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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 190137
Area Forecast Discussion
National Weather Service Honolulu HI
337 PM HST Fri Aug 18 2017
Trade winds will decrease a bit as a weakness develops in the
ridge to our north, allowing development of local sea breezes
across leeward areas into early next week. Leeward areas will see
an increase in clouds and showers each afternoon and evening,
while diminished trade flow will continue to supply both to
windward and mauka areas. Moisture from an old tropical cyclone
will increase shower coverage and bring muggy conditions to the
islands late Saturday through the first half of next week. Expect
drier and windier conditions to return late next week as the ridge
Upper troughing along 150W is beginning to put a dent in the
surface subtropical ridge to our north and a recent ASCAT pass
showed that winds have begun to subside across local waters.
Satellite loop shows scattered to patchy broken low clouds
embedded within trade flow moving across the islands, with little
change noted upwind as far as 145W. Radar shows only isolated
showers moving across the islands within this cloud cover. Latest
soundings showed a dry and neutral airmass, with about an inch of
PW and near zero lifted index values. Strong inversions persist
between 6000 and 7000 feet. The upper trough to our north has yet
to raise the inversion or significantly destabilize our airmass.
Models show the upper trough to our north will dig southwestward
over the next few days, placing the base of the trough over the
islands, and the trough axis near Kauai and Oahu, by Sunday. This
roughly coincides with the arrival of an influx of moisture from
now-defunct Jova and a general decrease in background trade flow.
While trade flow may rebound slightly Monday and Tuesday around
the Big Island, the overall statewide result will likely be
increased trade shower coverage across windward and mauka areas,
the introduction of afternoon sea breeze clouds and showers
across leeward areas, and an overall muggy feel to the air from
late Saturday through Tuesday. There is still noticeable model
disagreement concerning a potential thunderstorm threat on Monday.
GFS continues to push enough cold air aloft southward to trigger
this threat across the western islands on Monday, but ECMWF keeps
this cold air too far west of the main islands to be threatening
during the same time frame. We will keep thunderstorms and heavy
rainfall out of the forecast for now, but will reconsider if and
when model solutions converge.
By Wednesday, both models show the subtropical ridge recovering,
with trade winds increasing back into the moderate to locally
breezy range once again. Drier conditions will move in as well.
A ridge of high pressure north of the State will keep a moderate
to locally breezy trade wind flow in place through tonight, with
clouds and showers favoring windward and mauka areas. Brief MVFR
cigs may affect windward areas as showers move in with the trades,
particularly during the overnight and early morning hours.
Predominantly VFR conditions are expected at the TAF sites through
No AIRMETs are currently in effect. AIRMET Sierra may be needed
for mountain obscuration across some windward areas later
Winds and seas are expected to remain below Small Craft Advisory
levels through the weekend as a trough aloft moves over the area
and helps weaken the pressure gradient. The trough may make
showers more active, and could even produce some thunderstorms
early next week. Wind speeds will increase again later next week.
Surf along all shores is expected to remain well below the High
Surf Advisory threshold. Surf along south facing shores is up a
bit due to a swell generated in the Southern Hemisphere. Surf
along the south facing shores will gradually subside over the
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 the weekend. See the Special Weather Statement for more
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office