829 
FXHW60 PHFO 301942
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
942 AM HST Fri Aug 30 2019

.SYNOPSIS... 
Gentle trades will maintain through the holiday weekend with
localized afternoon sea breezes setting up over some of the
blocked leeward areas. Increasing moisture from the east will 
bring the potential for brief heavy showers Saturday and Sunday.
It is also expected to be very humid through the weekend. Drier 
air with gentle trades will begin the week with a possible
strengthening of the trades late next week.

&&

.DISCUSSION...
Water vapor imagery this morning shows a tropical upper 
tropospheric trough west of the main Hawaiian Islands. Scattered 
to broken cirrus clouds associated with the subtropical jet are 
passing over the state, which will likely continue at least 
through the next 24 hours given the current satellite/model trend.
At the low levels, a weak ridge is located northeast of the 
islands, with a weak stationary front to the north and a stronger 
high even further to the north. The highs are driving gentle to 
moderate trade winds. Expect another warm summer day today with 
highs around 90 F plus and lows in the upper 70s. Sea surface 
temperatures near the islands are around 82 degrees F. 

A surface trough 200 miles east of the Big Island will drag a 
plume of tropical moisture across the state beginning early this 
evening in the east and then across the smaller islands by 
Saturday morning. Guidance shows precipitable water (PW) values 
jumping from the current normal 1.4 inches to around 1.9 inches. 
We may also see a slight weakening of the winds through the 
weekend as the trough passes over the islands. Both GFS and ECMWF 
models show gentle trades maintaining through the weekend and the 
winds should just be strong enough to focus more of the clouds and
showers windward, however, localized sea breezes may set up 
Saturday and Sunday afternoons in blocked leeward locations 
leading to shower development both windward and leeward. Dew
points will increase to the mid 70s through the weekend which 
could push the afternoon heat index over 100 in a couple of spots.

With PWs this high, locally heavy rainfall and will be possible
this weekend. Highest confidence is across the interior Big 
Island Saturday and Sunday afternoons. It's possible that other 
areas of the state could get brief heavy showers too.

As the trough departs on Labor Day, relatively drier air will 
fill in leading to a more comfortable and somewhat less showery 
weather pattern. Winds will remain in the gentle category into 
midweek as a slowly-moving weak disturbance passes northeast of 
the islands. Expect the warm summertime temperatures to continue
through the week. Winds may strengthen by the end of the week as
long range models hint at a possible disturbance passing south of
the islands next weekend, though a bit early to tell this far out
in the forecast period.


&&

.AVIATION...
Gentle to moderate trade winds will deliver limited amounts of low
clouds to most windward areas through the day today. However, an 
area of increased moisture moving in from the E is expected to 
fuel increased windward showers over windward Big Island by this 
evening, spreading to the other islands overnight into Saturday. 
Cloud and shower coverage could potentially increase to the point 
that mountain obscuration may occur, but for now no AIRMETs are 
posted. High clouds streaming over the area from the SW will have 
little impact on sensible weather.

VFR will prevail at most TAF sites, with MVFR CIG expected at 
PHNY through mid-afternoon, and MVFR CIG/VIS in SHRA possible at 
PHTO later today. 


&&

.MARINE...
A weak trough will move through from east to west over the 
weekend while high pressure remains north of the area. The trough 
will bring an increase in showers mainly over windward waters 
during the weekend. 

Small long-period southwest swells will hold into the weekend. 
This will keep surf heights near the seasonal average through 
Saturday before dropping Sunday. A compact low, south of Easter 
Island on 8/26, had a fetch of gale force winds aimed at Hawaii 
that may bring a small boost to southeast facing shores Monday 
night through the middle of next week. A south- southwest swell 
will be possible through for the second half of next week. 

A small north swell is expected over the weekend with its peak 
likely on Sunday. A low in the NW Pacific is expected to 
strengthen this weekend as it passes south of the Aleutian 
Islands. This may bring a small to moderate north-northwest swell
starting Tuesday.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$


DISCUSSION...Foster
AVIATION...Birchard
MARINE...Morrison