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Hawaiian Islands Synoptic Discussion and Guidance

604
FXHW60 PHFO 200634
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
834 PM HST Fri Jul 19 2019

.SYNOPSIS...
A high pressure ridge will remain north of the Hawaiian Islands
into the middle of next week. Expect moderate to breezy trade
winds through the period with showers favoring windward and
mountain areas. Additional moisture may move up from the deep
tropics from Sunday into Tuesday, possibly increasing showers
across the state for the first half of next week. More typical
trade wind weather returns from Wednesday onward.

&&

.DISCUSSION...
A more typical trade wind weather pattern continues to unfold with
a high pressure ridge north of the islands into the first half of
next week. Regional satellite infra-red imagery shows a fairly
active Inter Tropical Convergence Zone around 10 degrees north
latitude south of the state. Cloud bands with embedded showers
are slowly drifting in with the trade winds. In the upper levels
of the atmosphere a deep upper low appears on on satellite water
vapor imagery.

Large scale lift and instability associated with this upper low
continues to weaken the stable subsidence inversion over the
islands allowing deeper clouds to form and more shower activity
mainly along windward and mountain slopes in the overnight through
early morning hours through Sunday. Brief periods of showers are
possibly over leeward slopes with stronger shower bands moving in
with the trade winds.

Global models continue to show a weak trough developing south of
the Big Island this weekend. Both the American (GFS) model and the
European (ECMWF) model are moving this trough in a general westward
after it forms. The GFS model has a more northwesterly track,
which would result in advecting deeper tropical moisture into the
islands for the first few days of next week. The latest GFS model
run brings this deeper moisture and enhanced showers into the
southern slopes of the Big Island on Sunday night, pushing
northward into the island of Maui by Monday afternoon. Meanwhile
the ECMWF keeps us in a drier air mass solution with the deep
tropical moisture passing well south of the state.

We are keeping a close watch on these model differences and are
blending both solutions to produce our current medium range rainfall
forecast grids. Confidence in enhanced showers for the eastern
half of the state remains fair at this time. More typical easterly
trade wind weather returns from Wednesday onward with passing
shower bands in the easterly wind flow.

&&

.AVIATION...
Strong high pressure far northeast of the Main Hawaiian Island
will continue to drive mostly dry trade wind flow over the area.
Bands of low-topped stratocumulus embedded in the trade wind flow
will impact mainly east and northeast facing slopes and coasts.
Shower activity will be limited, but is expected to increase
overnight. AIRMET Sierra for mountain obscuration has been
dropped, but could become necessary again towards morning.

Light to locally strong east winds will continue into Saturday and
likely beyond. AIRMET TANGO will remain in effect for moderate
turbulence below 7,000 ft.

&&

.MARINE...
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 may 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 moderate period small
southeast swell is expected through this time.

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

&&

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

&&

$$

DISCUSSION...Bohlin
AVIATION...Bedal
MARINE...Gibbs

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