Flash Flood and Heavy Rain in Hawaii, February 1 to 6, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
348 PM HST Thu Feb 1 2018

.SYNOPSIS...
Kona winds and abundant low level moisture will keep our local
weather warm and muggy the next several days. Bands of showers and
a few thundershowers are expected to continue developing,
especially near the smaller islands. Rainfall could be heavy
tonight and Friday mainly near Kauai and Oahu. For most of the
weekend, the highest chances of rain will shift back to the west
to be near Kauai. A strong mid- level trough will approach the
islands late this weekend, sending a front with active showers and
thunderstorms down the chain starting Sunday night.

&&

.DISCUSSION...
We continue to be in a very muggy and relatively unstable airmass
generally supportive of active showers due to persistent kona
winds. There have even been a few lightning strikes in the coastal
waters west of Kauai. The kona winds are the result of a surface
ridge over the Big Island, and a broad area of low pressure far to
our NE. Dewpoints at sea level continue to be in the low 70s,
quite high for this time of year.

The models have continued to gradually trend wetter over the next
24 to 36 hours as a weak shortwave interacts with the low level
moisture convergence ongoing near Kauai and Oahu. However, there
remains considerable uncertainty as to where the axis of heaviest
rainfall might set up--be that over Kauai, Oahu, or perhaps the
Kauai Channel. Even though the mid-levels are actually warming
somewhat, suggesting a slightly more stable atmosphere than in
recent days, the CIMSS MIMIC Total Precipitable Water (TPW) shows
lots more low level moisture lurking south of the islands headed
our way, with TPW values over 1.75 inches S of 19N. Relatively
unidirectional flow with height oriented roughly parallel to cell
motion makes training showers and thundershowers a distinct
possibility, and with all that said there appeared to be enough
evidence to hoist a Flash Flood Watch for Kauai and Oahu overnight
into Friday.

The axis of best low level moisture convergence will then slosh
westward Friday night and Saturday, taking higher rain chances
back toward Kauai. A stronger mid-latitude trough will begin to
push east over the weekend, which will once again destabilize the
airmass over the state and send the moisture band slowly back down
the island chain early next week. Heavy showers or thunderstorms
appear possible near Kauai later Sunday, with the threat spreading
to Oahu and Maui county Sunday night. One or more heavy rain
bands could get hung up over the islands after that, with the
highest risk expected to be near Maui county and the Big Island by
Monday night and Tuesday. There is considerable uncertainty
starting on Tuesday, as models disagree on how far east a surface
trough forms, but the trend into midweek should be for the wetter
weather to shift E of the islands and for cooler, drier air to
return.

At the summits, a belt of strong southwesterlies on the periphery
of the deep, large low to the NW is traversing across the Big
Island late this afternoon, bringing warning level winds to the
summits. Mid-level ridging is expected to build in from the E
late tonight and Saturday which should allow the winds to
gradually ease there later on.

&&

.AVIATION...
Southwesterly flow will persist through tonight, occasionally
pushing showers across the islands. Isolated thunderstorms are
possible. VFR conditions are expected to prevail, with pockets of
MVFR and isolated IFR conditions possible in clouds and
showers/thundershowers. No AIRMETS are in effect now, but AIRMET
SIERRA may be needed later for mountain obscuration.

&&

.MARINE...
The new northwest swell has now reached 11 feet at the Hanalei
buoy and 9 feet at the Waimea Buoy. The swell is expected to peak
this afternoon and evening. A High Surf Advisory (HSA) is in
effect through Friday evening for most north and west facing
shores of the smaller islands. As this swell subsides Friday, a
larger northwest swell will arrive and likely push surf above
warning levels on north and west facing shores Friday night.
Warning-level surf will persist through Saturday night and likely
remain above the advisory threshold even as it lowers through
Monday. The persistent north and east swells will fade out by
tomorrow night.

A Small Craft Advisory (SCA) remains in effect for waters exposed
to the rising northwest swell through tonight for seas above 10
ft. A trough to the west and a ridge to the east will combine to
produce winds from the south and southeast through this weekend,
including the waters southwest of the Big Island in the advisory
for winds around 25 kt. The southeast winds blowing over the
northwest swell may also make for steeper waves steeper and rough
seas.

A moist and unstable airmass will remain in place over the area
through most of this weekend, producing a chance of thunderstorms
and locally heavy rain at times, mainly over the western half of
the island chain. A strong front is expected to approach the
islands from the northwest Sunday night.

See the latest Collaborative Surf Discussion for more information
on swells and surf.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Friday afternoon for Niihau-Kauai-Oahu.

High Surf Advisory until 6 PM HST Friday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

High Wind Warning until 6 AM HST Friday for Big Island Summits.

Small Craft Advisory until 6 PM HST Friday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward
Waters.

&&

R Ballard/Powell/TS

Special Weather Statement
National Weather Service Honolulu HI
442 PM HST THU FEB 1 2018

HIZ002>004-020345-
Kauai Windward-Kauai Leeward-Kauai Mountains-
442 PM HST THU FEB 1 2018

...A STRONG THUNDERSTORM WILL AFFECT KAUAI IN KAUAI COUNTY...
...SEVERE THUNDERSTORM WATCH FOR KAUAI UNTIL 8 PM...

At 440 PM HST, a strong thunderstorm was located 9 miles southwest of
Barking Sands, or 32 miles west of Lihue, moving northeast at 40 mph.

Winds in excess of 40 mph are possible with this storm as it tracks
across Kauai between 5 and 6 pm. There is a possibility this storm
could intensify...and a Severe Thunderstorm Watch is in effect for
Kauai until 8 pm.

Locations impacted include...
Lihue, Kapaa, Kalaheo, Kekaha, Princeville, Kilauea, Anahola, Mana,
Hanalei, Omao, Barking Sands, Wainiha, North Fork Wailua Trails,
Moloaa, Wailua Homesteads, Wailua, Hanapepe, Koloa, Lawai and Haena.

LAT...LON 2221 15934 2215 15930 2205 15934 2197 15933
      2188 15943 2191 15960 2189 15961 2196 15967
      2198 15976 2202 15979 2206 15979 2223 15957
      2221 15950 2223 15949 2222 15943 2224 15940
TIME...MOT...LOC 0240Z 237DEG 34KT 2192 15986

R Ballard