Current Conditions
Temp5.4 C
RH47 %
WindN 9 mph
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 190628

Area Forecast Discussion
National Weather Service Honolulu HI
828 PM HST Wed Sep 18 2019

Trade winds will continue through Friday with a high pressure
ridge north of the region. Clouds and passing showers will favor
windward and mauka slopes of each island. Wet and humid weather
remains in the forecast from Friday through Monday as tropical
moisture moves over the islands as an upper level low drifts from
east to west across the state. A more stable trade wind weather
pattern develops next week Tuesday and Wednesday as high pressure
builds back in over the island chain.


The satellite imagery this evening shows a low pressure area
roughly 300 to 500 miles northwest of Kauai moving northward away
from the Hawaiian Islands. Icy cirrus clouds from this low
pressure area are drifting over the western half of the state. One
thousand miles to the northeast an upper level low is deepening
and drifting slowly southwestward. Bands of low clouds are piling
up along east facing slopes of all islands this evening. Local
radar imagery shows scattered showers produced by convergent
cloud bands downstream of each island. East to southeasterly wind
flow over the western islands is causing these showers to
occasionally drift into southern sections of Oahu and Kauai this

A fairly dry trade wind pattern will continue into Friday across
the region. Clouds and showers will favor windward and mauka
areas trending into the scattered range in the overnight and early
morning hours. The large scale pattern begins to change on Friday
as an upper level low approaches Hawaii from the northeast
direction. Wind speeds will also decrease from Friday into the
weekend as the lift associated with this upper low weakens the
surface pressure gradients around the islands.

Friday through Monday...Expect a turn towards cloudy wet weather
conditions as a cold core upper level low tracks from east to west
across the islands over a plume of warm moisture moving in from
the deep tropics. Winds will become light and variable from
Saturday to Sunday as this upper low weakens the surface pressure
gradients. Precipitable Water (PW) levels within this tropical
moisture plume remain in the abundant 2.0 to 2.5 inch range. PW is
a measure of the total water content of the atmosphere if all the
water in the column were to instantly fall as rain. The
combination of these cold and warm systems will result in a very
unstable atmosphere with clouds and numerous showers spreading
across the state. Rainfall with this event will spread equally
into both windward and drier leeward areas. Convective clouds will
form over island interiors in the afternoon to early evening due
to island heating effects. Locally heavy rain is likely with this
setup along with a few thunderstorms. Still too early to add
thunderstorms or heavy showers into the forecast grids, as the
timing and location impacts will likely evolve as forecast time
period grows shorter.

Tuesday through Wednesday...The upper low will drift northward
away from the islands allowing the high pressure ridge to build
back in over the state. Moderate trade winds and drier trends will
return as higher pressure builds into the region with increasing
stability spreading from east to west across the Hawaiian Islands.
PW levels drop back down into the 1.0 to 1.5 inch range. Isolated
to scattered shower activity will favor the typical windward and
mauka areas of each island.


A predominate VFR situation is expected through Thursday morning,
with the winds becoming more trades on Thursday while holding at
moderate to locally strong.

Current low level wind flow is southeast over on Kauai, east over
the central islands, then east- northeast over on the Big Island.
Speeds are light to moderate, except locally strong over the near-
shore waters of Maui County and the Big Island. Winds are expected
to become true trades, out of the east on Thursday, at moderate to
locally strong.

Cloud distribution is basically scattered to broken area-wide,
including the surround waters. There is overcast coverage, mainly
over the windward and leeward waters and coast of the Big Island,
between Waipio Valley and the Puna district, and from Kawaihae to
South Cape. Scattered showers are present with these clouds,
mainly over the waters but also over land including the Kailua-
Kona area of the Big Island and windward Kauai. We expect little
change overnight with these clouds and showers favoring the
windward and mountain areas of all of the larger islands. The
showers on the Kona coast are expected to linger to shortly after
midnight before dissipating. Tops of these showers are around 9k
feet, and will cause brief MVFR ceilings and vis.

No AIRMETs in effect and none is expected through Thursday


Fresh to locally strong, east to east-southeast winds will continue
due to high pressure to the northeast and a tropical disturbance
moving northward to the west of the state, but shift back to a
more typical east to northeast direction Thursday through Friday.
A Small Craft Advisory remains 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 through Friday. A small northwest swell is possible late
in the weekend into early next week. A small swell from Tropical
Cyclone Kiko should fill in Thursday along exposed east facing
shores before dropping over the weekend for only a slight boost in
surf heights. 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
Southeast Waters.




Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office