920 
FXHW60 PHFO 211400
AFDHFO

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

.SYNOPSIS...
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.

&&

.DISCUSSION...
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.

&&

.AVIATION...
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.

&&

.MARINE...
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 
increase.

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. 

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
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.

&&

$$

DISCUSSION...Bohlin
AVIATION...Bedal
MARINE...Gibbs