Preparing for Irma: September 6, 2017
Charleston enters the Cone of Uncertainty
Major Hurricane Irma, which continues to set the benchmark for high-end Category 5 longevity by maintaining max sustained winds of 185 miles per hour for the last 33 hours, has devastated the small island of Barbuda and made a very uncomfortably close pass to San Juan, which fortunately missed out on Irma’s ferocious eyewall by a hair.
11 PM Advisory
The 11 PM advisory on Hurricane Irma continues to show a very powerful major hurricane moving through the Bahamas on the way to the southeast coast of Florida by Saturday night. On the forecast track, the center of a category 2 Hurricane Irma with 110 MPH winds would be very close to the GA/SC border at the Savannah River entrance by Monday evening. Tropical storm force wind probabilities are steadily increasing and are now up to 50% over the lower SC coast, with the onset of tropical storm force winds possible on Monday.
The main questions for Charleston continue to revolve around the expected northward turn and intensity of the storm as it approaches.
Northward turn
The timing of the northward turn is still very much the main concern at this juncture. Model guidance continues to have varying ideas about when the turn will happen and to what degree the turn will happen. Overall, the model consensus continues to indicate a turn very near or over Miami, with the storm then moving north toward a landfall somewhere along the GA/SC coasts. There is still a lot of room for change, and the devil is in the details, but this is a concerning trend and will need to be watched carefully.
Intensity forecast
In the latest advisory, NHC went down to a high-end Category 2 storm (110 MPH max sustained winds) on day 5 with the storm center roughly around the Savannah River entrance. This expected weakening is due to the increasing effects of wind shear on the storm as it gains latitude. We will see if this forecast trend continues — it certainly would be a welcome bit of good news from a storm that so far has been devoid of any.
With all this in mind, though, intensity is the hardest thing to predict. Just ask any forecaster if they foresaw Irma maintaining 185 MPH max sustained winds for almost 36 hours. While we can hope for weakening, we need to be preparing for a major hurricane strike.
State of Emergency
Governor McMaster declared a state of emergency in the state of South Carolina today. All this means is that increased collaboration and resource sharing begins between state agencies to meet the needs of the emergency. It does not necessarily mean an evacuation. Local emergency operations centers went to their next levels of readiness to begin preparations for the hurricane.
Evacuation?
There is currently no evacuation order in effect. However, this may change. Take the time now to become familiar with the coastal evacuation zones and evacuation routes in case an order does come down.
A major point of confusion during Matthew was the difference between a voluntary and mandatory evacuation. South Carolina has no such distinction anymore — an evacuation order means “go.” If and when that evacuation order is given, listen for instructions from officials on how to evacuate quickly and safely. You do not want to be on the road when winds begin to pick up.
Timeframe
Based on the current Hurricane Center forecast and the experimental tropical storm force wind timing products, tropical storm force winds should begin in the Charleston area by Monday morning. Note that any change in forward speed or expansion of the wind field beyond what is forecast could accelerate that timeline. My general advice is to have your preparations wrapped up by Saturday night (or sooner if you are ordered to evacuate). If you’re still working on prep on Sunday, you’re pushing your luck, in my view. Once tropical storm force winds (39–73 MPH sustained) set in, attention should turn to sheltering in place — the time to prepare will have ended.
As always, notes on this timeline could change dramatically based on track and forward speed of the storm. The devil is most certainly in the details and we will need to watch very closely.
The Next Seven Days
We are getting a few precious days of calm before the storm to prepare and potentially evacuate. High pressure building in behind a cold front will drop temperatures into the low 80s for Thursday’s high, with just a few clouds. Expect similar weather for Friday as well. On Saturday we could start to see an increase in moisture ahead of Irma’s arrival.
Based on Irma’s current forecast track, Sunday may feature periodic rain bands, with the worst weather (high winds, heavy rain, and possibly a tornado threat depending on track) coming Monday into Tuesday.
To say this forecast is subject to change is analogous to saying that Elon Musk has some spare change lying around.
Next Update
Look for another brief Periscope video Thursday morning followed by a more in-depth Medium post that evening.
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