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Hawaiian Islands Synoptic Discussion and Guidance

172
FXHW60 PHFO 141323
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
323 AM HST Tue Jul 14 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 an upper trough and a lower level
disturbance 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
lingering into the weekend. Higher pressure builds back in on
Saturday allowing moderate to locally breezy trade winds to
return.

&&

.DISCUSSION...
In the big picture we see an upper level low roughly 600 miles
north of Oahu moving slowly northeastward and a low level 700 mb
trough moving through the center of the state. Unstable low clouds
along this trough are enhanced by an upper disturbance producing
enhanced showers statewide. These showers will diminish in
coverage later this morning. A subtropical jet stream continues to
bring a layer of thin high cirrus clouds over the state. More
stable stratocumulus clouds are shown on satellite moving in
behind the low level trough. These clouds are fairly stable and
support a drying trend due starting on Wednesday, as a weak ridge
increases stability aloft, and reducing cloud heights.

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 through the remainder of today. Sufficient
moisture in the lower levels, along with an upper level feature
passing to the north, will help to generate scattered to numerous
showers along the windward coasts and slopes. This activity may
produce MVFR ceilings and visibility at times. A relatively deep
inversion will allow for some showers to push over the terrain and
affect the leeward areas as well.

AIRMET SIERRA remains in effect for north thru east sections of
all islands above 2500 feet due to tempo mountain obscuration
from clouds and showers.

&&

.MARINE...
High pressure far north-northeast of the islands will continue
into the middle of the week. Winds dropped a little late yesterday
and overnight. An ASCAT pass showed winds 20 kts or less.
An small uptick is possible today, but not expecting winds to
reach Small Craft Advisory levels at this time.

A weak disturbance is expected to move to the northeast of the
islands Thursday and Friday which could further 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

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RH		27 %
Wind		E 8 mph
Road		Open (4x4)