Tropical Depression Four-C (2006)

Tropical Depression Four-C was a short-lived tropical depression the 2006 Pacific hurricane season. The depression formed from from a persistent low-pressure area south of Oahu on October 13, 2006. The depression did not last long, dissipating from upper-level wind shear on October 14. The remains of the depression caused heavy rainfall and thunderstorms on October 17 in Big Island. The depression proved difficult for the Central Pacific Hurricane Center to forecast. After the depression had dissipated, the system, along with a cold front also affected the island of Maui.

The remains, joined with an earthquake on October 15, caused heavy rainfall, flash flooding, and moderate damage to several of the islands. A number of effects from the flooding caused several highways along the islands to close. These included rivers overflowing its banks, debris being spread along the road, high water and flooding on the road, and sudden flash flooding. Although the storm caused heavy rains and closed roads, no damage or fatalities were reported on any of the islands, which also had to cleanup from the earthquake on October 15.

Meteorological history
Tropical Depression Four-C formed from a persistent low-pressure area on October 13 at 1500 UTC. The depression formed from several days of slow development. The depression had registered winds of 40 mph (60 km/h) north of the system's center and remained stationary. There was an unusual steering flow was on top of Tropical Depression Four-C, which eventually progressed the system slowly to the southwest. The hurricane center predicted that the storm would not strengthen any further than what it started with, 35 mph (55 km/h). Six hours later, when the next advisory had come out, the depression retained its strength of a tropical depression. However, the stationary system was beginning to move to the east, a rare occurrence. Wind shear increased by the evening of October 13, starting to pull the system apart.

The depression continued to survive the wind shear, causing difficulty in forecasting from the Central Pacific Hurriane Center. The forecast changed by the night of October 13. QuikScat runs missed the depression, and the hurricane center defied the models that were showing a well-defined center. The forecaster instead kept the depression at the same strength of 35 mph (55 km/h). However, the shear had already done its damage as the radar showed the system falling apart. The depression however, did not survive the strong shear, eventually turning into a low-level circulation center. The depression had been sheared apart strongly in the prior 24 hours, that the hurricane center ceased issuing advisories on the depression and forecasted that it would not survive much after that. People were still advised by the government to watch the system as it still posed a threat to the nearby Hawaiian Islands. The depression became a low-level swirl of clouds and moisture, moving to the northeast into a trough. This trough brought the system to the Big Island of Hawaii, and three days later, the remains affected Hawaii.

Preparations
On October 14, as the shearing tropical disturbance approached Hawaii, the state's government issued a flash flood warning for the islands. The hurricane center had predicted that the system would bring heavy rainfall, strong thunderstorms and flooding to the islands. On October 16, the National Weather Service issued a flash flood warning for the island of Maui. At around 1800 UTC, the remains of Tropical Depression Four-C and a nearby cold front were already causing rainfall in parts of the island. With waters in rivers already rising, the warning was extended, with the predictions that mudslides could occur along with flooding. They also warned that people should evacuate to higher grounds due to the flooding.

Coincidentally, as the remains of the depression were approaching the islands of Hawaii, the Big Island was struck by the 2006 Hawaii earthquake. The earthquake had cause moderate damage along the island, and with the threat of torrential rains possible from the depression's remains, the Hawaiian government left the flash flood warning in effect. On October 16, the remains of the depression did pass over the island of Hawaii, causing heavy rainfall and flooding to the island.

Impact
According to the National Weather Service Forecast Office in Honolulu wrote in a November 2006 report that the week of October 16–October 20 was the most significant in the month. On October 14 and October 15, a cold front, partnered with an upper-level trough. The cold front and trough, also the remains of Tropical Depression Four-C, caused minimal impact until moving towards Maui on the morning of October 16. The thunderstorms caused flash flooding in Kihei and along a stretch of slopes from Hana to Kipahulu in eastern Maui. The intensifying line of storms progressed, reaching the Big Island (Hawaii) on the evening of October 16.

On October 17 and October 18, the remains of the thunderstorms had left the islands, who had to clean up after an earthquake around the same time as the thunderstorms. However, the remains had left their mark, with 21.36 in falling from 5 PM HST on October 16 to 5 AM HST October 17 on the Big Island. The flooding on Kauai during October 14 peaked at 3.33 in at Kalaheo.

The flash flooding caused the government to close several highways in Hawaii. Flash flooding caused Hawaii Route 11 and the Keaau-Pahoa Highway to be closed at Kawa Flats. Some localized flooding also caused the Akoni Pule Highway in Kohala and Hawaii Route 180 at Holualoa to be closed. The Piilani Highway had to be closed on October 16 because of high water. The junction of the Pillani and Mokulele Highways were closed because debris and high water levels at the junction. The Akoni–Pule Highway had to be closed until October 17 from high water. The government closed Hawaii Route 130 on October 16, when the nearby Waipahoehoe Stream overflowed its banks.

No damage total was reported from the depression's remains, and there were no reports of fatalities.