Flash Flood Warning 16, NWS Honolulu, HI

634 
WGHW70 PHFO 130329
FFSHFO

Flash Flood Statement
National Weather Service Honolulu HI
529 PM HST THU OCT 12 2017

HIC009-130338-
/O.CAN.PHFO.FF.W.0016.000000T0000Z-171013T0445Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
Maui HI-
529 PM HST THU OCT 12 2017

...THE FLASH FLOOD WARNING FOR THE ISLAND OF MAUI IN MAUI COUNTY IS
CANCELLED...

Weather radar showed that heavy rain has ended. Therefore the Flash 
Flood Warning has been cancelled.

LAT...LON 2102 15656 2090 15648 2095 15632 2094 15630
      2082 15630 2078 15646 2080 15648 2078 15654
      2081 15663 2086 15667 2093 15670 2101 15667
      2104 15660

$$

Jelsema

Area Forecast Discussion
National Weather Service Honolulu HI
350 PM HST Thu Oct 12 2017

.SYNOPSIS...
The trade winds will slowly strengthen through Friday night, as a
trough of low pressure shifts westward across the island chain.
The trades will then become breezy on Saturday and locally windy
Sunday through much of next week as a strong area of high pressure
remains to the north of the State. An unstable airmass will
remain over the island chain through Friday night and possibly
into the first half of the weekend, with some locally heavy
rainfall and thunderstorms possible across portions of the island
chain. More typical trade wind weather will return Sunday through
much of next week, with showers favoring windward and mauka
areas, and occasionally reaching leeward areas due to the strength
of the trades.

&&

.DISCUSSION...
Currently at the surface, a weak and difficult to discern surface
trough is in the vicinity of the central and eastern islands, with
the trough extending northwestward to the north of Oahu and Kauai.
Meanwhile, a strong 1041 mb high is centered around 1700 miles to
the north-northeast of Honolulu. Despite the strong high to the
north-northeast of the State, the trade winds remain in the light
to moderate range this afternoon due to the surface trough in
place. Visible satellite imagery shows partly to mostly cloudy
skies across the State. Meanwhile, radar imagery shows scattered
to numerous showers affecting windward and mauka areas of Maui
County and the Big Island, with not much shower activity over Oahu
and Kauai at the moment. Additionally, a few thunderstorms have
developed over Maui County and the Big Island this afternoon, as
well as the adjacent coastal waters to the northeast of these
islands. Main short term concern over the next couple of days
revolves around the potential for thunderstorms and heavy
rainfall.

Tonight through Friday night,
The weak surface trough will slowly shift westward across the
island chain, as the strong high to the north-northeast settles
southward and closer to the State. As a result, we should see a
slow and steady strengthening of the trade winds through the
period. Aloft, an upper level trough will linger over the island
chain, keeping conditions unstable. The unstable conditions in
combination with the surface trough in the vicinity of the
islands, along with plenty of deep layer moisture with
precipitable water (PW) values in the 1.5 to 2.0 inch range, will
result in conditions that are favorable for some locally heavy
rainfall and even a slight chance of thunderstorms.

The airmass will be most unstable over Maui County and the Big
Island early this evening, with the unstable conditions then
shifting over the entire island chain later tonight through
Friday night. The unstable airmass will then exit off to the west
and southwest of the islands by Saturday morning. As a result,
a slight chance of thunder remains in the forecast across the
entire island chain tonight through Friday night. Next concern
revolves around the potential for flash flooding. We certainly
can't rule out flash flooding given the deep moisture and
instability in place, but given the background trade wind flow
over the islands, the probability of flash flooding is not high
enough to warrant a flash flood watch at this time.

Saturday through Wednesday,
The surface trough is expected to shift southwestward away from
the islands, while a strong surface high remains in place to the
north and northeast of the State. This will bring a return of
breezy trade winds on Saturday, with locally windy and quite gusty
conditions expected Sunday through the middle of next week. In
fact, some areas may reach wind advisory thresholds Sunday through
the middle of next week with the GFS and ECMWF both indicating
that the trades will really get cranking. As for sensible weather
details, the most unstable weather will shift southwestward with
the surface trough by Saturday morning. Upper level troughing and
deep layer moisture will remain over the state through Sunday
however, so fairly wet trade wind weather is expected through the
weekend. Although an upper level weakness will remain in place
across the State through the middle of next week, PW values drop
considerably Sunday night through Wednesday, so we should see a
more typical trade wind shower regime return to the island chain.
Showers will continue to favor windward and mauka areas through
the period, with showers occasionally reaching leeward areas due
to the strength of the trades.

&&

.AVIATION...
Unsettled weather will continue tonight and tomorrow. An upper
level trough and a swath of moist air with precipitable water
values nearing 2.0" and above will aid in locally heavy showers
and a slight chance of thunderstorms across the island chain. So
far today, thunderstorm activity has taken off along a weak
surface trough through the central islands and north of Oahu.
Average CB tops are between FL400-450 with isolated tops to
FL500. The surface trough will continue to weaken and move west
over the western islands tomorrow. The slight chance for
thunderstorms will remain in the forecast area wide.

Moderate to locally strong trade winds will begin to build back
in tonight, strengthening into this weekend. Windward areas,
especially Maui County and the Big Island will see widespread MVFR
conditions to isol IFR conditions. Most terminals in these area
could have prolonged MVFR CIGS/VIS. AIRMET SIERRA for mountain
obscuration is posted over the Big Island, Maui, and Molokai.

Convective SIGMET NOVEMBER series is posted over the northeast
adjacent waters from Oahu to the Big Island. A convective SIGMET
covers all threats associated with thunderstorm activity from
turbulence to icing.

&&

.MARINE...
Strengthening high pressure north of the area will build trade
winds across the region tomorrow. An ASCAT pass earlier today
showed some areas of 25 knots northeast of the islands. Some of
the windier areas may reach Small Craft Advisory (SCA) levels
tomorrow, but will hold off on issuing the SCA at this time.
Thunderstorms developed over the windward coastal waters this
morning and continue this afternoon, in response to an upper level
trough moving closer to the islands.

Winds are expected to increase over the weekend and into early
next week, so expect SCA conditions to expand to all waters. In
addition to the increase in winds, rough seas are expected, along
with some heavy showers and some thunderstorms as the above
mentioned upper level trough moves over the islands from the
northeast.

Surf along north facing shores will remain small into Saturday
before a new northwest swell begins to arrive. It is expected to
fill in Sunday, and then decline through early next week. Expect
surf to remain below advisory levels, but reach their peak along
north and west facing shores on Sunday.

Small, long-period, south and south-southwest swells will keep
surf from going flat through Friday. A new southerly swell
arriving this weekend will peak late Sunday and into Monday near
advisory levels.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

DISCUSSION...Jelsema
AVIATION...Eaton
MARINE...M Ballard