USA National Forecast

Tidal Onslaught Brings Flooding to 7 States, Highest Tide Since Hurricane Hugo in Charleston, South Carolina

A days-long onslaught of tidal flooding, high surf and beach erosion along the Atlantic coast of the United States will finally begin to wind down by Wednesday. Strong onshore winds have already brought coastal flooding to oceanfront communities in seven East Coast states.

The coastal flooding is occurring despite Hurricane Joaquin tracking well away from the East Coast.

Where these winds are blowing onshore, they're piling ocean water into the coastline, resulting in coastal flooding. Those same winds are also whipping up high surf on top of the high water, causing waves to break on sections of beach not typically affected by waves or tides during quiet weather.

Saturday afternoon's high tide in Charleston, South Carolina -- about 8.29 feet or 1.29 feet above flood stage -- was the highest measured there since Hurricane Hugo over 26 years ago. Combining with torrential rain, major flooding up to waist deep was seen in parts of the South Carolina Lowcountry.

Many areas from the Outer Banks to the Delmarva region saw their highest tide so far this past weekend, including Chesapeake Bay in Virginia.

Several streets were flooded in downtown Morehead City, North Carolina, during Sunday afternoon's hide tide. Monday afternoon, homes were reported to be surrounded by two feet of water in River Road, North Carolina, due to coastal flooding.

Adding to the high water, swells from Hurricane Joaquin propagated to parts of the East Coast. Joaquin, itself, also indirectly chipped in to help maintain the stubborn onshore flow as it moves north and northeast in the western Atlantic Ocean.

Periods of onshore winds will continue to impact portions of the Mid-Atlantic coast into Tuesday, before surface winds trend either offshore or diminish.

The National Weather Service has issued coastal flood advisories or warnings for the Atlantic coastline from Maryland to eastern North Carolina, the Lowcountry of South Carolina and northeast Florida.

(INTERACTIVE: Coastal/Marine Alerts)

According to the warning bulletins, additional coastal flooding may occur with Tuesday afternoon's high tide along the Virginia Tidewater. This includes the Hampton Roads metropolitan area encompassing Norfolk, Virginia Beach, Portsmouth, Newport News, and other nearby cities in southeastern Virginia.

(FORECASTS: Norfolk | Kitty Hawk | Atlantic City | Charleston)

This means flooding may inundate coastal roads and some properties, as well.

(MORE: Check High/Low Tide Times)

Minor to moderate coastal flooding was also expected in North Carolina's Outer Banks and the Delmarva Peninsula.

Large breakers and strong rip currents affecting the Atlantic coast will still be dangerous for anyone caught in them.

Rip currents claim an average of 100 lives in the U.S. each year. Last week, rip currents killed two and injured two others along the coast of Georgia.

Additional beach erosion is also expected.

Coastal Flooding Reports So Far

The coastal flooding has already destroyed one home, collapsing into grassy sound waters north of North Wildwood, New Jersey, late Friday night.

Flooding prompted closure of Route 1 at the Indian River Inlet bridge, as well as several other roads in Lewes, Delaware, early Saturday. Two roads were also closed by flooding in Frederica, Delaware, about halfway up Delaware Bay from the Atlantic Ocean.

Coastal flooding was reported in numerous locations from Massachusetts to South Carolina during the midday high tide Friday.

Among these was Ocean City, Maryland, where an estimated dozen blocks were flooded from the back bay, according to Tom Costello of NBC News.

A tidal gauge at the south end of the Chesapeake Bay Bridge Tunnel in northeast of downtown Norfolk, Virginia, recorded its eighth highest water level in 40 years, early Friday afternoon, according to hurricane specialist, Michael Lowry.

Water was reportedly knee-deep in parts of Norfolk and Hampton Roads, Virginia, around Friday afternoon's high tide.

Parts of the Outer Banks of North Carolina experienced minor coastal flooding during the day Friday. Coastal flooding also closed several streets in Milton, Delaware, and submerged roads in Wildwood, New Jersey.

Minor flooding was reported on city streets as far north as Scituate, Massachusetts, and as far south as Charleston, South Carolina, during the Friday high tide cycle. Both communities are notorious for their low-lying and easily-flooded neighborhoods, but their wide geographic separation is nonetheless a sign of how large an area is being affected by onshore winds and above-normal tides.

High tide spilled water over onto dunes and into some low-lying streets in Ocean City, Maryland Saturday, leaving many roads impassable for a time during the early afternoon high tide.

Meteorology 101: The Stubborn Setup

The setup for this coastal mess is a case of persistence and a blocked atmosphere.

Strong surface high pressure is anchored over eastern Canada, as the tangled web of non-tropical surface low pressure near the Southeast coast and Hurricane Joaquin dominate farther south.

Differences in pressure drive winds. The greater the pressure difference, the greater the winds.

Surface setup for coastal flooding/beach erosion event. Generally higher surface pressure is shaded in blue, lower pressure in red. Position of Hurricane Joaquin is a model forecast for this past weekend, and does not constitute an official forecast position by either the National Hurricane Center or The Weather Channel.

Strong winds are one major factor in generating large ocean waves. Another is the distance that winds of a similar direction travel over water, known as the "fetch."

Numerical forecast models suggest east to northeast winds reaching the Mid-Atlantic Seaboard during the event may travel an impressive distance of roughly 900 miles from off the southeast Newfoundland coast.

The dominant factor, in this case, is the pattern's longevity. Why is it lasting so long?

High pressure aloft over the north Atlantic Ocean, and a reinforcement of high pressure aloft that will push in from eastern Canada, is blocking the normal west-to-east progression of weather systems, forcing low pressure aloft to simply sit and spin, if you will, over the southeastern U.S.

Only when the block releases later this week will the stubborn onshore winds relent.