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Hawaiian Islands Synoptic Discussion and Guidance

291
FXHW60 PHFO 181726 AAA
AFDHFO

Area Forecast Discussion...UPDATED
National Weather Service Honolulu HI
726 AM HST Mon Nov 18 2019

.UPDATE...
Increased thunderstorm coverage statewide through early evening
given the current unstable environment and expected dynamic
forcing from an upper low over the area. Increased chances for
rainfall for leeward Big Island and Maui.


&&

.SYNOPSIS...
An unsettled weather pattern will persist across the island chain
through Tuesday, with locally heavy rainfall and thunderstorms
possible statewide. The airmass will stabilize Tuesday night and
Wednesday, but a showery wet trade wind pattern is expected to
hold through the weekend. Breezy to windy conditions are expected
Thursday through Saturday.

&&

.PREVIOUS DISCUSSION...
Currently at the surface, a 1026 mb high centered around 1200
miles north-northeast of Honolulu is driving moderate trade winds
in unsheltered areas across the island chain early this morning.
Infrared satellite imagery shows partly cloudy conditions in most
areas, with mostly cloudy skies in a few windward locales. Radar
imagery shows scattered showers across Kauai and the Big Island,
with isolated showers or rain free conditions elsewhere. Water
Vapor imagery shows a closed upper level low spinning around 100
miles northeast of Kauai, and it is this feature that will keep
unsettled conditions in place through the first half of the work
week. Main short term concern revolves around the potential for
heavy rainfall and thunderstorms over the next couple days.

Moderate trade winds are expected to hold in place today, before
easing into the light to moderate range tonight through Tuesday
night as a new front approaches from the northwest. Model
guidance is in good agreement showing the trades strengthening
Wednesday and Wednesday night, with breezy to windy conditions
expected Thursday into next weekend.

The moist unstable airmass will linger over the state through at
least Tuesday, as the upper level low moves slowly southwestward
and over the western islands. Unsettled conditions, including the
possibility for locally heavy rainfall and thunderstorms, will
continue through Tuesday. A Flash Flood Watch remains in effect
for the entire state through 6 PM today. The watch may need to be
extended into Tuesday for all or portions of the island chain, but
confidence is not high enough to extend the watch at this time.
Models indicate that the upper low will open into a trough Tuesday
night, then linger over the state through the remainder of the
week. As a result, we should see conditions stabilize a touch
beginning Tuesday night, but a showery wet trade wind pattern will
likely continue into next weekend.

&&

.AVIATION...
The potential for heavy showers and a few thunderstorms will
remain in the forecast today due to an upper low that has drifted
into the area. Although clouds and showers will be most active
over north and northeast facing slopes with the moderate trades
in place, some will manage to spill over into leeward areas
periodically. Trades will gradually trend down later today through
tonight with light and variable winds possibly returning for Oahu
and Kauai by Tuesday morning. MVFR conditions are possible in and
around showers. AIRMET Sierra for mountain obscuration remains in
effect over the Big Island and may be needed elsewhere as
conditions evolve through the day.

&&

.MARINE...
The current northwest swell will continue to subside and the High
Surf Advisory (HSA) has been cancelled. A small north-northwest
swell is expected to fill in late Monday and peak Tuesday well
below HSA levels. Currently in the north Pacific, we have a
captured fetch of strong gales with pockets of storm force winds
near 40N 170E that is traveling along the great circle route
toward Hawaii. A large west-northwest to northwest swell should
build throughout the day Wednesday and peak Wednesday night into
Thursday at solid advisory levels and possibly warning levels for
north and west facing shores of the smaller islands.

The combination of a surface trough near the Big Island moving
west slowly, and a disturbance aloft will continue to bring the
threat of thunderstorms through Tuesday.

The current Small Craft Advisory (SCA) for the typical windy zones
around Maui County and the Big Island should be cancelled sometime
this afternoon as winds drop below SCA speeds. Winds will ramp
back up to SCA levels towards the middle of the week as high
pressure strengthens north of the area. Winds may reach gale force
by the end of the week across the typical windy zones of Maui
County and the Big Island, with SCA level winds expected
elsewhere.

Overnight ASCAT pass revealed pockets of strong northeast winds
upstream of the state. This may bring a slight uptick in surf
along east facing shores today. The surf will increase further
along east facing shores during the second half of the week as the
trades become more robust. Expect choppy surf to increase along
east facing shores with surf heights possibly nearing advisory
levels this coming weekend. Minimal background swells are expected
along south facing shores through most of the week with a few
tiny southwest swells possible near the middle of the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through this afternoon for all Hawaii islands.

Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

UPDATE...Foster
PREVIOUS DISCUSSION...Jelsema
AVIATION...Gibbs
MARINE...Kino

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



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