Regardless if the sun is shining or not, or if severe weather is anticipated or not, the Storm Prediction Center (SPC) issues convective outlooks for each day of the week, every day of the week.  On days two and three, there is a categorical risk (none to high (moderate on day three)) and an accompanying percentage risk that is issued.  The same is true for day one, except each severe weather mode is broken down into it's own percentage.  The SPC issues these outlooks at various times throughout the day as data is collected by local weather forecast offices (WFO).  Based on the collection of this data, outlooks are refined and updated.  There are some instances where an area in a marginal risk zone will be increased to slight, enhanced, or even moderate (such as the 10 August 2020 event).  Other times, moderate or high risk areas may get downgraded.  The presence of any risk area never guarantees severe weather one way or another.  Think of these as a forecast built entirely on the chances of severe weather.

To understand that, you have to understand how precipitation chances would work.  The term "percent chance" generally refers to the number of times a model has run a scenario with a single event happening within X amount of distance from any given location.  So for example, a 20 percent chance of rain, means that 20 percent of the model output resulted in rain moving across any given location where that percentage is used.  The same is true for the severe weather outlook, except the SPC may increase or decrease this risk based upon known variables collected that models may not pick up on.  In the instance of the SPC outlooks, the percent chance term means that an event has been forecasted to impact any given area within 25 miles of a point.  So a 30 percent chance of a hail event means that of the model outputs provided after adjustment, 30 percent of those models resulted in a hailstorm within 25 miles of a location.

As previously mentioned, the SPC issues outlooks up to eight days out.  Days four thru seven will result in either a 0, 15 or 30 percent chance.  Days one thru three, however are treated differently.  The chart on the left denotes the general risk values for days one thru three.  Day one will use categorical plus the tornado, hail, and wind figures.  Day one does not use overall percent values.  Days two and three use overall percent values and the categorical values, but will not use tornado, hail, or wind.  Day three will never output a "high" risk - it is capped at moderate and all values will not exceed 45 percent until that time period moves to day two.

* - categories from enhanced thru high include "significant event" variables, which can increase the categorical index if present.  Therefore, some values for each categorical risk are a range rather than a definitive number.

Assuming you are in an area that has at least some favorability for the development of any kind of severe weather, you may find yourself progressing to step two.  The MCD is issued when conditions are showing some signs of organizing for either a localized or widespread severe weather event.  Many MCD's result in no further action beyond an isolated storm that builds up and weakens.  However, other MCD's are issued ahead of ongoing catastrophic severe weather events such as the one to the right.  There are different types of MCD's that are attached to different types of severe weather.  Convective MCD's are issued in association to thunderstorm activity such as tornadoes, large hail, and damaging winds.  They can also be associated with hurricanes, tropical storms, or outflow boundaries.  Rainfall MCD's are issued in connection to heavy rainfall where rates meet or exceed 1" per hour for an extended period of time.  These can also be associated with hurricanes and tropical storms.  Winter MCD's are often associated with ice storms, freezing rain, and snow squalls where snowfall rates exceed 1-3" per hour.  In the early Spring and late Fall, Winter and Convective MCD's can overlap.  Rainfall  MCD's can overlap either of the other MCD's at any time.

For the purpose of this article, we will be looking at the Convective MCD.  In general, these MCD's are issued with the verbiage of "watch possible" or "watch likely."  However, it is possible to see categorical enhancements, imminent weather watch messaging, and meso-level discussions on ongoing severe weather activity.  Meso discussions generally focus on increasing awareness of an increased severe weather threat within an existing weather watch.  Others will provided the basis for watch updates, such as cancellation, expansion, or extension.

In most cases, MCD's preclude a weather watch.  This is because the MCD is the SPC's means of notifying emergency management that lines of communication between the local WFO and the SPC have been opened and are discussing the immediate need for a weather watch.  The SPC may request increased radar scans, additional instrumentation, additional atmospheric soundings (such as weather balloons) and surface observations to base their decisions.  For example, if a WFO is constantly issuing a warning on a single cell and it continues to grow, that may influence the SPC to issue a watch.  However, if that cell is fluctuating and there are no confirmed events unfolding within those warnings, the SPC may not issue a watch.  The same may be true depending on the results of atmospheric sampling.  For this reason, an MCD may not be indicative of an imminent watch - but the opposite may also be true.  Thus, it is important to examine the headers of the MCD where it will indicate watch issuance likelihood or the overall basis for the discussion (i.e. Threat for WW"XXX" continues, "Watch Unlikely, etc - shown right).  A movement to a higher categorical risk, however, should be taken to mean that conditions may rapidly deteriorate and that a watch may be imminent.

