I’m in a position this evening where a detailed blog post is impossible, so I’ll share the thoughts of the National Weather Service Office in Norman, OK. The image above is very similar to my post here. The image below puts some numbers on just how hot it might be in Oklahoma.
I’m ready for Autumn…
As I mentioned last night, a substantial heatwave is forecast to grip a large portion of the country beginning this weekend and lasting into next week. Local National Weather Service (NWS) offices across the southern United States have already issued Heat Advisories or Excessive Heat Warnings in preparation. If you live across the southern half of the United States, please take this threat seriously. On average, more people are injured or killed by heatwaves every year than any other weather-related disaster. Please read the Center for Disease Control (CDC)’s webpage on Extreme Heat.
One last thing. Notice the “desert” southwest is blanketed with various forms of flood watches and warnings. This is fairly typical this time of year as the North American monsoon is in full swing. This is when a large portion of Arizona and New Mexico receive most of their yearly rainfall.
It’s been a while since I’ve done a forecast post of any sorts, so tonight I thought I’d give (most) everyone bad news: a very hot heatwave is potentially in the works for next week. The image above (and denoted below) is from this morning’s run of the Global Forecast System (GFS) Model and is valid for Monday evening. It depicts a very large subtropical ridge (high) over much of the United States. This ridge will bring with it very warm temperatures across a large portion of the United States, and not much potential for rain. The main storm track lifts north into the far northern United States and southern Canada, so cold fronts will be hard to come by.
Not shown here, but the GFS is hinting at the possibility of a tropical wave of some sort to develop off the Atlantic coast by late next week. It will certainly bear watching!
During the evening of 23 July 2010, Vivian, South Dakota experienced a severe thunderstorm no homeowner would enjoy; the thunderstorm packed winds estimated to be in the 110-120 miles per hour range. If that wasn’t problem enough, the thunderstorm was also producing hail that measured over 8″ in diameter and 18.5″ in circumference. (Anecdotal reports indicate that the hail was closer to 11″ in diameter when it fell!) Photographs from the region show holes in roofs. Not damaged roofs, but actual holes greater than 5″ in diameter! At least one hailstone was preserved and it officially weighed in at 1.9 pounds — even after substantial melting! You can read more about this thunderstorm, and see pictures of the hailstone(s) here! (Note, I would have blogged about this sooner, but I was waiting for official word that this hailstone is the world record holder, however that will take a while longer.)
If this hail stone is verified as legitimate, and I have no reason to believe it won’t, it will smash the previous record for largest hail stone. That hail stone measured 7″ in diameter and weighed closer to 1.6 pounds. It fell in Aurora, Nebraska on 22 June 2003. Prior to the Aurora, NE hailstone, the world record hailstone was from Coffeyville, Kansas.
Our current understanding of thunderstorms indicates that in order for a thunderstorm to generate a hailstone of this size, several things must come together. First, we need to have a lot of atmospheric instability. This allows for thunderstorms to develop very strong updrafts, which are necessary to suspend the hailstones aloft. Next, we need the freezing level to be relatively close to the ground. When the freezing level is close to the ground, more of the thunderstorm is below freezing. This means that most of the thunderstorm’s moisture is located in a part of the atmosphere that is conducive for ice formation. Lastly, we need the thunderstorm to rotate. This is because a rotating thunderstorm actually acts to make the updraft stronger than it would be if the updraft was not rotating. This allows for the thunderstorm updraft to suspend hailstones even longer, allowing them to become larger.
The radar images above clearly depicts a very strong thunderstorm, known as a supercell, shortly before the town of Vivian, SD experienced the potentially record hailstone. (Vivian is located in the “hook-like” appendage in the bottom-middle of the thunderstorm.) The two images above are both evaluating a height of 15,000 feet above the radar. At this height we can clearly see in the right image that this supercell thunderstorm had very strong rotation!
If we look at around 27,000 feet, we can see what is known as a Bounded Weak Echo Region (BWER) signature in the reflectivity (above/below left). (It looks like a donut-hole. If you are unfamiliar with this signature, the image below is annotated.) This signature is indicative of an extreme updraft. If we examine the velocity image, we see that the thunderstorm is still rotating, albeit, it is more difficult to discern.
The last set of images are a bit different from what I tend to show. Normal radar images depict a quasi-horizontal cross-section through a thunderstorm. These images are actually a vertical cross-section through the thunderstorm (beneath the vertical cross section is a typical radar image!). I’ve annotated a couple of important parts of the thunderstorm in the image below. This thunderstorm had a very strong forward-flank downdraft (which is where most of the heavy rain falls), a very strong rear-flank downdraft (strong winds, moderate rain, and hail1 are typically found here), and an extremely strong updraft (the largest hail fails around the updraft). At the time of the cross section, Vivian, SD was located almost directly beneath the strong updraft, which helps explain why the largest hail fell there.
Tonight a mid-level low continues to aid development of showers and thunderstorms across northern Texas. Additionally, a second mid-level low spins across the central Gulf of Mexico. Both of these lows are tropical in origin, which decreases the large-scale threat of severe thunderstorms across the aforementioned areas. (This is for reasons I won’t explain tonight – sorry!)
Across the northern United States and southern Canada, a mid-latitude cyclone is moving eastward. Severe weather has occurred and is continuing to occur across portions of the western Great Lakes in association with the passage of cold front. Tonight’s image depicts the showers and thunderstorms developing along the cold front. Thunderstorms that develop into long lines like this are often referred to as “squall-lines“. Strong winds are typically found along the leading edge of the squall-line, followed by intense rain. Squall-lines are a form of Mesoscale Convective Systems or MCS.
The mid-to-upper-level trough that was responsible for keeping Tropical Storm Bonnie at bay last week has continued to move around the southern periphery of a strong subtropical ridge located over the southeast United States. This has allowed the trough to move from the central Gulf of Mexico northwestward into northwest Texas. This tropical (short)wave trough has, once again, increased low-level moisture across the southern US and allowed for the development of widespread showers and thunderstorms.
Following on the heels of the trough over northwest Texas, a second upper-level trough/low is currently moving through the Gulf of Mexico. This low will continue along the same path as it’s predecessor, bringing more rain to the southern United States. (The blue arrow in the image below qualitatively indicates the path of these upper-lows.)
If the water vapor imagery above doesn’t convince you of the widespread nature of the showers and thunderstorms, the image below hopefully will. This is a radar mosaic from 22 UTC (5 PM CDT), depicting the widespread showers and thunderstorms. I expect more of the same tomorrow…
The hot weather across the southern United States, combined with the presence of ample low-level moisture and a weak surface boundary, led to the development of scattered clusters of showers and thunderstorms. Several of these clusters developed in and around the Oklahoma City metro area. Because the mid-to-upper-level wind shear was rather anemic, individual thunderstorms were unable to sustain themselves very long. This resulted in numerous outflow boundaries merging across the state.
In the image above (and depicted as blue lines with arrows below), several of these outflow boundaries were merging across the southern Oklahoma City metro area this evening. Ultimately, the outflow from the thunderstorms over and north of Oklahoma City became dominant and pushed the rain-cooled air south toward the Red River. Although it wasn’t the case this evening, oftentimes when outflow boundaries from thunderstorms collide, new thunderstorms develop near the point of the collision. (I was really hoping that would be the case today, but it wasn’t meant to be…)
