A strong upper level storm system along with a southward advancing cold front will slam into a strongly unstable, humid air mass on Thursday across North, Central, and East Texas. A severe weather event is anticipated tomorrow as a line of thunderstorms should develop along an advancing cold front by the mid-afternoon hours. Ingredients will be in place to support the risk of severe thunderstorms capable of producing damaging wind gusts and large hail. The Storm Prediction Center has placed portions of Northeast and East Texas in an enhanced risk of severe weather tomorrow running along and east of US-75 from Sherman to Dallas and north of Interstate 20 from Mesquite to Marshall. A larger area of severe weather risk includes much of North and Central Texas into the Hill Country along with the Brazos Valley and East Texas. Ingredients are coming together for the possibility of a severe weather outbreak with a line of severe thunderstorms.
An upper level jet will dip south into Texas by the late morning and early afternoon hours tomorrow. By the mid-afternoon this upper level jet should be favorably positioned in conjunction with an advancing cold front. Winds at 500 millibars or about 18,500 feet above sea level will be exceeding 45-55 knots. The minimum wind speed I like to see for more organized thunderstorms is 35-40 knots at 500 millibars so the upper level winds will support an organized convective mode. The reason we look for stronger winds in the upper atmosphere is that the winds will help blow precipitation away from a thunderstorm’s updraft thus allowing warm, unstable air to continue feeding into it. When upper level winds are weak precipitation has a tendency to fall straight down or into the updraft. When precipitation falls back into the updraft the warm/unstable air is cut off and the thunderstorm weakens. During the summer months we refer to those storms as popcorn storms since they go up quickly, produce a light of lightning and heavy rain, but quickly weaken and usually dissipate within 30-45 minutes often resulting in a localized microburst. Since winds tomorrow in the upper levels of the atmosphere should blow rain out of the updrafts we expected a more organized convective mode and longer lived storms.
If you’ve been outside today you know temperatures are quite hot and muggy. Temperatures are well above average across North Texas today and will be again tomorrow as surface temperatures climb into the lower 90s. One way to measure moisture in the atmosphere is the dewpoint temperature. The dew points this afternoon across the eastern half of Texas are in the 50s, 60s, and even 70s. Honestly today feels like an April or May afternoon which is usually not a good thing when you’re talking about a upcoming storm system in early October. With a very moist atmosphere in place ahead of the cold front tomorrow the atmosphere is going to become strongly unstable. Surface based convective available potential energy (CAPE) values will exceed 2,000 to 3,000 J/Kg by the afternoon hours Thursday. Those are fairly impressive values for October and in combination with the strong upper level dynamics and cold front could support an appreciable severe weather threat.
One unknown factor that is still causing some uncertainty is the actual placement of the cold front when thunderstorms begin developing Thursday afternoon. Some model solutions have thunderstorms developing east of I-35 and keeping the D/FW Metroplex and everyone west of I-35 will remain dry. Other models suggest thunderstorms will begin developing along a Nocona-Mineral Wells-LaLampasasine. The first storms will begin developing across Oklahoma and gradually build south along the cold front. Thus the first storms will develop near the Red River and build south into Texas as the cold front moves east. Hence where the cold front ends up when storms fire will determine exactly where the rain chances begin. Based on current data and the National Weather Service Fort Worth’s thinking the current forecast shows thunderstorms developing just west of the D/FW Metroplex and I-35/I-35W by the mid afternoon hours. Thunderstorm cocoverageill increase as the cold front/squall line moves into D/FW with a nearly solid line of storms by the time they end up east of D/FW and US-75/I-45 in the enhanced risk zone. This could chance and storms could fire further west or east and radically chance the overall storm chances for the D/FW Metroplex.
Due to the combination of favorable ingredients for severe weather once storms get going they will likely become strong to severe quickly with a threat for damaging straight line winds exceeding 60 MPH and hail up to the size of golfballs. Not everyone will be hit by thunderstorms tomorrow. Not everyone who gets a storm tomorrow will receive large hail or damaging winds. The overall setup favors a linear event (squall line) with low level wind shear unidirectional and marginal. The tornado threat will be low but not zero. Any individual thunderstorms that manage to stay by themselves for a while or interact with an outflow boundary could produce hail up to the size of hen eggs and exhibit some rotation. While weak low level winds will keep any tornado threat localized and marginal any supercell storms will have to be watched.