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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 210624
Area Forecast Discussion
National Weather Service Honolulu HI
824 PM HST Sat Jul 20 2019
A high pressure ridge will remain locked in place north of the
Hawaiian Islands through the next week, keeping the islands in a
trade wind weather pattern. A trough moving in from the deep
tropics will bring additional moisture and showers to the eastern
half of the state from early Monday morning through Tuesday. Some
of these clouds and enhanced showers may reach the western
islands by Tuesday. A more typical breezy trade wind weather
pattern returns from Wednesday onward with drier trends.
This evenings satellite imagery shows unstable clouds moving into
the windward and mauka slopes of each island in a moderate to
breezy trade wind pattern. The water vapor imagery shows a
seasonably strong upper level low roughly 500 miles north of Kauai
drifting slowly westward. Upper level divergence and instability
associated with this low will keep active windward and mountain
showers in the forecast through the overnight and early morning
hours. Convective activity continues to form around a developing
trough in the deep tropics south of the Big Island this evening.
This trough will drift northwestward changing island weather
conditions over the next three days.
In the big picture, the high pressure ridge north of the islands
will remain in place keeping moderate to breezy trade winds
flowing across the region through at least the first half of next
week. An upper level low north to northwest of Kauai will provide
some thermal instability and mechanical lift to enhance windward
and mauka showers across the region. A low level trough moving
northwest out of the tropical region south of the Big Island will
advect significant amounts of deep tropical moisture into the
eastern islands of Maui and the Big Island. Enhanced showers will
reach the southern and eastern slopes of the Big Island starting
early Monday morning. This deep moisture plume will have
Precipitable Water (PW) levels in the 2.0 to 2.5 inch range
producing widespread clouds and showers over both islands by
Monday afternoon. Isolated thunderstorms remain in the forecast
for the Big Island and adjacent waters. Global weather models have
improved their forecast consistency for enhanced rainfall over
the Big Island and Maui on Monday with good confidence at this
Enhanced clouds and showers will potentially drift further
northward to the western half of the state on Tuesday. The
American (GFS) and European (ECMWF) models are gradually narrowing
their differences and trending towards a wetter solution for the
islands of Kauai, Oahu, Molokai and Lanai. The challenge in this
forecast will revolve around precipitation amounts and timing for
these western islands; forecast rainfall impacts will be highly
dependent upon the interactions between the drier trade wind flow
moving in from the east, and the track of the low level trough
forecast to pass south of Kauai. As the trough drifts further
northward the trade wind speeds will increase and wind directions
will veer from east to southeast, potentially increasing the
south to north moisture advection over the western islands as the
trough axis passes. Stay tuned for changes to the rainfall
forecast for Tuesday as this weather pattern evolves.
The low level trough will start to drift west of the islands by
Wednesday with drier air keeping most shower activity limited to
typical windward and mountain areas favoring the overnight and
early morning hours..
A high will remain nearly stationary far north of the main Hawaiian
islands and maintain locally strong east winds over the area.
AIRMET TANGO is in effect for moderate turbulence below about 7,000ft
and this AIRMET will likely continue through Sunday.
Clouds and showers that developed over the southwest slopes of
the Big Island and Maui will dissipate later this evening. Low
level flow will continue to carry scattered to broken low clouds
over east sections of the islands. Isolated MVFR ceilings are
possible over these areas tonight. An AIRMET for mountain
obscuration could become necessary across windward portions of the
Big Island late tonight.
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 has
been extended through Sunday night, but will likely need to be
extended further into next week.
Rough surf will continue along east facing shores each day due to
strong onshore winds. Latest local buoy observations are showing a
gradually decrease in heights and we should continue to see a slow
downward trend through Tuesday due to the upstream trades
trending down. A slight uptick is expected once again Wednesday
into Thursday of next week as upstream trades increase.
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 southwest
swell moves through and peaks Monday through Tuesday. In addition
to this long-period source, a combination of small south to
southeast swells will be enough to keep the surf from going flat
along southern exposures through midweek.
A south-southeast (150-160 deg) long-period swell associated with
a compact gale that has developed southeast of the Tuamotus will
be possible late next week with a peak around Thursday night into
Friday. Surf will rise along exposed shores but should remain
below advisory levels.
Small Craft Advisory until 6 AM HST Monday 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
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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
Send comments to: Ryan Lyman