With severe weather season coming on strong, I figure I’d post something to elaborate on the certain alerts that are used. I’m basing this off what the National Weather Service (NWS) in the Quad Cities uses, so you can be assured that it’s accurate in the U.S.
For starters there’s the forecast, which I don’t think needs to be explained to you. It’s the anticipated weather for the next 24+ hours. Note, that I indicate it is over 24 hours. That’s because shorter term predictions are made as well. The short term forecast is one of these which spans out 1 to 3 hours. The remainder are outlooks, such as the NWS’s Hazardous Weather Outlook (HWO), Area Forecast Discussion (AFD), and an Area Weather Summary (AWS).
The next key in severe weather prediction that you should examine is the Convective Outlook (which spans from Day 1 to Day 8), the most current is the DAY1 product, DAY2 is for the following 24 hours, DAY3 is for the next, Day’s 4 through 8 are combined into a single outlook.
The DAY1 product should be your first go to in order to evaluate the risk of severe weather. An area that has a 30% risk of hail means that the probability is slight that hail will be present in any storm that develops that meets severe thresholds within 30 miles of an area. (we’ll talk about those thresholds in a minute). A tornado risk of 30% in contrast is high. Hatched areas are locations where significantly large or strong storms may produce larger hail, more destructive wind gusts, and strong tornadoes. The higher the risk of tornadoes, the higher the chance of a total tornadic outbreak.
Natively, Day’s 1 and 2 are the only categorical days where the NWS will use the term “HIGH RISK”. HIGH RISK means that any storms that do develop will likely strengthen to severe thresholds and a powerful severe weather outbreak appears likely. Each day that is associated with at least a “MODERATE RISK” will be accompanied by a Public Severe Weather Outlook (PWO), but it is imperative that you read the PWO when in a HIGH RISK area. Area’s can be upgraded during the middle of the day, but it is extremely rare and usually reserved for particularly dangerously developing conditions.
The next scale of severe weather forecasting is what is known as the Mesoscale Discussion (MD). Mesoscale Discussions differ from other severe weather forecasting products issued for convective purposes (that is thunderstorms), in that it is issued also for winter weather and flooding. This includes ice storms, blizzards, heavy snow, flash flooding, increased flooding potential up to five hours, sleet, freezing rain, and torrential rainfall.
During convective events such as during severe thunderstorm season, these discussions are the precursor to a watch. Mesoscale discussions basically break down the national short term forecasting that is plotted by maps, into a discussion concerning a smaller area in a nearer term. There are several stages of a convective MD.
- Watch Unlikely: Means that the probability of a watch being issued in this area is low, or under 20%. Usually this is reserved for when there are isolated severe storms moving through an area, but conditions are not widely favorable for the continued development of storms.
- Watch Possible: Means that the probability of a watch being issued in this area is moderate, between 20% and 50%. This is the most frequently used MD, and typically means that conditions are marginally favorable across the entire area. A watch may be issued if severe weather develops or conditions begin to intensify in the favorability for storms to develop.
- Tornado Watch Possible: Same as above, but reserved specifically for conditions becoming favorable for the development of tornadoes. Do not assume that because the word Tornado Watch is omitted from the previous MD that the watch will not be a Tornado Watch.
- Watch Likely: Means that the probability of a watch being issued in this area is high, between 50% and 90%. This is used when conditions are favorable for the development of severe weather, or severe weather is already ongoing in the MD area.
- Tornado Watch Likely: Same as above, with the same parameters of the former MD.
- Watch Needed Soon: Means that a watch WILL BE ISSUED within the next hour. This is usually used when strong severe thunderstorms are imminent or likely to occur. Severe thunderstorms may also be moving into the MD area and retaining strength or intensifying.
- Tornado Watch Needed Soon: Means that a watch WILL BE ISSUED within the next hour. Usually reserved for particular types of Tornado Watches.
What makes a thunderstorm severe?
A severe thunderstorm is characterized by meeting one or multiple criteria’s.
- Wind gusts in excess of 58 MPH (Cited as 60 MPH usually).
- Hail in excess of 1″ in diameter.
- A tornado.
What are the stages of convective watches?
Contrary to what most media outlets inform the public, there are actually four levels of convective severe weather watches. Watches are not issued by local weather offices. Only the Storm Prediction Center (SPC) can issue convective watches and MD’s.
- A Severe Thunderstorm Watch is issued when conditions are favorable for the development of severe weather through the end of the watch period. These watches usually run from 4 to 10 hours. This includes hail up to 3 inches in diameter, and thunderstorm wind gusts up to 80 MPH.
- A Particularly Dangerous Situation (PDS) Severe Thunderstorm Watch is issued when conditions are favorable for the development of severe weather through the end of the watch period. These watches can last from 5 to 12 hours. However, unlike it’s stage one form, the PDS variant means that conditions are favorable for rapidly developing severe thunderstorms (storms that form and become severe in as little as 35 minutes), life threatening hail in excess of 3.5 inches in diameter, and destructive wind gusts over 90 MPH. Severe thunderstorms can form in these areas with little or no advanced warning.
