Drought in Hawaii, May 1 to 16, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
845 PM HST Sun Apr 30 2017

.SYNOPSIS...
A strong upper low just southwest of the islands will interact with
a weakening stationary front near the Big Island overnight, keeping
showery weather around, along with a chance of thunderstorms, mainly
near the Big Island. The weather will gradually settle down Monday
and Tuesday, as the front dissipates and the upper low weakens. The
cool air mass over parts of the state will become increasingly warm
and muggy before trade winds return around mid-week.

&&

.DISCUSSION...
An unusually strong upper level low centered about 200 miles SW of
Kauai is moving SSW near 15 mph, and slowly weakening. A stationary
front is analyzed across the Big Island, and although it is
difficult to discern and likely washing out, a fairly significant
surface dew point gradient exists across the boundary. Isolated
thunderstorms are observed over Hawaiian Offshore Waters about 80
miles ESE of the Big Island, with thunderstorms in greater coverage
about 150 to 250 miles NE of the Big Island.

The cool and dry air mass over the smaller islands will be gradually
warming overnight and Monday, with surface dew points climbing out
of the upper 50's/lower 60's to near 70. This will occur as the
surface front dissipates and the low-level flow turns to the SE
statewide. Latest MIMIC-TPW imagery shows PWAT over the islands is
near 1.3 inches, with PWAT directly upstream of the islands is
closer to 1.8 inches. So, while the low aloft will be gradually
filling/warming the next day or so, deeper moisture will be moving
in on the islands. Lingering cold air aloft will remain over the
islands through Monday. As a result, the airmass will remain quite
unstable near the Big Island, with additional showers and
thunderstorms expected to develop overnight. The islands from Oahu
to Maui may see a transition to more showery precipitation.
We can't rule out some localized downpours there, but it appears
that the threat for flash flooding has diminished for all but the
Big Island, and a Flash Flood Watch remains in place overnight. A
Winter Storm Warning remains in place for the Big Island summits,
and latest ranger reports indicate that sleet and snow were the
primary precipitation types today, with 4-6 inches of snow and ice
observed on the upper slopes.

A gradual trend toward quieter weather will continue Tuesday/
Wednesday as the mid-levels warm and the low-level SE flow gradually
backs toward ENE. It looks to remain rather warm and humid through
next weekend however, with latest models indicating that the flow
will back toward the ESE late in the week/next weekend as a front
approaches the area from the NW. Current expectations are for the
front to remain N of the area, with a stable air mass (supported by
a mid-level ridge) capping incoming showers after mid-week.

&&

.AVIATION...
A compact but rather strong mid to upper level low centered just SW
of Kauai will produce widespread MVFR conditions across the eastern
end of the island chain through much of the night. The greatest
amount of moisture is expected to remain focused over the Big Island
and Maui. AIRMET Sierra for mountain obscuration will remain in
effect for the Big Island and Maui through at least the first half
of tonight, with some improvement expected on Maui. A shield of mid
level level clouds with bases around 070 will continue to generate
light to moderate rain over Oahu and Molokai through the first part
of tonight then break up as the low aloft stalls SW of the state. On
Kauai, only isolated MVFR conditions are expected.

The low aloft is also producing areas of turbulence and light icing.
AIRMET Tango for moderate low to mid level turbulence across the
eastern two thirds of the island chain will likely remain necessary
through Monday morning.

Heading into Monday, the mid to upper level low will gradually fill
to the SW of the state. Moist and somewhat unstable SE low level
flow will be continue to produce MVFR conditions over SE and S
facing slopes from the Big Island to Oahu.

&&

.MARINE...
The low level wind flow will be becoming mainly SE across the island
chain during the overnight hours, and remain in place through at
least Tuesday. The strength of the SE winds are expected to reach
small craft advisory (SCA) level of 25 kt 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, but the SCA will likely
be extended to at least late Monday afternoon.

As one south swell fades, another one should be arriving later
tonight and Monday. Expect surf to max out at or near advisory level
of 8 feet for the south facing shores late Monday. The lowering
process is expect to begin Tuesday afternoon. We will be evaluating
buoy data during the overnight hours to determine if a High Surf
Advisory is needed for the south facing shores. There is no S swell
for the rest of the period.

A moderate NW swell has peaked and will be lowering gradually
through Monday. No new NW or N swell until Wednesday night where a
small bump is expected.

Moderate trades are expected to return Tuesday evening and
strengthen to moderate to locally strong by Thursday, and persist
through the rest of the week.

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...
Winter Storm Warning until 6 AM HST Monday for Big Island
Summits.

Flash Flood Watch through until 6 AM HST Monday for Big Island.

Wind Advisory until 6 AM HST Monday for Big Island Summits.

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

&&

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