9AM CDT Update: At the 8AM forecast update, the SPC extended the high risk area to include a much larger area than it did last night. Oklahoma City, Tulsa, most of eastern and central Kansas, and a large chunk of Nebraska are now under the high risk zone, with areas just around that under the moderate risk zone. I've uploaded the new maps and included them below. Still looks like we're on track for a dangerous tornado outbreak with intense, long-lived tornadoes almost likely at this point.
Two points I want to make. The first is that someone in the comments asked if a derecho will form from these storms. It won't be a derecho, but once the storms do start to interact with each other and merge with one another, they'll likely form some sort of quasilinear convective system and move off towards the east, hence the heightened wind risk off to the east of the tornado outbreak today.
The second point that's been brought up a few times is "what if the cap doesn't break and the storms don't develop?" Well, if that happens, the best case scenario is that little to nothing happens, and the only effect is that people never listen to an urgent forecast like this again. The worst case scenario is...well, let's not think about that. The capping inversion will likely break, though. It's just a matter of when.
The original text of the diary continues below, with updated maps. The SPC updates their forecast again at 1130 CDT, at which time I will not be around (as I have a class), so please refer to the SPC for the latest charts at that point. Stay safe.
The 00z (700PM CT) models just came out, and things don't look too terribly different from how they've looked over the last few days. It still seems that we're on track for a major tornado outbreak across the central plains tomorrow, extending from the Texas/Oklahoma border straight up through eastern Nebraska and western Iowa.
The SPC took an extremely rare step of issuing not one, but two high risk areas for severe weather tomorrow two full days in advance of the outbreak. The SPC only issues a high risk like this when they're confident that the environment is capable of supporting a relatively large, intense tornado outbreak is expected.
Here's the SPC's latest severe weather outlook for Saturday. Anyone in the yellow, red, or purple shading needs to pay very close attention to the weather tomorrow. The areas at most risk are within the red (moderate) and purple (high) risk zones. This is where the Storm Prediction Center expected the most dangerous severe weather to occur.
Note: The SPC updates their outlooks at 100AM CDT, 800AM CDT, 1130AM CDT, 300PM CDT, and 800 PM CDT. The below maps were issued at 800AM CDT and are going to be outdated after 1130AM. Please use the Storm Prediction Center's website to see the latest forecast maps.
Many of these intense tornadoes will occur after dark. Please prepare yourself in advance for a nighttime tornado outbreak by having a way to be alerted of dangerous weather if/when you go to sleep Saturday night.
Categorical severe weather outlook:
Green means non-severe storms, yellow means slight risk for severe storms, red means moderate risk for severe storms, and purple means high risk for severe storms.
Probability of seeing a tornado within 25 miles of any point in the shaded area. Black hatching indicates a risk for significant tornadoes:
Green = 2%, Brown = 5%, Yellow = 10%, Red = 15%, Pink = 30%, Purple = 45%
Probability of seeing wind damage within 25 miles of any point in the shaded area. Black hatching indicates a risk for 75+ MPH thunderstorm wind gusts:
Brown = 5%, Yellow = 15%, Red = 30%, Pink = 45%
Probability of seeing severe hail (greater than 1.00" in diameter) within 25 miles of any point in the shaded area. Black hatching indicates a risk for hail larger than golf balls:
Brown = 5%, Yellow = 15%, Red = 30%, Pink = 45%
Jump the fold for some more discussion...
The basic synoptic setup is that a low pressure system is moving across the Rockies right now, and will ramp up across western Nebraska tomorrow afternoon. A strong upper-level jet (the jet stream is shaded in blue on the 250 millibar chart below) will continue to move east across the Rockies tonight into tomorrow, and allow that low pressure system to strengthen throughout the day.
A dryline (a moisture boundary denoting a sharp cutoff in the moisture in the air -- moist air lies to the east of the dryline, and dry air lies to the west of the dryline -- which serves as a focus for severe thunderstorms to rapidly develop under the right conditions) is expected to develop and move into Nebraska, Kansas, Oklahoma, and Texas by 7PM Central Time. In the forecast surface map below, the dry line is the tan/yellow line extending off the low pressure system.
