BMS Tropical Update 10/06/2017 12 PM CDT

By: - October 6th, 2017

Nate’s overall structure is finally beginning to improve, based on satellite imagery and observation from a hurricane hunter aircraft. Strong thunderstorms have developed over/near the center of the system, and organized outflow is developing in the upper levels. Banding structures are now beginning to organize around the system. This is a marked change from late yesterday afternoon when thunderstorms were disorganized and weak. The storm is now over the warm waters of the western Caribbean and further intensification is expected.

Nate’s track forecast continues to wobble around, but the general expectation remains consistent. The storm will either make landfall on, or barely miss, the Yucatan Peninsula today before tracking north-northwest into the Gulf of Mexico. A turn north and then northeast is expected on Saturday before a landfall somewhere between Lake Charles, LA and Panama City, FL around 1 am Sunday morning. The storm’s remnants will then race northeast and bring heavy rains to the Mid-Atlantic and parts of New England.

Nate’s Forecasted Intensity
The biggest question mark continues to be just how strong Nate will be at landfall. I focused on this in yesterday’s update, but it is worth discussing again since intensity is key to the overall insured impact at landfall.

As I have been showing, warm waters and low wind shear are the only two factors that suggest strengthening may occur. Dry air and land interaction with the Yucatan Peninsula are the two opposing factors that may cause Nate to be weaker as it moves into the southern Gulf of Mexico. It appears that the dry air will be the primary inhibiting factor in terms of Nate’s intensity prior to landfall in the U.S. The last several runs of the models show dry air wrapping into the system from the west and south as it moves closer to New Orleans tomorrow. If this dry air can wrap into the inner core of the system, it will keep a lid on Nate’s intensity. However, if an inner core can organize today before the dry air really kicks in, and if interaction with the Yucatan doesn’t disrupt this inner core, the storm may be able to continue strengthening despite the dry air.

The majority of ensembles suggest that Nate will have a pressure of 988 – 999 mb at landfall, which is what we would see in a weak hurricane or tropical storm.

Nate is expected to make landfall in 48 hours, so note the pressure forecast from the ensembles during this forecast time period.

Nate’s Forecasted Landfall

The last few model runs of the GFS suggest that land interaction with the Yucatan will be less of a factor as the track has shifted back to the east over the last 24 hours. This may cause Nate to be stronger than forecasted this afternoon, as it tracks close to the Yucatan but not over the Yucatan.

Notice over the last 12 hours a shift in Nate’s track (Green Line) versus what the NHC had forecasted (Red Line).  This may allow Nate to track between the Yucatan and Cuba and would limit land interaction.

 

Last night’s multi-model European, American, and Canadian ensemble’s strike probabilities for Nate.

The landfall location currently appears to be near Gulfport, MS, as the models shift back east away from a New Orleans landfall.  However, the range of possible landfall locations fall somewhere between Lake Charles, LA and Panama City, FL just before midnight on Sunday morning. I would not be surprised if the models continue to shift a landfall location to the Florida Panhandle.

Landfall Impacts
Nate’s impact will extend well away from the center regardless of the inner core’s organization, especially to the east side of the storm. I expect to see strong thunderstorm activity in Nate’s outer bands as it makes landfall, with the heaviest storms moving onshore along Florida’s entire west coast, hundreds of miles east of the center. Gusty winds, heavy rains, and tornadoes will be concerns in these outer bands.

This simulated satellite forecast from the ECMWF shows strong thunderstorm activity in Nate’s outer bands moving onshore in Florida.

Even the weakest of hurricanes can cause storm surge along the northern Gulf Coast. Note in the image below the wide expanse of onshore winds from New Orleans all the way east to the Big Bend of Florida. Even if these onshore winds barely reach tropical storm-force, they will pile up water along the coast. Due to the very flat slope of the Gulf Coast, even a small rise in water level is enough to cause flooding concerns.

The extent of the storm surge will depend on the storm’s intensity, but coastal locations vulnerable to storm surge flooding will likely see at least some level of storm surge. Mobile Bay, AL, Gulf Port, MS, Biloxi, MS and Pensacola, FL are all expected to see at least three feet in storm surge, with some isolated locations expected to see six feet.  Also worth noting that landfall will be close to High Tide, which means water levels will be even higher usual.

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

Several inches of rain may also cause freshwater flash flooding, but the storm’s fast forward motion should preclude any devastating flooding like we saw with Harvey. The map below shows total rainfall for the next 5 days across the U.S. Note the swath of higher totals along Nate’s forecasted path all the way up into New England, where the storm’s remnant low will enhance rainfall associated with a cold front.

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.

 

BMS iVision allows clients to run risks to better understand various impacts from Nate. This is the 3 sec wind gust in MPH, 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 (21 mph), concerned about higher inland wind potential in southeast as well.

 

Potential For Insured loss

The insured loss will depend on the intensity at which Nate makes landfall. It is likely that only minor insured losses will occur if Nate makes landfall as a Category 1 hurricane along the Gulf Coast (under $1B in insured loss). The best analog in recent memory would be Hurricane Cindy 2005 which made landfall over the Mississippi River delta as a Category 1 hurricane, tracking north-northeast with a second landfall just west of Biloxi, MS.  Cindy caused roughly $200M (in 2016 dollars) in insured loss across Louisiana, Mississippi and Alabama. Another analog might be Isidore 2002, which, in 2016 dollars, would have caused $279M in insured loss. Arlene 2005 made landfall as a tropical storm in a similar area but did not cause a PCS loss event.

 

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