299 
FXHW60 PHFO 260637
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
837 PM HST Wed Sep 25 2019

.SYNOPSIS...
The interaction between a surface trough west of Kauai and
a couple of upper level disturbances moving across the area has
resulted in a unsettled environment mainly over the western part
of the main Hawaiian Islands. Unsettled conditions with periods 
of heavy rains and thunderstorms are possible through Thursday for
Oahu but especially Kauai County. Improving conditions are in
storefor Friday and the weekend with light to moderate trades 
returning to the area.

&&

.DISCUSSION...
The islands is under a warm, humid, light southerly wind flow 
regime this evening. It has been a rather quiet Wednesday area- 
wide, although there was a burst or two of showers on Oahu this 
afternoon. Current satellite shows an eastward moving upper level 
impulse/disturbance about to move across Oahu. The impulse will be
taking away some of mid and high clouds over Oahu later this 
evening. But in doing so, it may lead to a brief flare-up of 
showers across Maui County in the next few hours. Radar is currently
showing some spotty, small showers across Maui County, and over 
the Kona area of the Big Island. As for Oahu, it is mainly dry 
over land, but radar shows scattered heavy showers off windward 
Oahu and a dying shower just off Barbers Pt. On Kauai, Skies are 
currently fair across Kauai and Niihau. But there are some
scattered to broken low clouds and a few showers associated with 
the surface trough sitting off the Na Pali coast and Niihau and to
the south of these two islands. We will be watching closely for a
flare-up of activity likely with this trough and south of 
Kauai/Niihau in the coming hours. 

We will reassess the idea of expanding the Flash Flood Watch to
Oahu for Thursday during the overnight hours.

All this weather is east of a front located 390 miles northwest 
of Kauai. The front is forecast to stall some 280 miles northwest 
of Kauai Thursday, and gradually fall apart there through Friday.

Light trades will be returning on Friday area-wide, but it will 
be light enough for the development of daytime sea breezes. The
upper trough will be lifting out and surface trough dissipating,
and a more stable air mass will be sliding in. With this more
stable air mass, expect just a few afternoon showers from these 
daytime generated clouds. The trades will continue to strengthen 
through the upcoming weekend, reaching moderate strength by 
Sunday, and locally strong about Monday. And with the return of
the trades, our rainfall pattern will transition to windward and 
mauka showers, especially nights and mornings.

&&

.AVIATION...
Tonight, rather deep moisture and instability associated with a 
trough oriented near Kauai will allow for weak southerly flow and 
scattered SHRA/SHRA+ over mainly Kauai and Oahu. A slight chance 
for thunderstorms exists, which would have the potential to lower 
cloud cover/visibility and produce MVFR or brief IFR conditions. 
Otherwise, expect light trade flow across the remaining islands 
with only isolated -SHRA. 

AIRMET Sierra will remain in effect for Oahu and Kauai for all 
areas above 2000 feet due to tempo mountain obscuration. 

&&

.MARINE...
Light south to southeast winds will continue for the next few 
days as a front approaches the islands from the northwest. The 
front is expected to stall over the far northwestern offshore 
waters on Thursday and dissipate through Friday. Light to moderate
trades will return this weekend as high pressure builds in in 
place of the dissipated front. An upper-level trough west of Kauai
will continue to support isolated thunderstorms over the western 
and southern waters of Kauai, including the Kauai Channel, through
tonight. 

Swells from the south-southwest and south will fill in tonight 
and Thursday, producing advisory level surf for south facing 
shores Thursday through Friday before slowly declining into the 
weekend. A High Surf Advisory (HSA) will go into effect Thursday 
morning. East facing shores will continue to see moderate surf 
through Thursday before slowly diminishing. A surface low well 
north-northwest of Kauai is generating a moderate swell that is 
forecast to reach Kauai's north and west facing shores Thursday 
morning and spread down the island chain through Thursday night. 
This swell should peak Friday below advisory level, then slowly 
decline this weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from 6 AM Thursday to 6 PM HST Friday for 
south facing shores of all islands.


Flash Flood Watch through Thursday afternoon for Niihau-Kauai.


&&

$$


H Lau/Thomas/Powell