The Dryline that Set the State Ablaze

I remember, when I was just a kid, that my father used to listen to a song whose lyrics went something along the lines of “I've seen fire and I've seen rain”. If you live in Oklahoma, then chances are you've lived out those lyrics this month. Between April 9 and April 12, both fire and rain swept the state in one of the more interesting storm systems this spring. This month we'll be diving into the nuances of wildfires and severe weather as we examine the story of the dryline that set the state ablaze, and the weekend deluge that doused the flames.

The commotion began on the morning of April 9, as a strong cyclone moved into the central plains. By late afternoon, the low was located near Woodward, OK, and featured a well-defined warm front, and trailing a cold front through central Oklahoma, and featuring a strong dryline extending across the state from north to south, as shown on the weather map below, issued by the Norman office of the National Weather Service (NWS). Thunderstorms began forming along the dryline by mid-afternoon, growing as the system moved east, later spawning numerous severe hail reports in eastern Oklahoma, as well as 42 tornado reports (according to Storm Prediction Center (SPC) preliminary data) in eastern Oklahoma, northeastern Texas, and northern Louisiana.

Though the severe weather itself was enough to make this system notable, conditions behind the dryline and cold front were what made it truly remarkable. Often the passage of a dryline is something of a letdown for weather enthusiasts—it means that the threat of storms has largely passed, and dry, hot, breezy weather can instead be expected. With this system, though, 'breezy' would have been the understatement of the year. This can best be seen from conditions on the Oklahoma Mesonet—the Mesonet is a network of more than a hundred weather stations spaced throughout Oklahoma to provide temperature, wind, rainfall, and pressure observations, just to name a few. A mesonet plot from 5:10pm April 9th (approximately the same time as the above satellite images) is shown below.

Note the abrupt wind shift behind the dryline, and the extremely strong winds behind the cold front. The peak observed gusts are noted in orange – at this time alone, gusts of up to 55 miles per hour were observed, with sustained winds at nearly 40 miles per hour out of the west. Peak wind gusts of over 70 mph were reported later in the evening. While such winds alone would have been a nuisance to high profile vehicles (not to mention cyclists), when combined with the dry conditions that have prevailed over much of Oklahoma since the middle of last year, the result was an outbreak of wildfires, which showed up vividly from space in infrared satellite imagery.

Infrared satellites work by observing upwelling infrared radiation from the earth below. Everything on earth and in the atmosphere emits infrared radiation, in an manner that varies predictably with temperature. Thus, by measuring the infrared radiation, the satellite can infer the temperature of the surface being observed – if clouds are present, this temperature represents the cold conditions of the upper troposphere (often -50 or -60C for deep clouds), but if the sky is clear, the surface temperature (perhaps 20 or 30C during daytime in the spring) is reported. Wildfires, however, can have temperatures well in excess of 100C, and as a result show up on infrared images as hot-spots. At least fifteen of these hot-spots can be seen in the headline image, part of a collection of wildfires that injured 60, destroyed more than 200 buildings, and resulted in a state of emergency being declared in 31 Oklahoma counties.

Amid all this devastation, there was, however, a silver lining. Those who have lived for any period of time in Oklahoma know, however, that spring is a season of flux whose weather is ruled by near-constant change; sleet following heat, and storms following calm, and in this case, rain following fire. In the wake of the mid-latitude cyclone of April 9, another system moved through Oklahoma on April 11 and 12, bringing a weekend deluge of more than two inches of rain to some of the areas hardest hit by the wildfires. As a result, fire danger is low this week, and more storms are expected.

Spring in Oklahoma can be quite an adventure, and storm season is just beginning—for the latest up-to-date weather information and severe weather alerts, tune in to your local NOAA weather radio station (see http://www.weather.gov/nwr/ for more information), or check the weather service website at http://www.weather.gov. And of course, if it's weather data you're looking for, there's no better place than right here on HOOT to find it!



Story is © Nate Snook, 2009