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Hawaiian Islands Synoptic Discussion and Guidance

081
FXHW60 PHFO 280143
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
343 PM HST Tue Jun 27 2017

.SYNOPSIS...
Breezy tradewind weather, with only limited windward and mauka
showers, can be expected through Thursday thanks to stable air and
a ridge of high pressure to our north. The trades will become
weaker and wetter by this weekend, as an upper level disturbance
with less stable air passes over the islands.

&&

.DISCUSSION...
A surface ridge about 700 mi N of the islands is maintaining
breezy to locally windy trades this afternoon. Tradewind rainfall
has been rather scarce today, with just a couple of gauges on the
windward side of the Big Island coming in with more than a tenth
of an inch of rain. The trade inversion slopes from near 5000 ft
on Kauai to near 7000 ft on the Big Island thanks to strong
ridging at the surface through mid levels of the atmosphere
immediately N of the islands. Not much change is expected through
about Thu with guidance maintaining a dry, stable airmass over the
state.

Some changes look to occur Fri through Sun. Models show the trade
wind inversion weakens and lifts Thu night into Fri in response
to a developing mid- and upper-level trough to the E of the
islands that will drift slowly W. This feature may cut off into a
weak but closed 500 mb low over the islands this weekend, while at
the surface, a weak and very broad surface trough passes E to W
across the islands. Interestingly, at the same time, the
subtropical ridge N of the islands will strengthen to a high,
helping to keep us from losing the trades altogether. Trade wind
speeds will probably reach a relative minimum this weekend, in the
gentle to locally breezy range. The models show a marked increase
in boundary layer moisture starting Thu night, with PW increasing
to 1.5-1.7 inches by Friday, along with some cooling in the mid-
levels. Thus, by Friday, we should see many more trade wind
showers, especially when compared to the current rather dry
weather. Depending on the strength of the surface trough, we could
see increasing showers in interior, leeward, or S shore locales
too, particularly if the low level flow to the E of the trough
veers more than currently expected.

Strong high pressure to our N will return breezy to locally windy
trades after the weekend and through July 4th. The mid level
features enhancing our showers should weaken and lift N away from
us, though lingering low level moisture may keep trade wind
showers somewhat active into Mon, with even fewer showers
expected for the holiday.

&&

.MARINE...
The Small Craft Advisory (SCA) has been expanded to additional
zones this afternoon to now include the Kaiwi Channel, Maui
windward waters and Oahu leeward waters. This is in addition to
the typical windy zones around Maui and the Big Island. The SCA
goes through the day Thursday, and it may continue for some of
these zones into Friday, followed by slightly weaker trade winds
this weekend.

The locally strong trades combined with a small northeast swell
generated by a fetch of gales west of Oregon and California, will
produce slightly elevated choppy surf along east facing shores
most of this week. Surf is expected to remain below the High Surf
Advisory criteria along east facing shores, however, some of the
northeast swell energy should wrap into exposed north facing
shores across the state. A small short-period northwest swell is
also forecast from Friday into this weekend.

Small pulses of swell energy from the southeast through southwest
will continue to produce near normal summertime surf along south
facing shores into mid-week. Reinforcing long-period south-
southwest and southwest swells will generate a slight bump in
surf heights along south facing shores from Thursday into this
weekend.

&&

.AVIATION...
A stable atmosphere and fresh to locally strong trade winds will
continue to support TEMPO moderate low level turbulence over and
immediately south through west of the mountains. AIRMET TANGO is
posted across the state, and will likely remain for most of the
work week.

Prevailing VFR conditions should be expected across the state.
Clouds and isolated showers will tend to favor the windward and
mountain areas. Any showers should remain light and brief.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Thursday for Oahu Leeward
Waters-Kaiwi Channel-Maui County Windward Waters-Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$
R Ballard/Foster/EATON

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



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