Tropical storm warnings issued as Cristobal churns toward U.S. Gulf Coast, bringing widespread flood threat

Tropical storm and storm surge warnings were issued Friday afternoon in anticipation of the system’s arrival. Tropical storm warnings span from the northern Gulf Coast east of Morgan City, La., to the Alabama-Florida border. This includes New Orleans, That’s where the greatest odds of encountering tropical storm conditions, with sustained winds of greater than 39 mph, exist beginning on Saturday evening.

A storm surge warning is in effect for areas from the mouth of the Mississippi River, outside of the Hurricane and Storm Damage Risk Reduction System, to Ocean Springs Mississippi. The risk reduction system was put in place to help protect New Orleans from surge-related flooding after Hurricane Katrina devastated the city in 2005.

Atlantic hurricane season forecast to be unusually active

A Storm Surge Warning means there is a threat of life-threatening inundation, from rising water moving inland from the coastline, during the next 36 hours.

Storm surge flooding will be an especially significant issue for areas east of the storm’s center. A storm surge watch is posted for a large real estate, from east of Morgan City, La., to the mouth of the Mississippi River, as well as in Florida between Indian Pass and Arepika. This includes Tampa, Fort Myers and Pensacola, Fla., and Mobile, Ala.

Despite relatively weak winds and a ragged structure, prolific rainfall and flooding have been a staple of Cristobal’s onslaught on Central America. Rain totals up to 35 inches were expected in Mexico, particularly in Chiapas, while more than two dozen were killed in El Salvador by flash flooding and devastating mudslides.

Tropical Storm Cristobal — the earliest third named storm on record in the Atlantic this year — is reemerging over the anomalously warm waters of the Gulf of Mexico, an elixir of life for the once-tepid system.

Even before reentering the Gulf, the National Hurricane Center upgraded Cristobal from a depression to a tropical storm at 2 p.m. eastern Friday. Its proximity to water allowed it to become better organized, and at 5 p.m. its peak winds stood at 40 mph. The Hurricane Center called for some additional strengthening over the next two days. Tropical storm force winds extended 240 miles from the storm’s center, which indicates that T.S. Cristobal will bring significant impacts to areas far from the precise location where it ultimately makes landfall.

Heavy rain and significant flood threat

Flash flood watches have already been hoisted for much of Louisiana, including eastern Acadiana, the Florida Parishes, Baton Rouge and Greater New Orleans. Those areas stand the greatest risk of seeing excessive rainfall as Cristobal’s tropical downpours arrive beginning late Saturday into Sunday, with rainfall rates of two to four inches per hour possible in the heaviest bands. Storm totals of three to six inches are likely, with some eight-inch amounts.

The Hurricane Center is forecasting rainfall totals through Wednesday morning of four to eight inches, with local amounts of a foot, for parts of the eastern and central Gulf Coast and lower Mississippi Valley.

“Isolated significant river flooding is possible along the central Gulf Coast,” the NHC states. Farther north, across the mid-Mississippi Valley, rainfall totals look to be lower, on the order of two to four inches, with locally higher amounts. “Rises along smaller-order streams are possible across the Mid-Mississippi Valley,” NHC states. “This degree of rainfall is expected to lead to flash flooding.”

Moderate to heavy rainfall could extend into coastal Alabama and the Florida Panhandle as well. Cristobal is unlikely to be a particularly formidable system from a wind perspective, but its wind field will be broad, which means it will impact a greater area.

Because of this broad structure, the Hurricane Center is emphasizing the importance of not focusing solely on where the storm’s exact center tracks, as “the strongest winds, highest storm surge, and heaviest rains could be well removed from the center of circulation.” There is a growing probability of significant rainfall making it as far north as the Great Lakes or Upper Midwest.

Much of the rain would be beneficial to a drought-stricken Florida. The Sunshine State saw its driest March on record, and despite recent extreme rainfall in the Miami area, the Big Bend and Panhandle are still dealing with dry conditions.

Wind and surge

The National Hurricane Center is forecasting Cristobal to maintain tropical storm strength as it makes landfall in the United States on Sunday night, with maximum sustained winds of 60 mph. However, forecasts of storm intensity are not as reliable as those of storm track, particularly at this time, so it’s possible it could be more intense or even slightly weaker.

“There is a risk of tropical storm force winds this weekend from Louisiana to the western Florida Panhandle and a risk of dangerous storm surge from Louisiana to the Florida Big Bend,” the NHC wrote.

However, a wide wind field can impact storm surge prospects. The storm surge is the storm-driven rise in water above normally dry land.

The combination of a dangerous storm surge and tides will cause water levels to potentially reach the following heights above ground if the peak surge occurs at the time of high tide:

Mouth of the Mississippi River to Ocean Springs MS including Lake Borgne...3-5 ft Ocean Springs MS to Indian Pass FL including Mobile Bay and Pensacola Bay...1-3 ft Indian Pass FL to Aripeka FL...2-4 ft Aripeka FL to Marco Island FL including Tampa Bay...1-3 ft Morgan City LA to the Mouth of the Mississippi River...2-4 ft

• Mouth of the Mississippi River to Ocean Springs, Miss., including Lake Borgne: 3-5 feet.
• Aripeka to Marco Island including Tampa Bay: 1-3 feet
• Indian Pass to Aripeka: 2-4 feet
• Ocean Springs to Indian Pass including Mobile Bay and Pensacola Bay: 1-3 feet
• Morgan City to the mouth of the Mississippi River: 2-4 feet

Surge may be most pronounced within about 100 miles east of Cristobal’s center, though elevated water levels are likely all the way down the Florida peninsula beginning early Sunday and lasting several tide cycles through Monday.

Historically, it is water, not wind, that is the greatest killer in tropical cyclones’ arsenal, including both storm surge flooding and heavy inland rains.

Severe weather risks

Because Cristobal’s structure will likely be more irregular — a comma-shaped storm rather than one resembling a doughnut-like spiral of rain and wind — the bulk of the greatest impacts from severe weather associated with thunderstorms will be to the east of the center. That’s where a few feeder bands of downpours could arc outward from Cristobal’s center.

Due to the presence of wind shear — or a change of wind speed and direction with height — generated by Cristobal’s landfall, a few of the downpours embedded within those bands may take on some rotation. That could result in a few tornadoes or waterspouts, especially from the Nature Coast of Florida westward.