819
FXHW60 PHFO 160157
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
357 PM HST Sun Sep 15 2024

.SYNOPSIS...
The general theme this week will be breezy winds and typical
showers, with higher rain amounts confined to windward exposures
and upper elevations, in a trade wind regime. A slug of higher
moisture passing through from the east tonight will increase
precipitation areal coverage with possible periodic heavy rainfall
primarily over windward Big Island and Maui County.


&&

.DISCUSSION...
A large area of surface high pressure approximately 1,400 miles
from Oahu will remain anchored in place through the week. The
pressure gradient downstream of the high will remain tight enough
to support areawide breezy to locally gusty easterly winds through
terrain gaps the next several days. The high will weaken late this
week and the gradient will slacken off just enough to allow
trades to slightly fall off. Precipitation will primarily be
focused along windward areas with the bulk of the rain falling
during the overnight hours. Night time into early morning windward
mauka showers will occasionally pass over the ridge tops and
wetten leeward slopes or provide sprinkles to leeward communities.

Higher mid-level moisture moving across Big Island tonight
(greater than 2 inch precipitable water air mass) is noted on
satellite by a wide swath of warmer broken to overcast clouds
moving onto Big Island and eastern Maui County this afternoon.
This area of deeper moisture and associated mid-level troughing
will pass across Big Island and windward Maui County tonight into
early Monday morning. The depth of this higher moisture increases
the probabilities that various windward Big Island and Maui County
communities may experience bouts of brief heavy rainfall tonight.
While the increase in moisture over the smaller islands is not
expected to be quite as deep, a higher frequency of (predominantly
windward) showers is likely. Once mid-level troughing washes out
west of the state Tuesday, typical trade wind weather will return.
Overall,less mid to late week diurnal rainfall is expected (in
both frequency and areal coverage) as a drier air mass advects in.
Windward slopes and smaller island higher elevations will still
pick up a daily half an inch to an inch of rain during the
overnight hours.

The extended forecast going into the final third of the month is
looking somewhat wet. Forecast guidance has an upper low moving
toward the state from the north northeast a week from now. While
rain probabilities will be on the rise, the chances for heavy rain
and isolated thunder (as upper levels cool a touch and mildly
destabilize the regional atmosphere) are low due to the
uncertainty of this low`s location or proximity to the islands.


&&

.AVIATION...
High pressure centered far north of the state will maintain
moderate to breezy trades over the next few days. Clouds and
showers will generally favor windward and mauka areas, though the
temperature inversion heights will remain elevated enough to allow
occasional showers to pass over island terrain into leeward
areas.

Tonight, a plume of deeper moisture will move in from the
southeast, affecting primarily east and southeast portions of the
Big Island and Maui. MVFR conditions, and possibly isolated IFR,
are expected in heavier showers. Elsewhere, VFR conditions are
expected to prevail.

With locally breezy trade winds prevailing across the state,
AIRMET Tango remains in effect for low-level turbulence over and
downwind of terrain. This AIRMET will likely continue through
Monday.

AIRMET Sierra will likely be needed by this evening for east and
southeast facing slopes of the Big Island and possibly Maui to
account for mountain obscuration due to low clouds and showers.


&&

.MARINE...
Moderate to locally strong trade winds will hold through
Wednesday, as a strong surface high meanders 1,000 to 1,400 nm
north of the islands. A Small Craft Advisory (SCA) remains in
effect for the typically windy waters around the Big Island and
Maui County through Tuesday night, though the advisory may need to
be extended through Wednesday. The high will weaken and move
eastward late Wednesday and Thursday, leading to a slight decline
in the trades.

Surf across most shores will be below seasonal average through
the week. Trade winds will drive short period seas of around 5 to
6 feet at 7 seconds into Wednesday, maintaining moderate east
shore surf. A decrease in trade winds will cause east shore surf
to decline Thursday and Friday. Moderate surf along south facing
shores will slowly decline Monday and Tuesday as the current south
swell drops. A small southwest pulse out of the Tasman Sea could
produce a slight rise in surf Friday. Aside from trade wind wrap,
surf along north facing shores will be minimal through Wednesday,
and a tiny northwest swell will be possible Thursday or Friday.

The combination of seasonally high astronomical tides and water
levels running around 6 inches higher than predicted will produce
localized coastal inundation each afternoon through Tuesday. A
Coastal Flood Statement has been issued to highlight these
conditions.


&&

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


&&

$$


DISCUSSION...Blood
AVIATION...Vaughan
MARINE...Wroe