Preparing for Irma: September 7, 2017
A significant shift in the forecast track is cause for optimism, but we can’t let our guard down yet
Major Hurricane Irma continues its rampage through the Atlantic this evening, lashing the Turks and Caicos while maintaining Category 5 strength on the Saffir-Simpson scale. This exceptionally dangerous storm will move through the Bahamas next as it targets South Florida for what could be the first Category 5 landfall in the United States since Andrew in 1992.
Shift in Track
The last 12 hours have shown quite a shift to the west in the forecast track for Irma, responding to a similar shift in the guidance suite. According to NHC, the expected turn back to the west is in response to a shortwave trough ejecting from the Central Plains to the southeast. This feature shows up on GOES-16 water vapor imagery quite clearly.
We are not out of the woods yet, though:
- There is still a lot of margin for error in the track this far out, especially with a northward turn that hasn’t really happened in earnest yet. 50 miles west or east can make a big difference. In fact, there is evidence in the 08/00z GFS that a small correction to the east is now underway. We will need to watch the model suite carefully for such a correction, as a correction in the NHC forecast could come with it. While tropical storm force wind probabilities have decreased significantly with the 11PM update, they are still in the 20–30% range in Charleston.
- We are still working with razor-thin margins as far as impacts go. 50 miles either direction will further change the calculus for Charleston.
- Irma will be a large storm, and significant rainfall is still on the table early next week, regardless of any other impacts.
With that in mind, continue your preparations, including for possible evacuation! If Irma skirts into the right side of the cone of uncertainty, that can significantly increase our exposure to strong winds, heavy rainfall, and severe weather, even if the center remains well away from Charleston.
Let’s Talk Impacts
Impacts will be very track-dependent; the further west the track bends, the less likely we will see severe impacts from Irma. So, take these possibilities with a grain of salt. However, it is important to note that Irma is a large storm, and may get even larger, with a tropical storm force wind field perhaps extending more than 200 miles from the center. Here are some of the impacts we need to consider for the Charleston area based on the latest NWS briefing.
Coastal Flooding
Coastal flooding will begin to be an issue at times of high tide starting on Friday, per the National Weather Service in Charleston. AHPS guidance is showing tide levels reaching over 8' by Sunday. At 8', major salt water flooding occurs in downtown Charleston. The forecasted height of 8.29' would fall in the top ten tides on record at Charleston Harbor.
Storm Surge
Storm surge impacts will be highly dependent on the track Irma ultimately takes; if the westerly trend verifies, surge impacts will decrease significantly. If the track adjusts back east, then we will have to take storm surge back into fairly serious account — one extratropical surge model suggests water levels rivaling Hugo on Monday. This must be watched very, very closely.
Rainfall
Finally, heavy rainfall will almost certainly be a concern. 5–10" of rain (with isolated greater amounts) will still likely fall, even with the westward shift in track noted in Advisory 36. We cannot let our guard down for the potential for flooding in downtown Charleston, especially if rain falls at high tide.
I highly recommend that downtown residents move vehicles into one of the parking garages for the duration of the storm, if at all possible. The City of Charleston will be opening up parking garages for free parking (excluding the Gaillard Center garage) starting at 5PM Friday, September 8, according to a post on their Facebook page.
Wind Impacts
The track shift in Advisory 36 also reflects a major shift in the tropical storm force wind probabilities for the Lowcountry, bringing them down in the 20–40% range from 60% or more. This is an encouraging trend, but we need to watch this carefully. Timing for tropical storm force winds, if they reach the Lowcountry, would still be by around 8AM Monday.
When winds reach tropical storm force, preparations become seriously impeded and evacuations become increasingly dangerous. Despite shifts in track, my advice continues to be to have preparations completed by Sunday morning at the absolute latest. If evacuations become required, you will need to accelerate your preparations, too. This gives plenty of room for error if the storm ends up arriving faster than forecast or the wind field expands.
Tornado Threat
While this will depend on the ultimate path of the center and how far west it trends, a tornado threat will exist late Sunday into Monday. Tornadoes from tropical systems are often weak and short-lived, but they can still do plenty of damage. Tropical systems can often spawn some of our most dangerous tornado outbreaks. Hurricane Frances in 2004 is a great example of this, when 20 tornadoes touched down within NWS Charleston, SC’s county warning area in SC and GA. This potential will be watched very carefully going into next week.
Evacuations
An evacuation order has yet to be issued by the Governor. If the track continues to keep its westward bias, I suspect that the order will not be needed.
If an evacuation order is eventually issued, it will take effect Saturday morning at 10am. At that time, lane reversals on I-26 would begin.
If you live along the coast or on a barrier island, are in a flood-prone area, a non-site-built structure (such as a mobile home), or otherwise are concerned for your safety as Irma affects the area, strongly consider evacuating ahead of time.
Remember, there are no such things as “voluntary” or “mandatory” evacuations in South Carolina anymore. Once an evacuation order is issued, it is considered mandatory. If you stay behind and get into trouble, first responders may not be able to reach you (or would be putting themselves at tremendous risk to try to do so). Weigh this carefully against the inconvenience of evacuating if you are put under the order.
Know Your Zone & Know Your Route
Ahead of the evacuation order, make sure you are familiar with your evacuation zone and the routes to take to leave your zone. Visit SCEMD’s website to learn more about how to safely evacuate.
The Next Seven Days
We still have a couple good weather days before Irma and its rich tropical moisture arrive in the Carolinas and GA. Expect heavy rainfall and gusty winds Sunday through Tuesday, with improvement Wednesday as Irma lifts away from the area.
As always, this forecast is subject to significant revision as track and intensity forecasts evolve. Check back frequently for updates.
Bottom Line
- Despite positive trends in the guidance and in the official NHC forecast, we cannot let our guard down. Just as quickly as the track shifted west, it could retrograde back east.
- Preparations should continue. The margin of error remains razor-thin for judging impacts on the Lowcountry, and I anticipate that there will be further track adjustments.
- Heavy, possibly flooding rainfall and tides remain a threat. Residents in flood-prone areas should continue to monitor forecasts closely.
- Dangerous rip currents are expected to continue. Do not enter the water! Rip currents will be particularly strong thanks to wave action from Irma.
The next video briefing will be a Facebook Live session with The Post & Courier sometime Friday evening (still working out the time). Stay tuned…and keep doing whatever it is you’re doing that is turning the track away!
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.
