Friday, September 25, 2015

Coastal Flooding, High Surf, Rip Current, Flash Flood Threat From the Carolinas to New Jersey (FORECAST)

Jon Erdman
Published: September 25,2015

The weather along parts of the East Coast from parts of the Carolinas to the Mid-Atlantic seaboard may resemble a tropical storm through the weekend.
However, any low pressure system will be too weak to be designed a tropical depression or storm.
Let's start out with the impacts and forecast, then we'll take you inside the meteorology of this windy setup.

Coastal and Inland Impacts

High surf with large breaking waves and rip currents can be expected through the weekend from the New Jersey shore southward to at least the northern Outer Banks of North Carolina.
High surf at the Rodanthe, North Carolina, fishing pier on September 23, 2015.
(Jeri Gauthier Theriault)

Wind/Wave Model Forecast

Rainfall Outlook



































Even experienced swimmers should stay out of the water due to the rip current threat. Look for surf hazard warning flags and heed the advice of lifeguards.
Over 30,000 rip current rescues are performed each year in the U.S., often occurring in waves of just 2-3 feet.
Breaking waves at the coast in the areas mentioned above will be higher, at least in the 6-9 foot range. Even the ocean beaches of Long Island are expected to see breakers in the 3-5 foot range. Farther south, the threat of rip currents is also in play from the South Carolina coast to Florida's First Coast.
(WATCH: The Formation/Danger of Rip Currents)
This long fetch, or distance, these onshore winds travel over the ocean plus the duration of these winds lasting through the weekend is setting the stage for not simply high surf or rip currents, but also coastal flooding and beach erosion.
These persistent onshore winds peaking Saturday into early Sunday, before slowly diminishing, will continue to buildup water at the coast with greater areal coverage and magnitude of coastal flooding, particularly during each high tide cycle.
If that isn't enough, astronomical high tides will rise as well this weekend with the full moon (a blood red "supermoon", no less). The risk of moderate coastal flooding is greatest into the weekend in the northern Outer Banks, the Virginia Tidewater and Delmarva Peninsula.
However, coastal flooding may also increase farther north to parts of the Jersey shore.
Some shallow tidal flooding is also possible as far south as Charleston, South Carolina, as occurred around high tide Thursday, as well as along parts of Chesapeake and Delaware Bays.
(INTERACTIVE MAPS: Marine Alerts | Flood Alerts)
Waves riding atop the higher water levels will lead to beach erosion in some areas, including highly vulnerable Highway 12 in the Outer Banks. Some coastal overwash was noted Wednesday in Buxton, North Carolina.
(MORE: Sea-Level Rise a Threat to U.S. Cities)
Here are the astronomical high tide times this weekend at Cape Henry in Virginia Beach, Virginia.
- Saturday: 6:42 a.m., 7:08 p.m.
Sunday: 7:35 a.m., 7:59 p.m.
(FORECAST: NOAA's High/Low Tide Times For Your Location)
Then there's the threat of pockets of locally heavy rain occasionally wrapping onshore. Bands of heavy rain will pinwheel through portions of the Carolinas into parts of Virginia. On Thursday, flash flooding impacted the Columbia, South Carolina area.
(RECAP: South Carolina Tornado)
Eastern North Carolina saw widespread heavy rain Thursday into Friday with several hefty 24-hour rainfall totals. Some of these include 6.77 inches at Beaufort and 5.07 inches on Hatteras Island.
With rivers, inlets and bays unable to drain as fast due to the pileup of water at the coast, any heavy rain would only compound any flooding in these areas.
(FORECAST: Kitty Hawk, North Carolina | Norfolk, Virginia | Rehoboth Beach, Delaware)

The Science Behind the Coastal Setup


Current Radar/Satellite

Coastal Setup Later This Week

































First off, any broad area of low pressure off the coast of the Carolinas is highly unlikely to develop into a subtropical or tropical cyclone.
As has been the case often this season, wind shear is currently too strong to allow any tropical or subtropical development.
(MORE: El NiƱo May End Hurricane Season Early)
Despite the low chance of development, a stuck weather pattern is shaping up to make a getaway to the beaches of the Carolinas and parts of the Mid-Atlantic states less than ideal, to put it mildly.
First, the polar branch of the jet stream will remain well to the north over eastern Canada or northern New England, ensuring little change in the weather over the eastern U.S. into the weekend.
(MAPS: Daily Jet Stream Forecasts)
At the surface, strong high pressure will build southeastward into eastern Canada and northern New England.
Meanwhile, stubborn low pressure will linger somewhere off the coast of the Carolinas possibly crawling slowly northward off the Mid-Atlantic seaboard into the weekend or early next week.
As the high pressure center moves south, the distance between the high and low pressure centers will become smaller – and the pressure difference, or gradient, between these two features will become stronger. This will maintain and even strengthen that persistent onshore wind from the Atlantic Ocean toward the Outer Banks of North Carolina, Virginia Tidewater, Delmarva Peninsula, and Jersey shore the next several days.
Those winds should peak in intensity Saturday and Saturday night, when a broad swath of sustained easterly winds between 30 and 40 mph is forecast over a stripe of ocean water just east of the Virginia and North Carolina coasts.
Check back with us at weather.com and The Weather Channel for updates.

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