Friday Update - July 12, 2019

Good morning from Cajun Country. “Red in the morning sailors take warning!”

Very red sunrise as the sun rays interact with high cirrus clouds ahead of Tropical Storm Barry. I’m en route to Southeast Louisiana to collect data.

Heavy rainfall will be the major story with Barry. The National Hurricane Center forecasts that more than 20 inches could fall in the region.

As of last advisory the tropical storm force winds remained offshore. Winds quite calm near Lake Charles now, but I expect them to pick up as I head east.

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Thursday Update - July 11, 2019

The big story with Tropical Storm/ Hurricane Barry will be widespread flooding from torrential rain. The National Hurricane Center forecasts more than 15 inches of rain will fall across portions of south central and south east Louisiana.

Although Barry has been struggling to get organized, the National Hurricane Center forecasts that conditions for development will improve over the next two days and the storm could make landfall in south Louisiana as a hurricane.

The entire Upper Texas Coast is outside the "cone of uncertainty" now....and models support a Louisiana landfall.

The risk for extensive salt water (storm surge) flooding is diminishing as Barry struggles to organize, because it takes time for tropical systems to push salt water. I published an academic paper in 2014 that proved storm surge levels correlate better with pre-landfall hurricane winds than the wind speeds at landfall. This means that storms that get organized just before landfall generate lower storm surges than storms that are well organized farther offshore. This is good news for south Louisiana.

However, persistent onshore winds will raise salt water levels and slow the drainage of heavy rainfall....a phenomenon known as compound flooding. So even if a community is not flooded by salt water, elevated salt water levels along the coast can contribute to flooding by slowing the drainage process.

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Wednesday Update - July 10, 2019

Development of Tropical Storm Barry is imminent.... The National Hurricane Center has released its first advisory on potential Tropical Storm Barry at 10AM Central Time.

However, until a center of circulation forms and the storm is named, we may expect the forecast track to shift considerably. The most likely scenario is for Barry to develop into a hurricane and make landfall in SW Louisiana, although people along the Upper Texas coast and the entire Louisiana coast should remain vigilant as this track may shift.

If Barry did form into a hurricane and approach the coast on a NW track, making landfall in SW Louisiana, the highest storm surge (salt water flooding) may occur near Vermillion Bay in South Central Louisiana.

In general, highest salt water flooding would occur just east of where the eye crosses the coast. Vermillion Bay could see substantial water levels because storm surge levels are enhanced in bays and inlets.

If Barry were to track farther east and make landfall near Grand Isle, storm surge flooding could be expected near the landfall location as well as the coastline east of the Mississippi River, Lake Pontchartrain and possibly Mississippi.

Heavy rain and wind impacts should be expected across a long area of coastline from Upper Texas Coast to Mississippi, including all of South Louisiana.

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Stay tuned for more updates.

Tuesday Evening Update - July 9, 2019

Good evening friends-

Here is my evening update on potential Tropical Storm Barry, which will likely form in the northeastern Gulf of Mexico tomorrow or Thursday.

SUMMARY

Tropical Storm Barry will likely form in the northeastern Gulf of Mexico on Wednesday or Thursday and track west across the northern Gulf Coast. Until a center of circulation forms, tracking and intensity forecasts are quite uncertain, although it appears that flooding from heavy rain, and possibly storm surge, are possible from Louisiana through the Florida Panhandle. Damaging winds are most likely to occur in Louisiana, however, people from Galveston through Pensacola should check back often as forecasts may shift considerably in the next 24 hours.

IN DETAIL

Tropical Storm Barry will likely form in the northeastern Gulf of Mexico on Wednesday. In the latest advisory, the National Hurricane Center forecasts a 70% chance that Barry will form in the next 48 hours. Until a center of circulation forms, considerable uncertainty exists regarding Barry's track and intensity forecast. It does appear that Barry would track west, across the northern Gulf Coast, with a landfall in Louisiana most likely at this time.

Both the European and American models have shifted EAST since yesterday. However, if the center of circulation sets up farther south than anticipated, Barry would likely make landfall farther west....possibly in Texas.

I think much of this uncertainty will be resolved in the next 24 hours. Hurricanes and tropical storms throw four hazards at us...1) Storm surge (saltwater flooding), which is the most deadly hazard; 2) Heavy rainfall; 3) Strong sustained winds; 4) Sometimes tornadoes.

Here's a look at how these hazards may stack up for Barry:

1) Storm Surge:

Storm surge, or salt water flooding, is the greatest threat where we find prolonged onshore winds, particularly in areas with shallow offshore water and where water can be "trapped", such as bays and along deltas.

The area of southeast Louisiana, east of the Mississippi River, and the entire coast of Mississippi, is one of the locations in the world where storm surge is generated most efficiently. This happens for three reasons:

A) The offshore water depth (bathymetry) is very shallow;

B) The Mississippi River Delta creates a "right angle" along the coast, thereby trapping water;

C) When hurricanes and tropical storms are centered in the central or north central Gulf of Mexico, the circulation pattern produces a prolonged east or southeast wind here, which enables water to pile up for a long time. Storm surge can pile up surprisingly well in this region, even if the center of the storm tracks well to the west. For this reason, people along coastal Mississippi and Southeast Louisiana, particularly east of the Mississippi River, should anticipate saltwater flooding....the duration and height will become more clear over the next 48 hours.

Places in South Central Louisiana, like Grand Isle and Morgan City, usually do not experience onshore winds as prolonged. For that reason, storm surge can actually be higher in places like Shell Beach or Biloxi than in central Louisiana, even if a storm makes landfall in central Louisiana.

That said, the entire Louisiana coast is vulnerable to storm surge, and places like Cocodrie, Grand Isle and the marshes south of Houma could definitely see some salt water flooding this weekend.

Incidentally, I put Lake Pontchartrain in with the more vulnerable area east of the Mississippi River, and a prolonged east/ southeast wind could efficiently pile up some salt water in the Lake with Barry.

2) Heavy Rainfall:

Regardless of Barry's exact track, plenty of moisture will be available for torrential rain. I posted the QPF rainfall potential map for the next 7 days. These maps are usually conservative in my perspective.

The map shows 5+ inches of rain across much of Louisiana...but keep in mind, this pattern could shift to the west or east depending on the exact track. I would not be surprised to see some areas exceed 10 inches of rain.

3) Strong Sustained Winds:

This depends on Barry's intensification, which is still quite uncertain. At this time it appears most likely that Barry would produce moderate to strong Tropical Storm force winds (sustained 50-73 mph) in Louisiana, but the area of impact and the intensity of winds could change.

If Barry tracks farther south it would have more time over warm water, so if atmospheric conditions were favorable for development, a more westward track could mean a stronger storm. We should know more tomorrow.

Even if Barry's winds do not reach hurricane force (74 mph+ sustained), tropical storm force winds can still cause considerable damage to buildings (like remove shingles and rip off siding), bring down a lot of trees, and cause long-term power outages.

Barry may generate the strongest winds from a tropical system in central Louisiana since Hurricane Gustav (2008).

4) Tornadoes:

Tornadoes are usually isolated in tropical systems and hard to predict the path even when we are much closer in time to landfall. They are often found well to the "right" of the storm track, and can occur at surprising distances from the eye. Insights into this hazard are usually uncertain until the day before landfall.

Stay tuned to your favorite sources of weather information frequently over the next 24-48 hours. I think we will have greater certainty in Barry's track and intensity forecast by tomorrow (Wednesday) evening.

- Hurricane Hal

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Dr. Hal Needham is a Flood Scientist at CNC Catastrophe & National Claims.

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