Far more MCD's are issued than any other weather product in the severe weather event chain.

Most of us have probably seen the meme of the Wicked Witch of the West and Almira Gulch being the difference between a watch and a warning.  That is to say, it's not totally removed from being accurate.  Watches are often preceded by MCD's as we just discussed and, as advertised when they are broadcast, mean that conditions are favorable for the development or continuation of severe thunderstorms (in this case).  A watch can mean that any kind of alerted weather can form or occur in a given amount of time.  Since we're talking about convective watches, that breaks down into three categories, each with two types of watches in them.  However, unlike what most people think, no two watches mean the same thing.  In addition, some watches have additional caveats and different meanings than others.  Some are significantly more serious than others as well and are far more ominous or serious when it comes to how dire a situation is or can become.

Before we get into that, below is the average annual watch issuance by county for tornado (left) and severe thunderstorm watches (right).

The oddball in the convective watches goes to that of the Flash Flood nomenclature.  These watches are handled exclusively by the local WFO and are not issued by the SPC.  Flash Flood Watches are also the odd ones out due to their duration and lead time of notice.  For example, these watches can be issued up to two days in advance and remain in effect for up to two days.  The average length for a watch, however, is generally about a day.

As their name suggests, Flash Flood Watches are issued when locally heavy rainfall is expected or imminent.  This may result in flash flooding, where water levels rise suddenly and rapidly.  However, a much more urgent version of this watch is the Particularly Dangerous Situation (PDS) watch.  The criteria for PDS Flash Flood Watches varies from WFO to WFO.  Generally these are reserved for major rainfall events where amounts exceeding four to six inches are possible in a very short amount of time.  PDS Flash Flood Watches are generally not coded differently from their more common standard Flash Flood Watch, and therefore may require some attention to retrieve the severity.

The Severe Thunderstorm nomenclature makes up two of four convective weather watches that are issued by the SPC.  These watches are issued when conditions are favorable for the development of storms that can produce hail over one inch in diameter or thunderstorm wind gusts over fifty-eight miles per hour (generally broadcast as sixty).  Storms do not necessarily have to have formed for a watch to be issued, conditions may favor development on the edge of the watch area and progress across the rest of it.  Other watches may be ahead of an ongoing storm system.  They are the most common watch to be issued by the SPC.  However, their PDS counterpart is one of the rarest issued.

A PDS Severe Thunderstorm Watch, as was issued on Monday, is issued when catastrophic damage is likely to occur from severe thunderstorms where tornadoes are not the main mode of destruction.  Generally, this will mean widespread wind damage over the entire watch area.  PDS Severe Thunderstorm Watches are therefore usually indicative of ongoing severe wind events such as derechos.  Most PDS watches of this variety include key notes for winds in excess of eighty miles per hour, with some (like on Monday) reporting up to 100 miles per hour or more.

Some systems are able to interpret the PDS identifier of these watches, and as such will display them or treat them differently.  Most news stations and media will advertise them in a different fashion.  While ever severe weather watch should be treated seriously, PDS watches are indicative of a more serious and ominous threat.  They may be issued for rapidly developing discrete cells that may form without any warning, widespread events where the probability of impact is greater than 95 percent, or indicate ongoing cell development where the chances of significant events are becoming more probable.  Statistically, a PDS watch is more likely to be indicative of an event that will produce an impact to an individual at any given location than it's standard counterpart.

Header text from a typical PDS Severe Thunderstorm Watch:

The NWS Storm Prediction Center has issued a

   * Severe Thunderstorm Watch for portions of 
     Eastern Iowa
     Northern Illinois
     Far northwest Indiana
     Far southern Wisconsin
     Lake Michigan

   * Effective this Monday morning and evening from 1125 AM until
     700 PM CDT.

   ...THIS IS A PARTICULARLY DANGEROUS SITUATION...

   * Primary threats include...
     Widespread damaging winds and scattered significant gusts to 100
       mph likely
     Isolated large hail events to 1.5 inches in diameter possible
     A couple tornadoes possible

Header text from a typical Severe Thunderstorm Watch:

The NWS Storm Prediction Center has issued a

   * Severe Thunderstorm Watch for portions of 
     Upper Michigan
     Eastern Wisconsin
     Lake Michigan
     Lake Superior

   * Effective this Monday afternoon and evening from 100 PM until
     700 PM CDT.

