Above: Enhanced infrared image of Tropical Storm Cristobal at 1836Z (2:36 pm EDT) Saturday, June 6, 2020. (NASA/MSFC Earth Science Branch)
Tropical Storm Cristobal is slowly gaining steam as it cruises northward across the Gulf of Mexico. On its expected course, Cristobal will make landfall Sunday afternoon or evening, most likely over or near southeast Louisiana. As it approaches, Cristobal will pull heavy rain, high surf, and storm surge well to the east of its center across the Gulf Coasts of Mississippi, Alabama and Florida.
As of 2 pm EDT, Cristobal was centered about 310 miles south of the mouth of the Mississippi River, moving north at 12 mph. Cristobal’s top sustained winds were 50 mph. A tropical storm warning was in effect from Intracoastal City, Louisiana, to the Okaloosa/Walton county line in the Florida Panhandle, including lakes Ponchartrain and Maurepas. A storm surge warning extended from the mouth of the Mississippi River in southeast Louisiana east to Ocean Springs, Mississippi, including Lake Borgne.
Mandatory evacuations were in effect for Grande Isle and lower Lafourche Parish, Louisiana, with voluntary evacuations increasing elsewhere in Louisiana, according to NOLA.com. See the weather.com article for more on Cristobal preparation and impacts.
Cristobal is a large, well-organized tropical storm. Yet as expected—and in a mode very typical of early-season tropical storms in the Gulf—Cristobal is also a highly lopsided system, with most of the active weather on the storm’s east (right-hand) side. Rainbands were already pushing onto Florida’s west coast by midday Saturday, and these will spread north and west on Saturday night into Sunday. The heaviest totals will likely end up just east of Cristobal’s center, somewhere between New Orleans and Mobile, where 6” – 10” totals are predicted and isolated amounts could range higher.
Satellite imagery on Saturday revealed the impact of widespread dry air to the west of Cristobal working its way into the system. Showers and thunderstorms (convection) wrapped around Cristobal’s center in a patchy fashion (see image at top of post), allowing the low-level center to show up clearly on visible satellite. By far the strongest convection was located in a large band moving through the eastern Gulf well away from Cristobal’s core.
The large zone of strong south winds east of Cristobal was already pushing up tides across the Northeast Gulf. Storm surge observed atop the midday Saturday high tide ranged from 1 to 2 feet from Louisiana to the west coast of Florida. This prolonged high-water event will increase into Sunday. Because this is a weaker long-duration event, rather than a sudden, intense spike, the highest inundations can be expected in many areas during the Sunday high tide, perhaps extending into Sunday evening. Tides may remain elevated on Monday and perhaps even Tuesday, as southerly flow continues streaming onshore toward Cristobal (and eventually its remnants).
In a blog post on Saturday, storm surge expert Hal Needham explained the many variables that dictate how expected inundation levels can mesh with site-specific details. For example, he notes, local estimates of elevation relative to sea level are often averaged over 19-year tidal cycles, but if that average is decades old (e.g., 1983–2001), then it won’t reflect sea level rise and/or local subsidence since then. In recent field work in coordination with Dauphin Island Sea Lab, Needham found that high tides on Dauphin Island are running about 21 inches above what one would expect from the Northern American Vertical Datum of 1988, abbreviated NAVD88.
“We can think of this as though the water level already has a 21-inch head start on flooding the land before a storm even arrives,” Needham said. “And that doesn’t even include the contribution to waves that can push additional water on top of the storm surge. These observations are important, and surprising, to many coastal homeowners, who assume the house level on their elevation certificate is the height above sea level. In many parts of the coast, high tide levels may be 1-3 feet higher than the NAVD88 datum, meaning the floor boards of people’s homes are 1-3 feet closer to the water level than they thought.”
Forecast for Cristobal
Cristobal’s best shot at any strengthening will be Saturday night, when wind shear will remain on the light side (5-10 knots) and the storm will be passing over sea surface temperatures of 27-28°C (81-82°F). Convection is trying to wrap around Cristobal’s center, but it may be too far a bridge for the storm to establish a strong inner core before it nears the coast. Cristobal’s large size, its lopsided structure, and infiltrations of dry air should keep any intensification modest, and Cristobal is expected to arrive on the Gulf Coast as a strong tropical storm, most likely making landfall in southeast Louisiana.
A few tornadoes will be possible on Sunday afternoon and evening as Cristobal pushes ashore, especially toward the north and east of Cristobal’s center across southeast Louisiana and southern Mississippi, as flagged in Saturday’s Day 2 outlook from the NOAA/NWS Storm Prediction Center.
Arriving on the heels of marginal Tropical Storm Bertha, which came ashore in South Carolina on May 27, Cristobal will give the United States its second named-storm landfall of the year—an impressive mark to achieve in early June!
Cristobal’s future after landfall
As it moves inland, Cristobal will be pulled northward at an increasing clip by a strong midlatitude storm moving across the Rockies early next week. Rainfall will be fueled by an extremely moist atmosphere near and just east of Cristobal’s center (see graphic above). A corridor of widespread 4” – 6” rains can be expected northwest across parts Louisiana, Mississippi, and Arkansas from Sunday night into Monday, with 2” – 4” possible further north into Missouri, Iowa, and/or Illinois from Monday into Tuesday.
Given that Cristobal will be moving at a good clip, inland flood risk will be tempered, although some areas of heightened concern may be flagged if Cristobal’s heaviest rains happen to coincide with areas of high soil moisture from western Arkansas northward.
Cristobal could maintain its strength even as it zips up the Mississippi Valley toward the Great Lakes while losing its tropical characteristics along the way. Both the GFS and European operational models project Cristobal to be centered somewhere close to Iowa as a post-tropical cyclone by Tuesday, with a central pressure on par with that expected around landfall. Eventually Cristobal will merge with the approaching front and continue into Canada as a strong midlatitude storm.