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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 190559

Area Forecast Discussion
National Weather Service Honolulu HI
759 PM HST Fri Sep 18 2020

The dry and stable trade wind pattern will continue into next
week, with the best chance for showers developing overnight
through the early morning periods across windward and mountain
locations. Moderate to locally breezy trade winds will continue
through the weekend, then relax early next week as the ridge
weakens to the north.


Latest surface analysis and satellite imagery showed the
subtropical ridge anchored north of the islands and patchy low-
level clouds moving through from east to west. Water vapor
imagery depicted mostly dry middle- to upper-level air over the
state between an upper trough to the northwest and an upper ridge
to the east and southeast. Some upper-level debris clouds from
earlier convection associated with a trough far southwest of the
area are streaming northward into portions of the islands. The
afternoon upper-air soundings at Lihue and Hilo exhibited mostly
dry and stable profiles with PWs values ranging from 1 to 1.4
inches. Showers have been sparse with little to no accumulations
over the past several days.

Guidance has initialized well with the current pattern and
depicts the stable and dry trade wind conditions continuing over
the weekend through early next week as the upper heights rise.
An area of moisture passing to the south will continue westward
and will have little to no influence on the local weather.
Dewpoints will dip toward 60 degrees each afternoon (below
predicted) for some leeward areas, which will drive relative
humidities into the upper 30s to lower 40s (see fire weather
section below for more details on the dry conditions). Shower
coverage will remain sparse with the best chance occurring
overnight through the early morning periods each day over windward
areas. Moderate to locally breezy trade winds will continue
through the weekend, then relax early next week in response to a
weakness developing in the subtropical ridge as a front passes to
the north.


High pressure ridging north of the islands will sustain moderate
to locally breezy trade winds through Saturday. VFR will hold over
the entire state, with only isolated areas of MVFR possible in
windward and mauka sections from time to time. Various amounts of
high level cirrus clouds, debris from earlier thunderstorm
activity south of the island chain, will be streaking over the
region through the night.

No AIRMETS are in effect.


Strengthening high pressure to the northeast of the islands has
tightened the pressure gradient across the island chain. Friday;s
ASCAT passes have shown solid moderate trade flow across both the
nearshore and offshore waters. There is a Small Craft Advisory
(SCA) in effect winds through the typically windy channels around
Maui County and waters south of the Big Island. These winds are
forecast to hold through Sunday afternoon.

Surf will is on the rise along north and south facing shores.
Smaller-period, north-northwest swells will arrive today from
strong to near gale force winds behind a northern Pacific cold
front. This should provide a small bump along north and west
facing shores tonight through Saturday. Long period, south-
southwest swells generated from last week's Tasman Sea storms will
drive surf heights above summer averages over the weekend, but
should stay just below the 8 foot advisory level. East shore surf
and wind waves will stay up in response to the recently
strengthened trade flow.


Provided the lack of any significant rainfall through the summer
months, fuels remain very dry across portions of the state.
Although the KBDI and relative humidity values will likely reach
criteria this afternoon, the trades should remain near/just below
red flag criteria.


Small Craft Advisory until 6 PM HST Sunday 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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