Tropical Storm Bill may bring near-record rainfall, flooding, & strong winds to DFW

This post is a carryover from the previous post that contained three days worth of forecast updates leading up to the formation of Tropical Storm Bill.  In an effort to improve readability, I thought it would be best to create a new post given that T. S. Bill will soon make landfall and subsequently move north into North Texas with significant impacts.

➡ Final Event Recap: As predicted, peak wind gusts of 50 mph and greater, and record rainfall hit North Texas. Read more.

Forecast Discussion Posted Tuesday 6/16 at 9:40 am

***Tropical Storm Bill Will Soon Make Landfall***

Here are a few noteworthy concerns that I have at this time:

● Atmospheric precipitable water content > 99th percentile for June
● 3 to 5 inches of rain (locally 8 to 10)
● Sustained winds 25 to 35 mph late tonight/Wednesday, gusts to 50 mph
● Some uprooted trees due to the saturated soil
● Power outages and flash flooding
● Near-record rainfall amounts for the month of June
● Embedded mesocyclones and isolated/brief tornadoes

Synopsis and Forecast

Tropical Storm Bill will make landfall within the next couple of hours and we will begin to see bands of heavy rain move northward later this afternoon. There are already very narrow outer bands of low-level convergence that are moving across North Texas, but they are just not generating very much rain as of yet; they will need daytime heating/destabilization to get things going. This afternoon, I believe that we will see bands of showers and thunderstorms develop within these narrow convergence lines (far too narrow to be detected by our observation network).

I still expect Tropical Storm Bill to track directly over North Texas, with the center of circulation passing over or just west of the metroplex. This has the potential to be a storm system unlike any we have seen in North Texas in a very long time. I have A LOT of analyses to do this morning, as a brand new model suite is trickling in as we speak. Please pay close attention to developments and any new information that I post throughout the day. I stayed up all night analyzing model simulations and historical data. Below, I will outline my immediate concerns.

1) Flash Flooding: This morning’s 7 am upper-air sounding from Fort Worth revealed that atmospheric precipitable water content now exceeds the 99th percentile for this time of year based on a 65-year PWAT climatology. If you’ve learned anything from the heavy rain events back in May, my forecast updates, and my forecasting methods/techniques, then you probably now realize the significance of that particular statistic. In a “normal” year, I would be concerned if we exceeded the 90th percentile; but the 99th percentile is extraordinary. By definition, 99% of all PWAT values in the historical record are less than the atmosphere’s moisture content today. Unfortunately, it doesn’t end there.

As T. S. Bill moves inland,  PWAT will continue to increase throughout the day today and through the night. Model-simulated thermodynamic profiles indicate that by tonight, PWAT will increase to near-record levels for the month of June. This morning’s sounding revealed PWAT at 2.01″. Model guidance suggests that PWAT will increase to between 2.45″ and 2.55″ tonight and I have some simulations that place PWAT above 2.60″ just before sunrise tomorrow. To put these numbers into a historical perspective for the month of June, the top-5 values in the 65-year climatology for DFW are: 2.26″, 2.30″, 2.31″, 2.32″, and 2.55″. Our atmospheric moisture content should easily end up in the the top 5 for June, if not the top 2. We’ve already seen that consequences of having this much atmospheric moisture in the presence of convective instability and strong forcing: rainfall rates that exceed 2 inches per hour. With a tropical system such as this, I expect rainfall rates within the more intense convective bands to be in the range of 2 to 4 inches per hour late tonight and tomorrow.

Although I’m forecasting total rainfall amounts to average 3 to 5 inches by the time this event ends, I wouldn’t be surprised to see localized amounts as high as 10 inches. Typically, the heaviest rain associated with weakening tropical storms post-landfall is located to right of the storm’s path (i.e., to the right of the storms center of circulation); in the case of a storm moving toward the north, the areas to the east of the storm’s circulation would receive the heaviest rainfall. If you experienced flooding in May or live in a flood-prone area, your planning/preparations should be rushed to completion. My predicted rainfall amounts would rival June records for both 1-day and 2-day totals, and may also break tomorrow’s daily record.

My Predicted Rainfall Amounts for Tuesday (6/16) through Wednesday (6/17)

Prepared Monday 6/15 11:20 am CDT: Our predicted rainfall amounts through noon Wednesday (6/17).

Prepared Monday 6/15 11:20 am CDT: My predicted rainfall amounts through noon Wednesday (6/17). There are no changes to my forecast reasoning and expected rainfall amounts, thus the graphic remains unchanged as of Tuesday 6/16 at 9:30 am.

2) Strong Winds: After Tropical Storm Bill makes landfall, weakening will ensue. This weakening is well-simulated by the numerical models that I have analyzed. However, despite the weakening wind speeds, friction causes the wind field to expand in size, with gusty winds spreading out over a much larger area relative to the more compact wind field that it had over the open ocean. Its interesting to note that at least one of the model simulations is showing some intensification just south of Waco late this afternoon, following a period of weakening, but whether or not that occurs remains to be seen. As the center of circulation approaches the metroplex late tonight through tomorrow, I expect sustained winds to increase into the range of 25 to 35 mph, with gusts to 50 mph possible. Winds this strong for a prolonged period can cause power outages. Additionally, trees are easily uprooted when the ground is super-saturated like it is right now. Therefore, I do believe that some trees will be uprooted in North Texas tonight.

3) Mesocyclones/Very Brief/Isolated Tornadoes: Tropical storms are notorious for causing embedded mesocyclones (rotation) and brief/weak tornadoes. I will not go into the physical processes that cause this to happen; but just like placement of the heaviest rainfall relative to the storm’s center/path, the favored area for mesocyclones/brief tornadoes is also to the right of the storm’s path (in this case, the eastern semicircle as Bill moves northward across the area). It would not surprise me at all to see a Tornado Watch issued for parts of North Texas tonight (especially the eastern half).

Summary of Threats:

  • Flash flooding and areal/river/lake flooding
  • Strong winds tonight, with gusts as high as 50 mph
  • Embedded mesocyclones and isolated tornadoes
  • Uprooted trees from weakened root systems in the super-saturated soil
  • Power outages

Timing:

  • Narrow bands of showers/thunderstorms will develop within pre-existing convergence zones spiraling northward across the area as the atmosphere destabilizes with daytime heating.
  • The main area of heavy rain will overspread North Texas late tonight, lasting through (at least) tomorrow morning and possibly into the afternoon.

Flash Flood Watch Remains in Effect Through Thursday 6/18

Posted Monday 6/15 3:30 pm: Flash Flood Watch in effect until 7 pm Thursday 6/18.

Posted Monday 6/15 3:30 pm: Flash Flood Watch in effect until 7 pm Thursday 6/18.

I think this discussion is long enough for now, but I will be going over brand new data throughout the day. I have been as specific as I can be, so I won’t be able answer questions about specifics on a street or city level. As always, you can use the tools that I provide on my website to track the storm on radar.