- A Tornado Watch is issued when conditions are favorable for the development of tornadic supercells or severe weather through the end of the watch period. These watches typically run 4 to 10 hours. The watch is issued for risks of tornadic thunderstorms, a few of which may be strong – producing winds of over 170 MPH, hail up to 3 inches in diameter, and thunderstorm wind gusts up to 80 MPH. Severe thunderstorms can form tornadoes with little or no advanced warning.
- A Particularly Dangerous Situation (PDS) Tornado Watch is issued when conditions are favorable for the development of tornadic supercells or severe weather through the end of the watch period. These watches typically run 5 to 12 hours. The PDS variant means that conditions are favorable for rapidly developing severe thunderstorms (storms that form and become severe in as little as 35 minutes), multiple large, destructive and life-threatening tornadoes, hail up to 5 inches in diameter, and destructive wind gusts in excess of 90 MPH. Severe thunderstorms can form in these areas with little or no advanced warning, and any that do can rapidly produce tornadoes without any warning.
What are the stages of convective warnings?
Contrary to popular belief, there are not three types of convective warnings, but in fact there are five convective warnings, three of which are tornadic.
- A Flash Flood Warning is issued when excessive rainfall has fallen over the warned area. These are the longest convective warnings, lasting up to 8 hours. Flash Flood Warnings are not indicative of severe thunderstorms over the warned area, but storms producing heavy rainfall. Flash Flood Warnings are also issued when any flash flooding is occurring or imminent, such as levee breaks, dam bursts, or ice jams.
- A Severe Thunderstorm Warning is issued when a storm has met the criteria for severe thunderstorms and is moving over the warned area. This may be Doppler radar indicated, reported by storm spotters, or local law enforcement. All severe thunderstorms are capable of producing tornadoes without any advanced warning. Seek shelter in a sturdy structure until the storm passes.
- A Tornado Warning (Variant: TOR) is issued when Doppler radar has indicated a severe thunderstorm capable of producing a tornado. Do not wait to see the tornado, seek shelter in a sturdy structure until the storm passes.
- A Tornado Warning (Variant: TORR) is issued when local law enforcement, or a storm spotter has reported a tornado on the ground. This also includes funnel clouds which are greater than half way to the ground. Do not wait to see the tornado, seek shelter immediately in a sturdy structure until the storm passes.
- A Tornado Emergency (Also known as a Tornado Warning variant TORE), is the least used convective warning. A Tornado Emergency is issued when one of the following criteria are met: 1) a tornado is on the ground and doing damage, and is approaching a largely populated area; 2) a large (wedge) funnel is on the ground and doing damage; 3) a multiple vortex system is on the ground and doing damage; 4) a tornado is on the ground in a populated area; 5) one or more fatalities have already been confirmed by the tornado. THIS IS AN EMERGENCY. Person’s living in mobile homes should flee them and seek shelter in a sturdy structure such as an industrial building. Person’s living in an area without access to a basement should move to the safest location in your home if you are unable to get to a stronger shelter. DO NOT IGNORE THIS WARNING, AT RISK TO YOUR LIFE. Do not leave your shelter until the emergency expires.
It should be noted, weather radio statements may not necessarily say that the issued product is a variant TORR or TORE.
TORR is identified by listening to the statement:
AT 634 PM CDT…A SEVERE THUNDERSTORM CAPABLE OF PRODUCING A TORNADO WAS LOCATED NEAR SARCOXIE…AND MOVING NORTHEAST AT 35 MPH.
HAZARD…TORNADO AND PING PONG BALL SIZE HAIL.
SOURCE…WEATHER SPOTTERS REPORTED FUNNEL CLOUD.
TORE is identified at the header:
…TORNADO EMERGENCY FOR MAYFLOWER AND VILONIA…
THE NATIONAL WEATHER SERVICE IN LITTLE ROCK HAS ISSUED A * TORNADO WARNING FOR… SOUTHWESTERN WHITE COUNTY IN CENTRAL ARKANSAS… NORTH CENTRAL PULASKI COUNTY IN CENTRAL ARKANSAS… EXTREME NORTHWESTERN LONOKE COUNTY IN CENTRAL ARKANSAS… SOUTHERN FAULKNER COUNTY IN CENTRAL ARKANSAS…
TORE also uses explicit wording at the close of it’s statements:
THIS IS AN EXTREMELY DANGEROUS AND LIFE THREATENING SITUATION. IF YOU CANNOT GET UNDERGROUND GO TO A STORM SHELTER OR AN INTERIOR ROOM OF A STURDY BUILDING.
So…be prepared when those annoying tones interrupt your television or radio program. Be aware of the alerts on your phone, and pay attention to what kind of alerts are being issued. Knowing these simple facts, as well as the difference between a watch and a warning could save your life.