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Temp1.7 C
RH88 %
WindNE 15 mph
RoadClosed
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Hawaiian Islands Synoptic Discussion and Guidance

118
FXHW60 PHFO 170214
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
414 PM HST Wed Oct 16 2019

.SYNOPSIS...
Moisture combined with low aloft over the state, will support
increased rainfall coverage across the western half of the state
tonight. Thunderstorms and heavy rain will be possible mainly over
Kauai and Oahu. Drier trade wind weather will return Thursday and
remain Friday. Increasing humidity and showers are possible this
weekend, particularly across the eastern end of the state.


&&

.DISCUSSION...
Water vapor imagery shows a slowly westward tracking upper low
centered just southwest of Oahu this afternoon. The low will
act to destabilize the atmosphere through tonight before shifting
west of the state Thursday. At the low levels, a 1028 mb high is
centered far north of the islands and driving moderate to fresh
trade winds across the area. The trades are pushing moisture
associated with an old, dissipated front under the upper low
causing the showers to intensify and develop a few thunderstorms.
GOES-17 total precipitable water (PW) and the 00Z Lihue sounding
shows above normal values of 1.9 inches along the front moving
onto Kauai and Oahu.

The combination of moisture and instability provided by the low
aloft will likely trigger heavy passing showers and a slight
chance for thunderstorms over the western half of the state
tonight, and possibly into Thursday morning for Kauai. The trades
will focus most of this rainfall across windward areas. A Flash
Flood Watch is posted for Niihau, Kauai and Oahu through 6 am HST
Thursday. Please see FFAHFO product on our website for more
information. 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.


&&

.AVIATION...
Showers continue across portions of Oahu and Kauai this afternoon
thanks to an upper low over the western end of the state.
Additionally, a slight chance of thunderstorms remains possible
through the evening. Conditions have improved somewhat from early
this morning which has allowed AIRMET Sierra for mountain
obscuration on Oahu to be cancelled. AIRMET Sierra for IFR on
Kauai has been changed to TEMPO mountain obscuration. Brief MVFR
conditions are still possible for windward portions of all
islands into tonight.


&&

.MARINE...
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 Thursday, allowing showers to subside
and winds to become stronger and more widespread into the weekend.
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.



&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through late tonight for Niihau-Kauai-Oahu.

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


&&

$$


DISCUSSION...Foster
AVIATION...Dye
MARINE...TS

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