A slow-moving cold front is forecast to bring heavy rainfall to much of North Texas beginning Friday and into the weekend. WPC is forecasting widespread totals of 2-3 inches of rain across most of North Texas over the next 72 hours, with locally higher amounts in excess of 6 inches possible in some locations. Given the recent heavy rainfall, flash flooding will certainly be possible. This heavy rain is expected to begin over parts of the Big Country and up into the Panhandle this evening, and spread east through the overnight hours.
Rather than simply stating the forecast however, I am going to use this blog to discuss what some of the ingredients are for heavy rainfall, and how this forecast comes to be. After all, this is supposed to be an educational blog.
Ingredient #1: Lift
Some of the necessary ingredients for heavy precipitation overlap with the ingredients for severe weather (discussed a bit here), with lift being one of them. In fact, lift is required for basically any type of weather that involves precipitation, so while it may seem obvious, it’s still important to discuss. “Lift” is a general term used to describe the process of imaginary parcels of air being lifted high enough so that they may cool sufficiently, causing any water vapor contained therein to condense into clouds and precipitation. In this particular event, lift is coming courtesy a surface cold front. This cold front will cause warm air to rise up and over the encroaching cold air mass, causing moisture within the “warm” air mass to condense as it rises and cools.
Where the front stalls will be crucial on Friday and over the weekend, as the strongest lift will be near the front, and thus will be the location of the heaviest rainfall. But it’s not just about what is going on at the surface. We must also look aloft. The 500 mb analysis from this evening reveals a trough moving off of the Rocky Mountains and into the Great Plains. There’s a lot to discuss about how troughs cause lift, but for now, just know that upper-level troughs are another big source of lift.
Ingredient #2: Moisture
Another ingredient that seems obvious is moisture. Obviously for rainfall to occur, moisture must be present. For heavy rainfall events however, we tend to quantify moisture differently than we do with severe weather events. With severe weather events, we like to look at surface dewpoint temperatures. For heavy rainfall events, we use a measure called “precipitable water”, which simply put, is the amount of rain that would fall if we condensed all water vapor out of a column of air. Since we are within 12-24 hours of the event, rather than looking at model data, we can look at observations (shocking isn’t it?). The sounding below shows temperature (red line) and dewpoint temperature (green line), as well as a bunch of other…stuff. Perhaps there will be a future blog post about what this other “stuff” is, but the main thing to focus on right now is “PW” (in the green box annotated below), which is precipitable water. This value was observed to be 1.96 inches. If we use SPC’s nifty sounding climatology website, we can see that 1.96 inches is very near the daily max for 0000 UTC on September 21. Put another way, this is one of the highest values ever recorded for this time of the year in North Texas. Now you’re probably thinking: 1.96 inches? I thought we were expecting 3+ inches of rainfall. And you would be exactly right. The reason the realized rainfall is much higher than the precipitable water amount is that convergence and lift cause this moisture to become locally concentrated in areas where rain is actually falling (plus the exact computation for precipitable water makes some assumptions that are valid in an “idealized” world, but nevertheless, it’s a good measurement).
High precipitable water implies that much of the atmosphere is near or at saturation, and indeed the Fort Worth sounding below shows that the atmosphere is basically saturated from the surface up to about 350 mb (about 34,000 feet). To double check, the sounding below tells us the low-level mean relative humidity (“LowRH”) is 76%, and the mid-level (“MidRH”) is 72%. This sounding isn’t totally saturated, but it’s pretty moist.
