Heavy Rain in Hawaii, March 27, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
353 PM HST Wed Mar 27 2019

.SYNOPSIS...
A front will move across the main Hawaiian Islands Thursday
before stalling near the Big island Friday into Saturday. Light
winds today will give way to post-frontal north to northeast winds
Thursday through Friday. Light winds will then return this
weekend into early next week as a second front approaches from the
northwest.

&&

.DISCUSSION...
Satellite loop shows the leading edge of a 150 mile wide frontal
band about 300 miles northwest of Kauai moving southeastward at 5
to 10 mph this afternoon. A weak ridge ahead of the front lies
across the main Hawaiian Islands, keeping the pressure gradient
slack and winds light. Sea breezes within daytime heating have
triggered an increase in cloud cover across the islands, leaving
adjacent waters mostly clear except around Kauai. A ragged band of
broken to overcast low clouds extends from northeast to southwest
across Kauai. Afternoon soundings show a rather dry but slightly
unstable airmass, with PW between 1.0 and 1.3 inches and small
negative lifted index values. An inversion layer has formed near
5700 feet above Hilo, while one with a base at 11700 feet has
formed near cloud top height above Lihue. This deeper layer below
the inversion over Kauai is corroborated by radar, which shows
scattered to numerous moderate showers across and south of that
island.

Light winds will continue through the rest of today, perpetuating
local sea breezes through the daylight hours, especially across
leeward areas. Showers will produce more rainfall across the
western islands, mainly Kauai, while islands farther to the east
should be drier. The front to our northwest will move across the
island chain from Thursday afternoon into Friday, ushering in a
brief period of post-frontal north to northeast winds. Models show
this front will increase clouds and showers mainly across
windward and north facing slopes of Kauai Thursday afternoon, then
across Oahu and Maui County Thursday night into Friday. The front
will then stall and begin to dissipate over Maui or the Big
Island by Saturday, possibly giving a boost to windward rainfall.
An upper trough is forecast to drop over the state late Friday
into early Saturday, possibly enhancing showers along the
dissipated frontal boundary.

Models depict a shallow cold front reaching the islands next
Tuesday. The post-frontal ridge north of the islands will remain
weak, re-introducing an extended light wind pattern across the
state from the weekend into the first half of next week. Expect
land and sea breezes to develop within this large scale light
trade wind pattern.

&&

.AVIATION...
An approaching cold front has allowed for very light flow over
the islands this afternoon. Thus, a sea breeze pattern has
developed, affecting many of the local airports. By tonight, land
breezes will develop over the islands.

Expect shower activity to be limited to mauka slopes and ridge
tops through this evening, then dissipate shortly after sun down
with the loss of diurnal heating.

Thursday evening into Thursday night, the aforementioned front is
forecast to approach from the northwest, increasing shower
activity and veering winds northerly.

No AIRMETS are currently in effect.

&&

.MARINE...
A front will continue to approach the region tonight, and till it
gets closer to the area, we will continue to see variable winds
with a local land breeze developing near the coast. The one
exception is for the waters around Kauai, where they should begin
to see northwest winds later tonight. We should begin to see an
increase of light to moderate northerly winds on Thursday and
Thursday night as the front moves through the western end of the
island chain and stalls and dissipates over the eastern end of the
island chain by Friday. A weak surface ridge is forecast to build
in just north of the state from Friday into Saturday, which will
maintain gentle northeast winds. Another weak front is expected to
approach the region from the northwest Sunday and possibly move
through the islands on Monday into Monday night.

No Small Craft Advisory (SCA) winds are expected through this
weekend. However, a large north-northwest swell may cause seas to
reach the SCA threshold over some windward waters from Friday
night into Saturday. The latest guidance is suggesting seas
peaking at 9 feet, which would be 1 foot below SCA thresholds.

The new northwest swell has filled in through the day today and
will continue to build this evening and tonight. Based on the
latest readings from buoys 51001 and 51101 northwest of Kauai, the
swell will peak tonight just below advisory levels and decrease on
Thursday. A larger reinforcing north-northwest swell arriving
early Friday, which was mentioned above, is expected to cause surf
to reach the HSA threshold along most north facing shores from
late Friday into early Saturday. Another reinforcing swell from
the west-northwest spreading down the island chain starting early
Saturday will likely maintain moderate surf along most north and
west facing shores from late Saturday through Sunday. This swell
is expected to gradually increase from Sunday night into early
next week, so a HSA may be required for some north and west facing
shores starting late Monday.

Small background surf will continue along south facing shores
today. A new small long-period south swell will produce a slight
bump in surf heights along most south facing shores from Thursday
through Friday. Surf along east facing shores will remain minimal
through this weekend due to the weak or nonexistent trade winds,
as well as the lack of a significant upstream easterly fetch.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

DISCUSSION...Powell
AVIATION...JT
MARINE...Kino