What the Charleston area can expect from wind and flooding through Monday from Hurricane Irma

As of 11 a.m., a Tropical Storm Watch has been issued for the Charleston Tri-County area and Charleston Harbor. A Tropical Storm Warning has been issued for the coastal waters off Charleston County out 20 nautical miles.

A Tropical Storm Watch means that tropical storm conditions (sustained winds 39–73 MPH) are possible within the watch area within the next 48 hours.

This could be upgraded to a warning later today or tomorrow.

Note: Jared Smith of @chswx will do a Facebook Live session on The Post and Courier’s page around 8:30 p.m. Saturday to talk about the impact of the pending weather.

Storm Surge Watch in effect

As of 11am, a Storm Surge Watch has been issued for the Charleston area through Isle of Palms. Storm surge inundation of 2–4′ will be possible in many locations, with locally higher amounts especially further south, where storm surge may reach 6′.

NHC has begun publishing storm surge inundation maps for the event; you can find them on the Hurricane Center’s website.

Impacts on Charleston

With increased certainty in the track — model agreement has become exceptional, with the ECMWF performing extremely well in particular with Irma — impacts for the Charleston metro area are starting to come into focus. These will continue to be refined as the storm approaches and confidence improves.

Expected impacts from Hurricane Irma for the Charleston metro area from Irma. Most significant: Flooding & storm surge.

Flooding & Storm Surge

AHPS forecast graphic for Charleston Harbor ending 9 a.m. Tuesday.

Flooding and storm surge look to be the most significant concerns from Irma. Tide levels will be steadily increasing throughout the weekend and into Monday, when a top-two tide is in the forecast. Coastal flood advisories will be issued with times of high tide.

Top tides on record. Monday’s forecast tide of 10.43′ in Charleston Harbor would be second only to Hurricane Hugo’s record tide of 12.52′.

Currently, it looks like 3–5′ of storm surge is going to be possible across the Charleston metro area. These predictions will continue to be refined closer to the event. I would prepare for Matthew levels at a minimum (especially the further southwest you go.) Also note that the track of the system keeps us in onshore flow the entire time, which will prolong the residence time of surge. In Matthew, surge waters were blown back out to sea as the back side of the storm moved through. No such luck this time.

Rainfall

We won’t get as much rainfall from Irma as we did from Matthew or from the tropical fetch from Joaquin during the October 1–5, 2015 event.

The current National Weather Service forecast is for 4–6" of rain across the area for the duration of the event (with locally higher amounts possible). However, the expected surge and tidal anomalies may make up for this from a flood threat perspective, so be aware. Remember, at high tide (and especially tides over six feet), it doesn’t take but a half-inch of rain to cause problems in downtown Charleston.

Wind

Wind impacts look to be moderate, and may be driven more by the interaction of high pressure to the north and Irma’s center as it makes its closest pass.

The current NWS forecast calls for a period of sustained 35–40 MPH winds for much of the day Monday with locally higher gusts. Winds will be strongest the closer to the coast you are, but inland areas should still see winds over 30 MPH with gusts to 50 during the height of the event.

Gusts will reach over 50 MPH at times in the Charleston metro area. This will be enough to blow loose objects around outside and cause treacherous driving conditions, especially on bridges. You should still bring in and secure loose objects ahead of the event — today is a great day to do so.

Tornado Threat

As Irma’s center approaches, wind shear in the northeast quadrant of the storm (where will be spending much of our time) will be sufficient for generating short-lived tornadoes. The tornado threat begins Sunday night and lasts through the day Monday.

With the risk of overnight tornadoes on Sunday night, it is imperative that you have a NOAA Weather Radio or smartphone app that can alert you to warnings and help you get to safety quickly. Leave phones off Do Not Disturb so you can hear warnings pushed through Wireless Emergency Alerts. If you’ve turned them off, you should turn them back on for the duration of Irma.

Rip Currents

Dangerous surf and rip currents will continue through the weekend and into early next week. Beach erosion is very likely with constant onshore flow during the event. There is serious concern with the potential for tides to undermine beachfront property. Your reports will be needed!

Follow Jared Smith’s Charleston Weather updates on Twitter and Facebook. An updated forecast and current conditions are always available at chswx.com, too.

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