Weathering Irma: September 10, 2017

Weather begins to deteriorate today, with the height of the storm Monday

The eyewall of Category 4 Hurricane Irma approaches Key West, FL as seen by the NWS Key West NEXRAD at 5:49am. Image: GR2Analyst

Hurricane Irma is about to make history in an ignominious fashion: It will be the first Category 4 storm on record to make landfall in the same season as another Category 4 storm, much less consecutively. As of this writing, the eyewall of Irma is lashing the Florida Keys with winds up to 135 MPH and locally higher gusts.

Based on the latest National Hurricane Center projection, Irma’s center is expected to continue to move NW and then turn a little more NNW along the Florida Gulf Coast today, with a second landfall possible around Tampa and a final landfall very near Saint Marks, FL tomorrow afternoon.

The Charleston area, as well as the rest of the Lowcountry, should not be fixated on the center of Irma; its impacts are being felt hundreds of miles from the center.

Impacts on Charleston

Expected impacts from Irma on the Charleston metro area, with flooding and surge the greatest concern.

Impacts from Hurricane Irma will gradually increase throughout the day today, peaking on Monday as the center makes its closest approach to the Lowcountry.

Wind

Charleston is already beginning to feel the effects on the periphery of this extremely large hurricane, as a pressure gradient has developed between a strong high pressure wedge centered over the Great Lakes and the very deep low pressure associated with Hurricane Irma. The interaction between the two pressure centers is driving gusty northeast winds across the area, similar to the effect one would feel standing between two large buildings.

850mb dewpoints and surface pressure plot, demonstrating the tightening pressure gradient over GA and SC. Image: GREarth/author

Very breezy conditions have been noted across SC this morning, with a gust to 29 MPH earlier this morning at Charleston International Airport. As Irma’s center draws more northward, the gradient will tighten further, increasing sustained winds and gusts to near tropical-storm force by Monday morning. The highest winds will be felt on the adjacent coastal waters and the beaches.

Wind forecast issued by the National Weather Service in Charleston for Monday.

Periodic wind gusts to near 50 MPH combined with heavy rainfall on top of what is pretty saturated soil already will be able to bring down some trees and power lines. Be prepared for scattered power outages as a result. At this point, it is not likely that there will be too much in the way of long-duration power outages, thankfully.

Tides & storm surge

High tide predictions from the Advanced Hydrologic Prediction Service as of 6:32am September 10, 2017.

Beginning today, major coastal flooding is expected around times of high tide as water levels steadily rise. Salt water flooding should start within 1–2 hours of the time of high tide and could last for several hours after the tide peaks.

Sunday’s tides will both be over 8', which is major flood stage in the harbor. The high tide at 11:55 PM should overtop the Battery seawall.

The most significant salt water inundation is expected Monday, when a storm surge of a little over 4' (as currently forecast) is expected to coincide with a 6.1' high tide. If the forecast verifies, this would be the second-highest tide on record in Charleston Harbor, second only to 12.52' set in 1989 when Hurricane Hugo crashed ashore. (The only other time the tide level in the harbor exceeded 10': August 11, 1940, when an unnamed Category 2 hurricane made landfall at Beaufort.)

Thus, for many of you — especially those new to the area or weren’t here for Hugo— you have never seen tides like this before. The salt water inundation in vulnerable coastal locations as well as parts of peninsular Charleston could be extreme. With heavy rainfall expected around that time, the flooding situation could worsen. Most importantly, the residence time for salt water flooding could be quite high. Unlike Matthew, where the track of the storm took the center north of us and helped turn the winds in an offshore direction, blowing out the surge, winds will remain onshore the entire time.

Storm Surge Warning outline for the Charleston County coastline.

Storm surge is not limited to the coast; its effects will be felt inland along rivers and creeks that are in the tidal zone. You can see which locations are vulnerable to surge using the new Potential Storm Surge Flooding Map on the National Hurricane Center website.

A storm surge warning remains in effect for much of the coastline, including downtown Charleston, through the end of the event. The warning means that significant salt water inundation is possible.

Rainfall and Flash Flooding

Expect the onset of rain in earnest later tonight. There is still plenty of dry air in place preventing much in the way of rain right now, but this will change as Irma’s center moves northward, pushing the tropical airmass up into the Lowcountry.

In the meantime, a few scattered showers will be possible at times, but the heaviest rain arrives late tonight into tomorrow.

A Flash Flood Watch will go into effect at 8PM Sunday, lasting through 2AM Tuesday. 3–6" of rainfall possible over the Tri-County, with greatest flood potential Monday.

A Flash Flood Watch will start at 8PM tonight and will last through 2AM Tuesday. The highest probability for flash flooding will come on Monday, as we will need to watch the colocation of heavy rain bands and the exceptionally high tide during the middle of the day.

Storm total rainfall forecast for Hurricane Irma. Source: National Weather Service, Charleston, SC.

The greatest rainfall totals are expected southwest of us in Georgia. Rainfall totals of 3–4" are expected for most in the Charleston metro, with isolated amounts to 6" or greater in spots. Combined with the tide, though, even these relatively low rainfall totals could still cause significant problems in vulnerable locations, including Downtown Charleston and the old Navy Base in North Charleston.

Monday’s not going to be a great day to go anywhere, but if you must travel, do not cross roads of unknown depth. It only takes two feet of water to sweep away a vehicle.

Tornadoes

As Charleston will be in the northeast quadrant of Irma as it moves to our west, we will be at risk for tornadoes.

September 10, 2017 severe weather outlook. Source: Storm Prediction Center

Sunday’s tornado risk will be confined to the coast and generally arrive after dark. If you live along the coast, have a warning source that can wake you tonight. Tornadoes at night are particularly dangerous because people are sleeping and they are hard to spot. Have your NOAA Weather Radio ready to tone-alert you, and leave phones on the charger, off Do Not Disturb and silent mode so you can hear Wireless Emergency Alerts kick in.

September 11, 2017 severe weather outlook. Source: Storm Prediction Center

Monday’s tornado risk will be higher and extend further inland as Irma’s center makes its closest approach to the Lowcountry, helping to maximize wind shear and allowing instability to better move inland. The tornado risk will be highest near the coast but the entire Tri-County area will need to be alert to the potential for severe weather on Monday.

Tornadoes in tropical systems are typically short-lived, but they can spin up with just a moment’s notice and often move very quickly. Be alert to warnings and be sure to move into an interior room quickly if one is issued for your location.

Bottom Line

  • Major tidal flooding will be possible for the next three high tides, with the worst effects expected midday Monday, when a top-three tide over 10' is expected.
  • Rainfall will begin in earnest tonight into tomorrow, with 4–6" of rain expected. Locally higher amounts will be possible. A Flash Flood Watch will be in effect from 8PM Sunday to 2AM Tuesday.
  • There is a tornado risk along the coast tonight; this risk expands inland on Monday as Irma’s center makes its closest approach.
  • Winds will continue to increase throughout the day Sunday, with tropical storm-force sustained winds over the waters and beaches for much of Monday. Tropical storm force gusts will be capable of knocking down trees and powerlines. Be prepared for power outages.
  • Conditions are expected to improve quickly late Monday into Tuesday, with calmer weather expected thereafter.

Next Update

I’ll be updating throughout the day on Twitter as needed as we start to shift into the event.


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