Flash Flooding and Heavy Rain in Hawaii, March 14 to 18, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
315 AM HST Sat Mar 14 2020

.SYNOPSIS...
Light winds and increasing moisture combined with a trough in the
area will keep the rainfall chances in the forecast across the
state through the weekend. Most of the shower activity will favor
interior areas of the islands through the afternoon hours as sea
breezes push ashore and near the coasts and adjacent waters
overnight through the early morning hours. Conditions will begin
to deteriorate late Sunday through early next week as this trough
shifts westward and a kona low evolves to the west. Strong
southerly winds along with an increasing potential for widespread
heavy rainfall and thunderstorms are expected.

&&

.DISCUSSION...
Water vapor imagery showed plenty of middle- to upper-level moisture
streaming over the region within the west-southwest flow and a weak
impulse approaching from the west. This pulse of energy moving into
the area today combined with a weak surface trough over the
islands will support a few heavy showers and even a thunderstorm
or two. Although the better source of moisture remains over the
eastern end of the state (east of the surface trough) this
morning, a westward shift back up the island chain is expected
beginning later today through Sunday with the trough. Most of the
shower activity will favor coastal and adjacent waters through the
morning and overnight periods, then across interior areas of the
through the afternoon hours as sea breezes push ashore.

Guidance remains persistent from cycle-to-cycle and continues to
advertise a transition period late Sunday into early next week as a
kona low evolves west of the state. This pattern shift will
translate to a wet pattern across the region that will persist
through a good portion of the upcoming week. Guidance shows a plume
of deep moisture pooling northward across the islands through this
time with precipitable water values surging to around two inches.
This moisture combined with upper height falls and the added
instability will support periods of heavy rainfall. Flash flooding
will become a possibility through this time, especially in areas
where bands of heavy rainfall setup for long periods of time.
Confidence in exactly where these bands will set up remains low
at this time. As a result, a Flash Flood Watch has been issued for
the entire state Sunday night through Tuesday. In addition to the
potential for heavy rainfall, a few strong-to-severe
thunderstorms can't be ruled out with the added instability and
deep layer shear expected.

&&

.AVIATION...
A northeast to southwest trough near Molokai is keeping winds
relatively light over the islands, with gentle to moderate winds
blowing from the north to northeast around Kauai and Oahu and from
the southeast around the Big Island and Maui County. The
prevailing wind flow has been weak enough to allow for land
breezes to develop. The land breezes have produced VFR conditions
over most areas of the islands, with showers just offshore where
the land breezes are converging with the prevailing winds. The
southeast flow is still pushing clouds and showers ashore on the
southeast side of the Big Island, so AIRMET Sierra remains in
effect for that area.

The atmosphere around the islands is somewhat moist and unstable
and will become more unstable as a trough digs west of the area.
Very isolated thunderstorms have been developing over water
southwest of the islands and may spread over the islands today. AIRMET
TANGO is in effect for Kauai through Maui for moderate turbulence
aloft.

Conditions are likely to deteriorate early next week as a low
deepens northwest of the area.

&&

.MARINE...
Light winds will continue through the weekend due to a surface
trough lingering in the area. The background flow will hold out
of a more northeast direction over the western end of the state
and out of the southeast over the eastern end through today. Land
and sea breezes will continue each day. A transition period
appears likely Sunday through early next week as a kona low forms
west of the islands. Strong southerly winds along with an
increasing potential for widespread heavy rainfall and
thunderstorms remain in the forecast late Sunday through early
next week.

Local buoys show swell energy has dropped sufficiently for
cancellation of both the Small Craft Advisory and High Surf
Advisory in effect for portions of the islands and surrounding
waters.

Surf along east facing shores will remain small through the weekend
due to the lack of trades. Surf along south facing shores will
remain small with mainly background south swells expected. An
upward trend is possible early next week due to increasing
southerly winds setting up over local waters. A small, long-period
south swell could become a possibility for Hawaii late next
weekend into the following week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from Sunday evening through Tuesday afternoon
for Niihau-Kauai-Oahu-Molokai-Lanai-Kahoolawe-Maui-Kona-South
Big Island-Big Island North and East-Kohala-Big Island Interior.

High Wind Warning until 6 PM HST this evening for Big Island
Summits.

&&

DISCUSSION...Gibbs
AVIATION...Donaldson
MARINE...Powell

Special Weather Statement
National Weather Service Honolulu HI
932 AM HST Sat Mar 14 2020

HIZ023-027-142000-
Kona-Big Island Interior-
932 AM HST Sat Mar 14 2020

...A LINE OF STRONG THUNDERSTORMS WILL AFFECT HAWAII IN HAWAII
COUNTY...

At 930 AM HST, radar indicated strong thunderstorms were located
along a line extending from 8 miles west of Kalaoa to 10 miles west
of Kailua-Kona. Movement was southeast at 15 mph.

Wind gusts up to 50 mph and nickel size hail are possible with these
storms as they move onto the Kona Coast from Kailua-Kona south to
Honaunau.

Locations impacted include...
Kailua-Kona, Captain Cook, Kealakekua, Honalo, Kalaoa, Honaunau,
Kahaluu-Keauhou, Holualoa, Kainaliu and Kona International Airport.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

LAT...LON 1936 15589 1949 15593 1952 15596 1959 15597
      1963 15599 1965 15603 1973 15606 1978 15605
      1981 15600 1976 15582 1944 15575 1930 15577
      1929 15589
TIME...MOT...LOC 1930Z 300DEG 15KT 1976 15610 1959 15614

Wroe