Flash Flooding and Heavy Rain in Hawaii, November 16 to 20, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
903 PM HST Sat Nov 16 2019

.SYNOPSIS...
An unsettled weather pattern will hold in place across the island
chain tonight through Tuesday, with locally heavy rainfall and
thunderstorms possible statewide. The airmass will stabilize a bit
Tuesday night and Wednesday, but a showery wet trade wind pattern
is expected to hold into next weekend. Breezy to windy conditions
could develop late next week into next weekend as well.

&&

.DISCUSSION...
Currently at the surface, a dissipating front is located just to
the east of Kauai, while a trough of low pressure is located just
to the east of the Big Island. Meanwhile, a 1023 mb high is
centered around 850 miles northwest of Kauai. The winds across the
island chain are becoming increasingly influenced by the building
high northwest of the state, with light trade winds present in
most areas this evening. Aloft a digging upper level trough is
evident in water vapor imagery several hundred miles north of
Kauai, and this is the feature that will bring a continuation of
unsettled weather to the state well into next week. Infrared
satellite imagery shows partly cloudy skies over Oahu and Maui
County, with mostly cloudy conditions over Kauai and the Big
Island. Radar imagery shows scattered showers and a few
thunderstorms over and in the vicinity of Kauai and the Big
Island, with northing more than isolated shower coverage across
the rest of the state. Main concerns over the next few days
revolve around the potential for heavy rainfall and thunderstorms.

The trough of low pressure east of the Big Island and the front
just east of Kauai, will merge into a broad trough over the
central islands tonight and Sunday. At the same time, high
pressure will build eastward well to the north of the islands.
This will bring increasing trade winds to the state, with moderate
to locally breezy conditions developing by late Sunday. The high
will strengthen as it shifts eastward well to the north of the
islands Sunday night and Monday. Moderate to locally breezy trades
will continue Sunday night, with a slight easing expected on
Monday. A further easing of the trades in the light to moderate
range Monday night through Tuesday night as a new front approaches
from the northwest. The trades should then strengthen again
Wednesday and Wednesday night, with breezy to windy conditions
expected Thursday into next weekend.

As for weather details, a moist unstable airmass will hold in
place tonight through at least Tuesday, as broad surface
troughing lingers over the state and an upper level trough closes
off into an upper level low over the island chain. Rather
unsettled weather over Kauai and the Big Island tonight, will
expand to include Oahu and Maui County on Sunday. The unsettled
conditions will then continue through at least Tuesday. With
plenty of instability in place, with surface based CAPE values of
1500 to 2500 J/KG and mid-level H7-H5 lapse rates of 6.5 to 7
C/KM, the potential for locally heavy rainfall and thunderstorms
will continue through the first half of the upcoming work week.
The limiting factors for widespread flash flood potential will be
the lack of deep tropical moisture, as precipitable water values
will generally hold around 1.5 to 1.6 inches through Tuesday,
along with the trade winds keeping showers moving along for the
most part. As a result, we are not anticipating a need for a
Flash Flood Watch at this time, but this will be monitored closely
as conditions evolve.

The upper level low will open up into a trough Tuesday night, but
broad upper troughing will linger over the state through the
remainder of next week. As a result, we should see conditions
stabilize a touch beginning Tuesday night, but a showery wet
trade wind pattern will likely continue right into next weekend.

&&

.AVIATION...
Although trades are forecast to return as the front near Kauai
dissipates, the potential for heavy showers and a few
thunderstorms will remain possible due to an upper disturbance and
surface trough moving into the area. Clouds and passing showers
will be most active over north and northeast facing slopes with
the trades returning. MVFR conditions are possible in and around
showers and an AIRMET remains in place for mountain obscuration
over Kauai (can't be ruled out for windward Big Island late
tonight through Sunday as the trough moves into the area).

&&

.MARINE...
Updated the current High Surf Advisory (HSA) to include the west
facing shores of the Big Island. A High Surf Advisory (HSA)
remains in effect for most north and west facing shores of the
smaller islands through Sunday afternoon. The current west-
northwest to northwest swell will peak tonight into Sunday morning
and steadily decrease over the next few days. Looking ahead to
the middle of next week, a new long period northwest swell
arriving early Wednesday may cause surf to reach the HSA criteria
along most north and west facing shores of the smaller islands
from late Wednesday through Thursday.

A weak and diffuse front in the vicinity of Kauai is nearly
stationary, while a surface trough just east of the Big Island is
moving slowly west. Thunderstorms developed over the coastal
waters near Kauai and the Big Island earlier this afternoon. Since
then, the thunderstorms are starting to gradually decline, but
the threat of thunderstorms will continue to persist over the
coastal waters during the next few days due to an upper level low
developing over the area.

Winds are now mainly out of the northeast across most of the
state and are expected to gradually strengthen as the front
dissipates and the trough moves over the eastern islands tonight.
Easterly trade winds will gradually strengthen from Sunday evening
through early next week. The trades may eventually approach SCA
levels over the typically windier waters adjacent to the islands
of Maui County and the Big Island by Tuesday or Wednesday.

Surf will remain small along east facing shores into early next
week due to the relatively light winds over and upwind of the
state. The surf will increase slightly along east facing shores
by Tuesday as the trades strengthen. As the trades continue to
strengthen later next week, expect choppy surf to increase along
east facing shores. Minimal background swells are expected along
south facing shores through most of next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Sunday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala-Kona-Kohala.

Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Big Island Windward Waters.

&&

DISCUSSION...Jelsema
AVIATION...Gibbs
MARINE...Kino