Ingredient 3: Instability, but not too much
In order to get thunderstorms, you need instability. Instability is the ability for parcels of air to rise freely once they are lifted enough (by a cold front for instance). Think of a cold front as the “macro-scale” lift, with instability being a more “micro-scale lift”. An unstable air mass occurs when parcel temperatures are warmer than the surrounding air (a more detailed discussion is another blog). In a moist environment, this occurs if temperatures cool with height at a rate of more than about 6 C/km. But notice I said “not too much” instability. This is because we want parcels to rise, but not too fast, and this has to do with how raindrops form. There are two ways raindrops can form. In one way, water droplets form and grow on ice particles (the so-called “cold rain process”), in which large drops form at the expense of smaller droplets. This is what typically occurs in most areas outside of the tropics and especially in severe thunderstorms. It’s a pretty complicated process, but Wikipedia has a good article on it. This process results in a lower concentration of large raindrops, and occurs whenever ice is present in clouds. Ice can become present in clouds either because strong updrafts lift water particles well above the freezing level (i.e. when there’s a large amount of instability), or if it’s just plain cold.
The opposite process is the “warm rain process”. This occurs when raindrops grow as the result of collisions with one another, resulting in a large concentration of smaller raindrops. This process occurs when instability is weak (i.e. updrafts are weak), but there’s still enough instability to get convection started. This can also happen if it’s just very warm (which is why it is common in the tropics and is why tropical cyclones produce very heavy rainfall rates). Going back to the sounding above, we can see that surface-based convective available potential energy (CAPE) is 1,252 J/kg. This is pretty much in the “high, but not too high” category, and this value should drop overnight as temperatures cool. This amount of CAPE is more than enough to sustain thunderstorm updrafts, but probably low enough to have at least a mix of cold and warm rain processes.
Ingredient #4: Warm Cloud Layer
This is related to #3 in the sense that it affects how rain drops grow, but is something different than instability. The cloud layer can be thought of as any layers of the atmosphere that are saturated (or are very near saturation). A good analog for the lowest level of the atmosphere where this occurs is the lifted condensation level (LCL). On the sounding above, the LCL is 285 m (or 935 feet for those of you stuck in the Medieval days), which is quite low. A low LCL suggests a very moist environment (though one must look at the entire sounding to make sure there is moisture depth, and as mentioned earlier, the low and mid relative humidity values are both north of 70%). The cold rain process described above occurs most effectively when temperatures are around -20 C, but can really happen anywhere temperatures are below freezing. Any portion of the atmosphere that is saturated, but is warmer than 0 C will allow the “warm rain process” to occur. If the atmosphere is saturated, but colder than 0 C, we’ll get the cold rain process. The warm cloud layer depth can then be defined as the vertical distance between the LCL and the 0 C level. On the sounding above, the freezing level is 14,184 feet, meaning the total warm cloud layer depth is 13,249 feet. Values above 10,000 feet are generally supportive of large amounts of warm rain processes.
Ingredient #5: Slow-Moving Rain
The first four ingredients above tell us that it will rain, and it will probably rain hard. But for flooding to be a concern, it has to rain for an extended period of time. This can happen through a combination of slow-moving rain and thunderstorms, or just very widespread rainfall. A good way to look at the speed of rain and thunderstorms in this situation is by looking at the LCL-EL (Cloud Layer) mean wind, which is about 21 knots (24 MPH). This is a fairly fast movement, but the rain is forecast to be very widespread, and it will probably rain for a lot of the day.
Putting it all Together
All of the ingredients listed above are going to be in North Texas for the next few days. We have a slow-moving cold front and upper-level trough to provide plenty of lift, we have near record levels of moisture, modest amounts of instability, and thermodynamics that will be very supportive of warm rain processes. This tells us that it will rain, and it will likely rain hard. And it will rain all day. The only question becomes where exactly will the front stall? Because this is where the most convergence and lift will occur, and where the heaviest rain will fall. All of these ingredients suggest widespread rainfall amounts of 2-4 inches are certainly likely, with some areas quite likely seeing more than 6 inches of rain by the end of the weekend. Given the recent heavy rainfall over the past month which has saturated soils and caused rivers to run high, it won’t take much more to get flooding to occur. None of this is to say we are anticipating widespread catastrophic flash flooding, but extreme caution will need to be exercised near any known trouble spots, because the amount of rain we’ll see will certainly cause them to fill with water.