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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 211400

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Sun Jul 21 2019

A high pressure ridge will remain locked in place north of the
Hawaiian Islands through the middle of next week, keeping the
islands in a moderate to breezy 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. Additional clouds and enhanced
showers may reach the northwestern islands by Tuesday morning. A
more typical breezy trade wind weather pattern returns from
Wednesday onward with drier trends.


Fairly unstable cumulus cloud bands are streaming into the
windward and mountain slopes of each island this morning. Radar
imagery shows the highest shower activity over typical windward
slopes. An active low level trough has formed south of the Big
Island this morning bringing up deep tropical moisture as the
trough axis tracks towards the northwest, passing just south of
the island chain. An upper level low roughly 500 miles north-
northwest of Kauai will produce lingering instability over the
state through Sunday night with shower activity favoring windward
and mauka areas.

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. The upper level low north to northwest of Kauai will
slowly drift westward over the next few days, and still provide
enough thermal instability and divergent lift to enhance windward
and mauka showers across the region. South of the state a low
level trough tracking in a northwest direction 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 tropical moisture plume will have Precipitable Water
(PW) levels in the 2.0 to 2.5 inch range producing widespread
clouds and showers over the Big Island and to a lesser degree over
Maui by Monday afternoon. Isolated thunderstorms remain in the
forecast on Monday over the Big Island and adjacent waters.
Forecast rainfall totals were increased for the Monday and Tuesday
time periods. Short range models are starting to show periods of
heavy rain along the south and eastern slopes of the Big Island.
Over the past 24 hours both global weather models have improved
their forecast consistency for enhanced rainfall over the Big
Island on Monday with good confidence at this point.

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
wet 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 northward push of deep
tropical moisture into the islands as the low level trough axis
passes south of Kauai. As the trough drifts further northward the
trade wind speeds will increase as it passes each island. Stay
tuned for changes to the rainfall forecast on 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. Breezy to locally windy trade winds remain in
the forecast for the second half of next week.


Strong high pressure far north of the Main Hawaiian Islands will
remain nearly stationary, continuing to generate locally strong
east winds over the area. AIRMET TANGO is in effect for moderate
turbulence below about 8,000ft and this AIRMET will likely
continue through Sunday.

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, especially in the morning hours.
Light sea breezes are expected this afternoon and early evening
across Leeward portions of the Big Island. Expect cumulus buildups
over the islands interior to redevelop. Localized heavy downpours
possible, especially in the afternoon and evening hours.


Fresh to locally strong easterly trades are forecast to continue
through the 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 tonight and now includes the Kaiwi Channel
and the Maui Windward Waters. The advisory may need to be extended
further into the week.

Rough surf will continue along east facing shores each day due to
strong onshore winds. A slight downward trend is expected through
Tuesday due to the upstream trades relaxing. A slight uptick is
expected once again Wednesday into Thursday as upstream trades

Surf along south facing shores will remain small through tonight,
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 Kaiwi Channel-
Maui County Windward Waters-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