Fair warning; this is a VERY lengthy post. However, the information within this post is EXTREMELY important, especially for those who live in areas that are prone to severe weather. I do ask you to read through this post, as it breaks down the steps taken to monitor severe weather; using today’s (11/17/13) severe weather outbreak as an example. Additionally, some of the links may change following today’s event (they are “live” pages) – however, archived information can be pulled from the links by entering the date for Nov. 17, 2013.
First of all, in order for chasers and spotters to be activated by National Weather Service officials, there must be a considerable chance of severe weather. Many spotters and chasers analyze model imagery for elevated chances of severe weather. When the models begin to come into agreement, the potential of an event arises and usually, the Storm Prediction Center (SPC) issues a statement on the threat. These statements can be found on the SPC’s Convective Outlook, which are detailed for days 1 through 3, and threat areas mentioned for days 4 through 8. At the bottom of each image is a synopsis and analysis of the threat (although with the extensive meteorological jargon, it may be hard to understand for some). Today’s Convective Weather Outlook is a prime example of what the product looks like for an outbreak.
With today’s event, many NWS offices have activated their storm spotter networks. The individual NWS offices release statements, commonly in their Hazardous Weather Outlooks, that have a statement regarding spotter information. For an example, here is the Hazardous Weather Outlook for today’s event from the NWS office of Lincoln, IL. Additionally, the SPC will issue Public Severe Weather Outlooks (PWO) which is aimed at state and local emergency managers, along with spotters, chasers, and the general public. The information within a PWO is explained in terms that are easier to comprehend compared to the convective outlook discussions, and also provides a statement of precautions for the public to follow. I suggest to take the time to compare today’s PWO with the discussion of the convective outlook hyperlinked in the prior paragraph; can you tell the difference? Additionally, Severe Weather Specialist Dr. Greg Forbes of The Weather Chanel updates his Tor:Con for events such as these, with today’s highest Tor:Con value as a 9 for central and eastern Illinois and Indiana.
Now for the day of the potential severe weather outbreak, Mesoscale Discussions (MDs or MSDs) are issued by the SPC regarding the threat and potential for watch issuance. A national list of MSDs can be found on a branch of the SPC’s website. To view the MSD, simply click one of the red areas or one of the items on the list below. For an example of a MSD, here is MSD #2011 that was issued for much of IN, central/lower MI, and northwestern OH. Within the MSD discussion, the threat is listed and described (meteorological jargon, yet again), along with the probability of watch issuance and sometimes, even the specific type of watch (which for #2011, was 95% for a tornado watch).
Continuing on from Mesoscale discussions are the watches that are often issued shortly after the issuance of a MSD. Watches mean that conditions are favorable for the occurrence of a specific weather event. For this example, I want to focus on the tornado watches that have already been issued for today’s event. Additionally, I can take you a step further by introducing you to a rare form of a watch; a PDS. First, all active convective watches can be found on another branch of the SPC’s website. First example is of a regular tornado watch, TW #563; along with an evaluation of the threat, there are many other sections of information along with safety precautions within the watch page. I also suggest taking the time to flip through the different tabs at the top of the watch image to familiarize with the different information.
Now, for the introduction to the PDS watch. A PDS watch is issued when the threat is extremely high for significant severe weather and/or tornadic activity. PDS stands for Potentially Dangerous Situation, and although it can be issued as a severe thunderstorm watch (usually for a high probability of damaging winds or extremely large hail), it is most commonly seen as a tornado watch. For this example, I will use PDS TW #561 (which was the resulting watch issuance of MSD #2011, linked above). The information within the watch is similar to that of the normal tornado watch. However, there is a considerable difference regarding the risk evaluations.
Following the development of a thunderstorm, there is the potential for a warning to be issued. Warnings mean that the event is imminent or already occurring. Although you can find a list of warnings on a branch of SPC’s page, I prefer the self-updating warning page from the College of DuPage’s Meteorology department. Within the warning texts are reports, radar information, hazard information, location of threat, path of threat, and precautions for the public to follow. Something to always remember is to heed all warnings issued by your local NWS office!
The last topic I wish to touch is storm reports. Storm reports are very important when it comes to producing new warnings or analyzing an event. Even though most storm reports are relayed by emergency managers, state/county officials, and trained storm spotters and chasers, the public is also encouraged to make reports to their local NWS offices. The SPC keeps a current list of storm reports, which are separated into unfiltered and filtered categories. Often times, there is a statement within warning text on how to make a report to your local NWS office. First of all, here are the guidelines, events, and criterion for making a report. Additionally, if there is no information regarding how to make a report, different methods and instructions for making a report can be found here.