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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 202010

Area Forecast Discussion
National Weather Service Honolulu HI
1010 AM HST Sat Jul 20 2019

A broad area of high pressure far north of the islands will
continue to feed the islands with locally breezy trade winds
through at least the middle of next week. There could be a boost
in enhanced trade wind showers between Monday and Tuesday,
especially the eastern end of the island chain, in association
with a low level disturbance passing south of the islands and the
interaction with an upper level low northwest of the islands.


A well anchored surface high north of the islands will be the
continuing source for trade winds for the area through at least
the middle of next week. These trade winds will be mainly be in
the moderate to locally strong range.

There has been some late changes with what the models have
instore for the islands in the next 72 hours or so. From what was
a benign, uneventful, trade wind pattern, models are now suggesting
a disturbance to pass south of the islands between Monday and
early Tuesday time frame. The disturbance will be accompanied by
tropical moisture that will sweep across the area from east to
west. Further more, an upper low at 500 mb is located 355 miles
north-northwest of Kauai now. It is forecast to deepen slightly in
the next 36 hours while remaining in that general vicinity
through early Monday morning, before moving further away from the
islands. By Tuesday afternoon the upper low is expected to be 700
miles west-northwest of Kauai. Both the GFS, ECMWF, and NAM, are
in pretty good agreement with this scenario, although the EC is a
few hours behind the GFS with the onset of the weather. Current
grids have thunderstorms slated for the Big Island Monday morning,
as per GFS solution.

Comparing the latest satellite imagery with the models, there is
a presence of low level low 460 miles south of South Point, fitting
the models solution. The models weakens this low to a strong
trough as it pushes west- northwestward through Sunday, then
northwest through Monday night, running parallel to the island
chain. The upper low northwest of Kauai, with its southerly winds
aloft, will be helping steer the trough's course. Perhaps the
question mark is how much tropical moisture will be accompanying
this trough. We hope to have a clearer picture by this evening.

Current grids have thunderstorms for the Big Island beginning Monday
morning, and keeping the smaller islands out of this unsettled
weather for now. With the upper low being weaker and some 900
miles away, the weather will return to regular trades by Tuesday
night or Wednesday.

Currently, satellite imagery shows broken low clouds coverage
upwind of the islands, especially upwind of Maui and the Hamakua
coast. Some dissipation is expected with these clouds, but not
all. So expect some passing showers for the rest of the day, and
perhaps this evening for Maui.


A high will remain nearly stationary far north of the main Hawaiian
islands and maintain locally strong east winds over the area.
AIRMET TANGO is in effect for moderate turbulence below 7,000ft
and this AIRMET will likely continue into Sunday.

The east winds will carry scattered to broken Low clouds over
windward areas. Later today clouds and showers will develop over
the southwest slopes of the Big Island and Maui. We expect VFR
conditions to prevail and we do not expect to have to issue


Fresh to locally strong easterly trades are forecast to continue
through next week as high pressure remains positioned north-northeast
of the area. The stronger winds will remain across the typically
windier locations from Maui County to the Big Island due to
terrain accelerations. The current Small Craft Advisory remains
in place for these areas through Sunday and will likely need to
be extended into early next week.

Rough surf will continue along east facing shores each day due to
strong onshore winds. Heights are forecast to slightly trend down
through the latter half of the weekend into early next week due to
the upstream trades trending down.

Surf along south facing shores will remain small through the
weekend, then trend up through the first half of the upcoming week
due to a recent system passing through/near the Tasman Sea.
Heights should remain below advisory levels as this south-
southwest swell moves through and peaks Monday through Tuesday. In
addition to this long-period source, a combination of small south
to southeast swells will be enough to keep the surf from going
flat along southern exposures through midweek.

A south-southeast (150-160 deg) long-period swell associated with
a compact gale that has developed southeast of the Tuamotus will
be possible late next week with a peak around Thursday night into
Friday. Surf will rise along exposed shores but should remain
below advisory levels.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.


Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay,
Pailolo and Alenuihaha Channels, and waters south of the
Big Island.



H Lau/Donaldson/Kino

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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