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Hawaiian Islands Synoptic Discussion and Guidance

704
FXHW60 PHFO 181330
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
330 AM HST Sun Aug 18 2019

.SYNOPSIS...
The ridge of high pressure north of the Hawaiian Islands will
weaken today resulting in lighter trades lasting through Monday.
Stronger trades will return over the second half of the week as
the high pressure ridge rebuilds north of the area. Showers riding
in with the trades will favor the windward and mountain areas,
especially during the nights and early mornings. Rainfall amounts,
though, will be light as a whole.

&&

.DISCUSSION...
A hybrid mix of light to moderate trades and sea breezes is
instore for today through Monday. This is made possible as the
ridge 400 miles north of Kauai is weakened by a passing front.
The scope of the sea breeze will be limited to only the lee areas
of the smaller islands, as models appear to back off on a widespread
scenario. More importantly, mid level ridging will limit the
vertical development to these daytime clouds to no higher than 10k
feet, thus, resulting in isolated, spotty showers. The weak gentle
trades will be strong enough to bring in a few showers to the
windward areas of all the islands. Leeward Big Island will go
through its normal phase of morning sun, then afternoon/evening
clouds and isolated to scattered showers each day. The Big Island
could get an uptick in trade showers on Tuesday night and
Wednesday, belonging to a passing tropical disturbance south of
the islands. Rainfall amounts are expected to be light, except for
moderate amounts for the Big Island Tuesday night into Wednesday,
with the passing disturbance.

The trades are slated to hold at moderate speeds between Monday
night and Tuesday before bumping up to locally strong by
Wednesday. This comes as the surface ridge rebuilds far north of
the islands. This pattern is expected to continue into next
weekend. Another disturbance may graze the Big Island this coming
weekend.

It should be noted the upper level pattern is changing, where a
trough is being replace by ridging this morning. This mean the
winds aloft are changing from becoming north and northeast this
morning, then to the east by Tuesday.


&&

.AVIATION...
A ridge of high pressure north of the state will keep moderate
trade winds in place through Sunday. Low clouds and showers will
bring brief MVFR cigs/vsbys to windward and mauka areas, mainly
during the overnight and early morning hours. Predominantly VFR
conditions are expected during the next 24 hours.

No AIRMETs are currently in effect.

&&

.MARINE...
Trades are forecast to weaken into the light to moderate category
later today through Monday as the gradient relaxes over the
state. Guidance remains in good agreement Tuesday through the
second half of the week and depicts a strengthening ridge north of
the islands, which supports a return of fresh to locally strong
trades (advisory-level winds in the typically windier locations
between Maui County and the Big Island) by Wednesday.

Surf along south facing shores will remain small through Monday,
with mainly background energy continuing out of the southwest and
southeast. An upward trend is anticipated beginning late Monday,
as one of the largest south swells of the season begins to build
across the Hawaiian waters. Surface analysis charts showed a
compact storm-force low bottoming out at 956 mb southeast of New
Zealand last Monday through Tuesday within Hawaii's swell window.
ASCAT passes reflected this and showed a pocket of storm-force
winds nosing northward through this period around the 190 deg
band relative to the islands. Altimeter passes within this area
showed seas ranging from 35 to 42 ft. This swell has already moved
through the nearshore PacIOOS buoy at Pago Pago, which showed
seas slightly higher than predicted by guidance. Energy levels
within the lower frequency bands closely resembled the levels
observed at this buoy for our recent July 14th warning-level
event. As a result, surf heights will likely near/reach warning
levels along south facing shores through the peak of the event
Tuesday through Tuesday night and may not drop below advisory
levels until Thursday.

The small westerly swell associated with recent typhoon activity
in the western Pacific will steadily trend down through Monday.
Surf will respond and lower along the exposed coasts from this
source.

Surf along north and west facing shores may trend up through the
second half of the week as a small northwest swell builds down the
island chain. Guidance remains in good agreement and depicts a
gale evolving around 2000 miles northwest of the islands near and
just west of the Date Line early this week.

Surf along east facing shores will trend up by midweek as the
trades strengthen locally and upstream of the islands.


&&


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

&&

$$


H Lau...Jelsema...Gibbs

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



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