[UPDATED] Preparing for Irma: September 9, 2017
Irma’s track shifts westward, but Charleston doesn’t get off scot-free
Major Hurricane Irma has been thrashing Cuba for the last 12 hours, making landfall as a Category 5 late last night. Interaction with land has weakened the storm to a 125 MPH Category 3 storm, but it is expected to rapidly reintensify as it gradually turns northwest and north today to make its approach to the Florida Keys and west coast.
Tropical Storm Watch now in effect
As of 11am, 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.
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.
Flooding & Storm Surge
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.
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!
Your questions, answered
I asked for your questions, and boy have all of you delivered :) Here are some answers…
On Irma’s Track
It is not out of the realm of possibility that the center could track offshore, but that is looking less and less likely. That being said, the exact point of landfall on the Florida peninsula is still difficult to pin down at this point.
Barring a complete failure of all of the available numerical weather prediction guidance, a direct hit on Charleston is all but ruled out at this point. Irma is a large storm, though, and we will still feel effects.
On Storm Surge
More concrete details on storm surge will be coming through later today as NWS and NHC work on the forecast, but right now it looks like storm surge of 3–5' will be possible for the Charleston metro area. Stay tuned for additional details.
As it stands right now, I would prepare for roughly equal impacts as Matthew. Keep in mind that with Matthew, the back side of the storm eventually moved through Charleston, blowing out the surge. We won’t get that luxury this time — onshore flow will continue for the duration of the event, prolonging storm surge impacts.
On Flooding
Flooding looks to be a significant concern for downtown Charleston, with forecasted tides exceeding Matthew’s levels. If the current forecast of 10.43' verifies, it would be the second highest tide on record at Charleston Harbor, second only to Hugo back in 1989. As mentioned above, we will not get the benefit of offshore winds from the back side of the storm, either. None of this takes into account rainfall, as well, with 4–6" of rain expected during the event.
The City of Charleston has opened parking garages for free (excluding the Galliard) to residents for safekeeping of vehicles that would otherwise be parked in vulnerable locations.
On Winds and Timing
Sustained onshore winds of 35–40 MPH with locally higher gusts will be possible starting Sunday night as the low pressure center of Irma interacts with a wedge of high pressure anchored to our north.
Hurricane force winds are not expected.
Tornadoes will be a possibility Sunday night into Monday, especially along the coast and on barrier islands.
Winds and rain should begin to pick up sometime Sunday evening.
On What To Do
Boarding up is probably overkill at this point, especially in North Charleston. There will be gusty winds, and perhaps some gusts into tropical storm force, but sustained high-end tropical storm force or hurricane force gusts are not expected.
Sunday will probably be the day to start hunkering in. Tides will be getting into major flood stage with the Sunday morning high tide, and the onset of stronger winds and rain should begin by Sunday evening. Worst of the weather will be Monday.
Absolutely. There are no lane restrictions or reversals in the Charleston area, so you may deal with traffic issues, but you should not feel like you have to stay. If you feel the need to get away, do so!
I’d consider inland generally west of Highway 17 for our purposes here. Coast would be the beaches and associated beach communities as well as barrier islands.
Wednesday looks great. Game on.
On Personal Preference
Can’t go wrong with a good cookies & cream ice cream.
Next Update
I’ll be along later this afternoon as details on storm surge and other impacts continue to come into focus for the Charleston area.
Follow Jared Smith’s Charleston Weather updates on Twitter and Facebook. An updated forecast and current conditions are always available at chswx.com, too. Bookmark it on your home screen! To support independent, hype-averse weather journalism, consider becoming a patron on the Charleston Weather Patreon page.
