Potential Tropical Cyclone #2

NHC this morning initiated advisories on Potential Tropical Cyclone #2 (PTC #2). As of the 10 p.m. CDT advisory, this area of low pressure is not yet a tropical depression, though NHC notes that it is quite close. Indeed, the minimum pressure is now down to 1,009 mb, and winds have ticked up slightly to 30 mph.

Nighttime "GeoColor" GOES-16 satellite image of Potential Tropical Cyclone #2  (10:06 p.m. CDT 7/10)

As for the forecast, there remains a bit of uncertainty, stemming largely from the fact that we do not yet have a well-defined low-level center of circulation. Nevertheless, the majority of model guidance brings PTC #2 towards the Louisiana coast by Saturday afternoon. In their 10 p.m. discussion, NHC noted that their forecast lies on the west side of the guidance envelope, so it is possible we see an eastward shift in the forecast with time, but one should not count on that, as there has been considerable run-to-run discrepancy with the models (i.e. each run of the same model has looked, sometimes markedly, different than the previous run).

7:00  p.m. CDT 7/10 forecast models. The black line labeled "OFCL" represents the official NHC forecast

As for the intensity, this is one place there has been a decent amount of model consistency, with the consensus generally being that PTC #2 will likely approach the Louisiana coast as a category one (74-85 mph) or perhaps low-end category two (86-110 mph) hurricane. Note that intensity forecasting is notoriously difficult however, and that folks in Louisiana should prepare for one category higher than is currently forecast.

The category/intensity only takes into account wind however. Perhaps the biggest concern with this tropical cyclone will be the rainfall potential. PTC #2 is forecast to parallel the Louisiana coast for a few days before turning north. Because of this, some forecast models are showing some rather prolific rainfall totals. Much of South and Central Louisiana, especially near and to the east of where the center passes, is likely to see at least ten inches of rain by the end of the weekend, with some places probably seeing more than twenty inches of rain.

NWS forecast rainfall from PTC #2

As eye-popping as these rainfall amounts may seem, it's all the more worse considering this is coming on the heels of a wet spring, with many rivers and streams in Louisiana still running high. Perhaps most notably, the Mississippi River in New Orleans is already just four feet below the top of its levees. The Lower Mississippi River Forecast Center's official forecast for the Mississippi River at New Orleans shows the river cresting at 20 feet on Saturday afternoon, which is basically the top of the levee system. This forecast considers the effects of both storm surge and expected rainfall. Dr. Jeff Masters at Weather Underground has an excellent piece on the river forecast and what it means for New Orleans.

Forecast river stage for the Mississippi River at New Orleans (issued 7 p.m. 7/10)

Here are the key takeaways:

• Don't focus on the exact track of the center at this time. Even for a well-defined, mature tropical cyclone, track errors at this stage can be off by more than 100 miles.
• Anyone along the Louisiana coast needs to prepare for the possibility of life-threatening, catastrophic storm surge.
• Anyone in South and Central Louisiana, particularly locations like Lafayette, Baton Rouge,  and New Orleans should prepare for very heavy rainfall, and the high likelihood of significant inland flooding.
• Hurricane force winds will be possible near and to the east of where the center eventually tracks. It is hard to say where that will be for sure right now, but it's hard to rule out anyone between Lake Charles and Baton Rouge right now for potentially seeing hurricane force winds.
• Hurricane Watches are in effect for virtually all of the Louisiana coastline, and for most parishes along (or just inland from) the coast. NOW is the time to put your plans into action.
• Impacts are likely to begin in far southeast Louisiana as early as Thursday night, and spread west through Friday. The worst conditions are expected on Saturday. Conditions will gradually improve through Sunday.