280 
FXHW60 PHFO 100213
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
413 PM HST Wed Oct 9 2019

.SYNOPSIS...
Winds will strengthen and veer southeast on Thursday in response 
to a low forming northwest of the islands and building high 
pressure to the east. Deep tropical moisture moving onto the Big
Island this afternoon will spread westward tonight and bring a 
chance of heavy rain and thunderstorms through Thursday night. 
Trade winds will build in Friday into next week and bring drier 
and more stable weather.

&&

.DISCUSSION...
A slow-moving cold front is located about 650 miles northwest of 
Kauai with a ridge axis about 250 miles north of Kauai. The parent
high associated with the ridge is located far to the northeast is
driving moderate winds, just south of due east this afternoon. 
Water vapor imagery shows an upper low about 1300 miles northwest 
of Hawaii digging southward. Current visible imagery over the 
state shows partly cloudy skies over the smaller islands. On the
Big Island, an area of deep tropical moisture moved in from the
east earlier this afternoon bringing moderate to occasionally 
heavy showers along with a few embedded thunderstorms.

The upper low is expected to induce a surface low along the front
tonight while high pressure east of the state begins to 
strengthen. As a result, winds will strengthen from the southeast 
tonight and spread the tropical moisture over the rest of the
state through Thursday night. Current MIMIC layered precipitable 
water (PW) imagery shows values of 2.5 inches just east of the Big
Island moving towards our area. Expect the rain to continue across
the windward and southeast sections of the Big Island through the
night.

Models indicate a decently strong convergence band setting up 
over the state by Thursday (along the eastern flank of the earlier
mentioned developing surface low). Thus, heavy rain and thunderstorms
will be possible for the eastern end of the state tonight, and 
for most of the state Thursday and Thursday night. Southeast 
winds will focus rain over windward and southeast facing slopes, 
however, with winds running parallel to the island chain tomorrow 
(and thus weakening the winds) leeward/interior portions of the 
islands could also see rainfall increase during the afternoon
tomorrow. Leeward plume showers could also anchor to any of the 
smaller islands. For now, we are not issuing a Flood Watch due to
mitigating factors such as the upper ridge over Hawaii and with 
some of the high-resolution models (ARW and NAMNEST) indicating
the rainfall deflected north and south of the island chain. We
still may consider a watch on Thursday depending upon how features
set up through the overnight.

Improving conditions are expected on Friday and Friday night as 
the southeast winds gradually backs to the east, or trade winds,
which will push the moisture band westward. Lingering heavy
showers will remain over the western end of the state during the
day Friday. The tropical airmass is expected to clear Kauai by 
Friday evening. Locally breezy trades will persist through the 
weekend and into next week with drier and more stable air tracking
in.


&&

.AVIATION...
Expect light to moderate east southeast winds through this 
forecast period. The best chance for rain through the forecast 
period, with the exception of the Big Island, is windward mauka 
areas but it should be just scattered showers with passing MVFR 
conditions. The Big Island showers have already begun and it looks
as if this activity will not end through this forecast period. 
Expect showers which could be heavy at times with a possible 
thunderstorm. The rest of the area should see a pretty nice day 
and VFR conditions except for a chance for interior island showers
during the afternoon. As the tropical moisture moves in from the 
southeast to the northwest, each island should get a good chance 
for rain during this event. We will monitor and issue AIRMETs if 
necessary.

There are no AIRMETS in effect or expected for this forecast
period.


&&

.MARINE...
A high pressure ridge north of the state continues to strengthen,
which will bring an increase in the winds over the coastal 
waters. As a result, a Small Craft Advisory (SCA) has been posted 
for the waters around the Big Island including the Alenuihaha 
Channel. Later tonight and into Thursday, a low pressure system will
be developing northwest of the area in conjunction with a 
strengthening of the high pressure area far to the northeast. This
will result in a veering of the surface winds to southeasterlies 
along with a tightening of the pressure gradient which will
further increase the wind speeds. The higher wind speeds will 
primarily affect the windward waters off of the Big Island and 
Maui County, and to a lesser extent Oahu. This shift will be 
short-lived as the trades return on Friday and persist through the
weekend. The resumption of trades should result in a shift in SCA
areas to include more of the channels.

In the short range, not much is going on with the swells and surf. The
existing north swell is decreasing across the area, but a new, 
long period swell is expected to arrive from the north-northwest
Thursday night. This swell will be small with surf well below the
advisory level. A west swell, generated by Tropical Cyclone 
Hagibis in the western North Pacific, may reach exposed coasts 
early next week. 

Surf along east facing shores will increase during the second half
of the week as east to southeast winds increase. Small south- 
southwest swells will keep the surf from going flat along south 
facing shores through the weekend. 


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Friday for Alenuihaha 
Channel-Big Island Windward Waters-Big Island Leeward Waters-Big 
Island Southeast Waters.


&&

$$


DISCUSSION...Foster
AVIATION...Chevalier
MARINE...Kodama