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What is a velocity report

what is a velocity report

1. SPC questions (top )

1.4 Is the SPC a part of the National Severe Storms Laboratory?

The SPC is not a part of the National Severe Storms Laboratory (NSSL ). We are co-located in Norman with them and other NWS organizations such as the Norman Weather Forecast Office (WFO). the Radar Operations Center. and the Warning Decision Training Branch.

1.5 How can the media or the public, tour or visit the SPC?


Tours of the National Weather Center (NWC) in Norman, located on the south University of Oklahoma (OU) campus, are offered weekdays with each tour lasting 60-90 minutes. The tour provides a comprehensive overview of the NWC and the units housed within including the Storm Prediction Center. the Norman WFO, and NSSL, as well as the OU School of Meteorology, the NWC observation deck, classroom, and laboratory facilities. You must make reservations for tours as space is limited. For further information or to request an NWC tour, please see the NWC Tour Information page.


Interviews with SPC staff, TV shoots, and photo shoots by members of the media should be arranged first through Keli Pirtle, NOAA Public Affairs, (405) 325-6933, or email If Keli is unavailable, the backup contacts for urgent media inquiries are Greg Carbin (SPC Warning Coordination Meteorologist) at (405) 325-2080, or Peggy Stogsdill (SPC Administrative Officer) at (405) 325-2067.

1.6 What is the SPC copyright information and web links exchange policy?

We are pleased that you find our web site useful and you are certainly welcome to link to any of our pages.

Information on government servers is public domain, unless specifically annotated otherwise, and may be used freely by the public. NWS data and products form a national information database and infrastructure which can be used by other government agencies, the private sector, the public and the global community. We encourage innovative and constructive uses of NWS data and products, particularly when they contribute to the protection of life and property and the enhancement of the national economy.

While you are free to use NWS data and products, a few words of caution are in order. First, NOAA and NWS logos and names cannot be used in a manner that implies an endorsement or affiliation with NOAA/NWS. As NWS information changes rapidly in response to weather and climate events, special attention should be given to the date and time of the data and products being displayed. In addition, NWS information may not be modified in content and then presented as official government material. NWS is providing this data as is and the user assumes any risk related to using this data. And in no event is NWS liable to you or anyone else due to your use of this data.

Finally, as required by 17 U.S.C. 403, third parties producing copyrighted works consisting predominantly of the material appearing in NWS web pages must provide notice with such work(s) identifying the NWS material incorporated and stating that such material is not subject to copyright protection.

We do not provide reciprocal links. Our linking policy requires that any links outside of the U.S. government meet specific standards and requirements. The data must not be available from Federal agencies, must be necessary for and material to the presentation of agency information, and that the contents of the linked pages meet Section 515 Information Quality standards and comply with Department of Commerce restrictions on lobbying.

1.7 How does SPC forecast severe weather?

That's a simple question with a complex and ever-changing answer. The ways we make a severe weather forecast vary from event to event, as much as the weather itself, and depends on the timing, location, and intensity of the hail, thunderstorm wind, and tornado threats. This PowerPoint poster (9 MB) gives a summary of some of the tools and concepts we use in making severe weather forecasts.

2. Watch Questions (top )

2.1 What is a Severe Thunderstorm or Tornado Watch?

A Severe Thunderstorm Watch outlines an area where an organized episode of hail 1 inch diameter or larger and/or damaging thunderstorm winds are expected during a two to eight hour period. A Tornado Watch includes the large hail and damaging wind threats, as well as the possibility of multiple tornadoes or a single intense tornado. Typical watches cover about 25,000 square miles, or about half the size of Iowa. Click to view a video clip on "What's a Watch"?

2.2 How many watches do you issue in a year?

The SPC issues approximately 1000 watches each year. Of course, the precise number varies from year to year, depending on weather.

The SPC issues all Severe Thunderstorm Watches and Tornado Watches. All warnings are issued by local NWS offices (see Question 2.3 )

2.6 Why are watches not issued for all severe storms?

Many severe thunderstorms affect only a small area for a short period of time, making watches impractical. Watches are issued primarily for areas where well organized or significant severe weather is possible, or the severe weather threat is expected to persist for several hours.

2.7 I noticed the wording "THIS IS A PARTICULARLY DANGEROUS SITUATION" in some of your watches. What does this mean? What is the criteria for a PDS watch?

The "Particularly Dangerous Situation" wording is used in Tornado Watches for rare situations when long-lived intense tornadoes are likely. This enhanced wording may also accompany Severe Thunderstorm Watches for widespread significant severe events, usually produced by exceptionally intense derechos. PDS watches are issued, when in the opinion of the forecaster, the likelihood of significant events is boosted by very volatile atmospheric conditions. Usually this decision is based on a number of atmospheric clues and parameters, so the decision to issue a PDS watch is subjective with no hard criteria. However, the SPC goal is to have 3 out of every 4 PDS Tornado Watches verifying with multiple intense tornadoes. PDS watches are most often issued with a High risk in Day 1 Convective Outlooks.