   * Primary threats include...
     Scattered damaging wind gusts to 70 mph possible
     Scattered large hail events to 1.5 inches in diameter possible

The popular watch amongst the convective watches is, of course, the Tornado Watch.  After all, when people think severe weather, it is easy for them to imagine a tornado roaring across the plains.  But, much like warnings and the severe thunderstorm counterparts, no single tornado watch is like another.  Tornado Watches have some unique characteristics about them that Severe Thunderstorm Watches do not, as well.  For example, Tornado Watches are frequently issued accompanying hurricane and tropical storm landfall as tornadoes can spin up in the rain bands of these storms.  A result of this is long-duration watches of up to twelve hours, sometimes stacked on each other.  In coastal areas, it is not uncommon during a landfall to be under a Tornado Watch for up to a day and a half.

Like the other convective watches, a Tornado Watch does have a PDS variant.  These are issued far more often than PDS Severe Thunderstorm Watches, but remain extremely rare.  PDS Tornado Watches account for ten or less convective watches issued every year on average.  They are usually indicative of strong tornadoes that are long-lasting and capable of doing major damage on their own.  Other uses for PDS Tornado Watches include rapid storm development where warnings may come too late, or instances where widespread tornadic activity is expected.

Tornado Watches do not necessarily mean that only tornadic activity is possible.  Damaging wind and hail are still very much possible, and are statistically more likely to occur than a tornado.  However, Tornado Watches do indicate an elevated risk of tornadic activity in the area.  Remember, any severe thunderstorm can produce a tornado with little or no advanced warning.  Therefore, it is important you treat all severe thunderstorms as potentially tornadic in nature.

The watch phase means to be prepared to move, be on the lookout, and keep in mind that your plans may need to change.

Header text from a typical PDS Tornado Watch:

 The NWS Storm Prediction Center has issued a

   * Tornado Watch for portions of 
     Central and Southern Alabama
     Southeast Mississippi

   * Effective this Sunday afternoon from 440 PM until Midnight CDT.

   ...THIS IS A PARTICULARLY DANGEROUS SITUATION...

   * Primary threats include...
     Numerous tornadoes expected with a few intense tornadoes likely
     Widespread large hail and isolated very large hail events to 3
       inches in diameter likely
     Widespread damaging wind gusts to 70 mph likely

Header text from a typical Tornado Watch:

 The NWS Storm Prediction Center has issued a

   * Tornado Watch for portions of 
     Eastern Illinois
     Southwest and central Missouri

   * Effective this Wednesday afternoon and evening from 210 PM
     until 900 PM CDT.

   * Primary threats include...
     A few tornadoes possible
     Scattered damaging wind gusts to 70 mph likely
     Scattered large hail events to 1.5 inches in diameter possible

Warnings, for all intents and purposes, mean that weather is happening now or will begin shortly in the warned area.  It is a call to action for persons in the path.  Warnings have their own hierarchies, structures, and characteristics that make each of them unique just like watches.  However, contrary to how watches function, warnings are much smaller in scope, shorter in duration, and can exceed the watch criteria.  Much of the confusion regarding warnings stems from radar appearance.  Public scrutiny of convective weather warnings generally stems from storms "not appearing that bad" when examining radar.  However, it is important to note that radar is a depiction of reflectivity, and not indicative of the intensity of all values of a storm complex.  For example, radar may occasionally not depict intense reflectivity, but it may return a large amount of vorticity in a storm system, thus prompting a Tornado Warning.  Low echoes may also be observed with high velocity, prompting Severe Thunderstorm Warnings.  One thing reflectivity is generally good at is depicting heavy rainfall and sometimes hail cores, but when it comes to wind and sometimes tornadoes, reflectivity is not the final authority on whether or not an event is ongoing.

Flash Flood Warnings can vary in severity and largely depend on your topographical location for significance.  However, they are universally known for being issued when incredible amounts of heavy rain fall in a short period of time.  This can be dangerous regardless of your location, as ground that may not be able to absorb water could result in rapidly rising water that could prove damaging or life threatening.  Flash Flood Warnings are typically issued for flooded streets, creeks, streams, and sometimes rivers.  They may also be issued when fields are filling with water from heavy runoff.

In addition to standard Flash Flood Warnings, PDS Flash Flood Warnings are also issued when it appears that there is a much more significant threat to life and property.  PDS Flash Flood Warnings are also sometimes interchanged and defined as "PDS Flash Flood Emergencies", as a means to underscore their significant risk to life and property.  For the purpose of this article however, PDS Flash Flood Warnings are generally left to particularly intense rainfall scenarios, such as storm training, tropical weather systems, and serial squall lines.

