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Hawaiian Islands Synoptic Discussion and Guidance

835
FXHW60 PHFO 041936
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
936 AM HST Sat Jul 4 2020

.SYNOPSIS...
Light to moderate east-southeasterly trades will continue through
the holiday weekend. Showers will favor windward and mauka areas,
although scattered showers will also affect leeward and interior
areas at times. A more typical trade wind pattern will resume on
Monday, with showers favoring windward and mauka areas along with
the stray leeward spillover. This trade wind pattern will hold
through late next week, with breezy conditions expected Tuesday
into next weekend.

&&

.DISCUSSION...
Currently at the surface, a weak trough of low pressure is located
in the vicinity of Kauai, a 1030 mb high is centered around 1600
miles north of Honolulu, and another weak trough is present
several hundred miles northeast of the state. The resulting
gradient is producing light to moderate east-southeasterly trades
across the island chain this morning. Visible satellite imagery
shows mostly cloudy skies in most locales, with a few pockets of
reduced cloud coverage scattered about the state. Radar imagery
shows numerous windward showers, and scattered leeward showers,
with coverage the greatest over Kauai and Oahu at the moment.
Main short term concerns revolve around rain chances and trade
wind trends.

The gradient will remain disrupted from the high far north of the
islands during the next couple days by the trough of low pressure
northeast of the state which will track westward. This trough
should then begin to dampen out on Monday, allowing high pressure
to become more dominant, with the high then sliding southward and
closer to the islands Tuesday through late next week. Light to
moderate east-southeasterly winds should hold in place through the
remainder of the holiday weekend, with localizes sea and land
breezes developing in the more sheltered leeward areas. Moderate
trades will then return through the day on Monday and becoming
breezy Tuesday through late next week.

Shower coverage is expected to be highest across the area today,
as a plume of high precipitable water (PW) values (1.4 to 1.6
inches) moves overhead. Showers will favor windward areas,
although scattered showers will also affect leeward/interior
areas. Shower coverage should diminish this evening as daytime
heating is lost and a pocket of lower PW values moves over the
islands. Scattered windward showers will remain possible through
the night and during the morning hours Sunday, with isolated
leeward showers remaining possible mainly near the coast in the
east-southeasterly boundary layer flow. Scattered showers will
remain possible in windward areas Sunday afternoon, with scattered
leeward/interior showers also expected to develop as a result of
sea breezes and daytime heating influences. Showers should
diminish in most leeward/interior areas Sunday evening, with
scattered showers continuing in windward locales. As trade winds
become more dominant on Monday, we should see a return of more
typical trade wind weather, featuring mainly windward/mauka
showers and a few leeward spillovers. This pattern appears to hold
in place through late next week.

&&

.AVIATION...
An area of moist air associated with a low level trough is moving
slowly west across the main Hawaiian Islands. A trough aloft is
making the atmosphere a bit unstable. This combination has been
producing extensive MVFR ceilings and visibilities and isolated
IFR ceilings over the islands. The low level flow is south of
east, so more showers have been moving ashore in the Kau district
of the Big Island than is typical. The east southeast flow also
means that cloud lines that usually extend over water west
southwest of the islands are extending west northwest and moving
over islands farther downstream. These factors will likely mean a
continuation of the extensive MVFR conditions through this
evening. AIRMET Sierra for mountain obscuration is currently in
effect for Kauai and Oahu and may need to be expanded.

Drier air will be spreading west across the area tonight and will
bring higher cloud ceilings and fewer showers.

&&

.MARINE...
A surface trough passing near Kauai has weakened the pressure
gradient over the state and will maintain light to moderate trades
through Sunday. After the surface trough departs the region, the
forecast guidance indicates surface high pressure will build far
north of the islands early next week. This will cause the trade
winds to gradually strengthen. Therefore, Small Craft Advisory
(SCA) conditions will likely develop across the typically windier
waters adjacent to the islands of Maui County and the Big Island
as early as Monday night, and continue through mid-week.

Surf is expected to remain below the High Surf Advisory criteria
along all shorelines through early next week. Expect a mix of
small background swells from the south-southeast and south-
southwest through Tuesday. A new longer period south-southwest
swell arriving Wednesday is expected to gradually build through
Thursday. This may cause a slight increase in surf along most
south facing shores by the middle of next week.

The weak trade wind regime will keep small surf along east facing
shores into early next week. As the trade winds strengthen,
expect choppy surf to increase along east facing shores from late
Tuesday through the end of next week. Surf along most north facing
shores will remain nearly flat for the foreseeable future.
However, small background easterly trade wind wrap and small
northwest swell energy may produce periods of tiny surf along some
north facing beaches and reefs.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Jelsema
AVIATION...Donaldson
MARINE...Kino

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



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