The Big Spring experiment continued today at the National Weather Center. It was not a very active day for severe storms, though there were still some severe storms and tornadic cells to track in the late afternoon. Since it wasn't too terribly active, I took some time to ask the NWS forecasters about the process they go through when issuing warnings!
There are three forecasters, each from a different weather forecast office (WFO), during the Big Spring experiment. This week, I've been working with forecasters from Buffalo, NY; Louisville, KY; and right here in Oklahoma City. As the only forecaster in the Experiment who has never actually issued a Severe Thunderstorm or Tornado Warning, I realize that I can learn about the thought process that goes into these warnings!
As it turns out, the process varies a bit from each WFO to another. But the one thing that's consistent is that radar data is almost exclusively used as the tool for issuing warnings. The reason that the process varies is because some aspects of a storm's structure will indicate severe weather in the Great Plains, for instance, that won't be important in our region. The forecaster from the Oklahoma City office, for instance, deals with large hail much more often than we do in the DC Metro. And in Buffalo, the climate is very different than ours. I found that the forecaster from the Louisville office had the forecasting protocol that was most similar to what we would have in the DC Metro.
The Louisville forecaster, Kevin, said that there are specific things they look for on radar data when they're issuing warnings. As I mentioned before, they are almost exclusively tied to observations that you can make in radar data.
1. There's a threshold value in the radar reflectivity- that's a measure of the intensity of the precipitation- which can indicate large hail when that reflectivity is observed at the freezing point in the cloud. If you are familiar with dBZ units in radar reflectivity, then this will make sense: they look for a value of 65 dBZ at the OC isotherm.
2. A strong gradient in reflectivity on the updraft side of the storm cell can help forecasters recognize the threat for severe-level wind gusts.
3. If a storm cell is moving quickly, it's almost certain to get a severe warning, because wind gusts inside the storm will be amplified by the speed of the storm's movement. This is called relative velocity. For instance, if a storm has 30mph wind gusts and is moving at 40mph, then the relative velocity of those wind gusts would be 70mph.
4. Radars don't just send out a beam at one level; instead, the radar scans several times to pick up variances in reflectivity and velocity throughout the vertical column of the atmosphere. This allows the radar to see areas of rotation. If there is rotation in the mid-levels of the atmosphere, this is often an indicator for strong, straight-line wind gusts. This factor is called mid-altitude radial convergence.
5. Finally, if there's rotation evident in the lower levels of a storm cell, a tornado warning will be issued.
The whole point of this week's experiment (actually, it's a 4 week program, with different forecasters visiting every week) is to work with experimental products that could help streamline the process of analyzing a storm cell and determining whether a warning should be issued for a particular cell. But before these products can become operational, they need to be tested and tweaked to ensure that they're actually making the warning process better. It's been an honor to be a part of this project, and I'm excited to see what Day 4 will bring!
In case you missed it on my Facebook or Twitter page today, here's a look inside the Hazardous Weather Testbed: http://hwt.nssl.noaa.gov/spring_experiment/tales/2014-wk2/