Scientific Forecaster Discussion
fxhw60 phfo 182003
Area forecast discussion
National Weather Service Honolulu HI
1003 am HST Mon Nov 18 2019
an unsettled weather pattern will persist across the island chain
through Tuesday, with locally heavy rainfall and thunderstorms
possible statewide. The airmass will stabilize by Wednesday, but
a showery wet trade wind pattern is expected to hold through the
weekend. Breezy to windy conditions are expected Thursday through
water vapor imagery shows an upper low centered about 80 miles
northeast of Lihue and slowly tracking westward this morning. The 500
mb temperature on the 12z Lihue sounding showed -10.5 c while the
GFS depicted the coldest air at -14.5 c within the core.
Precipitable water (pw) was a near normal 1.3 inches at Lihue with
a bit higher values across the eastern portion of the state with
1.7 inches at Hilo. Meanwhile, at the surface, a 1029 mb high
centered around 1250 miles north northeast of Hawaii is driving
moderate trade winds across the island chain. A broad surface trough
is located just over the islands which is disrupting the trade
wind flow slightly. Visible satellite and radar show some high
clouds associated with the upper low passing over Kauai and Oahu
this morning. A few heavier showers have popped up across the
northeastern end of Kauai as well as some thunderstorms a couple
of hundred miles south of the island chain. Elsewhere, partly
cloudy skies prevail this morning.
Daytime heating over the islands, as well as the unstable atmosphere
and dynamic forcing from the upper low, will contribute to the
potential for heavy rain and scattered thunderstorms to develop
today. Moisture parameters look better across the eastern portion
of the state, but the higher instability and forcing will persist
westward. Potentially, any part of the state could see heavy rain
or thunderstorms, but coverage will not likely be widespread as
the trade winds will be working against this development. A Flash
Flood Watch remains in effect through 6 PM HST today.
The upper low will continue to linger near Hawaii as it slowly
weakens into Tuesday while the low level moisture hangs around.
Thus, we will still have a slight chance for thunderstorms and
heavy rain in the forecast for tonight and Tuesday. By Wednesday,
the upper low will move northeast of the state with more stability
returning. Trade winds will also strengthen on Wednesday as the
surface trough moves west of the state and a new high pressure
system north of the state builds in. The trades will keep windward
and Mauka showers in the forecast through the end of the week. The
trade winds will become strong by Friday into the weekend as the
high moves northeast of Hawaii and strengthens. Leeward sides of
the smaller islands will likely get passing trade wind showers at
times as the winds strengthen.
the potential for heavy showers and a few thunderstorms will
remain in the forecast today due to an upper low over the area.
Although clouds and showers will be most active over north and
northeast facing slopes with moderate to breezy trades in place,
some will manage to spill over into leeward areas periodically.
Trades will gradually trend down later today through tonight to
light and variable for Oahu and Kauai by Tuesday morning. MVFR
conditions are possible in and around shra/tsra. Airmet Sierra
for mountain obscuration remains in effect over north and east
sections of Kauai due to low clouds and showers, but may also be
needed elsewhere as conditions evolve through the day.
moderate to locally strong trade winds are being supported by a
surface high passing well north of the islands, but a well-developed
closed low aloft is disrupting the flow somewhat, as are the
thunderstorms and heavy showers forming in response to the low's
instability. As the low tracks generally W, a weak low-level
trough is expected to sharpen slightly along the longitude of
Kauai (160w), leading to a reduced east-southeast flow beginning tonight,
while continuing to bring the potential for thunderstorm development.
A Small Craft Advisory (sca) posted through today for the windier
zones around Maui and The Big Island should be allowed to expire
as this occurs.
Strengthening trade winds are expected by the middle of the week
as the trough dissipates, and a new high pressure cell builds north of
the islands. The low aloft will also weaken, and the potential
for thunderstorms will diminish after Tuesday. Winds may become
locally strong, potentially reaching gale force in some of the
waters around Maui and The Big Island by the end of the week.
Surf along all shores will remain below high surf advisory (hsa)
levels the next day or two, but a new long-period northwest swell
arriving Wednesday will require a hsa for exposed shores of most
islands through Thursday. If peak swell heights are larger than
wavewatch guidance (which is pretty much the norm recently), then
surf could approach the 25 foot threshold for a high surf warning.
Another northwest swell arriving next weekend may also require a hsa. In
the meantime, a new northwest swell will build tonight and peak Tuesday,
with peak surf remaining below advisory levels. Strong trade
winds will likely drive an increase in short-period wind waves
Friday into the weekend, leading to high surf along east facing
shores. No other significant swells are expected.
Flash Flood Watch until 6 PM HST this evening for all Hawaii
Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island leeward waters-Big
Island southeast waters.