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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 182032
Area Forecast Discussion
National Weather Service Honolulu HI
1032 AM HST Wed Sep 18 2019
Easterly trade winds will prevail over the next few days thanks to
high pressure far north of the area. Clouds and passing light
showers will tend to favor windward and mauka areas. Lighter winds
along with a wetter weather pattern is expected beginning Friday
and continuing through the weekend as a moist and unstable
airmass moves in from the east. A more typical trade winds weather
pattern is expected to return early next week.
Surface high pressure is centered far to the north northeast of
the area while an area of low pressure is centered several hundred
miles to the west of Kauai. Radar and satellite imagery indicates
that the low level wind flow across the area is slightly south of
due east due to the lows proximity to the state. The airmass is
rather stable across the area due to a ridge aloft. The 12Z
soundings indicate inversion heights are around 6000 to 7000 feet.
Light passing showers can be expected across some windward and
mauka areas today but most locations will be generally dry. Some
thin high clouds may also pass over the area from time to time due
to deep convection west of the state.
The surface high will drift slowly eastward over the next few days
while the low to our west moves off to the northwest. The low
level wind flow will become more easterly as the low pulls away
from the area. Wind speeds will generally be in the light to
moderate range. The ridge aloft will remain over the area through
Thursday so shower activity will be highly dependent on incoming
moisture availability and primarily fall over windward and mauka
A change to this weather pattern is expected as we head into the
weekend. An upper low is forecast to move down over the area from
the northeast while high pressure will remain far to the north
northeast of the area. The upper low will destabilize the airmass
across the area. In the meantime, moisture associated with a weak
area of low pressure currently centered several hundred miles
southeast of the state will move up over the area. Rainfall
chances will increase beginning Friday over the east end of the
state, then gradually spread westward over the weekend. The
airmass may become unstable enough to allow for some
thunderstorm development to occur especially during the afternoon
and evening hours. Locally heavy rainfall is also a possibility
across some areas. Winds will also become lighter due to lows
proximity to the state.
By Monday, the upper low is forecast to move off to the west of
Kauai which will allow for a more stable airmass to move overhead.
Trade winds are also expected to pick back up to moderate levels.
Showers will then favor windward and mauka areas at that time.
Moderate east-southeasterly winds continue across the island chain
for today, but are expected to become more easterly to
northeasterly by tomorrow. Scattered showers will impact mainly
windward and mauka areas of the state throughout the day with
leeward Big Island seeing an uptick in afternoon shower activity.
Mainly VFR conditions are expected.
No AIRMETs are currently in effect.
Fresh to locally strong, east to east-southeast winds will
continue today due to high pressure to the northeast and a
tropical disturbance moving northward to the west of the state.
A shift back to a more typical east to northeast direction is
expected Thursday through Friday. A Small Craft Advisory is
now in effect for the typical windy areas around Maui and the Big
Island through Thursday night. Guidance shows a return of light
to moderate southeast flow late Saturday through Sunday due to an
approaching trough from the east-southeast, which would support
localized land and sea breeze conditions near the coasts.
A series of small swells from the southwest and south are expected
through the rest of this week. The largest of these looks to be
towards the end of the week as a long-period south-southwest swell
and short-period west-southwest swell combine across the area. This
will likely boost surf along south and west facing shores late
Thursday into Friday. A small northwest swell is possible late in
the weekend into early next week. A small swell from Tropical
Cyclone Kiko around Thursday along exposed east facing shores
before dropping over the weekend. Surf heights are expected to be
below advisory levels with all of these swells.
Small Craft Advisory until 6 AM HST Friday for Pailolo Channel-
Alenuihaha Channel-Big Island Leeward Waters-Big Island
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office