A low-level jet will form later in the afternoon into the evening, pumping in warm, moist air from the Gulf to help feed these storms and allow them to grow. The transition from southwesterly winds at the upper levels to southerly winds in the midlevels to southeasterly winds at the surface will provide a good amount of wind shear to allow the storms that do develop to become supercellular and produce tornadoes.
The storms are expected to develop late in the afternoon into the evening on Saturday along that dryline, and when they do, they will quickly become severe. Due to the amount of vertical wind shear in the atmosphere (winds changing direction with height), large, intense tornadoes are of grave concern, sparking the high risk for the aforementioned areas.
Here's a forecast hodograph for Omaha, NE at 9PM Saturday. For the non-weather geeks among us, here's a good explanation of what a hodograph is, and how to read it. Basically, it depicts how fast the winds are as you travel up through the atmosphere, and how fast they're going. Big, clockwise turning hodographs like the one pictured below are good for supercell development and tornadoes.
The tornadoes will be the big story tomorrow, although the threat of strong, damaging winds and hail larger than baseballs will be possible. A few of the variables that help forecasters predict tornadoes are the "Significant Tornado Parameter" (STP), and helicity.
The STP is a number that helps you visualize the tornado threat that takes into account wind shear, the amount of rotation a storm can tap into once it develops, the amount of CAPE available (Convective Available Potential Energy...the metaphorical unstable gunpowder sitting in the atmosphere that, once released, can form intense storms), and a few other variables. While it's not a perfect parameter, it's good at quickly visualizing the threat.
Here's an animation of the STP from 15z until 06z tomorrow (10AM CT Saturday until 1AM CT Sunday). A significant value that is one to cause concern to a forecaster is a value of 1, which is denoted by yellow in these images. The warmer colors are higher values, being capped off by red which denotes a value between 7 and 10. In other words...the environment is ripe for producing large, long-lived, intense tornadoes.
Another way to visualize the tornado risk is to measure the helicity in the atmosphere. Helicity is the amount of rotation that can occur in a thunderstorm given the environmental conditions in which the storm develops. A helicity value of 450 means that thunderstorms are capable of supporting violent EF-4 to EF-5 intensity tornadoes.
The values in Omaha NE at 9PM are predicted to be 896, Manhattan KS has a max value of 773, Tulsa gets up into the 700s, and OKC gets up to around 500-600 during the evening. That's insanely high. This is why the SPC has kept this whole area under a high risk for tornadoes tomorrow, and why I'm trying to drive the point home that tomorrow will be dangerous.
Late into the night on Saturday into early Sunday morning, the supercells will merge into a line of storms and the main threat will become damaging winds as the line moves off to the east.
I'm not writing this to scare you or try to make it sound like the world is ending. I'm writing this to let you guys know that the environment tomorrow/today is going to be absolutely ripe for producing large, violent, long-lived tornadoes.
Another reason I'm trying to drive home the point is that these tornadoes will predominately occur at night. The only thing worse than a large, intense tornado is a large, intense tornado occurring at night when A) you can't see it, B) people are sleeping, and C) people aren't paying attention to the weather anyway. Most tornado fatalities occur at night because people are sleeping or otherwise not aware of the danger around them.
It's imperative that you help get the word out. Share this diary if you'd like, share links to the SPC, share links to the NWS, or post in your own words about the situation. The more people who know what's coming, the better.
I'll have another diary with the latest information around 1-2PM on Saturday when I get out of class.
Keep track of the severe weather using the excellent tools provided by the Storm Prediction Center.
Wunderground has an excellent, free radar for you to keep track of the storms.
If you have an iPhone or an iPod Touch, I encourage you to purchase the iMap Weather Radio app. It's $9.99 and worth every penny. It will use your GPS location, as well as locations you program into the app, to instantly warn you the second the NWS issues a watch or warning for your location. It's an amazing app.
You can receive FREE severe weather text alerts from the following local news -organizations:
-NBC 6 from Omaha NE
-KFOR in Oklahoma City offers a text service for tornado warnings, that is free as far as I can tell.
If you know of any other free, reputable text alert services, please let me know.