5/27/17 – Southern Oklahoma

Launching a balloon
Releasing a weather balloon during a stop. Weather balloons were one of many tools used in RiVorS. (Photo by Matthew Mahalik/OU CIMMS)

For days, computer models had consistently shown near-record levels of heat and humidity today, with temperatures soaring into the upper 90s and dew points approaching a downright miserable 80 degrees. We could break a sweat just by opening the window. Ironic, since the day before, it was too dry for storms to develop.

Throughout the midday, the extreme instability made itself known in the form of unusually strong midlevel updrafts, producing the altocumulus cloud field so many storm chasers look for on outbreak days. Our first balloon launch of the day in the early afternoon just south of Norman showed extreme instability — on the order of 6000 J/kg of CAPE (Convective Available Potential Energy). For reference, CAPE over 2000 J/kg is considered high.

From there we drifted south, closer to the dryline itself. A couple hours later, another sounding revealed the cap was still quite stout but eroding. And all the sunlight was heating the surface layer, increasing the CAPE to an astounding 7000+ J/kg. This data was used by the NOAA Storm Prediction Center and the National Weather Service to update their forecasts and tornado watch, and even resulted in a special SPC mesoscale discussion that warned about imminent explosive storm development.

A little after 4 p.m., one or two small cumulus towers were just strong enough to punch through whatever cap was left. These updrafts were able to tap into the rich, energetic mid-levels, and from there it was off to the races. Our last sounding resulted in an incredibly large 7866 J/kg of CAPE with no remaining cap to suppress the storms. With this much energy available, the storm exploded.

We were extremely fortunate to have a road network directly in the storm’s path. For the next two-plus hours, we completed transects across the storm’s hook, inflow, and forward flank of heavy precipitation. Along the way, we noticed striking changes in wind direction and temperature. Early evidence shows these are signs of potential vorticity rivers feeding from the precipitation core toward the updraft. Some of these wind-shifts occurred right as the temperature spiked — at times enough to fog up our windows for a few seconds before returning to the same antecedent conditions. We found a handful of these features, but they disappeared about as quickly as we could find them.

The data collected is very encouraging and we were able to work in precisely the correct part of the storm without too much difficulty. After four days of traveling, we were ready for some sleep and a restful Memorial Day.

Tags: None

5/26/17 – Southeast Colorado/Southwest Kansas

NOXP in Colorado during RiVorS
The NOXP radar deployed in Colorado during RiVorS. (Photo by Ted Mansell/NSSL)

Sometimes, the safest play when conducting severe storms field work just doesn’t work out. Day two of our three-day deployment presented us with two options: drift west from our hotel in Hays, Kansas, for a possible repeat of yesterday’s intense storms — likely including a supercell or two — across northeast Colorado and northwest Kansas, leaving us with a long drive south for a mission in the highly-volatile airmass over Oklahoma; or, drop south to intercept potential storms coming off the mountains of southern Colorado — a much riskier play that would put ourselves in a better location for the next day.

Facing the prospect of possible early-day intense convection far south the next day, we made the difficult decision to forego the north option. We headed south into Lamar, Colorado, where we sat and waited for storms to develop. And we waited…and waited…and waited. We launched a few sondes with not much encouraging data. We were antsy enough to make a couple passes through a shower near Las Animas, but the shower met its demise at the hands of dry air fairly quickly.

It soon became clear the models just didn’t have it right this time. About 90 minutes before sunset, we knew it was time to fold and headed down to Woodward, Oklahoma, for the night.

Tags: None

5/25/17 – Western Kansas

The first multi-day mission of RiVorS departed from Norman late Wednesday afternoon, ferrying the two mobile mesonet crews and CLAMPS sounding team to Dodge City, KS, in preparation for the first of several severe weather days in the High Plains.

Thursday morning brought prospects of strong to severe thunderstorms in northeastern Colorado and northwestern Kansas. High-resolution model guidance was fairly consistent in producing an isolated, rotating storm near the I-70 corridor. However, as is often the case in this part of the Plains, the environment seemed dry enough that organized supercells would likely struggle to persist. Our sounding taken near Colby, KS, in the early afternoon showed that fact with a surface dew point of only 50 degrees. Around this time, a cluster of showers and storms quickly intensified in northeastern Colorado. We decided to focus our attention on an area of kinematic and moisture convergence further south, near the Kansas/Colorado border, south of I-70.

A supercell to the north had developed an intense low-level circulation, with a tornado probable within minutes, but at the same time, the drive might be too long, causing us to possibly miss the storm at its peak intensity. Hoping that clear inflow and more favorable low-level moisture would win out, we decided to intercept a southern cell, which appeared to be organizing quickly.

The northern cell did indeed produce a brief, weak tornado, but weakened substantially soon after as its massive pool of rain-cooled air gusted outward and cut off its warm inflow. This appeared to be the pattern for storms today – quickly build, become marginally severe, and “gust out” and dissipate soon after. Given such limited moisture, this was not surprising: drier air causes higher evaporation rates, which in turn cools the air in the area. We completed our first pass through our storm’s core as it crossed into Kansas near the town of Kanorado. Sensors on the mesonet measured very cold air inside the storm, and the wind flow was directly away from the storm. Although the outflow was far too strong, we noticed signs that it had an opportunity to mature.

The two mobile mesonets sampled the region around the storm’s rotating updraft and successfully transected its core in its early stages of development. Near Winona, we penetrated the hook of the storm, experiencing slushy, ping-pong ball-sized hail and measuring wind gusts well over 70 mph. An initial look at some of the data suggests that we sampled some intriguing small-scale wind features.

Beyond the impressive fact that a small shower evolved into an intense supercell in near 50-degree dew points, this storm was notable for its rapid evolution despite its outflow-dominant nature. We witnessed several instances of storm cycling and approached rain-wrapped, near-ground velocity couplets on more than one occasion. In addition, the rapid initiation of additional showers very near the hook and their subsequent interaction with the dominant storm made for an interesting and challenging scenario. Whether any vorticity rivers were observed will be determined after a closer inspection of the data.

Tags: None