000 
fxhw60 phfo 200136 
afdhfo 


Area forecast discussion 
National Weather Service Honolulu HI 
336 PM HST sun Aug 19 2018 


Synopsis... 
a rather humid airmass remains in place due to a weak trough of 
low pressure just west of Kauai. A drier airmass along with 
strengthening trade winds are in the forecast for Monday and 
Tuesday. For the remainder of the week, the forecast will be 
highly dependent on the track and intensity of Hurricane Lane. An 
increase in trade winds and shower activity will be the likely 
result based on the latest forecast for Lane. 


&& 


Discussion... 
areas of high pressure are currently centered far north-northeast 
and far northwest of the state with a weak westward moving trough 
of low pressure centered just west of Kauai. Instability associated 
with the trough did allow for a few thunderstorms to form over the 
leeward Big Island slopes this afternoon but latest radar imagery 
shows that these are now ending. Otherwise just a few showers can 
be expected through tonight mainly over windward and Mauka areas. 


For Monday and Tuesday, the two highs will gradually merge and 
take up a position far to our north. This will cause for an uptick 
in trade winds speeds across the area. The trough of low pressure 
will continue to move west away from the area and gradually 
dissipate west of the state. Thus a drier airmass is forecast to 
move into the area from the east though some passing light showers 
will still be possible over some windward and Mauka areas. 


During the Wednesday through Friday time frame, our weather 
pattern will be highly dependent on the track and intensity of 
Hurricane Lane. It remains too early to forecast the exact track 
of Lane and whether it will directly impact portions the state. 
The latest forecast track from the Central Pacific Hurricane 
Center has Lane passing by south of the main Hawaiian islands. On 
this track, the main impacts on the islands weather would be for 
an increase in trade wind speeds across the area due to a 
tightened pressure gradient between high pressure north of the 
state and Lane to our south. Moisture around the periphery of Lane 
would likely cause an increase in shower activity over east and 
southeast sections of The Big Island, and then eventually 
spreading up the island chain through the remainder of the week. 
Everyone is encouraged to monitor the latest advisories for Lane 
from the Central Pacific Hurricane Center. 


&& 


Aviation... 
airmet Sierra is in effect for mountain obscure on Kauai and Lanai. 


A high far northeast of the area will remain nearly stationary. 
A weak trough near Kauai will move west at 10 kt. The trough has 
been weakening the surface winds. Easterly surface winds will 
strengthen and become fresh to locally strong overnight. 


Moisture associated with the trough has produced heavy showers 
and isolated thunderstorms today near Kauai and over The Big 
Island slopes. As the trough moves farther west, the atmosphere 
will become drier and more stable this evening with VFR conditions 
prevailing area-wide. 


As winds strengthen and the atmosphere becomes more stable, 
turbulence in the Lee of the mountains may become moderate and 
airmet Tango may be needed. 


&& 


Marine... 
a weak trough of low pressure will continue to move through the 
western coastal waters tonight producing some locally heavy 
showers and isolated thunderstorms over the western half of the 
state. Light to moderate trade winds are expected to continue 
today then strengthen to fresh to strong speeds tonight. A Small 
Craft Advisory (sca) has been issued for the typically windy 
waters around Maui County and The Big Island through Tuesday. The 
latest forecast for Hurricane Lane has it passing by to the south 
of The Big Island Tuesday night through Wednesday. As a result, 
trade winds will likely increase further and the Small Craft Advisory will likely 
have to be expanded to additional zones around that time. It is 
still too early to determine the exact impacts that Hurricane Lane 
may have on the coastal waters, so mariners are urged to keep up 
to date with the latest forecasts. 


Long period energy from Hurricane Lane is expected to begin 
filling in along east and southeast facing shores of The Big 
Island and Maui tonight and Monday. As Hurricane Lane moves 
closer to the islands around midweek, swell is expected to 
increase and result in large surf for east facing shores of The 
Big Island and Maui during the early part of this week. A high 
surf advisory has been issued for east facing shores of The Big 
Island and Maui through Tuesday. After that time, the large surf 
may increase to warning levels for east and/or south facing 
shores as Lane progresses westward south of the state. We will 
continue to monitor the track and intensity of Hurricane Lane, as 
any changes in the track or intensity will significantly affect 
surf heights. A small, north-northwest swell is expected to 
arrive Monday and continue through Tuesday. A larger north swell 
is expected to arrive late Tuesday, peak Tuesday night and early 
Wednesday, then slowly fade through Thursday. 


See the latest Oahu surf discussion (srdhfo) for more details on 
surf and swell. 


&& 


Hfo watches/warnings/advisories... 
high surf advisory from 6 am Monday to 6 PM HST Tuesday for 
windward Haleakala-south Big Island-Big Island north and east. 


Small Craft Advisory until 6 PM HST Tuesday for Maalaea Bay- 
Pailolo Channel-Alenuihaha Channel-Big Island leeward waters-Big 
Island southeast waters. 


&& 


$$ 



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