A tropical disturbance in the Gulf of Mexico, with the potential to become the hurricane season’s second named storm, will bring a significant amount of tropical moisture and rainfall to the southern states this week — mostly to areas that are still saturated from the past couple months of above-average rainfall.
Beginning tonight and extending through Wednesday, eastern Texas and eastern Oklahoma could see widespread rainfall totals of five to seven inches, with isolated locations getting 10 or more. Needless to say, this could turn out to be a very dangerous situation for flash flooding in these areas that were inundated by excessive rainfall events time and again in May.
Flood and flash flood watches have already been issued as the tropical disturbance, which could be named Bill by the National Hurricane Center some time on Monday, tracks north toward Texas. The National Weather Service in Houston has issued a special weather statement warning residents in southeast Texas of a dangerous flood threat over the next few days.
To put that forecast into perspective, consider these maps that highlight just how wet it has been there already this spring. The first one shows the rainfall anomaly as a percent of normal (if a location received 10 inches in the past 60 days and the average over the same period is five inches, that would be 200 percent of average).
The second one shows the monthly soil moisture anomaly, and it is easy to spot which areas are not hoping for more heavy rain.
As the disturbance tracks north through the eastern U.S., the heavy precipitation plume will include Missouri and Illinois, east into New Jersey and New York. The Washington, D.C., area will likely see some of the tropical moisture late this week and into the weekend.
The tropical disturbance in the Gulf has been very slow to organize into a fully-formed tropical cyclone, though the lack of an impressive appearance on satellite does not diminish the flooding threat. Two hurricane hunter missions are planned for today, and based on the appearance and the overall strengthening trend, it seems likely that this disturbance will be declared Tropical Storm Bill sometime on Monday.
The exact track of the system does not matter too much in terms of impacts — the “blob” will continue to move toward the northwest, heavy rain and thunderstorms will reach the coast later today, and the center should cross the coastline overnight.
Something worth pointing out is how well this system was forecast by the leading global models, the American GFS and European ECMWF. The plot shows cyclone forecast tracks made back on the evening of June 4, and extending out 10 days. The GFS model absolutely nailed it!
In terms of climatology, June is actually not an uncommon month for tropical cyclone landfalls on and near the Texas coast. Since 1851, there have been 26 landfalls, from tropical depressions up to a category 4 hurricane.
The most noteworthy comparison is probably Tropical Storm Allison from 2001, which was the only time a name was retired from the list for “just” a tropical storm, after it caused widespread flooding in southeast Texas and Houston in particular, killing over 40 people and doing $9 billion in damage. Allison remains the deadliest and costliest tropical storm to make U.S. landfall.
You can also see from the historical track map below that June favors storms developing in the southern Gulf of Mexico.
As far as the northeast U.S. goes, the rainfall associated with this system will stream into Pennsylvania, New Jersey, New York and the D.C. area starting late-week and likely lasting through the weekend. It is too soon to think about exact amounts, but just be aware that the potential exists for heavy rainfall in that period.
The last time the Atlantic reached the “B” storm on or before June 15 was 2012, when Subtopical Storm Beryl formed on May 26.