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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 260205

Area Forecast Discussion
National Weather Service Honolulu HI
405 PM HST Tue Sep 25 2018

Low pressure north northwest of Kauai will move east during the
next couple of days, dragging a band of showers eastward over
islands. High moisture levels with southerly winds ahead of the
trough will bring the potential for flooding rains tonight for
Kauai. Muggy conditions are expected to continue across the state
through the next couple of days. A more typical trade wind
pattern should return by Thursday night or Friday, with trades
becoming breezy during the weekend.


Surface analysis depicts a stalled, elongated low pressure trough
about 75 miles to the west of Kauai. Light southeast to south
winds ahead of this trough are bringing a muggy airmass northward
over the state, with dewpoints well into the 70s to 80 on Kauai.
Aloft, water vapor imagery shows a cutoff upper low about 500
miles north north of Kauai which is lifting to the northeast.
Meanwhile, an upper ridge lies just to the east of the state. The
00Z soundings showed a moist airmass in place, with PW of 1.86
inches at Lihue and 1.95 inches at Hilo. MIMIC-TPW imagery depicts
greater than 2 inch PW values extending northward from the deep
tropics over Kauai, and another area of high PWs near 2 inches
moving into the Big Island from the southeast. Somewhat lower PWs
of 1.5 to 1.8 inches extend from Oahu eastward through Maui.

Showers and thunderstorms associated with the surface trough were
concentrated across Kauai and the Kauai Channel last night and
this morning. The shower activity has waned through the day, but
models do show it flaring back up over these areas tonight. A
Flash Flood Watch will remain posted for Kauai and Niihau through
tonight. The upper low to the north will continue moving away
from the islands tonight and should bring an end to the flooding
rain threat. On the Big Island, a few sea breeze driven showers
and thunderstorms fired up over the interior section, and should
dissipate around sunset. The rest of the state has been mostly

Model guidance shows above normal moisture sliding eastward and
lingering over much of the state from Wednesday through the
remainder of the work week. Heavy rains will be less likely, as
the upper low moves away resulting in the loss of forcing over the
islands. Surface troughing also to our north will maintain light
southerly flow through at least Thursday, therefore expect warm
and muggy weather to continue through then. A slight chance of
thunderstorms is forecast for the Big Island for both Wednesday
and Thursday afternoons due to the abundant moisture and daytime

Trade winds should resume by Thursday night or Friday as weak
high pressure builds to our north, and a tropical low pressure
area possibly develops to our south. Longer range guidance shows
the tropical low developing further during the upcoming weekend as
it tracks off to the southwest and west of the islands, with
breezy trades and a drier airmass returning to the state.


Winds are mostly out of the south-southeast today from the Big
Island to Oahu. Afternoon sea breeze/night land breeze scenario.

Rain should continue from the northern half of Oahu over the
Kauai Channel and over Kauai through tonight and into Tuesday
before it moves a little farther south. AIRMET Sierra for MTN
obscuration over Kauai should remain in place through today and
probably into Tuesday. Expect MVFR conditions across Kauai and
into Oahu at the end of this period and VFR elsewhere in the

AIRMET Sierra is in effect for mountain obscuration over Kauai.


Light to moderate southerly winds will continue across the
Hawaiian waters through midweek as low pressure to the northwest
slowly lifts north-northeastward. Plenty of moisture and
instability will support moderate to heavy showers at times,
especially over the western waters. The pressure gradient will
begin to tighten over the weekend as high pressure builds to the
north and a potential tropical cyclone develops and passes to the
south. Seas will quickly respond and build, potentially reaching
Small Craft Advisory levels this coming weekend.

Surf along south facing shores will remain small through midweek
with mainly background south-southeast energy. A small to
moderate south-southwest swell is expected to fill in Thursday
night through Friday, then hold into this weekend before easing.
A slightly larger reinforcing south-southwest source will be
possible early next week.

Surf along north facing shores is expected to reach moderate
levels this weekend. Guidance continues to show a large area of
strong north-northwest winds setting up over the central Pacific
later tonight through midweek as low pressure slowly lifts
northeastward. Proximity of this source to the islands will
result in shorter periods upon arrival (10-12 sec), which will
support surf heights remaining below advisory levels.

See the latest Oahu Surf Discussion (SRDHFO) for more details on
surf and swell.


Flash Flood Watch through late tonight for Niihau-Kauai.




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