National Weather Service Raw Text Product

779 
FXHW60 PHFO 091403
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
403 AM HST Tue Jul 9 2019

.SYNOPSIS...
A high pressure ridge far north of the state will keep trade 
winds in the forecast for the next seven days. The remnants of 
former tropical cyclone Barbara will continue to drift westward 
today with enhanced showers lasting through the day. More stable 
drier trends are expected from Wednesday through Friday with wet 
trade wind weather returning to Maui and the Big Island by next
weekend.

&&

.DISCUSSION...
A high pressure ridge far north of the Hawaiian Islands will keep
trade winds in the forecast through the upcoming weekend. The
low level remnants of former tropical cyclone Barbara are roughly
300 miles south of Kauai this morning moving towards the west 
around 15 MPH. Convergent cloud plumes forming off the leeward 
waters of the Big Island and Maui are interacting with a deep 
tropical moisture pool and producing thunderstorms south of the 
island chain. 

Upper air soundings in Hilo and Lihue show high subsidence 
inversion heights between 10000 to 15000 feet with Precipitable 
Water levels on the soundings around 1.8 inches. This unstable and
humid air mass will continue to produce showers across the state 
today with a higher percentage of rainfall favoring windward and 
mauka areas. Strong trade winds enhanced by the passing of this 
system will gradually decrease today as the trough axis associated
with Barbara moves westward away from the state. However, a 
second trough shown on 700 mb streamlines will keep enough 
tropical moisture and instability around for periods of wet trade
wind showers through the overnight hours.

Wednesday through Friday, the large scale weather pattern changes
as a stronger ridge aloft causes increased downward vertical 
motions and lower trade wind inversion heights. This means shower 
activity will slowly trend lower on Wednesday with clouds and 
showers favoring windward and mountain slopes. By Thursday, the 
trade wind inversion heights will fall into the 5000 to 7000 foot 
range, capping cloud development and shower activity. A drier 
trade wind weather regime will continue for all islands through 
Friday afternoon.

On Saturday and Sunday, another tropical disturbance develops far
southeast of the Big Island. Long range model solutions continue 
to track this system northwestward into the Hawaii region on 
Sunday. Another plume of deep tropical moisture will advect into
Hawaii and Maui counties, likely heralding another round of wet 
trade wind weather for at least the southeastern islands. Both 
global models are showing a wet weekend weather pattern for the 
Big Island and windward Maui lasting into the first few days of 
next week. 

&&

.AVIATION...
High pressure north of the state and the remnants of former 
Tropical Cyclone Barbara passing by to the south, will keep
moderate to breezy easterly trade winds in place today, with the
trades easing tonight. AIRMET Tango remains in place for low 
level turbulence over and downwind of the terrain of all islands,
and will likely remain in place through the day.

Deep tropical moisture will linger over the islands today, 
bringing MVFR cigs/vsbys to windward areas in passing showers. 
AIRMET Sierra remains in effect for mountain obscuration across 
windward sections of Kauai, Oahu, Molokai, Maui and the Big 
Island. Conditions should improve in many if not all areas later
this morning.

&&

.MARINE...
The northern end of a surface trough, which is the remnant of 
former East Pacific Tropical Cyclone Barbara, appears to be about 
100 nm south of Kauai early this morning. This feature has been 
moving westward at around 15 kt, so its direct influence on 
weather across the state continues to diminish. The wind speeds 
have dropped below gale force overnight in the Alenuihaha Channel
and the waters south of the Big Island. However, the pressure 
gradient remains tight enough to keep Small Craft Advisory (SCA) 
conditions through early Wednesday morning across the typically 
windier waters adjacent to the islands of Maui County and the Big 
Island. The current forecast indicates the trade winds may briefly
drop below the SCA across these windier waters on Wednesday. 
However, a new surface high pressure system is forecast to build 
far north of the state later this week. Therefore, SCA winds may 
return to the typically windier waters again on Thursday.

The locally strong trade winds continue to produce elevated rough
surf along east facing shores early this morning according to the
nearshore buoy observations. Therefore, a High Surf Advisory 
(HSA) remains in effect for east facing shores of most islands 
through mid-afternoon today. Surf is expected to gradually lower 
along east facing shores from late this afternoon through 
mid-week.

The combination of a declining south-southwest swell and a small
southeast swell generated by strong southeast trade winds in the 
South Pacific last week will likely maintain surf heights near or 
slightly above the summer-time average along exposed south facing
shores through mid-week. New long period south-southwest swells 
arriving starting Friday are expected to cause surf to reach the 
HSA criteria along south facing shores from this weekend into 
next Monday.

Elsewhere, a small northwest swell is expected to arrive late
Wednesday. This swell may produce a slight bump in surf along 
north and west facing shores of some of the smaller islands from 
Wednesday night through Friday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 4 PM HST this afternoon for Kauai 
Windward-Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-
Windward Haleakala-Leeward Haleakala-South Big Island-Big Island 
North and East.

Small Craft Advisory until 6 AM HST Wednesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big 
Island Southeast Waters.

&&

$$

DISCUSSION...Bohlin
AVIATION...Jelsema
MARINE...Houston