2.8 Do you forecast hurricanes and tropical storms?

We do not forecast hurricanes and tropical storms. That is done by the National Hurricane Center. We do issue Tornado Watches for tropical cyclones striking the continental U.S. when conditions favor multiple tornadoes or a single intense tornado.

2.9 Where can I get a list of the points used for watch locations?

You can find a table containing these locations (sorted by lat/lon) here or (sorted by station ID) here.

2.10 What is the watch status line (line with an arrow inside a watch) and how is it interpreted?

When a watch is issued, the original threat is contained within the entire watch area. When the SPC determines where the severe weather threat continues within a Severe Thunderstorm or Tornado Watch, usually on an hourly basis, we issue a Watch Status Report. Unless the whole watch area remains valid, the Watch Status Report includes points on a line that indicates where the severe weather threat remains from our perspective. The severe weather threat is to the right of that line, as in our outlooks. As adjustments are made to the watch, you will see that yellow "status" line shift across the original watch area. This gives local NWS offices, emergency managers, media, and the general public guidance as to where the threat remains for severe weather.

2.11 Who clears watches?

It is up to the local NWS forecast offices -- not SPC -- to clear or keep counties within the watch once SPC has issued it. Our status lines (described above ) are just for guidance purposes to describe the remaining threat area.

2.12 What do the watch probability numbers mean? How are these different from outlook probabilities?

Watch and outlook probabilities have different meanings and should not be compared to each other. Probability numbers for watches tell you the chance that a certain kind of severe weather or combination of severe events (say, 2 or more tornadoes, or 6 or more combined severe hail/wind events) will happen in the whole watch. This is much different from outlook probabilities, which describe the chance of severe weather within 25 miles of any point. For more details on outlook probabilities, see Using the Convective Outlooks on the SPC Products page.

Watch probabilities are tagged as Low, Moderate, or High. This is different than how the outlook risks are tied to their probability numbers. For watches, the "Low" category contains probability values ranging from less than 2% to 20% (F2-F5 tornadoes), less than 5% to 20% (all other probabilities), "Moderate" from 30% to 60%, and "High" from 70% to greater than 95%.

2.13 Do you have an email or text messaging service for watches and warnings?

SPC does not offer this type of service. However, there are several private weather companies and other federal agencies that do take NWS watches, warnings, and other official weather bulletins and send them to individual e-mail addresses or cell numbers. More information can be found at

3. Outlook Questions (top )

3.1 What is this "Z" time

that you use all over your outlooks and also in other SPC forecasts?

The time you see on our products is Universal Coordinated Time (UTC) (also known as Greenwich Mean Time (GMT) or Zulu in military parlance -- hence the "Z" abbreviation). We do this for simplicity since the SPC forecast domain covers multiple time zones across the entire 48 contiguous U.S. states, and since all official weather records for scientific use are stored by Z time. It doesn't change with time zone or with daylight versus standard time, so Z makes an efficient and standardized way to communicate a common time reference, no matter where you are. Please visit this link for a conversion table. Another good site to visit regarding the UTC/GMT/Z time and your local time is US Naval Observatory Master Clock Time (Javascript must be enabled).

3.2 Where can I find a list of the abbreviations and contractions that I see in the Convective Outlooks?

We have a page devoted to the abbreviations and acronyms that you'll find in our products.

3.3 What do the Marginal, Slight, Enhanced, Moderate, and High risk categories in the Convective Outlook mean?

Marginal, Slight, Enhanced, Moderate, and High risks represent progressively larger threats for organized severe storm episodes. These risks, along with their numerical, abbreviated labels, and colors (1-MRGL-dark green, 2-SLGT-yellow, 3-ENH-orange, 4-MDT-red, 5-HIGH-magenta), are based directly on the numerical probabilities of severe weather that we provide with every outlook. See the Severe Weather Risks section on the SPC Products page for more information.

3.4 What do the Convective Outlook probabilities mean?

For Day 1, the percentage lines provide the chance that the given type of severe weather (tornado, hail, or damaging thunderstorm wind) will happen within about 25 miles of a point. For Day 2 and Day 3, the probabilities cover all severe storm hazards together. Categorical (1-MRGL, 2-SLGT, 3-ENH, 4-MDT, 5-HIGH) labels are tied directly to the probability numbers as defined in this table for Day 1. in this table for Day 2 and this one for Day 3 Convective Outlooks.

3.5 Why do the probability values on the Convective Outlooks seem so low?

The probability values represent the chance of severe weather within about 25 miles of a point, which is about the size of a major metropolitan area. Though severe storms tend to receive a large amount of media coverage, severe weather is uncommon at any one location. Your chance of getting a tornado on any random day are very small, climatologically speaking. Put in that context, even a 10% chance of a tornado within 25 miles of a point means a much bigger threat than usual, and should be taken seriously. Think of how often tornadoes normally happen close to you on any given day, and those small-looking probabilities start to seem large by comparison!

3.6 What is that nameless light green area on the Day 1-3 Convective Outlooks?

The light green area on the categorical graphic is for general, (non-severe) thunderstorms, with a 10% or greater probability of thunderstorms to the right of the green line.

