Flash Flood Watch 12, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
955 AM HST Tue Dec 26 2017

.SYNOPSIS...
Showers and thunderstorms ahead of an approaching front will
impact the smaller islands today and tonight. A cold front will
shift eastward across the state Wednesday and Wednesday night,
bringing improving conditions and a decrease in showers in its
wake. Trade winds along with drier and more stable conditions will
begin to return on Thursday, with locally breezy trade wind
weather feature mainly windward and mauka showers then expected
through early next week.

&&

.DISCUSSION...
A 1006 mb closed surface low is located 425 miles northwest of
Kauai moving east at 25 mph, and dragging a cold front towards the
state. Far to the northeast of Hawaii, a a nearly stationary 1026
mb high is centered. These two features are driving gentle to
moderate south to southeasterly winds over the state, which is
expected to bring an increase in moisture over the islands today.
Visible satellite imagery shows cloudy skies across Kauai and
Oahu and mostly sunny skies over Maui County and the Big Island.
Radar loop through the morning shows heavy showers with some
embedded thunderstorms over and around Kauai and Oahu. Water vapor
imagery and satellite derived upper level winds indicate an upper
level trough associated with the surface low with an axis 225
miles west of Kauai this morning.

There is a potential for strong to possibly severe thunderstorms
in association with today's storm system. Increasing moisture in
combination with strong forcing for ascent provided by the upper
level trough, is expected to lead to thunderstorm development
across the western and central islands through tonight. Although
conditions are not as ominous as the models depicted 24 hours ago,
ingredients still remain favorable for organized convection over
the western and central islands. Surface based CAPE values are
expected to climb into the 1000 to 1500 J/KG range today, with
700 mb to 500 mb lapse rates of 6.5 to 7.0 C/km, and 0-6 km bulk
shear values of 30 to 40 knots. As a result, we expect increasing
thunderstorm development continuing through tonight as the
deepest moisture and strongest forcing for ascent most closely
align. Some of the thunderstorms may be strong and possibly severe
from Kauai to Maui County. The main hazards from the strongest
storms that move through the area are expected to be straight line
damaging winds and hail.

Some locally heavy rainfall will be possible with the thunderstorms
as they move through the area as well, but given the strong deep
layer flow, the thunderstorms should be progressive and not linger
in any particular location for an extended period of time. As a
result, flash flooding is not anticipated.

Additionally, University of Hawaii's Vog Model shows the plume
from Kilauea remaining over the smaller islands today, resulting
in hazy conditions. Vog will decrease over Kauai and Oahu tonight
as showers scour out some of the vog and the flow turns more
southwesterly. The southwesterlies are then expected to pull the
vog plume off of Maui by daybreak Wednesday, but it will continue
to affect windward Big Island through at least Wednesday night.

Winds are expected to be advisory levels over the Big Island
summits through tonight due to the enhanced wind aloft caused by
the upper trough. A Wind Advisory is now in effect.

The earlier mentioned surface low will track north of the state
tonight and Wednesday and drag the front across the island chain
Wednesday and Wednesday night. A trailing band of clouds and
showers associated with the front, along with another shortwave
trough moving overhead, will keep conditions somewhat unsettled,
particularly from Oahu eastward to the Big Island.

Thursday through next Tuesday,
More settled weather will overspread the island chain as high
pressure builds in to the north of the state, and the cold front
continues to shift eastward and away from the area. Moderate
north-northeasterly winds are expected behind the front, with
moderate trades then returning Friday through early next week.
More typical trade wind weather is expected statewide, with
showers favoring windward and mauka areas through the period.

&&

.AVIATION...
A cold front northwest of the state will continue to advance
towards Kauai today. Winds have veered out of the southeast and
south for most of the state. Kauai and Oahu will continue to see
prefrontal effects, from periods of heavy showers, lower
visibilities, and a chance of thunderstorms through the day and
into tonight. Any thunderstorm can bring periods of IFR CIGS/VIS,
lightning, and turbulence. There is also a possibility of small
hail.

AIRMET SIERRA for mountain obscuration is posted for Kauai and
Oahu. PHJR and PHNL will be susceptible to prolonged periods of
MVFR CIGS/VIS to isol IFR as showers fill in from the south. Any
layered clouds around Kauai and Oahu also pose the threat for
isolated light icing. The most significant upper level support
looks to reach the western islands this afternoon and evening.
Conditions will then start to improve after midnight for Kauai.

Tempo MVFR CIGS/VIS are likely on Molokai and Lanai through this
afternoon, before the threat of thunderstorms spreads east this
evening. AIRMET SIERRA will be expanded as needed today. Maui and
the Big Island should see predominately VFR conditions today,
with showers and possible thunderstorms spreading to Maui late
tonight.

&&

.MARINE...
Winds over the coastal waters have been veering from the
southeast to south as a low northwest of the area tracks to the
east. A front associated with the low will affecting at least the
western half of the state through tonight. The feature will bring
widespread showers and periods of thunderstorms to much of the
area, which may include lightning, low visibilities, and strong
and gusty winds.

A reinforcing north-northwest swell is turning more northerly.
Therefore, surf along east facing shores with a more northern
exposure will likely see elevated surf for the smaller islands,
and later this morning for the Big Island. A High Surf Advisory
(HSA) is posted for east facing shores of Kauai, Oahu, Maui,
Molokai, and the Big Island through this afternoon.

The low mentioned above is forecast to move to a position north
of the state around Wednesday. Model runs have backed off a bit on
the strength of the winds in the fetch directed at the islands.
Thus, surf is now expected to stay just below HSA criteria for
north facing shores. An HSA for the exposed east facing shores
would become more likely. The swell, however, is highly dependent
on the position and strength of the low.

Most south facing shores should see a slight bump in surf this
week from a series of small long-period south-southwest and
southwest swells.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for east facing
shores of Niihau, Kauai, Oahu, Molokai, Maui, and the Big Island.

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

&&

DISCUSSION...Foster
AVIATION...Eaton
MARINE...TS