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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 200143
Area Forecast Discussion
National Weather Service Honolulu HI
343 PM HST Sat Aug 19 2017
Light to moderate trade winds will prevail through early next
week as a ridge of high pressure north of the State remains weak.
The lighter winds will allow local sea breezes to be more
prominent across leeward areas, with an increase in clouds and
showers each afternoon and evening. Meanwhile, low clouds carried
by the trades will still continue to affect windward and mauka
areas at times. Tropical moisture from the east will bring an
increase in shower coverage and muggy conditions beginning
tonight and continuing through Tuesday. Expect drier and windier
conditions to return Wednesday through late next week as high
pressure strengthens north of the island chain.
Currently at the surface, a 1029 mb high is centered around 1900
miles northeast of Honolulu, with a ridge axis extending southwest
from the high to a location around 800 miles north of Kauai. The
resulting gradient is driving light to moderate trade winds across
the island chain this afternoon. Visible satellite imagery shows
partly to mostly cloudy skies across the state. Radar imagery
shows scattered trade wind showers moving into windward areas,
with a few showers also affecting leeward locales. Main short
term concern revolves around rain chances over the next few days.
Tonight through Tuesday night,
Light to moderate trade winds will continue through early next
week, with some slight strengthening of the trades expected late
Tuesday and Tuesday night as high pressure strengthens to the
north of the State. Aloft, an upper level low will drift
southwestward and closer to the islands tonight through Sunday
night, then pivot off to the west and further away from the state
early next week. This will act to de-stabilize the airmass over
the islands a bit, particularly over the western end of the State.
At the same time, some deeper moisture will begin to work its way
into the area from the east tonight, with precipitable water
values climbing into the 1.5 to 2.0 inch range Sunday through
As for sensible weather details, we should begin to see an
increase in trade wind showers beginning tonight, with periods of
showery weather expected Sunday through early next week,
particularly in windward areas. In leeward areas, we should also
see an increase in showers, with some localized downpours possible
each afternoon into the early evening in association with sea
breeze enhancement. Additionally, due to the more unstable airmass
over the islands, isolated thunderstorms will be possible over
the Big Island, primarily the interior and leeward sections, each
afternoon into the early evening. Isolated thunderstorms may also
affect the waters north of Oahu and around and including the
island of Kauai Sunday night through Monday evening as the upper
level low makes its closest approach. Thunderstorms may also
affect interior and leeward sections of Oahu Monday afternoon,
particularly near the Waianae mountains, so have included a
slight chance of thunderstorms in the forecast here as well.
Conditions will also become quite humid Sunday through early next
week, as the trades lighten up and dewpoints climb into the lower
and middle 70s across the area.
Wednesday through next Saturday,
High pressure will strengthen north of the State, bringing
moderate to breezy trade winds back to the island chain through
the end of the work week. Drier more stable conditions are also
expected, with more comfortable trade wind weather expected.
Showers will continue to favor windward and mauka areas, with a
stray shower reaching leeward areas from time to time due to the
strength of the trades.
VFR conditions prevail at all TAF sites with brief MVFR conditions
being reported at PHTO and PHNY. Occasional MVFR conditions are
likely occurring over windward slopes for most of the state as
moisture riding in on the trades interacts with island terrain.
No AIRMETS are currently in effect but one may be needed later
tonight for mountain obscuration over eastern portions of the
state as an area of enhanced moisture moves in from the east.
Winds and seas are expected to remain below Small Craft Advisory
levels through early next week as a trough aloft helps weaken the
pressure gradient. The trough may make showers more active, and
could even produce some thunderstorms Sunday night through Monday
night. Wind speeds will increase again later next week.
Surf along all shores is expected to remain well below the High
Surf Advisory threshold through the middle of next week.
The peak high tides for the month will occur over the next few days.
Water levels are running as much as a foot above the predicted
levels due to an eddy moving westward through Hawaiian waters.
Coastal flooding is possible around the times of the high tides
through early next week. See the Special Weather Statement for
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office