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Hawaiian Islands Synoptic Discussion and Guidance

526
FXHW60 PHFO 131946
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
946 AM HST Mon Jul 13 2020

.SYNOPSIS...
Trade winds will weaken tonight and Tuesday, then remain on the
lighter side for the next several days before strengthening over the
weekend. The trade winds will deliver passing showers that will
primarily favor windward areas during nights and mornings, but the
lighter winds will allow some afternoon clouds and showers to
develop over leeward areas.

&&

.DISCUSSION...
Trade winds will diminish tonight and Tuesday, then remain on the
lighter end of the spectrum for the next several days, with wind
speeds increasing next weekend.

The local winds are expected to diminish and veer slightly toward
the ESE over the next day or so, even though a surface high pressure
cell far NE of the islands moves/changes very little. Instead, the
decrease in winds will, in part, be due to weak low-level features
passing between the islands and the high, acting to loosen the low-
level pressure gradient.

One such feature is a weak low-level trough currently fueling
increased trade showers over and near Kauai and Oahu, while Maui
county and the Big Island have fewer showers arriving in the short
term. An update to the short term forecast was issued to account for
latest satellite/radar trends. Another weak low-level trough to the
N of the islands (associated with a low aloft) will move W and
sharpen to the NW of the islands from Tuesday into Thursday, while a
trough marking the remnant of TS Cristina moves between the islands
and the high later in the week. Additionally, a front will dig
toward the islands from the NW toward the end of the week. All these
features will act to keep the flow over the islands veered and
diminished.

The lighter winds will allow afternoon leeward sea breezes to drive
cloud and shower formation that will diminish after sunset. Warming
mid-level temperatures are expected in the short term as the low
aloft to the N moves W the next couple of days. A stabilizing mid-
level ridge will then prevail into the weekend, and shower coverage
and intensity will be limited, especially since most of the moisture
slug associated with the remnants of Cristina will pass well N of
the islands. As the low aloft moves to a position NW of Kauai, it
will continue to deliver some periods of high clouds. Latest
guidance has the bulk of the high clouds over Kauai and Oahu,
potentially becming thick at times from Tuesday into Thursday.

&&

.AVIATION...
A high pressure system far to the north northeast will drive
moderate to fresh trade winds across the state through at least
tonight. Low clouds and showers will impact mainly windward portions
Kauai and Oahu into this afternoon, with drier conditions expected
across the rest of the state. Mountain obscurations, occasional MVFR
ceilings and visibility are expected around these showers. AIRMET
SIERRA is posted for tempo mountain obscurations above 2500 feet
along north through east sections of Oahu.

The clouds and showers should pass west of Kauai this evening. The
next area of moisture is expected to arrive from the east tonight
potentially impacting most windward areas.

AIRMET TANGO remains in effect below 8000 feet for south through
west of mountains on all islands due to tempo moderate turbulence.

&&

.MARINE...
High pressure far north-northeast of the islands will keep fresh to
strong trades in place through this afternoon. As a result, a Small
Craft Advisory (SCA) remains posted for the typically windy waters
around Maui County and the Big Island. A couple troughs of low
pressure will pass by to the north of the state Tuesday through
Friday, easing the trades into the moderate range in most areas,
with fresh trades periodically affecting the typically windier
channel waters. Trades should then ramp back up to locally strong
levels next weekend as high pressure strengthens far to the north of
the island chain.

There will be a series of small southerly swells over the next
several days that will keep south shore surf near of just above
background levels. A slightly larger long period south swell may
fill in over the weekend. East facing shores will continue to
experience short period surf produced by the trade winds locally and
upstream of the state. A small but slightly longer period east swell
produced from former tropical cyclone Cristina, may provide a slight
uptick in surf heights during the late Tuesday through Thursday time
frame. No other significant swells are expected.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION...Birchard
AVIATION...Foster
MARINE...Kino

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



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