While any convective warning can be issued without a watch issued before hand, Flash Flood Warnings tend to be the most notorious for happening virtually at any time or anywhere.  Heavy rainfall events on their own do not make a thunderstorm severe.  Thus, as a result, Flash Flood Warnings are omitted from SPC tracking and monitoring.  Instead, heavy rainfall events are monitored by the Weather Prediction Center (WPC) who also is responsible for issuing Rainfall MCD's.  Note, local WFO's issue Flash Flood Watches and Warnings - not the WPC.

By far the most common warning issued by WFO's, the Severe Thunderstorm Warning is the most well known of the convective warnings. In the last ten years, Norman, Oklahoma's WFO has issued over 7,000 of the warnings - ten times more warnings than it has issued of either other warning set.  The reason for such a wide use is that the Severe Thunderstorm Warning covers virtually everything.  Winds in excess of sixty miles per hour, hail over one inch in diameter, and even wall clouds will result in hoisted warnings.

Unlike it's counterparts above and beneath it, however, the Severe Thunderstorm Warning does not have a recognized PDS identifier.  It has been discussed at length, mainly due to the existence of intense discrete cells and derechos that are capable of producing more widespread damage.  Variations on the PDS text for Severe Thunderstorm Warnings have included "extremely dangerous situation", "life threatening situation", and "serious injury with significant property damage" have been used.  Recognizing that these are all simple variations on the PDS text gives evidence that new PDS identifiers may be provided with the warning at a later time.

It is important to note that depending on how a Severe Thunderstorm Warning is coded, and city ordnances, severe weather sirens may tone for an event.  This is due to the extent of damage the storm is capable of producing and the risk to life to those who may be caught outdoors.  It should be noted that storm sirens are intended to alert people who are out side, not those who are indoors.

The most often discussed warning, and possibly the most misunderstood by the public.  Tornado Warnings are triggered by various events that unfold in the sequence of a thunderstorm.  They also have multiple levels and tiers that reflect how significant they are.  All Tornado Warnings should be taken seriously, regardless of its place in the TOR hierarchy.  Simple Tornado Warnings can be issued for radar observed rotation or reported funnel clouds that haven't made it to the ground yet.  Tornado Vortex Signatures (TVS) often is detected with a hook echo, or a notch in a squall line.  Spin-up tornadoes often result in both Severe Thunderstorm and Tornado Warnings.

Tornado Warnings where there is a confirmed tornado will carry a "confirmed" identifier to elevate the storms severity.  There is a difference between the terms "radar indicated" and "radar confirmed."  Radar confirmed tornadoes generally are identified by the presence of a pronounced TVS or a debris ball signature.  Any radar confirmed tornado will be treated as a confirmed tornado - the same as if it were reported by the public.

Lastly, the PDS Tornado Warning is issued when a larger tornado is on the ground.  They may also be used when a tornado is confirmed on the ground and moving towards a populated area (such as a town or city).  PDS Tornado Warnings elevate the significance of a tornado based upon how much damage it can do to life and property in an observed foreseeable future.  However, the PDS Tornado Warning is generally reserved for smaller towns and cities.  The size and density of the funnel cloud will also contribute to whether or not the PDS identifier is used.

Unlike other hazardous products in this article, most Flash Flood Emergencies are issued for non-convective reasons.  These are largely associated with catastrophic water retention failures, such as levee breaks, dam breaks, and barrier breaches.  Ice jams on rivers, creeks, and streams can also cause emergencies to be issued.  Flash Flood Emergencies could also be issued in conjunction with Tsunami Warnings, but generally the messaging in Tsunami Warnings is sufficient to not warrant an accompanying product.  Ultimately, they can be characterized as a hazard that is observed where the risk of rapid rise in water is expected imminently from a confirmed cause.

Flash Flood Emergencies are issued within Flash Flood Warnings, and typically carry mixed-in messaging to the rest of the messaging.  For this reason, they may not be formally recognized as actual convective weather products.  However, their function is that of emphasizing a major risk to life and property that will occur.

The underscoring message is that if you find yourself in one of these areas, seek higher ground immediately.

Tornado Emergencies are the pinnacle of severe weather products, in that there is no higher priority warning that can be issued.  Sometimes identified as TORE, Tornado Emergencies are issued to accompany large, devastating, long-tracked, tornadoes that have, are, or will do damage to populated areas.  This also encompasses multi-vortex systems.  The first one was issued in 1999 when an F-5 tornado streaked through Moore, Oklahoma.  Since, there have been 191 Tornado Emergencies issued by the National Weather Service.

Statistically speaking, Tornado Emergencies result in tornadoes over ninety percent of the time, with less than five percent being false positives.  Forty-two percent of them produce significant tornadoes of EF-3 or greater strength.

