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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 090130

Area Forecast Discussion
National Weather Service Honolulu HI
330 PM HST Mon Aug 8 2022

Moderate to locally breezy trade winds are expected to prevail into
next weekend, except for locally windy conditions Wednesday and
Thursday. Clouds and brief showers will focus over windward slopes
and coasts, with afternoon and evening clouds over leeward Big
Island slopes.


Seasonable trade wind weather is expected through the forecast
period, with persistent high pressure far NE of the islands
supporting a steady trade wind flow. Winds are expected to increase
through midweek as surface pressures lower to the S of the islands,
with a tropical disturbance potentially developing within a broad
area of low pressure. Regardless of development, a tightening of the
local pressure gradient is expected, with locally windy conditions
possible. There continues to be a greater than normal amount of
uncertainty with this part of the forecast, as global model guidance
appears to be overestimating the strength of the disturbance.
Associated moisture may increase shower coverage near the Big Island
Wednesday and Thursday, but the forecast currently anticipates the
bulk of this moisture will remain S of the islands. Winds will ease
slightly toward the end of the week as the disturbance moves quickly
westward away from the area, with moderate to locally breezy trade
winds expected into next weekend. Long term guidance indicates a
trend toward lighter winds early next week.

A relatively dry trade wind weather pattern is expected to prevail
overall, as a mid- and upper-level ridge extends over the area from
a high centered NE of the islands. Little change to the ridge's
strength is expected through Friday as it moves slowly N, with some
weakening possible by next weekend. A strong subsidence inversion
will limit shower intensity, with brief showers primarily favoring
windward areas, although leeward Big Island will receive a few
afternoon and evening showers. In the short term, satellite data and
model guidance indicate that the trade wind flow will be a little
drier than normal, especially over Kauai and Oahu. A modest increase
in PWAT is expected to move from E to W across the chain overnight
and Tuesday, likely leading to a few more showers for most windward

A jet max associated with a low aloft NW of Kauai is sending some
cirrus clouds northeastward near Kauai, but the low is forecast to
move N over the next couple of days, and most high clouds will lift
N of the area. Some increase in mid- and high-level clouds is
possible Thursday and Friday as a new low aloft develops NW of
Kauai, with southerly flow aloft potentially advecting some moisture
over the islands from the tropics.


High pressure well northeast of the state will maintain moderate to
locally breezy trade wind flow across the islands through Tuesday
night. A dry airmass across the islands will limit the areal
coverage and intensity of showers through the TAF period, while
strengthening trades focus limited clouds and showers along favored
windward and mauka areas. Mainly VFR conds are expected across most
TAF locations through Tuesday. However MVFR cigs will continue to
occasionally drift across windward Big Island and Maui through
tonight, continuing the need for AIRMET Sierra due to tempo mountain

AIRMET Tango continues from Molokai through the Big Island as
strengthening trades generate low level mechanical turbulence
downwind of the mountains. There is a potential period of tempo
moderate turb above FL360 near Kauai tonight as upper dynamics
linger in the region, though currently these conditions are expected
to remain below AIRMET criteria.


The gradient around the southern periphery of northeast Pacific high
pressure is being enhanced by an area of broad low pressure
positioned well southeast of the islands. This is driving locally
strong trades in the typical windier locations around Maui County
and the Big Island today. The Small Craft Advisory remains in effect
for these areas with no changes needed. Moderate to fresh trades
will then become established around Oahu and Kauai during tonight
into early Tuesday as this area of low pressure slowly advances
westward. An expansion of the SCA may eventually be required by late
Tuesday or early Wednesday, especially if trades further strengthen
in response to any increased organization or developing low pressure
within this trough.

Strengthening trades will build short-period wind waves over east-
facing shores through mid-week, peaking near or slightly above the
seasonal average Wednesday into Thursday. A very small, long period
south swell arrives tonight into Tuesday, fading by Thursday. No
significant swell is expected through the forecast period.


Small Craft Advisory until 6 AM HST Thursday 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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