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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 251350

Area Forecast Discussion
National Weather Service Honolulu HI
350 AM HST Mon Sep 25 2017

A gentle to moderate trade wind flow will be with us for the new
work week as a surface ridge of high pressure remains north of the
islands. The trades will be light enough to bring on daytime sea
breezes favoring trade wind shelter areas such as the leeward and
southern shorelines. This will lead to some afternoon clouds over
the lee and interior areas. Meanwhile, the trades will bring in a
few showers to the windward areas. A boost in the showers is likely
the second half of the week as passing surface trough ushers in a
wetter trade wind flow. More changes are expected over the weekend
with southerly winds replacing the trades, which will bring on the
vog and muggier conditions.


Gentle to moderate trades will grace the new work week with
limited moisture during the first few days of the week. A
weakening surface trough then moves through the islands from the
east on Wednesday, and likely usher in a wetter trade wind flow
for the rest of the week. Also, the trades may strengthen slightly
to mostly moderate speed during this second stretch. Over the
weekend, a drastic change takes place with the trades giving way
to light southerly winds.

The air mass is currently rather stable with a low level inversion
between 6k and 65 hundred feet. It is not a robust inversion over
Hilo, likely due to a nearby upper level trough that is expected
to be replaced by upper level ridging by tonight. This weakness
in the inversion could be responsible for some of the evening and
late night showers along the Kona coast of late.

The latest satellite imagery shows generally fair to partly
cloudy skies. Radar shows a few showers riding in with the NE
trades, mostly concentrated Oahu. There is a small pocket of
showers located just off Keahole Airport as well.

While the trades blow in a few showers into the windward areas
during the next 2 days, the winds will be weak enough to allow for
daytime sea breezes, favoring the leeward coasts and interior
areas of the smaller islands. The stable mass, however, will limit
these afternoon showers to isolated. As for leeward Big Island
this afternoon, we anticipate some showers. They should be ending
early before midnight due to a more stable air mass.

We anticipate a boost in the trades showers as early as late
Tuesday night as a weakening surface trough moves through the
islands from east to west. This surface trough is currently 260
miles E of the Big Island, and is supported by a stationary upper
level low 450 miles NE of Hilo. The upper trough is expected to
weaken in the next 24 to 36 hours. In wake of the trough's
passage, the second half of the week appears wetter, which is
supported by both the EC and GFS solutions. The precipitable water
values jump up from the current 1.15 to 1.3 inches, to between
1.3 and 1.6 inches east of the trough. A slight bump in the trades
is also expected Wednesday night, and last through Friday. If
this pans out, daytime sea breezes will be severely limited.
Although the windward and mountain areas will rake in most of the
rain, a few showers will venture into the leeward districts of the
smaller islands. Leeward Big Island will continue to observe
afternoon/evening clouds with scattered showers.

The surface winds become light SE on Friday night, then S on
Saturday. Under this wind flow, expect the discomfort level to
rise with increasing humidity, while vog spreads across the
region. This change in wind direction is in response to a front
stalling 350 miles NW of Kauai Saturday night as per GFS.
Differences between the EC and GFS continues with the location of
both the surface front and the accompanying upper level trough,
with the EC being a couple of hundred miles further west with both
features. Both models, however, agree in maintaining a wet

On Sunday, the southerly wind starts to turn more easterly. This
comes about as the front weakens, allowing ridging to rebuild N of
the islands. So, by Sunday night, all islands should be under a
light to moderate trade wind flow.


VFR will predominate across the island chain. Only brief MVFR
conditions are possible over windward and mauka sections in
passing showers, being carried by the light to moderate trade
winds. Also, wind speeds will be light enough during the afternoon
and early evening hours to allow for cloud buildups, and possible
showers, over interior and leeward areas of each isle.

No AIRMETs are currently in effect and none are anticipated.


Light to moderate trades are expected to persist through the first
half of the week as a weak surface trough approaches the islands
from the east. The wind flow will be light enough for land/sea
breezes to setup over some leeward areas through this time. A
return of moderate to fresh trades are forecast through the second
half of the week as the trough moves through from east to west.
Some of the typically windier channel waters may even near
advisory levels Thursday through Friday.

Surf along south facing shores will remain small through the week
with mainly a mix of background southeast and southwest swell energy
expected. The highly zonal pattern with systems racing eastward
across the southern Pacific continues, with no significant sources
shown setting up within Hawaii's swell window over the next several
days. The next long-period pulse out of the southwest (210-220 deg)
from the Tasman Sea should fill in Tuesday night and continue
through midweek, which will lead to a slight increase in surf. This
source is from a strong- to gale-force low that moved across the
Tasman Sea last week.

Small, short period (10-12 seconds), northwest swell energy will
continue and should be enough to keep the surf from going flat along
north facing shores through the week. Recent surface analysis and
satellite data showed a 998 mb low centered around 1200 nm north-
northwest of the islands with a pocket of near gale-force winds
focused toward the islands within the 330-340 deg band and seas up
to around 10 to 13 ft. A reinforcing north-northwest swell should
reach the local waters late Wednesday and hold through Thursday.
A compact storm-force low currently east of Japan is forecast to
continue eastward over the upcoming few days toward the Date Line
around 35N. Although the GFS/WAVEWATCH III solution suggests a
favorable setup for a west-northwest (310-320 deg) swell to fill
in across the local waters this coming weekend, the ECMWF is not
as aggressive with this system. As a result, swell forecast
confidence from this source for the upcoming weekend remains low
at this time. Guidance should come into better agreement over the
upcoming couple of days.

Surf along east facing shores will remain small through the first
half of the week as the trades locally and upstream of the state
remain light. Once the aforementioned surface trough moves through
from east to west by midweek, small and choppy surf will likely
return as the trades fill back in through the second half of the





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