Flash Flood and Heavy Rain in Hawaii, March 22 to 26, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
325 AM HST Thu Mar 22 2018

.SYNOPSIS..
Cloudy conditions with breezy trades will continue through Friday.
Showers will focus over windward areas with highest chances across
the western portion of the state over the next couple of days. An
upper trough will approach from the west and bring another wet
period along with a few thunderstorms across the state late Friday
through Saturday night. Improving conditions will be possible Sunday
into early next week.

&&

.DISCUSSION...
Cloudy and breezy conditions continue across the state this
morning due to strong high pressure to the north and an upper
trough positioned several hundred miles west of Kauai near Midway.
Water vapor imagery showed plenty of middle- to upper-level
moisture being drawn northward east of this upper trough around a
mid-level ridge, which supports the cloudy conditions in place.
Although most of the shower coverage expected will continue to
focus over the typical windward and mountain locations today,
heavier showers over Kauai and Niihau are expected as a weak
pulse of energy moves through aloft around the mid-level ridge.
Latest radar showed a shield of moderate to heavy showers with a
few storms embedded moving in from the west over Niihau and Kauai.
Expect this trend to continue through the morning hours today.

Guidance remains in decent agreement through the short-term period
(through Friday) and appears to be initializing well with the
current trends and patterns. Consensus supports breezy trades
holding as strong high pressure far north of the area shifts
southeastward across the northeast Pacific. The aforementioned
upper trough positioned to the west is forecast to begin shifting
eastward toward the region today. This upper pattern will continue
to support cloudy skies. Highest rainfall chances will remain
over the western end of the state where the best source of deeper
moisture and upper support exists. Elsewhere, showers will remain
focused mostly over windward and mountain areas with a few passing
showers leeward periodically.

Conditions will begin to evolve Friday into the upcoming weekend as
a shortwave trough digs southeastward into the area bringing a
threat for heavy rainfall and even a few storms statewide.
Despite the model spread and inconsistencies between various
solutions that have been shown in each cycle, most continue to
depict low pressure developing along an approaching trough in the
vicinity of the islands late Friday into Saturday with an
abundance of deep moisture being drawn northward (2" precipitable
water values). A combination of falling upper heights, 0-6 km bulk
shear (40-50 kt) profiles, lift, and deep tropical moisture will
lead to a wet weekend with a few thunderstorms, especially late
Friday through Saturday. The one limiting factor appears to be the
instability (high shear/low instability event), which may end up
being out of phase with where and when the heaviest activity ends
up developing. The best chance for the heaviest rainfall and
storms will remain over the western end of the state late Friday
into Friday night, then shifting eastward to Maui County and the
Big Island Saturday and Saturday night.

Sunday through early next week, improving conditions are expected as
drier air fills in and the upper trough and surface low lift
northeastward and away from the area. Guidance depicts light west
winds, potentially giving way to overnight land breezes and
afternoon sea breezes by early next week. Forecast confidence,
however, remains low through this extended period provided the
spread between solutions shown.

&&

.AVIATION...
Surface high to the north will gradually move SSE and slowly
weaken the E winds, but these are still strong enough to justify
AIRMET TANGO for low-level turbulence to the lee of the higher
terrain into the morning. Clouds and a few light showers continue
to create MVFR conditions over N and E sections of the Big Island Maui
Molokai and Oahu with more widespread rain showers around Kauai,
so AIRMET SIERRA is still in place for mountain obscuration. This
should continue into this morning. Light icing possible around
Kauai due to thick layer clouds.

&&

.MARINE...
The large NE swell has peaked, but is slow to diminish. According
to the buoy reports, warning level surf continues along the north
and east facing shores this hour. So, the High Surf Warning has
been extended to 6 pm HST this afternoon. The swell is expected to
diminish at a faster pace starting tonight. Expect the warning to
be lowered to an advisory sometime this afternoon.

No other significant swells are expected through the rest of the
forecast period. The fading NE swell will be reinforced by a
small to medium pulse of 3 feet from the NNE between Saturday
night and Monday. This swell will be followed by a small to medium
NW swell between Sunday night and Tuesday. South- facing shores
can expect a slight rise in the surf on Friday when a small
southwesterly swell arrives.

An approaching trough from the west will stall in our western
offshore waters Saturday evening, while a surface low forms over
the western islands. This will cause a shift in the winds to
mainly southeast and south across our eastern waters, and light
variable winds west of Kauai. The area of stronger 25 kt winds
will be relocated to the offshore's northeast waters. The low
will move north and out of our offshore waters by Sunday. The
trailing trough from the low will move east of the Big Island on
Sunday, taking the heavy weather with it. The stalled trough will
gradually dissipate. Expect a thunderstorm or two with this trough
and low pressure system.

With this scenario, the breezy trades will linger across our
waters into Friday. The Small Craft Advisory (SCA) has been
extended to until 6 am HST Friday morning. With the diminishing
trades across our coastal waters, and the dying NE swell, we will
begin scaling back the SCA either tonight or Friday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...

High Surf Warning until 6 PM HST this evening for north and east
facing shores.

Small Craft Advisory until 6 AM HST Friday for all Hawaiian
waters-

&&

Gibbs