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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 250618

Area Forecast Discussion
National Weather Service Honolulu HI
818 PM HST Sun Mar 24 2019

A strong high pressure center far to the north of the Hawaiian
Islands will keep moderate to breezy trade winds in the forecast
through Monday. Windward and mountain showers will continue
tonight with increasing trends for these areas on Monday. The high
pressure ridge north of the islands begins to weaken on Tuesday
with trade winds decreasing in strength becoming light and
variable on Wednesday ahead of an approaching cold front. Clouds
and showers along the frontal band will reach the northwestern
islands from late Wednesday night into Thursday. Cool northerly
winds will develop after frontal passage.


Satellite imagery this evening shows bands of cirrus clouds with
transverse banding forming over the region under the influence of a
subtropical jet stream. The high clouds will move east of the state
by Monday morning as the upper level pattern begins to change. A
curved band of low level cumulus clouds and showers, remnants of
an old convergence boundary, lingers just a few hundred miles
north and northeast of the state. Some of these clouds and showers
will move into the islands with the trade winds on Monday.

In the big picture, a strong 1030 mb high pressure center roughly
1000 miles north of the Hawaiian Islands will continue to produce
moderate to breezy trade winds across the state through Monday.
This high pressure system will slowly weaken and drift southeastward
through Tuesday as another cold front approaches the islands from
the northwest.

The cold frontal trough will weaken the ridge north of the islands
and also decrease the strength of the trade winds on Tuesday. By
Wednesday, medium range models show the cold front roughly 400 miles
northwest of Lihue, wind speeds will become light and variable across
the state with onshore sea breezes each day and offshore land breezes
in the overnight hours. Models are also showing an upper level trough
supporting a band of low level convergence producing elevated clouds
and showers ahead of the front over Kauai on Wednesday night.
Expect enhanced showers spreading southeastward to Oahu by early
Thursday morning with additional showers drifting over Maui and
Hawaii Counties later on Thursday. Our confidence in the arrival
timing on these showers remains only fair at this time due to the
relative weakness of the frontal boundary and the four to five day
forecast time period. Cool northerly winds will move in behind
the front with temperature dropping statewide by a few degrees on
Thursday and Friday.

The longer range solutions for both the American (GFS) and the
European (ECMWF) models show the forward motion of the front
stalling out over the central islands. We continue to forecast
light to moderate trade winds with elevated clouds and showers
mainly over windward and mountain slopes from Friday into next


High pressure centered far north of the Hawaiian Islands will
drive moderate to fresh trade winds through Monday night.
Scattered high clouds will exit the area by morning. Low clouds
moving along the trade wind flow will bring periods of showers
with brief MVFR cloud ceilings over the windward mountain slopes
of each island. Otherwise, VFR conditions will prevail.


High pressure north of the state will keep fresh to strong trades
across the coastal waters through Monday night. The trades will
ease Tuesday and Tuesday night as a front approaches from the
northwest, with winds becoming light and variable by Wednesday.
The front is expected to move into the islands late Wednesday
night and Thursday, then slowly dissipate across the central or
eastern islands Thursday night and Friday. Moderate to fresh
northerly winds are expected to fill in behind the front, with the
winds returning to a more typical trade wind direction at light
to moderate speeds over the weekend as weak high pressure builds
north of the state.

A Small Craft Advisory (SCA) remains in effect for the typically
windy waters around Maui and the Big Island through 6 AM HST

The current north-northwest swell will gradually decline late
tonight through Monday night. A small reinforcement out of the
north is expected Monday night through Wednesday, followed by a
small northwest swell Tuesday night and Wednesday. Surf will
remain below advisory levels during this period. A new moderate
sized northwest swell will build Wednesday and Wednesday night,
and could approach low end advisory levels Thursday through
Friday. Surf will remain along north and west facing shores
Friday night through the weekend as new overlapping moderate
sized north-northwest and northwest swells move through the
island chain.

Surf along east facing shores will see a small increase tonight
through Monday due to a slight uptick in trade wind speeds. A
downward trend is expected Monday night through Wednesday with
small surf expected through the rest of the week.

Surf along south facing shores will remain small but several long
period south swells will produce background surf throughout the
week. A slight bump is expected Wednesday night through Friday,
followed by more background south swell over the weekend.


Small Craft Advisory until 6 AM HST Tuesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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