Like other emergency usage, Tornado Emergencies have varying and loosely defined criteria from WFO to WFO.  All requirements include a tornado being on the ground, doing damage, has a long tracked history, and a significant population in it's path.  That is to say, the tornado is confirmed on the ground either by radar or trained spotter, and is approaching a populated space.

427
WGUS53 KDVN 302054
FFWDVN
IAC163-010245-
/O.NEW.KDVN.FF.W.0005.190430T2054Z-190501T0245Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/

BULLETIN - EAS ACTIVATION REQUESTED
Flash Flood Warning
National Weather Service Quad Cities IA/IL
354 PM CDT Tue Apr 30 2019

...FLASH FLOOD EMERGENCY FOR DOWNTOWN DAVENPORT...

The National Weather Service in the Quad Cities has issued a

* Flash Flood Warning for...
  South central Scott County in east central Iowa...

* Until 945 PM CDT.

* At 350 PM CDT, emergency management reported weaknesses in the
  temporary flood walls that are protecting the downtown area of
  Davenport, and failure of this temporary levee system is likely to
  occur soon.

THIS IS A FLASH FLOOD EMERGENCY FOR DOWNTOWN DAVENPORT. This is a
PARTICULARLY DANGEROUS SITUATION. SEEK HIGHER GROUND NOW!

If the flood walls do breach, water from the Mississippi River will
move quickly into areas of downtown Davenport, near the river, that
have previously been protected.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Move to higher ground now. This is an extremely dangerous and life-
threatening situation. Take immediate action if you are in an area
subject to flooding or under an evacuation order.

&&

LAT...LON 4151 9060 4150 9060 4149 9061 4151 9061
      4154 9055 4153 9054

$$

799
WFUS53 KILX 012315
TORILX
ILC021-012345-
/O.NEW.KILX.TO.W.0028.181201T2315Z-181201T2345Z/

BULLETIN - EAS ACTIVIATION REQUESTED
Tornado Warning
National Weather Service Lincoln IL
515 PM CST SAT DEC 1 2018

...TORNADO EMERGENCY FOR TAYLORVILLE...

The National Weather Service in Lincoln has issued a

* Tornado Warning for...
  Central Christian County in central Illinois...

* Until 545 PM CST.
* At 514 PM CST, a confirmed large and destructive tornado was 
  observed near Taylorville, moving northeast at 30 mph.

  TORNADO EMERGENCY for Taylorville. This is a PARTICULARLY
  DANGEROUS SITUATION. TAKE COVER NOW!

  HAZARD...Deadly tornado.

  Source...Radar confirmed tornado.

  IMPACT...You are in a life-threatening situation. Flying debris 
           may be deadly to those caught without shelter. Mobile
           homes will be destroyed. Considerable damage to homes,
           businesses, and vehicles is likely and complete
           destruction is possible.

* The tornado will be near...
  Taylorville around 525 PM CST.
  Stonington around 540 PM CST.

Other locations impacted by this tornadic thunderstorm include
Willeys, Taylorville Airport, and Sharpsburg.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

To repeat, a large, extremely dangerous and potentially deadly
tornado is on the ground. To protect your life, TAKE COVER NOW! Move
to an interior room on the lowest floor of a sturdy building. Avoid
windows. If in a mobile home, a vehicle or outdoors.. Move to the
closest substantial shelter and protect yourself from flying debris.

&&

LAT...LON 3944 8932 3950 8944 8926 3967 8914
      3966 8914 396 8910 3964 8907
TIME...MOT...LOC 2314Z 212DEG 27KT 3950 8934

TORNADO...OBSERVED
TORNADO DAMAGE THREAT...CATASTROPHIC
HAIL...1.00IN

$$

There are numerous aspects and variables that go into a severe weather forecast and storm prediction.  Here, we have examined the steps from the outlook to the emergency.  There are of course steps in between, such as the Public Severe Weather Outlook (issued when a Moderate risk or greater of severe weather is forecast) and ongoing severe weather MCD's.  The messaging in each product is a key factor in determining the scale of severe weather that is impacting your area.  The system in place now has been there since 1995, and revised a dozen or more times since.  Activation of the Emergency Alert System (EAS), adHoc cellular notification, and television or radio break-ins have significantly reduced fatalities since the 1960s.  Still, there are major misunderstandings about the function of various weather terms, systems, and signals that the public often expresses exasperation about.  This continues in spite of ongoing attempts to educate.

Tornadoes, derechos, squalls, and more...none of them are new terms, nor are they new to any given area.  The fact that the public is unaware is not for a lack of trying, but rather due to a lack of interest by the public.  If and when I return to this topic again, we'll cover some of those other terms, their origins, and some fancy shmancy weather history too.