BMS Tropical Update 10/07/2017 11 AM CDT

By: - October 7th, 2017

Last night at 11:30 PM EDT, the NHC upgraded Nate to hurricane status about 18 hours ahead of when it was originally predicted to become a Category 1 storm. This is now the first time since 2012 that more than nine hurricanes have developed in one season. This is also the first time since 1893 that nine Atlantic named storms have consecutively reached hurricane status (Franklin-Nate). But, it should be noted that, obviously, vastly different systems are now in place than in the 19th century, which means weaker systems likely were missed before satellite and aircraft observation.

Since Nate is now a hurricane, there is no doubt it will continue to strengthen and be a Category 2 storm as it makes landfall around Gulfport or Biloxi, MS.  There is even a possibility that Nate could be a major hurricane at landfall if it undergoes rapid intensification over the next 12 hours. However, this is more difficult to predict with a fast-moving storm like Nate. Regardless, a stronger storm at landfall will increase the insured losses expected from this event, as yesterday’s estimates were based on a weak hurricane or tropical storm at landfall.

Nate continues to improve its satellite presentation as it cruises north/northwest this morning. With winds at 90 mph, Nate has maintained its gradual strengthening overnight. Nate is located 180 miles south/southeast from the mouth of the Mississippi River. Intense thunderstorms have wrapped all the way around the center, and distinct spiral bands have developed both northwest and southeast of the center. Strong outflow is noted in all directions, which indicates low wind shear. It will be a race against time to see how much Nate can intensify before it moves onshore and weakens.

Nate’s Landfall Impacts
The general landfall area continues to narrow with an expected location between New Orleans, LA and Mobile, AL with the consensus of a landfall location near Gulfport or Biloxi, MS. The main question continues to be: how much strengthening can happen in the next 12-15 hours before landfall?  Nate is already close to a Category 2 hurricane.

Wind Impacts
What we do know is based on observations from the hurricane hunter – the overall wind structure is quite lopsided. This is likely due to Nate’s fast forward motion of 26 mph. This will cause Nate to have a small core of hurricane-force winds only to the northeast of its center.

Cross section of Nate showing pressure and wind. The wind profile clearly shows that the stronger winds are to the right side of the storm with weaker winds on the left side of the storm center. This is likely due to Nate’s rapid forward motion.

There will be little, if any, hurricane force winds to the west of the storm which would be good for New Orleans and cities to the west side of the landfall location. Tropical storm-force winds will extend well east into Alabama, and possibly the Florida panhandle, as Nate makes landfall around 7 pm CDT.  This is also much sooner than expected based on Nate’s rapid forward motion.

Because of Nate’s swift forward motion east of its center, locations further inland could experience higher winds, versus a typically slower-moving storm, as Nate will be further inland before it weakens over land.

As with all landfilling hurricanes, weak tornadoes are the primary threat away from the center of the storm. However, due to the incredibly warm, moist nature of hurricanes, hail is not expected to be an issue.

BMS iVision allows clients to run risks to better understand various impacts from Nate. This is the 3 sec gust wind speed, which shows only coastal areas will see the strongest winds. However, tropical storm-force winds will be widespread, particularly along the eastern side of the storm. Given current fwd speed (26 mph), concerned about higher inland wind potential in southeast as well.

 

BMS iVision allows clients to run risks to better understand various impacts from Nate. This is the 1 minute wind speed, which shows only coastal areas will see the strongest winds. However, tropical storm-force winds will be widespread, particularly along the eastern side of the storm. Given current fwd speed (26 mph), concerned about higher inland wind potential in southeast as well.

Surge Impacts
As with any northward moving hurricane, the onshore winds will cut across a wide swath of the Northern Gulf Coast from southeast Louisiana to the Florida panhandle. These onshore winds will pile water up against the coastline, resulting in storm surge flooding. As I mentioned yesterday, the slope of the Gulf Coast is quite flat, meaning that it doesn’t take much storm surge to cause issues. The factor that could limit the amount of storm surge is Nate is an extremely fast-moving system, meaning that it won’t have prolonged winds along the shore, thus limiting the storm surge. Currently, Mobile Bay seems to be the target for the highest storm surge amounts, which could be close to nine feet. One unfortunate coincidence is that the moon is full and high tide will occur around midnight locally, which coincides with landfall and peak storm surge, maximizing coastal surge.  At this time this does not look to be a big surge event for New Orleans as higher impacts should be greater in MS and AL coastal locations.

NHC Potential Storm Surge Flooding Map. These will get updated here: http://www.nhc.noaa.gov/refresh/graphics_at1+shtml/091740.shtml?wsurge#contents

Rainfall Flooding Impacts
Rainfall will be heavy as this system moves onshore, especially near the center, but could extend well east of the center to the Florida peninsula. As with the surge, the fast motion of the storm precludes any extreme impacts from rainfall, but flooding should be expected whenever 3-8″ of rain occurs. Also note that the heavy rain will continue north into the Appalachians.

Forecasted rainfall over the next 5 days. This shows up to 8″ of rain is now forecasted for parts of the eastern slopes of the Appalachian Mountains. This could create isolated flash flooding.

Insured loss
Over the last few days, I have provided a few analogs that were from tropical storms like Lee in 2012 to Cindy in 2005 and even Isidore in 2002. All of these analog events resulted in insured losses of under $300 million. However, with Nate now likely a Category 2 at landfall, a comparison to a higher analog storm such as Ethel in 1960 seems more appropriate, which today would be closer to a billion dollar insured loss event. Fortunately, it’s highly unlikely Nate will be worse than Opal was in 1995, which today could be close to a 3 billion dollar insured loss event. The intensity at which Nate makes landfall will dictate the amount of insured loss, but an insured loss closer to a billion dollars is more credible with a stronger storm at landfall.

Next Tropical Trouble

A strong suppressed Kelvin wave is forecast to push across the Atlantic this week. After Nate, there is a good chance of a lull in activity until late Oct.  There is a chance that  something manages to spin up in the sub-tropics, which are less impacted by suppressed Kelvin wave passages.  Overall Nate looks to be the last U.S. landfalling hurricane at least for the next two weeks.

 

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