This graphic shows the probability of experiencing severe weather within 25 miles of any given point (such as your house) tomorrow afternoon into tomorrow night. The Storm Prediction Center has issued the standard risk of severe weather roughly along and east of a line from San Angelo to Childress. The eastern extend of this risk runs from San Antonio, Austin, northeastward to Fairfield and Mount Vernon. All of western and central Oklahoma is also included in this risk. The primary timeframe for severe weather will be from 5 PM Monday and continuing into the overnight hours into Tuesday. With the initial supercells, very large hail will be the primary risk. As storms grow into a line or complex later in the evening hours, damaging winds will also become an increased possibility along with large hail. One or two brief tornadoes will also be possible. If all you care about is if its going to rain on you tomorrow, go ahead and stop reading here. If you’re a weather weenie and want to know the details of the weather setup, it’s nerd time!

Surface dewpoints at 21Z/4 PM CDT Sunday

At the time of this blog post a diffuse dryline was located just east of San Angelo and Abiliene. To the east of the dryline, dewpoint values were well above 60° to 65° degrees while very dry air was located to the west of the dryline. As head into the overnight hours, the dryline will retreat, or move west, back into West Texas before advancing eastward once again on Monday. Locations along and head of the dryline will have the threat of convection while very dry air will result in some blowing dust to the west of the dryline. Winds will also be gusting on either side of the dryline tomorrow as our storm system approaches and a low pressure begins to deepen over the Texas Panhandle.

12Z GFS: Surface pressure and dewpoint values at 7 PM Monday

This graphic shows the Global Forecast System (GFS) model output of nearly the same graphic at 7 PM on Monday. A surface low pressure around 1000 millibars is progged over eastern New Mexico, which backs surface winds and allows for southeasterly winds ahead of the dryline. The dryline is located from Vernon to Abiliene. East of the dryline, dewpoint values are above 65° while behind it winds are out of the west and dewpoint values are in the twenties.

12Z GFS: 500 millibar winds and heights at 7 PM Monday

Weather models have had a really hard time with this storm system. As of right now, the consensus is that a weakening upper level low pressure system will be located across eastern New Mexico around 7 PM tomorrow evening. A strong southwesterly flow aloft will be in place across West Texas and into the western sections of North Texas. These winds are at 500 millibars or 18,500 feet above sea level. The reason these are so important is that if they are a few hours late, thunderstorms may be delayed in developing. In addition, these winds help organize thunderstorms. Without at least 30 to 35 knots of mid-level flow, thunderstorms will remain in a disorganized storm mode and not allow for organized convection. Generally speaking in terms of the severe threat, the eventual position and arrival time of this weakening upper level low pressure system will dictate when we begin to see thunderstorms develop. In addition, we want to keep winds out of the southwest or west. Remember that we’ll have southeast winds at the surface, so if we have southwest winds at 18,500 feet above the surface, that creates the turning with height that also allows storms to become more organized. If winds at 500 millibars are out of the south or southeast, that doesn’t create the turning with height and lowers the threat of organized thunderstorms. Weather models were showing that solution two days ago for Monday, but have generally come into agreement that we should have a southwesterly flow aloft at 500 millibars. Once again, where this system ends up tomorrow afternoon will dictate if we’re able to see supercells develop off the dryline tomorrow afternoon or if we’re going to have to wait for the energy to arrive later tomorrow night and develop a line of thunderstorms.

12Z GFS: 850 millibar winds and heights at 7 PM Monday

As we lower down to 850 millibars or 5,000 feet above sea level, we see that winds ahead of the dryline are generally out of the south. While the low level jet is fairly weak, with only 20 to 30 knots of wind, it will be high enough to support slow moving, organized thunderstorms. Generally speaking, the weak low level wind fields will prevent a major tornado threat on Monday. However, a brief tornado or two is certainly not out of the question since instability amounts will be on the high side. ‘Super-Size’ hail is a good bet with any sustained supercell along with a damaging downburst wind threat. Notice winds west of the dryline are out of the west/southwest, which may help increase convergence along the dryline and allow for a few isolated supercells to form tomorrow afternoon. Lots of mesoscale factors will determine if and where storms will develop tomorrow afternoon, so theres no point in trying to be specific on that. It is a good bet that a line of thunderstorms will form tomorrow evening as the upper level energy really begins to kick in and that line will slowly move east tomorrow night with the attendant threats of hail and damaging winds, not to mention a pretty wicked light show.

12Z GFS: Convective Available Potential Energy at 7 PM Monday

In terms of instability values, we should have plenty to work with on Monday as surface temperatures should be well into the 80s with dewpoint values in the mid to upper 60s. I expect that we’ll have a fairly large zone of at least 3,000 joules per kilogram of instability on Monday, which is considered high. The graphic I’m showing you now is off the GFS, which is a lower resolution model, so instability values will likely be higher then what is shown on this graphic. Instability values don’t drop much as we head into tomorrow night, so don’t think the severe weather potential will be eliminated just because the sun goes down. While the values will drop slightly, they will still be around 2,500 joules per kilogram.

When storms first fire up tomorrow, they will likely be in the form of slow moving supercells. Due to the high instability values in place, very large hail will be likely with those storms. Baseball and perhaps up to Softball size hail will be big issues with the strongest storms. After a few hours, storms will likely begin to grow into a complex or line. That will help reduce the extremely large hail threat, but the threat for damaging winds will increase. The tornado threat is not high tomorrow, but one or two brief tornadoes can’t be ruled out. We’ll be chasing tomorrow so be sure to check in for our live chasercam video! I’ll try to post a brief update when the evening weather models come in. Look for that update to be posted by about 11 PM.