Written by cuweathernerd    Tuesday, 03 January 2012 08:22   
Chasing 101: Section 1:1 Forecasting: The Storm Prediction Center (SPC)

This is the first in a series of discussions about chasing. They are meant to be accessible to people completely new to chasing, and provide an agar for veteran chasers to share their knowledge. Terms in bold are meteorologically significant, and defined in a glossary at the bottom of the post. Because chasing is anchored in the physical, real world examples will be used whenever possible. For today's reading, we'll be using the May 24th 2011 tornado outbreak for examples.


There can be no place to start a discussion on chasing besides understanding what professional meteorologists see in any given situation; their expertise, education, and experience gives them an edge over all but the best storm chasers in understanding the synoptic set up.

When it comes to severe convection, the most widely used website, and the most important to understand, is the Storm Prediction Center, or the SPC. They employ the premier scientists that deal with severe weather, and their entire job is to monitor the United States for severe weather setups, and issue forecasts in the form of convective outlooks, mesoscale discussions, and watches.

Let's take a brief look at their website. You can access it at spc.noaa.gov and you should memorize or bookmark this address -- you're going to use it a lot.

Anyway, for now just click around and become familiar with the interface. Let's familiarize ourselves with the three main products of the SPC using a simple case study.


We'll start with the product with which you are most likely familiar: watches.

The SPC is the only entity which issues severe weather watches in the US. For our purposes, we are only concerned with severe thunderstorm and tornado watches.

A watch is issued in advance of severe convection, when atmospheric conditions are most conducive to severe weather. In general, they are issued after convective initiation is visible on satellite imagery, and also generally after a mesoscale discussion (discussed later in this post).

A watch on the SPC's page looks like this.

You'll see the radar when the watch was issued, with the included counties outlined. A short rationale for the watch is provided below it. Right now, that looks jargon filled and imposing, especially because it is in all caps. Future posts will explain the mass of what is being said in that text, and help you to capitalize on that information in the field.

One important thing to recognize in this watch is that it is a particularly dangerous situation or PDS watch. These are relatively rare, and indicate situations which are particularly prone to produce violent, long track tornadoes, or (more rarely) significant severe winds associated with a derecho. You can learn more about PDS watches here.

Finally, you will notice that there are probabilities for each of the severe criteria listed happening within the watch's bounds -- these probabilities give you a quick judgement of how the SPC sees the situation playing out.


With that in mind, let's take a look at a typical mesoscale discussion:

Mesoscale discussions, like watches, are issued by the SPC on the day of the severe convection. They are a short analysis of an area likely to see severe weather develop, or of an ongoing event. They cover an area larger than a single storm cell, and are generally more meteorologically intensive than most discussions. They also tend to precede a watch by 1 to 3 hours, and generally discuss the likelihood of issuing or continuing a watch.

You can see what an MD looks like in it's entirety here.

Again, you see a block of all caps text. That is the most important part, and it is likely more or less jargon to you now. The hope of this series is to help make that information usable to you to chase more safely and productively.

For now, know MDs exist. As you get ready to chase, these will probably be amongst the most important tools you have available.


Finally, we are going to discuss convective outlooks:

These are forecasts made by the SPC in the days before an event. They are categorical probabilities of an event happening within 25 miles (40km) of a point, and a meteorological synopsis of why that outlook is being issued.

Convective outlooks are issued in three 'flavors' -- slight, moderate, and high risks. Each relates to a specific percentage probability, as dictated by this table (from wikipedia)

On the current day, the outlooks are made for wind, hail, and tornadoes instead of severe weather events as a whole.

These resources will be a great starting point if you are just learning to forecast, or a nice reference if you are more experienced. When chasers start really getting excited, it tends to line up with strongly worded convective outlooks, for obvious enough reasons.

Let's take a look at the day 1 outlook for our May 24th event:

caveat -- this isn't exactly how this page will look in on the day of an event, but it's the best we can do for now -- the data itself is identical

You can see this is a pretty rare day -- a high risk has been issued for OK and KS. You can see a large area of 30% tornado risk -- meaning the SPC thinks that any given point in that high risk has a 30% risk of a tornado within a 25 mile radius. That's pretty significant! (Higher numbers can happen, but they are rare).

What's more, you can see the hatching -- areas shaded with lines. Those indicate areas where particularly severe weather is possible with greater than EF2 tornadoes or very large hail, or both.

Again, there is that prevalent block of all caps text. Like the watches and MDs, this is the heart of this product, and the hardest part to use. If you want to learn a little more about convective outlooks click here. For a little more in depth look at any of the SPC's products, check out their website.


How did the SPC do this day? Pretty well -- there were 57 tornadoes and 590 severe weather reports, including one EF5.

How did I do? I chased, but in southern KS, which I chose out of necessity (distance, chasing alone, etc) That area, for complex reasons, busted in terms of tornadoes. Mess of wind, hail, and fast moving storms. Here's one picture, the flank of a severe thunderstorm which I just punched through trying to get to a better position.



 

 

 

Glossary

Convection - Meteorologists like to use this word as an analogue for thunderstorms. In discussions, convection is most likely being used in this way. Sometimes, you will see convection separated between shallow and deep. Cumulus (cu) towers tend to be associated with shallow convection, while deep convection is marked by cumulonimbus (cb).

Convective Initiation - The onset of deep convection. Generally, this means environmental inhibition to thunderstorm development has been overcome, and that a convective event is likely.

Convective Outlooks - 1 to 192 hour forecasts made by the storm prediction center that highlight severe weather possibilities categorically.

Mesoscale Discussions - Short time-limited discussions of a specific severe weather set up on the day of the event. Usually precedes the issuance of a severe thunderstorm or tornado watch.

Potentially Dangerous Situation - A specific kind of watch reserved for the most violent severe weather set ups.

Synoptic - "seen at the same time" -- a branch of meteorology which deals with weather on large scales -- fronts, cyclones (low pressure areas) and jet streams. Events tend to be on the scale of thousands of kilometers and have a duration of weeks.

 

 

 

 

 

 


 

The above is an article written by Reddit user cuweathernerd and has been modified by Ryan Lehms. The original article and discussion can be found here:

http://www.reddit.com/r/stormchasing/comments/o0w0o/chasing_101_section_11_forecasting_the_storm/

Last Updated ( Tuesday, 06 August 2013 18:55 )
 

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