Day 2 Severe Weather Outlook

Lets start off by taking a look at the latest severe weather outlook from the Storm Prediction Center. This outlook was issued at 11:30 AM on Sunday. The next outlook will be issued around Midnight tonight and we’ll bring it to you then. At this time, a standard risk of severe weather has been issued for much of Central and Eastern Texas. Major cities in the severe weather risk include Victoria, San Antonio, Austin, Waco, D/FW, and Houston. The standard risk of severe weather can be identified by the orange line. Areas within this orange line have a 15% chance of experiencing severe weather within 25 miles of their location on Monday. Outside the standard risk of severe weather is a low chance of severe weather, identified by the dotted brown line. This includes much of the state of Texas. Severe weather is defined as tornadoes, hail larger then one inch in diameter, and winds in excess of 58 MPH. Lightning is not included in the severe weather criteria. I wouldn’t be surprised to see this outlook shifted a bit with the new issuance at midnight.

0Z NAM: 500 Millibar winds at 6 PM Monday

Without adequate mid to upper level support and associated wind shear, storms really aren’t able to get past the multicellular phase. That’s why we always start off by looking at 500 millibars, which is at about 18,500 feet above sea level. When looking for the possibility of organized convection, we like to see winds out of the west or southwest with speeds at or above about 40 knots. In the spring and summer time, we normally have issues with upper level support. We do not normally have that issue in the cool season. Tomorrow’s system is quite strong and should have more then enough upper level support for organized thunderstorms. Winds should be at or above 60 knots, approaching 80 knots in some locations.

0Z NAM: 850 Millibar winds at 6 PM Monday

Descending down to 850 millibars, which is around 5,000 feet above sea level, we analyze the lower parts of the atmosphere. Wind direction makes a world of difference in the low levels as it helps determine the amount of turning in the atmosphere. When looking for the potential of low level rotation and tornadoes, we want to see a sustained low level jet. Wind speeds normally need to be at or above 30 knots to support sustained supercells and their attendant low level mesocyclones. Wind direction is also important. If winds are out of the southwest, that normally supports a more linear mode (squall line) and lessens the potential tornado risk. If winds are out of southeast, that creates turning with height and thus the potential of low level circulations. Looking at tomorrow, we have a very strong low level jet that should be out of the south/southeast. Winds will be in excess of 50 to 60 knots. In terms of wind shear, this system is very strong. We do see systems like this during the winter time, but we don’t normally have the instability necessary to support organized thunderstorms. In fact, we had a setup like this about two weeks ago when parts of North Texas had snow. It all depends on the positioning of the weather features. If this storm was about two hundred miles further south, we would end up be dealing with a snow storm in North Texas most likely, but since we’re on the southeastern edge of the system, we’re in the warm sector.

0Z NAM: Surface dewpoint values at 6 AM Monday

0Z NAM: Surface dewpoint values at 12 PM Monday

0Z NAM: Surface dewpoint values at 6 PM Monday

Right now, dewpoint values across most of the state are around 40 degrees with higher values closer to the Gulf Coast as moisture return has already begun early this evening. As the storm system responsible for all this crazy weather across the state continues to approach, a very strong low level jet will develop. This low level jet will rapidly move moisture northward across the state. In fact, this may be one of the fastest moisture return scenarios I’ve seen. Depending on how high the dewpoints get will determine how significant Monday’s severe weather threat will end up being. Current data suggests that dewpoints will be sufficient to support organized severe weather, including surface based thunderstorms. The primary ingredient that is still in question are the instability amounts. These three graphics show how quickly the higher dewpoint values will be moving across the state. Each of these graphics are six hours apart, beginning at 6 AM and ending at 6 PM. Unless something significant changes which prevents the moisture from moving northward, I’m not concerned about a lack of moisture.

0Z NAM: Surface Based Instability values at 6 PM Monday

As is often the case with cool season events, determining how much instability will be present during an event is the primary complication to a forecast. Several factors affect instability amounts. My concerns for tomorrow’s instability values are going to be how far north the 60 degree dewpoint line gets and if we have any breaks in the cloud cover tomorrow. A few days ago, it looked like instability values would be below 500 joules per kilogram, thus limiting the severe weather threat. Models have increased the expected instability values and are now showing between 750 and 1,000 joules per kilogram across portions of North and Central Texas. At 6 PM, the NAM (North American Model) has the primary instability axis along the Interstate 35 corridor from San Antonio north to Fort Worth. With the strong upper level disturbance moving in and a very favorable wind field, it will not take a lot of instability to support organized severe weather. It’s always difficult to determine the exact ratio needed in terms of wind shear and instability, but 750 joules per kilogram is definitely enough to support severe weather on Monday.

Lets talk about specifics. The overall pattern is supportive of a linear complex of thunderstorms, known as a squall line. If we didn’t have sufficient instability in place, this line would likely only be your normal line of thunderstorms with lightning and thunder, nothing else. However, it appears we will have the instability needed to support severe weather across Central and North Texas on Monday. Around 12 PM, the line will likely be located from Wichita Falls south to Abilene, maybe a bit west of there. By 6 PM, this line will be on or approaching the I-35 corridor. The instability axis will be moving east along the line of storms. Because of the moist atmosphere and the strong winds aloft, it won’t take much to transport down the strong winds located just a few thousand feet above the ground down to the surface. Thus, I do expect damaging winds to be the primary threat on Monday. How widespread the damaging wind threat will be is going to depend on the instability amounts, which will be affected by any breaks in the clouds plus mesoscale features that we can’t determine until a few hours out. In the area of highest instability, generally between Austin and D/FW, is an area which I’m concerned about in regards to a tornado threat. If we we’re dealing with discrete supercells tomorrow instead of a linear squall line, it would be a major tornado outbreak across the area. At this point, it appears that will not be the case. However, any storms that form ahead of the squall line will have the potential to become supercellular and produce a tornado. Brief tornadoes are also possible within the squall line.

Honestly, I’ll be able to be much more specific about the threats in the morning. At the least, we should have a threat of damaging winds with the squall line along with a tornado risk across Central Texas. We’re going to be cloudy most of the day. It may not feel like a severe weather day when you wake up in the morning, but trust me, that is going to quickly change. One final thing to note about tomorrow’s setup. Storms will be moving very quickly, especially out ahead of the squall line. We may be dealing with 60 to 65 MPH storm motions, so if you go under a warning, take action immediately because you will not have much time. We’ll be posting plenty of updates tonight and tomorrow regarding any developments on the severe weather threat. I know you normally expect severe weather a week before Christmas, but it can happen. Don’t lower your guard because we’re in December.

In terms of my chasing plans, I am planning on chasing tomorrow. My current thinking is that I’ll head south on Interstate 35 towards Waco, but I’ll likely refine my plans when I wake up in the morning. I’ll have my live streaming video up and running so be sure to stop by! Those who have watched before know I can be entertaining with the audio running.