N-W Storm Chasers' Guide

[Nick F]

Here's a guide to storm chasing in the US Mid-West, this mainly covers supercell storms - how they form and their potential to spawn tornadoes:

Supercells - and what causes them to develop

A supercell is a severe thunderstorm with a deep, persistently rotating updraft (a mesocyclone). Supercell thunderstorms are the largest, most severe class of thunderstorms.

Instability Thunderstoms that become supercells need very unstable environments in which to develop the necessary rotating updraft. A cap or lid is a layer of relatively warmer drier air above cooler moister air which prevents parcels of air from rising unless the parcels become warmer than the capping layer. The cap is often present in the morning and early afternoon and allows the low-level moisture and temperature to increase which ultimately enhances severe weather potential for those stronger convective cells that are able to break the cap. The cap can build up very large amounts of SBCAPE or surface based Convective Available Potential Energy (up to 3000-4000 j/kg) in the boundary layer between the cap and the ground. Thunderstorms which develop rapidly within or near an area of significant capping likely will become severe and become supercells. Conversely, the lack of a lid allows many storms to develop which then compete for the available moisture and storm-relative inflow so are unlikely to become supercells. So it is necessary that there is sufficient advection of warm and moist air to build up beneath the cap for the chance for supercells to develop. Mechanisms which break the cap are normally strong surface heating in hot sunshine - where rising parcels of hot air are warmer than the capping layer and so continue to rise breaking the cap, and/or a synoptic scale forcing mechanism in the form of a cold front, trough or dry-line mixes out the warm dry capping layer.

12z sounding from Norman, OK on 04/05/2007 displaying a capping inversion (where a layer or relatively warmer and drier air at around 800mb prevents parcels of air from rising and creating thunderstorms)

Forcing Mechanism Surface based thunderstorms often develop in late afternoon and early evening when ground temperature are at their hottest and when parcels of rising warm air are at their strongest and most likely to break the cap as discussed above. Supercell thunderstorms need a continuous inflow of moist air at the surface to maintain them or they will dissipate (eventually this happens anyway) by entrainment. Entrainment is the (relatively) dry air that is pulled from the surrounding environment which eventually manifests itself into downdrafts and the cool gusts you feel when the thunderstorm approaches. Entrainment can be counterbalanced by the removal of air from the top of thunderstorms. This is known as divergence which allows more moist air from the lower levels of the thunderstorm to be sucked up into giant updrafts and then diverge away from the top of the thunderstorm which allows it to maintain itself and intensify. Upper air divergence is accomplished with jet stream winds splitting or diverging which creates a void that has to be filled by air from below and hence a vacuum effect in the upper atmosphere which contributes to the formation of strong updrafts. Upper divergence is often found in the left exit region or right entrance region of a jet streak. As well as favourable upper air dynamics, other forcing mechanisms are required to actually initiate the storm in the first place, this tends to occur along an approaching cold front - where denser colder drier air undercuts warm moist air along the boundary and creates forced ascent and cumulonimbus clouds; also: severe storms can initiate in a warm sector well away from frontal boundaries when either a trough (often an upper trough) embedded in the troposphere forces air to rise due by introducing colder air aloft steepening lapse rates and creating forced ascent, or wind convergence - where areas of winds with different direction meet or converge along a line forcing warm moist air upwards. Another common forcing mechanism for supercells, particularly tornadic ones, is the dry line - this is discussed in more depth further down.

Supercells derive their rotation through tilting of horizontal vorticity (an invisible horizontal vortex) caused by vertical wind shear. Strong updrafts lift the air turning about a horizontal axis and cause this air to turn about a vertical axis. This forms the deep rotating updraft, the mesocyclone.

Strong vertical wind shear is usually present when there is both strong jet streak aloft creating strong 'speed shear' and also where winds change direction through different levels or directional shear, the strong speed shear is shown by strong winds on the jet charts or 0-6km deep layer shear charts below. Strong winds aloft allows the updrafts to be kept seperate from the downdrafts and makes sure the precipitation and the accompanying downdraft doesn't falll back down into the updraft cutting it off and killing the storm. Also the strong winds aloft vent the storm and so air taken away from the top has to be continuely replaced by warm moist air flowing into the base of the storm prolonging its life - this is why suprcells which form in these strong wind shear environments have a long-live span:

Directional shear can often be seen on actual or forecast soundings by looking at the wind speed direction barbs on the right hand side of the sounding 'skew-t' diagram. Below is an example of a sounding that is likely to lead to supercells with rotating updrafts and associated severe weather ... the sounding is from Norman (Oklahoma City) at 00z UTC or 18z Oklahoma time and shows winds that are strongly backed towards the surface compared to the winds aloft ... i.e winds above 700mb are predominately from the W or WSW, but at the surface they are from the SE:

Low-Level Jet differs from the polar front and subtropical jet streams in that it forms around 850mb rather than 250-300mb, and the low-level jet moves south to north rather than west to east. The LL jet occurs over the Plains of the mid-west USA during spring and early summer when temperature contrasts at night between the rapidly cooling air over the High Plains in the west contrasts with the still warm Gulf air further east and SE towards the Mississippi and GoM - this steepening thermal gradient at night creates a strong Serly wind about 1000m/800-850mb above and this wind transports copious warmth and moisture northwards mainly at night and morning from the Gulf of Mexico across the Plains when the synoptic conditions are favourable - i.e. when a trough lies N-S over the Rockies with high pressure east of the Mississippi. The transport of warm moist air north aids in creating sufficient moisture return north to allow severe thunderstorms develop overt he Plains should a dry line, trough or cold front interact and cause it to rise. It also is responsible for increasing night-time thunderstorm activity and nocturnal tornadoes

Outflow boundaries - these tend to form when the sinking cold outflow of a thunderstorm forces warm air to rise and create new convection in an adjacent area to the original storm ... sometimes when two outflow boundaries from different storms intersect they can lead to severe t-storms developing

Supercells have the capability to deviate from the mean wind. If they track to the right of the mean wind (relative to the vertical wind shear), they are said to be "right-movers." Alternatively, if they track to the left of the mean wind (relative to the shear), they are said to be "left-movers." Right movers typically are associated with a high potential for severe weather. (Supercells often are right movers) and can sometimes go on to produce tornadoes due to the storm moving east and increasing the directional wind shear and chance of tornadoes

Features of a supercell

Diagram of supercell from above. RFD: rear flank downdraft, FFD: front flank downdraft, V: V-notch, U: Main Updraft, I: Updraft/Downdraft Interface, H: hook echo

Overshooting top

This "dome" cloud feature appears above the anvil of the storm, the updraft overcomes the level of equilibrium above the storm anvil (where air develops negative boyuancy and sinks). It is a result of the powerful updraft. If too close it cannot be seen from the ground. A persistent overshooting updraft can be a sign or precursor to the storm having or developing a tornado.

Precipitation-free base

This area, typically on the southern side of the storm in North America, is relatively precipitation free. This is located beneath the main updraft, and is the main area of inflow. While no precipitation may be visible to an observer, large hail and rain may be falling from this area. It is more accurately called the main updraft area. Often the mesocyclone (rotating updraft area) of a supercell will be preciptn free.

Wall cloud

The wall cloud forms near the downdraft/updraft interface. This "interface" is the area between the precipitation area and the precipitation-free base. Wall clouds form when rain-cooled air from the downdraft is pulled into the updraft. This wet, cold air quickly saturates as it is lifted by the updraft, forming a cloud that seems to "descend" from the precipitation-free base. Wall clouds are common and are not exclusive to supercells: Only a few actually produce a tornado. Wall clouds that persist for more than ten minutes, wall clouds that seem to move violently up or down, and violent movements of cloud fragments (scud or fractus) near the wall cloud are indications that a tornado could form.

Photo of a rotating wall cloud I took in Kansas

Shelf Cloud

A shelf cloud is a low, horizontal arcus cloud, associated with a thunderstorm gust front or occasionally with a cold front. Unlike a roll cloud, a shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent and wind-torn.

Occasionally people see a shelf cloud and think they have seen a wall cloud, which is an easy mistake, since an approaching shelf cloud appears to form a wall made of cloud. Generally speaking, a shelf cloud appears on the leading edge of a storm, and a wall cloud will usually be at the rear of the storm.

Shelf-cloud in Texas I took last May

Mammatus clouds

Mammatus (Mamma, Mammatocumulus) are bulbous or pillow-like cloud formations extending from beneath the anvil of a thunderstorm. These clouds form as cold air in the anvil region of a storm sinks into warmer air beneath it. Mammatus are most apparent when they are lit from one side or below and are therefore at their most impressive near sunset or shortly after sunrise when the sun is low in the sky. Mammatus are not exclusive to supercells and can be associated with developed thunderstorms and cumulonimbus.

Precipitation area

This is the area of heaviest precipitation. Between the precipitation-free base and the precipitation area, a "vaulted" or "cathedral" feature can be observed. In high precipitation supercells an area of heavy precipitation may occur beneath the main updraft area.

Flanking line

The flanking line separates cool storm outflow from warm moist storm inflow and sits above the gust front. New storms form along the flanking line as the moist inflow air rises as it approaches the cool surface air.

Radar features of a supercell

Hook echo on doppler radar of supercell which produced EF5 Greensburg tornado on 04/05/2007

Hook echo or Pendant

The "hook echo" is the area of confluence between the main updraft and the rear flank downdraft (RFD). This indicates the position of the mesocyclone. These hook echoes sometimes display a Tornado Vortex Signature (TVS) - or ETVS (Elevated Tornadic Vortex Signature) - and is a signature of a potential tornado indicated by the NEXRAD radar. NEXRAD will indicate "TVS" or "ETVS" if the rotational shear that has been calculated by the NEXRAD radar is judged strong enough that the storm may produce a tornado or "NONE" meaning no tornado vortex signature is detected (there is either no rotation or the rotational shear is not of sufficient strength to result in a tornado) as judged by the NEXRAD radar. The National Weather Service will issue tornado warnings based on the strength of the TVS or ETVS signature.

Bounded weak echo region (or BWER)

This is a region of low radar reflectivity bounded above by an area of higher radar reflectivity. This is evidence of a strong updraft.

Inflow notch

An "notch" of weak reflectivity on the inflow side of the cell. This is not a V-Notch.

V Notch

A "V" shaped notch on the leading edge of the cell, opening away from the main downdraft. This is an indication of divergent flow around a powerful updraft.

Supercell variations

Supercell thunderstorms are sometimes classified by meteorologists and storm spotters into three categories. However, not all supercells fit neatly into any one category, and many resemble all three at different times during the lifespan of the storm. The standard definition given above is referred to as the Classic supercell. All types of supercells can produce severe weather.

LP Low Precipitation

LP supercells contain a small precipitation (rain/hail) core separate from the updraft. This type of supercell may be easily identifiable with "sculpted" cloud striations in the updraft base or even a "corkscrewed" or "barber pole" appearance on the updraft, and sometimes an almost "anorexic" look compared to classic supercells. This is because they often form along dry lines, thus leaving them with little available moisture despite high upper level wind shear. They usually dissipate rapidly rather than turning into classic or HP supercells, although it is still not unusual for them to do the latter, especially if they happen to collide with a much moister air mass along the way. Although these storms usually produce weak tornadoes, they have been known to produce strong ones. These storms can produce large hail even with little or no visible precipitation core, making them hazardous to storm chasers and people and animals caught outside. Due to the lack of a heavy precipitation core, LP supercells can sometimes show weak radar reflectivity without clear evidence of a hook echo, when in fact they are producing a tornado at the time. This is where observations by storm spotter and storm chasers may be of vital importance. Funnel clouds, or more rarely, weak tornadoes will sometimes form midway between the base and the top of the storm, descending from the main Cb (cumulonimbus) cloud. Lightning is rare compared to other supercell types, but it is not unknown and is more likely to occur as intra-cloud lightning rather than cloud-to-ground lightning. In North America, these storms almost exclusively form from the Rocky Mountains to the Mississippi River in the spring and summer months. They can occur as far north as Montana, North Dakota and even in the provinces of Alberta and Saskatchewan in Canada.

LP supercells are quite sought after by storm chasers, because the limited amount of precipitation makes sighting tornadoes at a safe distance much less difficult than with a Classic or HP supercell.

- LP Supercell, NW Oklahoma 4th May 2007

HP High Precipitation

The HP supercell has a much heavier precipitation core that actually can wrap all the way around the mesocyclone. These are especially dangerous storms, since the mesocyclone is wrapped with rain and can hide the tornado from view. These storms also cause flooding due to heavy rain, damaging downbursts and weak tornadoes, although they are also known to produce strong to violent tornadoes. They have a lower potential for damaging hail than Classic and LP supercells, although damaging hail is possible. It has been observed by some spotters that they tend to produce more cloud-to-ground and intracloud lightning than the other types. Also, unlike the LP and Classic types, severe events usually occur at the front (southeast) of the storm. The HP supercell is the most common type of supercell in the United States east of the Mississippi River and in the southern parts of the provinces of Ontario and Quebec in Canada.

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Dry Line A moisture boundary - and its role producing supercells

During the spring and early summer, forecasters in the south-central USA know to watch for the formation of a "dryline" - the narrow boundary separating hot, dry, southern Rocky Mountain air from very moist Gulf of Mexico air.

The dryline's daily eastward movement, under the right conditions, can trigger supercellular severe thunderstorms and tornadoes.

As the sun heats the barren ground of the Southwest, particularly New Mexico, a hot, dry wind begins blowing down the east slopes of the southern Rockies right into the face of warm, humid air nosing northward from the Gulf of Mexico.

The most pronounced collision of these two different air masses commonly occurs over Texas and Oklahoma, creating a narrow moisture gradient known as the "dryline."

Air rising from the ground and overturning in the lower atmosphere to the west of the dryline bring southwesterly winds at about 1,000 feet above the ground down to the surface, shifting the morning's southerly winds to southwest in eastern New Mexico.

East of the dryline in Texas or Oklahoma, however, winds continue to blow from the south under a mix of low clouds and sun.

By afternoon, the strong, dry southwest winds converge with the weaker, humid south winds along the dryline, squeezing the moisture boundary into a very narrow area. Dewpoints, which are a measure of the air's moisture, could be in the 60s ahead of the dryline but drop sharply to the 20s just a few miles farther west.

The stronger winds blowing in from the southwest shove the dryline to the east. But the dryline's movement at the surface often is not as fast as it is aloft. Hot air at about 1,000 feet overruns the warm, moist air at the surface, creating a capping inversion. This "cap" acts like a lid on a pot of boiling water; it keeps the heated air from rising.

On some days this lid is very thick preventing the blossoming of storms. But on other days, the lid holds the unstable air down for only so long before it the inversion weakens. Once it breaks, billowing, moist air can explode upward at more than 100 mph, growing into 50,000-foot thunderstorms in minutes.

The strongest begin rotating as their updrafts twist with the shifting low-level winds, which can lead to powerful tornadoes The thunderstorms most likely to produce tornadoes are called supercells and are often a product of the Southern Plains severe weather trigger known as the dryline.

States like Texas, New Mexico, Oklahoma, Kansas, and Nebraska frequently experience drylines in the spring and summer, while east of the Mississippi River, drylines are extremely rare. The dryline is represented on surface maps by a dashed yellow line (see example below).

Dryline is shown over W Texas by the line with unfilled semi-circles

Dew points (in green figures) east of the dryline shown above range from the upper 50's to low 70's, with winds from the southeast. West of the dryline, dew points are much lower, in the 20's and 30's, which is almost 50 degrees less than those found east of the dryline.

Air temperature ahead of the dryline is generally in the 70's and 80's. Behind the dryline, temperatures are hotter, ranging from the mid 80's to mid 90's. The drier air behind the dryline lifts the moist air ahead of it as it advances, which could lead to the development of thunderstorms along and ahead of the dryline in a manner similar to how thunderstorms develop along cold fronts.

Dry-Line Bulge - a bulge in the dry line, representing the area where dry air is advancing most strongly at lower levels. Severe weather potential is increased near and ahead of a dry line bulge.

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Tornadoes from Supercells

Supercells contain mesocyclones, an area of organized rotation a few miles up in the atmosphere, usually 1–6 miles (2–10 km) across. Most intense tornadoes (EF3 to EF5 on the Enhanced Fujita Scale) develop from supercells. In addition to tornadoes, very heavy rain, frequent lightning, strong wind gusts, and hail are common in such storms.

Most tornadoes from supercells follow a recognizable life cycle. The cycle begins when increasing rainfall drags with it an area of quickly descending air known as the rear flank downdraft (RFD). This downdraft accelerates as it approaches the ground, and drags the supercell's rotating mesocyclone towards the ground with it.

Tornado formation

As the mesocyclone approaches the ground, a visible condensation funnel appears to descend from the base of the storm, often from a rotating wall cloud. As the funnel descends, the RFD also reaches the ground, creating a gust front that can cause damage a good distance from the tornado. Usually, the funnel cloud becomes a tornado within minutes of the RFD reaching the ground.

Initially, the tornado has a good source of warm, moist inflow to power it, so it grows until it reaches the mature stage. During its mature stage, which can last anywhere from a few minutes to more than an hour, a tornado often causes the most damage, and can in rare instances be more than one mile across. Meanwhile, the RFD, now an area of cool surface winds, begins to wrap around the tornado, cutting off the inflow of warm air which feeds the tornado.

Aswell as there presence of a mesocyclone, strong tornadoes tend to be favoured by strong low-level wind shear (change in speed/direction of low-level winds). When the vertical wind shear in the lowest kilometre of the troposphere is around 10 m/s of shear or greater, tornadoes are more likely with supercell storms. A typical 0-1km Low-level wind shear chart is shown below:

Also, high storm-relative helicity (SRH) which is calculated froM low-level wind shear vectors (strong veering winds) in the lowest kilometre tends to be related to strong tornadoes aswell, here's a typical 0-1km SR Helicity chart:

Nonsupercell Tornadoes

Tornadoes also form from non-rotating storms, normally on convective squall lines along cold fronts. Even though supercell thunderstorms are responsible for the biggest and deadliest tornadoes, a significant number of tornadoes form under nonsupercell clouds and storms.

These nonsupercell tornadoes (NST) are normally short-lived and weak, but from time to time can become strong enough to damage property and kill people, a few have been documented up to F2/F3 in intensity.

NSTs can form due to:

-A slow-moving or stationary wind shift boundary with pockets of vertical vorticity

-Developing thunderstorm updrafts (in the convergence directly over the boundary) that can stretch the low-level vertical vorticity into tornadoes if the updrafts are properly co-located with the available vorticity.

[Nick F]

Here's some good links to some charts, such as used to predict supercell/tornado probability using the various pararmeters mentioned above:

SPC Hourly Mesoscale Analysis: http://www.spc.noaa.gov/exper/mesoanalysis/s4/index2.html

Surface Prog Charts: http://adds.aviationweather.gov/progs/

SPC Convective outlooks: http://www.spc.noaa.gov/products/outlook/

US Model forecasts: http://www.wxcaster.com/conus_0012_us_models.htm

State weather pages: http://kamala.cod.edu/

NOAA weather radio: http://www.weather.gov/nwr/streamaudio.htm

Not forgeting a usefull selection of N-W GFS charts for the US: http://www.netweather.tv/index.cgi?action=usacharts;sess=

Feel free to add anymore usefull links

[johnholmes]

thanks for all that Nick, it will be really handy for us back in the UK to be able to have that easily to hand.

suggest to mods(yourself!) or whoever that this thread is put in the Guides and locked so only yourself can input data into it.

[Nick F]

Thanks John, may be adding a few bits to the guide over next few days, will see if I can pin it anyways.

Feel free if anyone wants to add, as it's always difficult to know if everything's covered.

Topic now pinned ...

[pottyprof]

Fantastic that Nick. Must have taken a while to piece together. Thank you!

This is for various media coverage

http://www.netweather.tv/forum/index.php?showtopic=36945

[AudMun]

Nick

Thank you so, so much.

I have just spent the last 3 weeks trying to find something that I can understand but being unable to and you have created exactly what I was looking for.

A very, very grateful

Stewart

[NL]

Nice work there Nick, Really good explanation of how everything comes together.

I was wondering you have GPS,?..Just for when you get there how will we follow you? I know of a site

http://www.spotternetwork.org/register.php .. I have GPS tool on Google Earth too havent sussed that out yet though! ..

Bet ya getting really excited now, i know i would be, Have a great time ,stay safe and remember if ya happen to be in the

bible belt no screaming out religious banta (Jesus Christ etc) coz they'll string ya up!!

[Paul Sherman]

Nice work there Nick, Really good explanation of how everything comes together.

I was wondering you have GPS,?..Just for when you get there how will we follow you? I know of a site

http://www.spotternetwork.org/register.php .. I have GPS tool on Google Earth too havent sussed that out yet though! ..

Bet ya getting really excited now, i know i would be, Have a great time ,stay safe and remember if ya happen to be in the

bible belt no screaming out religious banta (Jesus Christ etc) coz they'll string ya up!!

[NL]

Will do Paul, someone has spent a lot of time plotting out

WiFi hotspots on Google Earth and as far as i know they're all free!!

No doubt you'll know most of the hotspots Paul, Mcdonalds being an obvious one.

[ian cameron]

Nick,

as someone thats not all that clued up on whats what in weather I think its a great tutorial for and am definatly more enlightened and now have a better understanding of what we should expect so would just like to say a big thank you

Ian

[Paul Sherman]

Just edited the post John, Ian is very new to the site so probably did not know, will brief him later!

Paul S

[Andy Bown]

Wall cloud

The wall cloud forms near the downdraft/updraft interface. This "interface" is the area between the precipitation area and the precipitation-free base. Wall clouds form when rain-cooled air from the downdraft is pulled into the updraft. This wet, cold air quickly saturates as it is lifted by the updraft, forming a cloud that seems to "descend" from the precipitation-free base. Wall clouds are common and are not exclusive to supercells: Only a few actually produce a tornado. Wall clouds that persist for more than ten minutes, wall clouds that seem to move violently up or down, and violent movements of cloud fragments (scud or fractus) near the wall cloud are indications that a tornado could form.

[Paul]

Just wanted to bump this, Nick wrote it for last years chase and it's a fantastic guide to pretty much everything you ever needed to know about the storms we're likely to encounter.

[Mesosphere]

thanks for the info Nick , very helpful and well explained

[Azores Hi]

That's absolutely top tackle Nick. I've printed it off for some bedtime reading. I'm seriously thinking of going on the chase next year, my missus has just given me the green light!

[Paul]

We're taking bookings now, as there has already been a lot of demand for next year, so don't delay!

We'll be running 3 tours again in 2009..

http://www.netweather.tv/index.cgi?action=...tormchase;sess=

[kold weather]

thats awesome Nick, I know how much effort you put into it after making my hurricane guide a few years back...I'll have to have a little read up as I'm afraid I've gone a little stale as I didn't watch much last year due to college commitments, which thankfully are much reduced this year and so going to do much deeper virtual chasing.

[Cookie]

great read nick http://nwstatic.co.uk/forum/public/style_emoticons/<#EMO_DIR#>/laugh.gif

[Nick F]

I have expanded the original guide to incorporate more info on how severe storms form and the environment they require, plus some other additions, so hopefully it should give a fairly comprehensive idea of what we look for when forecasting severe storms for our chases and the structure of the storms:

http://www.netweather.tv/forum/index.php?s...st&p=972442

Will probably post more stuff on how actually chose a target for each day ...