Monday, June 15, 2015

Even the tropics are aiming the faucet for the central US

After the eastern Pacific got off to an early, rapid start with its tropical storm season (which isn't unexpected in an El Nino year), we're looking at our first tropical storm developing in the western Gulf of Mexico in the next 12 hours.  On satellite, this storm has been getting its act together:
And the National Hurricane Center thinks we'll likely see Tropical Storm Bill by tonight.  This storm is forecast to run out of ocean really fast, though, and hit the Gulf coast of Texas tonight and tomorrow.   From Weather Underground, here are the recent 18Z GFS ensemble forecasts for the track of this cyclone over the next several days.
You can see that there's pretty high agreement that the storm will cross central Texas before curving north through Oklahoma then back northeastward across Missouri and Illinois and into the midwest and mid-Atlantic states.  Of course, almost as soon as this storm hits land in Texas it will stop being a tropical cyclone and turn into an extratropical cyclone.  Though this means weaker winds, the storm will still bring a lot of moisture into the heart of the country.   Here's an animation of the satellite-derived total precipitable water in the atmosphere over the last five days.  You can see towards the end of the animation that the developing storm in the western Gulf of Mexico (with its characteristic spiraling motion) is bringing in lots of moisture towards the Texas coast.
That moisture will move inland over the next several days.  Here's the 18Z NAM forecast precipitable water for tomorrow afternoon (from HOOT):
 Lots of moisture over Texas and Oklahoma, with residual moisture from the current frontal zone  across the Ohio River Valley.  Looking ahead to Wednesday afternoon there is still a lot of moisture forecast over eastern Texas and Oklahoma.
And even by Thursday afternoon, east Texas is still looking at a ton of moisture, though we're also seeing a lot of liquid move north into Arkansas and Missouri.
Not unsurprisingly, the Weather Prediction Center (NOAA/WPC) is forecasting some intense rainfall amounts over the next three days, with a swath over 6 inches from Texas into Oklahoma and western Arkansas, and over 3 inches from Missouri into central and southern Illinois.
It's interesting to note that, despite this storm being a tropical cyclone, its ability to influence the larger-scale synoptic pattern is somewhat limited.  Notice how the storm path follows a nice, sweeping curve around the southeastern United States.  Why is that?  Because we have a relatively modest ridge of high pressure/high heights sitting over the southeastern US, as seen in this 24-hour 500mb height forecast from the NAM:
Though that ridge looks so weak (it only has a single height contour!), there's actually still more potential energy sitting in that ridge than kinetic energy in a tropical cyclone.  So, even though it seems to be a strong storm to us, our tropical storm is still a slave to the synoptic pattern, and that "weak" ridge over the southeast will "shield" that area from most of the effects of this storm.

Unfortunately, it's that same ridge that is directing the storm over areas that really don't need any more rain right now.  Here's a map of the percentage of normal precipitation during this May.  Much of Oklahoma and Texas received 200-500% of their normal precipitation for May.
Soils in that area are also saturated.  Here's a map of soil moisture percentiles as of the end of May.  Much of eastern Texas and southern Oklahoma is in the 99th percentile for soil moisture...that is, soils likely contain nearly record amounts of moisture for this time of year.
Add on top of this the fact that several major rivers throughout that corridor (and also into Missouri and Illinois) are currently at flood stage from all the rain we've already gotten recently (the orange and red dots on this map)...
And you have a recipe for a flooding disaster across a large swath of the central US.  Flood watches are already up for most of eastern Texas and eastern Oklahoma, with hydrologic outlooks suggesting at future flood watches going up throughout Arkansas, Missouri, Illinois and Indiana as well.  If you live in these areas, you need to be paying attention to the weather this week and be alert for possible flooding.  This is going to be the weather story of the week.