totals at
some windward locations during the past six hours. Surface
analysis depicts an east-west oriented high pressure ridge located
about 1100 miles north of Kauai, with a weak and disorganized
tropical disturbance (Invest 91C) located about 750 miles
southeast of the Big Island. Breezy trade winds prevail over the
state to the south of the ridge. Aloft, a lengthy mid/upper level
ridge extends from the Northeast Pacific to the WSW over Hawaii,
while a large and persistent Tropical Upper Tropospheric Trough
(TUTT) is located to the west and northwest of the state. The
ridge aloft is providing stable conditions over Hawaii. The 12Z
Hilo sounding appeared to be affected by a passing shower at
release time, but the 12Z Lihue sounding showed a strong inversion
based at 5500 feet with measured PW of 1.53 inches. MIMIC-TPW
satellite imagery shows similar PW values over and to the east of
the state, with a more moist airmass exiting to the west of Kauai.

Little overall change is expected during the next several days,
with moderate to breezy trade wind weather continuing, and mainly
passing showers focused over windward areas. Model consensus is
in good agreement with the surface ridge holding far north of the
state during this time. Fairly dry trades should continue through
this evening, as mid-level ridging overhead keeps the inversion
strong and fairly low and PW values remain near normal. Invest 91C
is forecast to continue tracking westward, passing about 350
miles south of the Big Island on Wednesday. An increased pressure
gradient between this disturbance and the ridge will cause trade
winds to increase a bit more over the state. While the bulk of the
deep tropical moisture is forecast to remain south of the state,
enough additional moisture may pass over the islands to cause some
increase in shower activity, mainly over windward/mauka areas,
from late tonight through Wednesday evening. A dry airmass
spreading in from the east should result in lower rain chances
over the islands from Thursday through Saturday. Another surge of
tropical moisture associated with a disturbance to our southeast
and south may arrive over the state late in the weekend, but there
is still considerable uncertainty as the latest GFS and ECMWF are
not in agreement that far out in time.


Breezy trade winds are forecast over the next 24 hours with a
persistent high pressure ridge far to the north of the Hawaiian
Islands. Low clouds and scattered showers moving through the trade
winds today will bring mostly VFR conditions developing during the
daylight hours with periods of MVFR conditions forming along some of
the windward and mountain slopes during the overnight periods. The
windward slopes of the Big Island will see persistent MVFR conditions
and increasing showers tonight as deeper moisture moves in from the

AIRMET Sierra continues for Tempo Mountain Obscuration with MVFR
ceilings over north through east sections of Kauai, Oahu, Molokai,
Maui, and the Big Island. This AIRMET will diminish later this

AIRMET Tango issued for Tempo Moderate Turbulence over mountains
and south through west sections of each island including coastal
waters. Breezy trade winds will keep these conditions in the
forecast through this evening.


Broad high pressure far north of the islands will continue to
produce locally breezy trade winds across the state through mid-
week. In addition, an area of low pressure, which is expected to
pass south of the islands Wednesday and Thursday, will also
likely cause trade winds to strengthen and seas to build. A Small
Craft Advisory (SCA) has been issued for all of the waters east
of Oahu, except the Big Island Windward Waters, through Wednesday
night. In addition, the Oahu Leeward Waters have been added to
the SCA starting tonight, and continuing through Wednesday night.
Additional waters may need to be added to the SCA later today or
tonight. Trade winds are forecast to gradually weaken later this

The locally breezy trade winds combined with a small short-period
northeast swell may cause choppy surf along east facing shores to
approach the High Surf Advisory criteria from Wednesday into
Thursday. Surf along along east facing shores is expected to
gradually subside from Friday through this weekend as the trade
winds weaken.

A series of reinforcing south and southwest swells will maintain
small to moderate surf along south facing shores across the state
through Friday. Slightly larger and longer period south-southwest
swells spreading across the state this weekend may cause surf to
approach the High Surf Advisory along south facing shores from
Saturday into Monday.


Small Craft Advisory until 6 AM HST Thursday for Kaiwi Channel-
Maui County Windward Waters-Maui County Leeward Waters-Maalaea
Bay-Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-
Big Island Southeast Waters.

Small Craft Advisory from 6 PM this evening to 6 AM HST Thursday
for Oahu Leeward Waters.




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