3.7 Where can I get a list of the points used for the Convective Outlook Areal Outline product?

3.8 What do the points in the Convective and Fire Weather Outlook Areal Outline product mean?

4. Weather Questions (top )

4.1 How does the NWS define severe weather?

4.2 How does the NWS define a severe thunderstorm?

A severe thunderstorm refers to a thunderstorm producing hail that is at least 1 inch in diameter or larger, and/or wind gusts of 58 mph or greater, and/or a tornado. Although lightning can be deadly, the NWS doesn't use it to define a severe thunderstorm. If it did, every thunderstorm would be severe, by definition. Also, excessive rainfall may lead to deadly flash flooding, but heavy rain is not a severe criterion either. The flood threat is handled through a separate set of watches and warnings from your local NWS office.

4.3 How does the SPC define a significant severe thunderstorm?

A significant severe thunderstorm refers to a thunderstorm producing hail that is at least 2 inches in diameter or larger, and/or wind gusts of 75 mph or greater, and/or a tornado that produces EF2 or greater damage.

4.4 What is organized severe weather and why is it more important than non-organized severe weather?

To some degree, all thunderstorms are organized. We refer to organized severe storms as those which persist for several hours, are capable of multiple and/or significant severe weather events, and which we are able to forecast consistently. Organized severe storms tend to form in areas where the vertical wind shear is relatively strong and has particular characteristics, while non-organized severe storms are most common when vertical wind shear is weak. The most long-lived, intense tornadoes and largest hail are usually produced by supercell thunderstorms, while the most serious convective wind storms are produced by bow echoes.

4.5 What environmental parameters are most often observed with organized severe weather?

In general, severe thunderstorms form in areas where moisture, instability, and lift are present. Additionally, long-lived severe thunderstorms are often associated with strong vertical wind shear (e.g. winds that change direction and increase in speed with height). Unfortunately, thunderstorm initiation is not well-understood, and weather observations are too widely spaced to detect all important small-scale features that influence storms.

4.6 Do you have any educational information about lightning?

NSSL has a great FAQ about lightning. NWS Pueblo also provides handy links to web resources on lightning.

4.7 Where can information be obtained on the occurrence of lightning from a past storm event?

The SPC does not maintain a database for past lightning strikes. While the NWS does not endorse any commercial lightning vendors, there are numerous vendors of lightning detection systems and data. A list of commercial vendors can be found at

4.8 Can you give me details on a particular hailstorm, thunderstorm or tornado which hit on a certain date? Or can you tell me about hail or wind damage for the last 20 years in a certain place?

Unfortunately, no. We don't have the resources or staffing to fulfill every local weather information request we receive. However, the National Climatic Data Center (NCDC) does provide local weather event information and for severe weather events, in a publication called Storm Data or interactively through the Storm Events Database.

4.9 What is a derecho and where do I learn more about them?

4.10 Can you tell me what the weather will be like on a specific date? Or is it going to rain today?

SPC doesn't produce local weather forecasts, and doesn't have the staffing to handle individual forecast requests like this. The SPC home page. along with the main page of any other NWS center or office, will give you local forecasts by city, state, or ZIP code, using an entry box at the upper portion of the page.

5. Tornado Questions (top )

5.1 What is a tornado?

5.2 How many tornadoes and tornado deaths occur in a year?

5.3 How does SPC forecast tornadoes?

5.4 Do you have some tornado safety tips?

Many issues of tornado safety are also covered in the Tornado FAQ under its Safety section.

5.5 How many tornadoes have there been in my state or county?

6. Other Questions (top )

6.1 Why did some severe weather not show up on your reports list? How do I get a list of reports for a particular place or period of time?

6.3 Where can I get information on building a safe room in my house to help protect our family from tornadoes?

6.4 What is NOAA Weather Radio and how can it help to protect me?

6.5 How does someone become a meteorologist (at the SPC or elsewhere)?

For example, in the following report:

the latitude is 37.42 (or 37.42N) and longitude is 95.68 (or -95.68W)

6.9 What are the 3-letter IDs in the Storm Reports comment section?

6.10 What are the differences between the regular and the raw Storm Reports CSV files?

6.11 What are the differences between the filtered and the unfiltered Storm Reports?

Unfiltered. On March 8, 2011, the SPC started providing unfiltered storm reports by removing space/time filtering on incoming NWS LSRs. This approach is consistent with NWS storm-based verification methods. However, identical reports are removed from SPC logs and should not appear on the preliminary maps and lists.

Filtered. The space/time filtering had been used by SPC in an attempt to reduce duplicate reports and limit artificially inflated initial estimates of severe weather events when many reports arrived for the same event. The filtering is applied to each of the tornado, hail, and wind reports separately.

The logic implemented for filtering is as follows:

6.12 How do I unsubscribe from email/SMS weather alerts?

6.13 What is the SPC/University of Oklahoma Career Experience Program?

6.14 What is the difference between the experimental and the operational Storm Reports pages?

7. Fire Weather Questions (top )

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