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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 270152
Area Forecast Discussion
National Weather Service Honolulu HI
352 PM HST Wed Jul 26 2017
Trade wind weather will prevail for the next week, thanks to high
pressure persisting far to the northeast of the area. Clouds and
passing showers will favor windward and mauka areas with just a
few brief showers expected over leeward locations. More showery
trade wind weather is expected beginning Thursday night and
continuing into Saturday as an area of enhanced moisture moves
across the island chain. Drier and more typical trade wind
weather is expected to return Sunday through the middle of next
Currently at the surface, a 1031 mb high is centered around 1400
miles northeast of Honolulu, while the remnant low of Post-
Tropical cyclone Greg is located around 800 miles east of Hilo.
Visible satellite imagery shows mostly sunny to partly cloudy
conditions in most areas, with some enhanced cloud cover over
mountain slopes and leeward sections of the Big Island.
Meanwhile, radar imagery shows nothing more than a few isolated
showers, with predominantly rain free conditions across the
State. Main short term concern revolves around rain chances over
the next few days.
High pressure will remain to the northeast of the State through
the middle of next week, keeping trade winds blowing across the
island chain. Moderate to locally breezy conditions are expected
to prevail through Thursday, with a slight easing of the trades
expected Thursday night through Friday night. The trades should
then increase a bit over the weekend into early next week as the
remnants of Greg pass by to the south of the State.
As for sensible weather details, the airmass over the islands
remains dry and stable this afternoon, with precipitable water
(PW) values from the 00Z sounding at PHLI and PHTO coming in at
1.26 and 1.15 inches respectively. Meanwhile, inversion heights at
both locations were around 6 kft. This dry and stable airmass
will remain in place this evening, with some slight moistening and
a slight increase in inversion heights expected after midnight.
Overall, we should see a fairly dry evening, with an uptick in
trade wind shower activity after midnight and into the morning
hours on Thursday. Shower coverage should diminish for the
afternoon and into the evening on Thursday due to diurnal effects.
A more showery trade wind pattern is expected to move into the
region after midnight Thursday and continuing through at least
Saturday morning, as some deeper moisture associated with the
remnants of Greg shifts into the island chain. This in
combination with a disturbance aloft swinging through the region
helping to elevate inversion heights, may result in some heavier
showers across portions of the island chain. The GFS lingers the
deeper moisture across the eastern islands through Sunday, while
the ECMWF suggests the deepest moisture will clear the island
chain Saturday afternoon. For now will stick with forecast
continuity and show a drying trend in the forecast beginning
Saturday afternoon. More typical trade wind weather is then
expected Saturday night through the middle of next week. Showers
will focus primarily over windward and mauka areas through the
period, but showers will reach leeward locales more regularly
after midnight Thursday through Saturday morning as the deeper
moisture associated with the remnants of Greg moves through.
VFR conditions prevail at all TAF sites and these conditions are
expected through the remainder of the day and through the
overnight hours. There may be some brief MVFR conditions over
windward slopes as moisture riding in on the trades interacts with
AIRMET Tango is in effect for moderate low level turbulence over
and south through west of all mountains below 8000 feet.
Moderate to locally breezy trade winds will persist through most
of the week. A Small Craft Advisory (SCA) is posted for the
typically windier waters around the Big Island and Maui through
A distant typhoon in the West Pacific is expected to send a small
mid to long period west swell towards the islands Friday into the
weekend. However, Niihau and Kauai will help effectively block
most of the remaining smaller islands from this swell.
Another round of distant East Pacific tropical cyclones could
send a series of mid to long period east swells to the islands
late Saturday into midweek. This will need to be monitored over
the next several days for more specific details on timing and
Otherwise, the locally strong trade winds will keep rough choppy
surf along the east facing shores. A series of small south,
southeast, and southwest swells will produce small background surf
through the period.
Small Craft Advisory until 6 PM HST Thursday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office