Heavy Rain in Hawaii, May 1, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
345 AM HST Mon May 1 2017

.SYNOPSIS...
A strong upper low stalling just southwest of the islands will
interact with increasing moisture to keep an unsettled weather
pattern in place through tonight. While heavy showers are expected
to be active near Maui and the Big Island, heavy showers and
thunderstorms could potentially occur statewide. The weather will
gradually settle down on Tuesday, as the upper low weakens. A more
typical trade wind weather pattern is expected by mid-week, with
stable weather continuing through next weekend.

&&

.DISCUSSION...
Unexpectedly quiet for most of the night over most of the islands -
considering that a potent upper level low is centered about 300
miles SW of Kauai, and clusters of strong thunderstorms persisted
about 125 miles NE, and 200-300 miles SW, of the Big Island for most
of the night. The low has been moving SSW near 15 mph, and these
clusters appear to have formed where the flow around the low was
most divergent. These remained away from the islands and the coastal
waters for the most part, but have recently begun to increase
near/over the Big Island and Maui. Radar and satellite show a line
of thunderstorms moving over windward Big Island and adjacent waters
from the SE, an area of rain and embedded thunderstorms approaching
both Maui county and the Big Island from the SSW, some heavy showers
over E Maui, a line of heavy showers over waters N of Oahu, as well
as a few small showers over leeward waters from Oahu to Maui.

Latest analysis features a surface high to the distant NE, while the
surface reflection of the low aloft has weakened to a trough about
450 miles SW of Kauai. However, a weak surface low may exist SW of
the Big Island, and the combined effects of these features place a
light to locally strong SE flow over the area, as the surface front
that was over the islands yesterday appears to have dissipated. With
the surface front now dissipated, the cool and somewhat dry low-
level air mass over the smaller islands has moistened overnight,
with the dew point at PHMK now at 70F as compared to 59F at 4pm
Sunday. However, Oahu and Kauai remain in the cooler air for now,
with surface dew points still near 60F. So, while the low aloft will
be gradually filling/warming the next day or so, deeper moisture
will be moving in on all islands. This appears to be fueling the
renewed heavy shower and thunderstorm development seen on radar.

With the lingering cold air aloft, the increasing low-level
moisture, and model indications that the low will stall near it's
current location, the Flash Flood Watch (FFA) and Winter Storm
Warning for the Big Island summits have been extended through the
day. Given latest trends, the FFA was expanded to include the island
of Maui, due to the potential for terrain-anchored showers along
windward E Maui, and due to the recent rains saturating the island.
Localized downpours can't be ruled out anywhere across the chain
today, and potentially tonight, and the short-term forecast remains
filled with uncertainty.

A front passing N of the islands is expected to lead to lighter SE
winds later tonight through Tuesday night, even as the trough(s)
near the islands weaken and move W. In this regime, showers will
favor windward areas during nights and mornings, and with the
smaller islands in the wind "shadow" of the Big Island, winds may be
light enough to allow afternoon sea breezes to drive cloud and
shower formation over leeward areas. A mid-level ridge building N of
Kauai will likely limit shower coverage and intensity by Tuesday,
but some lingering instability associated with the gradually filling
low aloft may allow some deep convection to develop on the leeward
Big Island slopes. A new high building N of the front will bring
gradually building trade winds Wednesday into Thursday as the
associated moisture remains N of the islands. This same high will
support a trade wind flow through next weekend. By week's end and
next weekend, the mid-level ridge will move over all islands and the
island atmosphere will be strongly capped and a benign weather
regime is expected.

&&

.AVIATION...
A compact mid to upper level low centered approximately 180 nm sw of
Oahu will continue to generate widespread instability even as it
gradually weakens. The remains of a stalled front draped over Maui
County and the Big Island will continue to act as a moisture source
for convective activity. Daytime heating combined with the proximity
of the upper level low will likely generate enhanced afternoon
showers statewide today. The potential for thunderstorms is greatest
near the Big Island this morning but there remains a chance
elsewhere across the chain.

An AIRMET for mountain obscuration remains in effect for Maui and
the Big Island. Ceilings are expected to lift over Maui by late
morning and Leeward Big Island by late afternoon. Low ceilings as
well as periods of heavy rain will likely persist across Windward
Big Island throughout the day--and possibly into the evening.

The low aloft is also producing areas of turbulence and light icing.
AIRMET Tango for moderate low to mid level turbulence across the
eastern half of the island chain will likely be cancelled later
today as conditions gradually improve from the west.

&&

.MARINE...
The low level wind flow will be becoming mainly SE across the island
chain this morning, and continue through Tuesday. The strength of
the SE winds are expected to reach small craft advisory (SCA) level
of 25 kt this morning across the windward and leeward coastal waters
of Maui County and the Big Island. A SCA is in effect for these
waters until 6 am HST Tuesday.

As one south swell fades, another one should be arriving this
morning. Expect surf to max out just below the advisory level
criteria of 8 feet for the south facing shores this afternoon or
evening. The swell is expect to start lowering Tuesday afternoon.
No other south swell is expected for the rest of the week including
the upcoming weekend.

A moderate NW swell has peaked and will be lowering gradually
through Monday. A series of northwest swells is on tap for the rest
of the forecast period. The first of two is due in on Thursday with
a small to medium size swell. A larger moderate swell rolls in
Saturday night and Sunday. The anticipated return of the trade winds
later this week means the surf will rise slightly along the east
facing shores. Choppy waters will return as well.

Higher than normal high tides are expected to continue for the next
couple of days. This above normal tide coupled with an re-enforcing
swell from the S later tonight will maintain elevated total water
levels through Tuesday. The greatest chance for beach inundation is
during the evening hours when the high tides will be at their
highest. A return to normal tides is expected by Wednesday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through this afternoon for Maui-Big Island.

Winter Storm Warning until 6 PM HST this evening for Big Island
Summits.

Small Craft Advisory until 6 PM HST this evening for Maui County
Leeward Waters-Big Island Leeward Waters.

Small Craft Advisory until 6 AM HST Tuesday for Maui County Windward
Waters-Alenuihaha Channel-Big Island Windward Waters.

&&

Discussion...Birchard
Marine...Lau
Aviation...Bedal