Friday: A line of severe storms during the evening
This is certainly a week of extremes. We couldn’t ask for better July weather today, while we may in for the worst on Friday.
For today, the low humidity and gentle north breeze will be absolutely divine for the 18th day of July. You can’t order better weather than this in the Mid-south this time of year. Be sure you get out and enjoy it. If you’re not feeling it this morning you’re not alone, the dewpoint is still at 67 degrees but it is slowly falling. I just watched it drop another tenth of a degree in the last half hour. That sure beats it rising! Drier air will continue to slowly filter in as we go through the day. We always say it’s not the temperature that matters this time of year, it’s the humidity!
Tomorrow will feature much of the same, with low humidity and light winds. There will be a bit of a disturbance over in the Smokies and that could destabilize our atmosphere enough to cause an isolated shower or storm. I’m putting that chance at about 20%. The most likely recipients of that would be those of you east of Highway 127 and south of I-40.
Then, a pattern develops that could spell trouble for Friday afternoon/evening. The concerns for this pattern have increased this morning and I want you all to stay tuned for that Friday forecast. Hopefully, models will back off of the scenario they are playing out this morning, but I have a feeling they won’t, especially since they keep ramping up the threat with each new model run and they haven’t been backing down much at all. A line of strong to very severe storms appears likely across Illinois and Kentucky during the late afternoon. This line of storms will be steered our direction and we could be looking at widespread damaging winds from what we call a derecho. This is like what we had come through last year over Memorial Day weekend. I’ll keep a very close eye on this and I will certainly keep you updated, as always.
The Storm Prediction Center (SPC) is already issuing a slight risk for Friday for portions of Kentucky. An expansion southward is likely. The SPC only issues “slight risk” areas three days out if their confidence is especially high about severe weather occurring. I wouldn’t be surprised to see an upgrade to the risk as we get closer to Friday.
The wind shear and instability associated with the system coming in on Friday is impressive for this time of year. We almost always have an unstable atmosphere this time of year, with so much heat and humidity around, but to get shear this time of year is much more difficult. All the systems that would produce shear are usually well to our north by now, taking those parameters with them. But, this year is a bit different.
Storms need shear to stay organized. Normally, July t-storms just fire up, drop a lot of rain and lightning, and then fizzle out. Sometimes they do all this in a matter of minutes on one neighborhood. But, if there’s plenty of wind shear storms can organize and become severe. Wind shear is the change in direction and speed of wind with height. In the spring time, if we have southeast winds here at the surface that increase in speed as they swing to the south at about 5,000 feet up, then increase even more as they swing to the southwest even higher up than that, we get worried about tornadoes. That change in wind speed and direction gives storms what they need to stay organized and to survive longer.
A storm that is moving north, while surface winds are from the east, will be continuously resupplied with fresh, warm/humid air as it travels north. This will keep it sustained and able to track for as long as it wants to (this is how we get long-track supercells in the spring). If, however, the storm is moving north and surface winds are from the south, the storm will have its own rain-cooled air blown right back into it, which will soon choke it off and the storm will dissipate. This is why a change in wind speed and direction is necessary for storm organization and survival.
And wind shear will certainly be present here on Friday. Tornadoes will not be the main threat, but damaging straight-line winds associated with a derecho certainly could. Friday’s shear is more directional in nature, meaning that winds increase with height more than they change direction. For example, winds aloft will be roaring from the northwest, while surface winds will be light from the west-southwest. Tornadoes aren’t out of the question, for sure, but this type of pattern often favors more of a screaming squall line of storms.
Whenever you have strong winds aloft, the thunderstorms that develop in that atmosphere may push those winds down to the surface with their downdrafts. The rain is the downdraft and as it falls it drags the air down with it. Winds that may have been 60 mph one mile up off the surface may suddenly be pushed to the ground with the heavy rainfall of the t-storm. This is how we get severe t-storm winds. If we know winds are really strong up above us, and we see a storm with very heavy rainfall, we know to watch that storm because it may push those strong winds aloft down to the surface. That often prompts the National Weather Service to issue a severe t-storm warning.
Whew! I probably explained more than you bargained for! But, seriously, this is a very active pattern we’ll be going into for Friday. Hopefully, the models will back off this scenario. In the meantime, I’ll be watching it all and you know I’ll always be online as long as there is a threat, no matter what day of the week it is.
Since I said so much here, I’ll send an afternoon update out with star gazing info for tonight, as I promised in yesterday’s blog. It’s going to be a good night for going out and looking up!
You all have a great day and get out and enjoy this weather!