Current Conditions
Temp6.0 C
RH65 %
WindNNE 16 mph
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 161913

Area Forecast Discussion
National Weather Service Honolulu HI
913 AM HST Wed Oct 16 2019

A dissipating front approaching from the north, combined with a
developing upper trough, will support increased rainfall coverage
across the western half of the state today and tonight. Thunderstorms
and heavy rain will be possible mainly over Kauai and Oahu. Drier
trade wind weather will return Thursday and remain into Saturday.
Increasing humidity and showers are possible Saturday night and


Water vapor imagery shows a nearly stationary upper low centered
over Kauai and Oahu this morning and will destabilize the atmosphere
through early Thursday morning before shifting west of the state
Thursday. At the low levels, a 1029 mb high is centered far north
of the islands and driving moderate to fresh trade winds across
the area. Infrared satellite shows a weak front with associated
clouds and showers caught in the trade wind flow moving across
Kauai and Oahu this morning. GOES-17 total precipitable water (PW)
shows above normal values of 1.9 inches along the front moving
onto Kauai and Oahu. The combination of moisture and instability
will likely trigger heavy passing showers and a slight chance for
thunderstorms over the western half of the state through tonight,
and possibly into Thursday morning for Kauai. The trades will
focus rainfall windward, however, all areas of both Kauai and Oahu
can expect wet conditions over the next 24 hours. A Flash Flood
Watch is posted for Niihau, Kauai and Oahu from noon today through
tonight. Please see FFAHFO product on our website for more
information. Showers will also increase (to a lesser extent)
across windward sides of Molokai, Maui and the Big Island today,
as well as the typical shower buildups over the Kona slopes.

Tonight, the upper low will induce a weak surface trough west of
Kauai allowing winds to veer slightly south of east, however, wind
speeds will maintain moderate to fresh strength. By Thursday
morning the moisture from the front is expected to push west of
the state, along with the upper level low, likely alleviating the
threat of widespread heavy rain. Trade winds will carry in drier
and more stable conditions, with some windward and mauka showers,
but fewer showers overall.

On Saturday, a weakening front over the Northwest Hawaiian Islands
will push the high pressure generating our trades far enough east
to shift the trade winds from the east-southeast. This slight shift
in wind direction may be enough to allow some tropical moisture
to flow over the eastern half of the state Saturday afternoon and
Sunday. This would increase humidity and trade wind showers mainly
for the Big Island and Maui County. The upper level forecast
shows a stable atmosphere remaining overhead, so widespread heavy
showers are not expected at this time.


The combination of an upper level low over the western end of
Hawaii and moisture from a weak front will bring the potential for
widespread showers and a slight chance for thunderstorms over the
western end of the state. AIRMET Sierra is posted for IFR
conditions over Kauai and for tempo mountain obscurations over
Oahu. Brief mountain obscurations will be possible for windward
sides of Molokai, Maui and Big Island as showers increase in these
areas today. Light icing may occur within some of the heavier
showers or thunderstorms this afternoon and tonight.


Strong high pressure north of the area will maintain moderate to
locally strong easterly trade winds over the coastal waters
through at least tonight, but likely continue through the rest of
this week. A Small Craft Advisory remains in effect for Maalaea
Bay and the Pailolo and Alenuihaha Channels through tonight, but
may need to be extended. A weak front between the high and the
main Hawaiian Islands is keeping the winds from being stronger
than they otherwise would be.

A low aloft and remnants of the front will provide a slight chance
of thunderstorms through tonight, mainly around Kauai and Oahu. The
low is forecast to move away beginning Thursday night, allowing
showers to subside and winds to become stronger and more widespread.
Winds may slightly weaken again on Sunday as a new front passing
northwest of the state weakens the pressure gradient.

Surf is expected to remain below advisory levels along all shores
through early next week. A series of northwest, north, and south
swells will produce small surf through early next week. Surf along
east facing shores will trend up toward the end of the week as
the trades strengthen.


Flash Flood Watch from noon HST today through late tonight for

Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel.




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