Weathering Irma: September 11, 2017
Height of the storm arrives today
Irma has been downgraded to a tropical storm as of 8am, with max sustained winds of 70 MPH; it continues to move NNW over the Florida peninsula and will continue to weaken with time. This does not change expected impacts on Charleston.
Irma will make its closest pass to Charleston today, bringing the worst of its weather to the area. Already, wind gusts well into tropical storm force have been recorded along the coast, and there are reports of scattered power outages and trees down. I expect these reports to continue as the day progresses.
Storm Surge & Tides
All eyes will be on the Charleston Harbor tide gauge today, as a tide topping out between 10–12' is looking likely just after midday today (12:23 PM, to be exact). This will produce major salt water inundation in downtown Charleston and coastal communities! Preparations to protect from flooding should be rushed to completion over the next couple hours.
Unless you were here for Hugo or were alive during the 1940 hurricane, you’ve not seen flooding like this before. This tide should top out perhaps a foot or two higher than Matthew’s 9.28'.
As salt water flooding from tides begins at 7', expect the onset of flooding at least 2 hours ahead of high tide today, and even sooner if rainfall coincides.
But why? It’s just going to be a tropical storm when it gets here!
Check out the wind field as of the 5am advisory:
The yellow shading shows the extent of Irma’s tropical storm force wind field. It is massive — over 415 miles wide. To boot, this wind will remain onshore through the event, allowing plenty of water to build up and push ashore during high tide today.
Anomalies at this morning’s low tide were a little over 2.8', indicating that surge is already having an effect. The National Weather Service is expecting a peak surge somewhere between 4–6' coinciding with high tide.
Storm surge is not only a function of wind speed, but also the breadth of the storm. To boot, Irma has been sending us swell for days, and we will see the ultimate result of that today. Moral of the story: You do not need a strong hurricane to generate storm surge.
A Storm Surge warning remains in effect for the duration of the storm. If you live in the warning area, you could face effects from salt water inundation and should take action to protect yourself and your property.
Rainfall
On top of this tide, it is looking likely a period of heavy rain from Irma’s squalls will dump several inches of rain on Charleston. This rain will fall very quickly and in a very short period of time, and additional aggravation of tidal flooding is expected in coastal areas, particularly if the heaviest rain coincides with the exceptionally high tide that is predicted.
A Flash Flood Watch is in effect today for all counties. Be alert to potential Flash Flood Warnings; these will trigger the alert function on your phone.
Tornado Threat
A tornado threat will be with us for much of the day today as Irma continues to lift north-northwest and the better wind shear overspreads us. The key question is to see how much instability can develop over land areas. Right now the air over Charleston is still rather stable, with temperatures in the mid-60s. Offshore, buoys are sampling air temperatures in the upper 70s. This trend will be watched closely today, and if tornadoes begin to look likely, the Storm Prediction Center will issue a tornado watch. Stay tuned.
Tornado Safety
If a tornado watch is issued:
- Stay tuned to local media or NOAA Weather Radio for watches and warnings
- Know where you are going to go: Interior room on the lowest level of a site-built structure
- Be prepared to abandon mobile homes for more substantial shelter
If a tornado warning is issued:
- A tornado has been sighted or is likely to develop based on Doppler radar
- Take immediate action to protect yourself: Move to an interior room, hall closet, or bathroom on the lowest level of your structure
- If you are in a mobile home, leave and go to a site-built structure
- Remain sheltered until the all-clear comes over the media or NOAA Weather Radio
Winds
Expect sustained winds of 35–45 MPH today over much of the area, with higher winds possible on bridges. Gusts could reach over 60 MPH at times. This will be more than enough to down trees and power lines. Expect the potential for power outages to increase as the day goes on. (For reference, the National Weather Service warns thunderstorms capable of winds over 58 MPH as severe thunderstorms.)
The bridges will not close, but after 40–45 MPH sustained winds, patrols will cease. Keep this in mind if you must travel today.
Bottom Line
- Worst weather from Irma will be during the day today.
- 10–12' tide possible in Charleston Harbor just after noon today. Salt water flooding will occur at least 2 hours before the onset of high tide!
- 4–6" of rain is expected, with the vast majority of it falling during the day today in heavier squalls.
- There is a risk of tornadoes across the area for much of the day through sunset, dependent on how much instability can develop. If conditions become favorable for tornadoes, a tornado watch will be issued.
- Expect improvement overnight and better weather for Tuesday.
Next Update
I’ll have incremental updates throughout the day; follow @chswx on Twitter for breaking weather. I’ll also be breaking in on Periscope if a tornado warning is issued anywhere in the Tri-County 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.
