Severe Weather Forecasting Secrets: Pt. 3: Nowcasting
In "Severe Weather Forecasting Secrets: Pt. 1: The Maps" and "Severe Weather Forecasting Secrets: Pt. 2: The WRF" we talked about forecast resources on the Internet that can predict the time and place of thunderstorms up to a week in advance. But what if it's the day that severe weather was predicted and it has (or hasn't) broken out?
If you're less than 8 hours from predicted severe weather, you're doing what's referred to as "nowcasting." By this point, it's too late to be looking at the models because they have an 8-hour delay. Your choices now are to monitor AccuWeather.com, SPC Advisories, PredictiveRadar and your radar scope. Here's how:
1. AccuWeather.com: During major severe weather outbreaks, AccuWeather.com has a team of meteorologists showing radar loops and talking about how the thunderstorms will progress via videos. However, there's no guarantee that we will be doing a video for your area at the time you need it. Check our video page or News Page to see if we have something timely. Look for "Breaking Weather" or a headline talking about specific storms or areas. If there's nothing there for you, check Henry Margusity's Facebook Fan Club - he will be issuing updates throughout the day on severe weather (starting this week), even if he only blogs once in the morning.
2. SPC Advisories: The Storm Prediction Center, who we talked about in Part 1, has a complex "Mesoanalysis" page showing many severe weather indices (calculations of risk) overlaid with radar, satellite, advisories, and their forecasts. This page can be helpful if you have studied meteorology but if you're the Average Joe that most of this tutorial is geared towards, it's overkill.
There are really only two things that you need to monitor from SPC when nowcasting; both are shown in the example above. The first is their MCDs (Mesoscale Convective Discussions). They issue these when they are monitoring a specific area for severe weather but have not issued a Watch yet. Even if you can't understand their abbreviations in the summary, this means they are paying close attention to your area and it's usually a good sign. Often they state whether or not they will issue a watch hours in advance.
The second is their Watches. The SPC issues a Severe Thunderstorm or Tornado Watch when severe weather is about to break out. It's nearly a guarantee that big thunderstorms will happen near you. Read the text in the summary for additional information.
3. PredictiveRadar: If severe weather has already broken out, you can get a good idea on the timing of when it will hit you by using AccuWeather.com RadarPlus PredictiveRadar (a subscription service with a 30-day free trial), which (unlike forecast models) takes the current radar picture and moves it 6 hours into the future (here's an example)! Here are a couple of case studies that I picked at random this week:
Taking a look at the 2.25-hour forecast from PredictiveRadar in Alabama, Storm (1) is a little eastward of its prediction, but Storm (2) is dead on. If you had been making a forecast for Birmingham 2 hours ago, you'd have been right on the money. Like any forecast, the farther you get out, the less accurate PredictiveRadar becomes.
Above is another more extreme example, looking at the 5-hour PredictiveRadar for Virginia. The storm marked at (1) is in about the right place, but the time is off by 1 hour. If attempting to forecast more than 2 hours in advance based on PredictiveRadar (or really, using any method that I have taught you) it's best to put 30-60 minutes on either side of your prediction. One final example: This is something I have run into before. You're having a yard sale in Nashville and the neighbors see clouds gathering. They ask you when they should start bringing their stuff back in the house. If you had predicted at 1:30 PM that it would start raining in Nashville "around 3", you'd be congratulated by your neighbors when their yard sale stuff was safely in their dry house.
There is one caveat: it doesn't generate or dissipate precipitation, so while it's a much better tool to estimate the arrival of storms than measuring on your screen with your finger, the storms could still dissipate (or more could spark up) before they reach you. To determine whether storms will dissipate, see if the WRF dissipated the storms in its forecast, and check if the storms are running out of an SPC Risk Area (Part 1) or Watch (see above). It will help to know your geography too -- for example, more often than not here in Central Pennsylvania, lines storms dissipate as they move eastward down the mountains (which dries them out).
4. The Radar: If you don't have access to Predictive Radar, you can still do a pretty good job watching the radar. You will be able to detect whether storms are dissipating or getting stronger (if they are) based on the radar loop. Any radar will do, but you may want something that shows the Storm Tracks (say, RadarPlus or Premium). This is a primitive form of PredictiveRadar, which doesn't show the movement of the entire radar picture; it just assigns 1-hour track windows to discrete strong storms, and it's fairly accurate (especially if you show the tracks in a loop to see if they are trending longer, shorter, or in a different direction). Below is an example from RadarPlus (the straight line covers 1 hour, the "pie slice" is the margin of error.
Unfortunately, if the automated government radar algorithms are not tracking your storm, you'll have to use "the finger method" to find out when storms will arrive (measure with your finger how far they went during the radar loop, then place your finger on the map to see how far they will go in the next timeframe). Using a radar system that plots the National Weather Service Warnings for your area on top of the radar (such as RadarPlus) is also helpful although it's usually too late for planning purposes if you're going to try to catch the storm.
Thanks for reading this series. Got additional tips or tricks for your fellow armchair meteorologists? Leave me a Comment below or on Facebook.
