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Flooding across Oahu is now the worst Hawaii has seen in over 20 years. More than 230 people have been rescued so far. Some 5,500 people remain under evacuation orders in part because of rising water levels at the Wahiawa dam.

Dozens — if not hundreds — of homes were damaged Friday but officials haven't been able to fully assess the destruction, Honolulu Mayor Rick Blangiardi said. Damage estimates have hit $1 billion and are climbing, with homes, roads, schools, airports and a Maui hospital all impacted.

Emergency crews are still conducting search-and-rescue operations.

The National Guard and Honolulu Fire Department airlifted 72 children and adults who had been attending a spring break youth camp in affected areas.

This round of flooding rain is being caused by a Kona low located north of Hawaii, but repeated rounds of rain and their resulting rainfall runoff since last week's heavy rain continue to escalate flood worries and concerns that the Wahiawa dam will be overtopped even as rain slows.

Kona lows are low pressure systems that reverse the winds across the Hawaiian islands, causing heavy rainfall and gusty winds in communities that don't typically have such foul weather.

(MORE: What Is A Kona Low?)

At least 10 people have been hospitalized for hypothermia.

Parts of Oahu have seen 8-12 inches of rain with several more inches expected this weekend.

Officials are monitoring the 120-year-old Wahiawa dam, which is near critical levels and at risk of failing.

Officials have told residents to leave the area immediately as the dam "may collapse or breach at any time." The warning adds that residents should carpool because of heavy traffic on the island.

The area north of Honolulu has been inundated by heavy rain and catastrophic flash flooding has hit the island of Oahu.

Emergency sirens blared along Oahu’s famed North Shore, where rising waters also damaged homes. Honolulu officials issued a “LEAVE NOW” evacuation order at 5:35 a.m. Friday for Waialua and Haleiwa: “Extremely dangerous flooding and Wahiawa Dam is high.”

The North Shore of Oahu has been under flash flood warnings since early Friday morning.

A series of low pressure systems continues to dredge up tropical Pacific moisture and fling that moisture over the Hawaiian Islands. Given the very heavy rainfall in the first of these Kona lows last weekend in addition to these smaller system, the rainfall has really added up. Without an extended break, this flooding could continue for a while.

According to senior digital meteorologist Jonathan Belles, "This flooding in some ways is connected to the heat dome entrenched into the Lower 48. We often talk when one area of the country is seeing heat, another is seeing cooler, wetter weather." Belles adds, "Right now, the West is seeing heat and the East is seeing slightly stormier weather. Well, that pattern continues west, too. That cooler, wetter weather persists over Hawaii and into Alaska. Until some major storm system can shove this pattern into a different gear or the pattern can slowly break down, this flooding and heat will persist."

The Hawaii National Guard has been activated in response to the flooding. Gov. Josh Green says, "The storm of course is very severe right now, particularly on the northern part of Oahu." He added, "It's going to be a very touch-and-go-day."

Most of the state remains under a flood watch.

Portions of this report are from the Associated Press.