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Temp0.8 C
RH16 %
WindN 12 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

129
FXHW60 PHFO 140641
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
841 PM HST Mon Jul 13 2020

.SYNOPSIS...
Expect a gradual decreasing trend in the trade winds through the
week. In the short term, enhanced shower activity will continue
across all islands as both upper and lower level disturbances will
combine forces to increase shower coverage. The highest coverage
for showers will develop along the windward and mountain slopes of
each island with more isolated to scattered shower coverage over
leeward areas, especially in the overnight and early morning
hours. Drying trends are expected from Wednesday into Thursday
under a weak ridge aloft. Another low level trough passes through
the islands on Friday with increasing shower trends into the
weekend. Higher pressure builds back in on Saturday allowing
moderate to locally breezy trade winds will return.

&&

.DISCUSSION...
The satellite picture this evening shows an upper level low
roughly 600 miles north of Molokai with a lower level trough
passing over the state. Both of these disturbances will combine
forces and keep the atmosphere fairly unstable into Tuesday
morning. Expect numerous shower coverage over windward and
mountain areas as the trade winds will converge with the mountain
slopes and focus much of the shower energy over windward areas.
However, there is enough instability for isolated to scattered
shower activity reaching the drier leeward slopes with scattered
shower trends forecast more in the overnight to early morning
hours.

Wednesday and Thursday...Lighter winds will develop as the high
pressure center over the Central Pacific basin moves northeast
away from the islands and local pressure gradients decrease. These
lighter wind speeds will allow local scale land and sea breezes to
develop mainly over sheltered leeward areas. Although some
additonal showers may develop along southeasterly wind and sea
breeze convergence zones in the afternoon hours, a general drying
trend remains in the forecast for this time period.

Friday through Sunday...Light and more variable winds with local
scale land/sea breezes will continue on Friday. Shower trends will
increase on Friday as another low level trough moves into the
state from the east. High pressure will build back in from the
northeast heralding a return to a moderate to breezy trade wind
regime on Saturday and Sunday. Wind speed convergence with the
returning trade winds will keep shower activity enhanced into the
weekend, especially over windward and mountain areas.

&&

.AVIATION...
High pressure far north of the state will once again lead to
moderate trades tonight. Sufficient low level moisture, caught up
in the flow, will bring scattered to numerous showers along the
windward coasts and slopes. MVFR ceilings and visibility are
expected at times with this activity. Some showers will push over
the terrain and affect the leeward areas due to a relatively deep
inversion.

AIRMET TANGO remains in effect for north thru east sections of
Kauai above 2500 due to tempo mountain obscuration from clouds and
showers. Conditions are expected to deteriorate for the other
smaller islands later tonight, around 09Z, and AIRMET TANGO will
go into effect then above 2500 feet for north thru east facing
sections of the islands.

&&

.MARINE...
High pressure far north-northeast of the islands will continue
into the middle of the week. Winds have dropped a bit, with
another uptick expected Tuesday afternoon. A Small Craft Advisory
could be needed, depending on how much the winds pick up.

A weak disturbance is expected to move to the northeast of the
islands Thursday and Friday which will once again disrupt the
trade wind flow. Expect the trades to ramp back up over the
weekend as that disturbance dissipates and the high pressure
strengthens to the north of the islands.

Overall, no significant swells are expected through the forecast
period. Small background surf will continue along south facing
shores over the next several days. A small south-southwest swell
is possible over the weekend. East facing shores will continue to
experience small short period surf produced by the trade winds
locally and upstream of the state. An increase is expected over
the weekend as the breezy trade winds return. Surf along north
facing shores will remain nearly flat through the period.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Bohlin
AVIATION...JT
MARINE...M Ballard

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office