(Image source: NOAA)
In the southeast this year, rain follows rain follows rain. Now a tropical system may be preparing to add its own moisture to the already very wet mix.
The river of upper level airflow called the Jet Stream begins an almost due south movement at the Arctic Circle near the Northwest Territory and Canadian Arctic Archipelago. The atmospheric river dives down over Central Canada and into the Great Lakes region. Continuing ever southward, it finally encounters a wall of warmer air setting up near the south Tennessee border where it speeds up and turns eastward, joining another Jet Stream flow coming in over the Rockies.
Riding along these convergent Arctic and Pacific Jet Stream flows are a number of wet and stormy impulses. Because this deep north to south dip has been in place over the Eastern and Central US for much of the summer, storm after storm has continued to impact a large region from Missouri to New Jersey southward through the Carolinas and then again southwestward along through Georgia and the Gulf Coast states. This persistent stormy pattern, a result of a slow, wavy, stuck Jet Stream, which is, in turn, caused by sea ice and snowfall loss in the Arctic, has pushed rainfall totals 20 inches or more above the yearly normal in some locations. It has delivered extraordinarily moist storms that dump inches of rainfall in just a few hours. And it resulted in some regions of Missouri receiving four months worth of rainfall in just one week.
In the above water vapor image, provided by NOAA, you can see the storms riding along the south frontal boundary of this Jet Stream trough through the Dakotas, Montana, Nebraska and Colorado then down into Texas, turning eastward through the Gulf Coast States and then surging north along the US East Coast.
Yesterday, this pattern delivered a powerful storm complex to Pennsylvania and New Jersey — flooding roads, knocking out power and dumping as much as six inches of rain. Now, the Jet Stream has driven further south, pushing the frontal boundary over the Gulf Coast and US Southeast.
(Image source: California Regional Weather Server)
But as if the convergent flows of the Jet Stream delivering very powerful storms, as if the stuck weather pattern keeping this storm-delivering trough over the Eastern US for months, and as if the human-added atmospheric heat amping up the hydrological cycle and spurring more intense rain events were all not enough, we now have a large, moist tropical weather system moving in and threatening to become entangled in an already very wet pattern.
As of this morning, the National Hurricane Center had issued an advisory for a tropical system in the western Caribbean predicting a 60% likelihood of tropical storm formation over the next several days. The system is a large, sprawling and disorganized mass of thunderstorms and moisture moving toward the north-west at 10 to 15 mph. A circulation appears to be trying to form in a region near the densest thunderstorms. The system is predicted to move toward the northwest until it is eventually captured by the frontal boundary at the leading edge of our deeply sagging Jet Stream. Should this mix-up occur, a tropical weather system and potential tropical cyclone will have again combined with an Arctic originating air-mass over the Eastern US, setting the stage for a rather intense and widespread rainfall event.
If there are some reading this analysis and thinking that it rhymes somewhat with set up for Superstorm Sandy, they wouldn’t be entirely off the mark. Until recently, it was less likely that tropical systems would combine with polar originating air masses over the US. The troughs originated and faded rapidly, only infrequently coming into dramatic collisions. But now, with the Jet Stream increasingly settling into a stuck pattern (spurred by human caused warming and sea ice loss) and seeming to favor trough development over the continental US and not over the ocean, such collisions are far more likely.
With the current system, unlike Sandy, rain appears to be the primary concern for now — not storm hybridization, expansion, and superstorm development. These are not on the radar. But we are still very early in storm development and we don’t yet know how powerful the slowly organizing tropical system will become. What we have right now is a large, and potentially strengthening package of tropical moisture setting its sights on an already soaked US southeast.
(Image source: The National Hurricane Center)
So the risks at the moment are for a potential major flooding event on the five to seven day horizon as the tropical system continues to track northwestward until it is pulled into the stormy flow of the Jet Stream trough. At that point, it is predicted to dump a heavy load of moisture and rain over the US southeast.
Current predictions from NOAA reflect uncertainty in storm development and track and, at the moment, call for 3-5 inches of rainfall from the Gulf Coast States through Georgia and the Carolinas over the next 5-7 days. Local amounts, however, could be much higher, on the order of 8 inches or more. Such an event would intensify an already severe flood problem over this large area, likely resulting in major and widespread flooding.
(Image source: NOAA)
Any significant wind field of 40 mph or more would also likely result in a mass of fallen trees. The ground in this region is saturated with moisture, making it harder for tree root systems to grip the soil. So it takes much less wind to blow them down. Widespread power outages due to trees falling over power lines is, therefore, also a potential threat for this system.
Given the current position of the Jet Stream and uncertainty over potential storm strength and track, this situation could rapidly develop into a dangerous event for the southeastern US or we could end up with a storm system making landfall closer to Texas and Mexico. So we’ll be closely watching storm strength and path over the coming week.
Tropical weather system 92 L is starting to exhibit some cyclonic turning: