Current Conditions
Temp4.1 C
RH39 %
WindWNW 4 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 241951

Area Forecast Discussion
National Weather Service Honolulu HI
951 AM HST Mon Feb 24 2020

Trade winds will move in across the islands this afternoon as
high pressure builds behind a rapidly weakening front just north
of the state. Trade winds will strengthen through the remainder
of the week with clouds and showers favoring windward slopes.
Showers will increase in coverage and intensity across the eastern
end of the state as an upper trough digs in from the north.


A rapidly weakening cold front is positioned just northwest of
Kauai this morning. Ahead of the front, light winds and land
breezes overnight allowed for sunny skies and cool temperatures
over the islands this morning, while a few clouds and showers
persist over the coastal waters.

High pressure building in from the northwest behind the front
will push in trade winds across the area today, which will
strengthen to moderate to fresh breezes on Tuesday as the high
settles northeast of the area. A gradual increase in clouds and
showers across windward areas is expected over the next 24 hours
as the trades strengthen. Ridging aloft will keep the atmosphere
stable and showers light over the next couple of days.

The stable pattern starts to change late in the week as an upper
trough digs across the eastern end of the state on Friday and
erodes the trade inversion. This trough is expected to cutoff into
a low and drift westward this weekend. Temperatures with the low
are quite cold; GFS and ECMWF both show 500mb temperatures below -17C
(near 3 standard deviation below normal) and 700mb temperatures
near 0C. Moisture may be somewhat limited with precipitable water
values near an inch to an inch and a quarter. However, with the
degree of instability with the cold low, we could see a wet
pattern with active showers and thunderstorms, as well as snow at
high elevations.

In addition to the weather associated with the low aloft, strong
high pressure at the surface building north of the state late in
the week will also lead to a further increase in trade wind
speeds. Winds could likely exceed wind advisory thresholds for
portions of the state, and may even approach high wind warning
thresholds for some areas during the Friday through Sunday time


High pressure building N of the area will lead to a strengthening
trade wind flow today, thereby focusing clouds over N through E
facing slopes, with isolated MVFR CIG/VIS in a few showers. No
AIRMETs are posted and none are expected through tonight, but
AIRMET SIERRA for low-level turbulence to the S through W of the
terrain may be required on Tuesday.


A stalled front will dissipate northwest of the Kauai today. Meanwhile,
high pressure will be passing by far north of the area. This will result
in moderate trade winds returning across the area. This combined with
a building northwest swell will likely require the issuance of a Small
Craft Advisory later today across most areas. These conditions will
linger on into Friday. A stronger high is forecast to build in north
of the area over the weekend. This may result in gale force winds across
some areas.

The current small short period northwest swell will linger through
tonight. A larger and rather long period northwest swell will fill
in Tuesday morning, peak late Tuesday and Tuesday night, then lower
gradually Wednesday and Wednesday night. Current guidance indicates
high end advisory level surf with this swell. Recent altimetry
passes northwest of the state indicate that the swell could be a
bit higher than model guidance. We will wait till we get data
from the NOAA buoys northwest of the state to decide if the swell
results in warning level surf.

Another long period, but slightly smaller, northwest swell is
expected to build Wednesday night and Thursday, peak Thursday
night, then lower gradually Friday through Sunday. Advisory level
surf is expected with this swell.

Short period choppy surf along east facing shores will gradually rise
through the week and become rather large by the weekend due to
strengthening trade winds. Advisory level surf is likely. There
will also be a series of small background south swells through the
week and on through the weekend.






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