Storm Type

Storm TypeMteor 417 – Iowa State University– Week 8Bill Gallus

Three Major Types of Storms Single Cell Multicell Supercell

Single Cell (Ordinary Cell) A) Forecasting Hints1. Generally occur with instability (CAPE 1000 J/kg) and a lack of other NORMALsevere weather parameters like shear orforcing2. Weak wind shear/random hodographs3. May occur with inverted-V soundings4. Common during warmer, more humidmonths of year over much of US(anywhere atmosphere is unstable)

Single Cells B) Severe Potential1. Can cause “pulse” severe with largehail and damaging winds2. Tornadoes almost NEVER happen3. Because severe intensity is so randomand brief, very difficult to warn much inadvance4. Inverted-V sounding might be bestindicator for microburst severe winds ifstorms form

Single Cells C) Structure1. Has a three stage life-cyclea) Towering Cumulus Stage (upwardmotion everywhere, any precip issuspended aloft)b) Mature Stage (rain core developsand falls to ground, downdraft and updraftexist side-by-sidec) Dissipating Stage (entire storm rainsout, downdraft everywhere)

Images taken from Lyndon State College web site

Single Cells2. Entire lifetime is only 30-60 minutes3. Peak updraft speed usually around 10m/s

Multicell (can be organized into bowechoes, derechoes, squall lines, andnonlinear events)A) Forecasting Hints1. Occur with moderate to high instability(CAPE 1500 J/kg)2. Moderate wind shear, mostly due to speedchanges and not directional ones (0-6km shearvectors 35-40 knots)3. Hodograph is a straight line4. If a cyclone gets too “wound up” oroccluded, and flow at all levels is from roughlysame direction, multicells dominate oversupercells

(continued) Fronts or boundaries can help givemulticells even if shear is really weak andconditions more favorable for single cells Orientation of 0-6 km shear vector relativeto boundaries like dry lines and cold frontsis important – if the shear vector isrelatively parallel to boundary ( 45degree angle), any initial supercells arelikely to evolve into a multicell system.

MulticellsB) Severe Potential1. Large hail rather common (usually 2 inches indiameter)2. Strong wind gusts (can 100 mph in derechoes)3. A few tornadoes may occur, but they are usually weakand found in parts of multicell systems having*enhanced horizontal shear (such as from a boundary) or*when very strong low-level shear ( 15 m/s change overlowest 2-5 km) exists which can result in downdrafts tiltingvorticity that creates small hooks or appendages alongleading line, usually north of apex of system

MulticellsC) Structure1. Organized clusters of at least 2-4 short-livedcells2. Each cell generates a cold outflow, and theoutflows combine to form large gust front3. Convergence on gust front helps create newcells every 5-15 minutes, usually on south end ofsystem4. Storms move with mean wind, but area motionwill appear to deviate significantly due toDISCRETE PROPAGATION (new cells form alonggust front)5. Lifetime can be many hours, although individualelements within multicell have short lifetimes

DISCREET PROPAGATIONMeanwindStorms follow meanwind toward east, butas they go throughtheir short lives, newstorms arepreferentially formingto the south so that the“SYSTEM” movementappears to be towardthe southeast.

MulticellsD) Organizations of multicells1. DERECHO: classic, straight-line windstorm, occurs with very strong speedshear, often in a “ring-of-fire” zone. Canbe very long-lived and travel quickly overlong distances

Progressive Derechoes CAPE 4000 J/kg (even after dark), withLI -8 W to NW flow Warm advection induced over weak frontalboundary Upper-level jet streak/transient weak shortwave Rapid propagation 40 knots

orWarm frontor otherboundary

Serial Derechoes Less instability (LI -4 to -8, CAPE1500-3000 J/kg) 700 mb winds 50 knots STRONGER 500 mb winds 70 knots DYNAMICS Straight-line hodographs

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HDerecho paths (from Stu Ostro, Weather Channel) during 1995 –around the ring of fire subtropical high located over the Midwestleading to a heat wave

MulticellsD) Organizations of multicells2. BOW ECHO: Can also containsupercells, has a distinct bow shape.Strongest winds are where echo movesfastest – often happen with very highamounts of CAPE and a good bit of speedshear in lower-levels. Most derechoesprobably consist of one or more bowechoes

Fairly large-scale bow echo, from a wikipedia site

Multicells3. SQUALL LINE: Can be extremelylarge area of multicell convection, withstrongest storms in line, and big areas ofsteady “stratiform” rain behind them. Mayhappen when shear is relatively weaker,and area is ahead of a cold front. Bigproblem for aviation since they can bevery long (so it is not possible to flyaround)

Squall line extending fromsouthern Missouri intonorthwestern Mississippi

Supercells (include classic, HP,LP)A) Forecasting hints1. Moderate to high instability (CAPE 1500J/kg)2. Moderate or strong speed and directionalshear (0-6km shear vector 35-40 knots)3. Hodographs have lots of curvature in lowlevels (usually true but not always)

SupercellsB) Severe Potential1. Large to extreme hail (“softballs”)2. Damaging wind gusts (due to variousstorm-scale jets)3. Biggest producer of tornadoes (especiallyviolent ones)

SupercellsC) Structure1. Consists of one quasi-steady rotating updraft2. Can form as a multicell initially3. Initially may move with the mean wind, butgenerallly they move to the right of the meanwind at a slower speed4. Combination of strong CAPE and shear createsperturbation pressure fields which produce therotation, longevity and rightward drift

Supercells5.Storm-splitting occasionally occurs into a leftmoving and right-moving storm. Although bothcan produce severe weather, the right-movingone usually becomes more intense6. FLANKING LINE of towering cumulus generallyoccurs on the right flank of storm7. Base of the updraft is marked by a lowered,slowly rotating nearly precipitation-free cloudbase called a WALL CLOUD

Supercells8. TAIL CLOUD may form, feeding into the wallcloud from the precipitation core, which isusually off to the N or NE9. WALL CLOUD generally has a smooth, stratuslike appearance10. Smooth striations may also appear in midlevels around the storm cloud – a good indicatorof rotation11. OVERSHOOTING TOP usually appears abovethe anvil top. This indicates the strongest, mostactive updraft, and is often above the areawhere the tornado will form (although things maybe tilted some)

Supercells12. Storm may have a greenish appearancedue to hail and the lack of precipitation inthe updraft13. Upward motion can exceed 50 m/s14. Tornadoes generally form under the wallcloud15. Mammatus clouds may be seen onunderside of anvil

mammatusstriationsFlankingLineTail cloud

SupercellsD) Variations of Supercells1. CLASSIC: produces standard hook echoradar echo. It has a substantial rain corethat is slightly removed from main updraftregion of storm where wall cloud islocated. This type of storm allows thetornadoes to be seen from some distanceif off to the right of the direction towardwhich storm is moving.

“Classic” Supercell Thunderstorm

SupercellsD) Variations of Supercells2. HP (High-Precipitation): Rain falls muchcloser to the tornado, effectively making ithard to see. Now believed that stormrelative winds at 9-10 km level determinetypes of supercells. HP occur if thesewinds are 20 m/s

High-precipitation Supercell

SupercellsD) Variations of Supercells3. LP (Low-precipitation): Usually occursnear dry line. Except for some very largehail, there is little precipitation in thesestorms, except several km downstream.Storm structure can be easily seen – andsometimes called in as UFOs. Tornadoesare not as common. LP sups are likely ifthe 9-10 km SR wind 30 m/s.

LP-Classic-HP Also the supercell type may be affected by“seeding” from upstream storms. If stormsdevelop upstream (usually SW or W), andtheir anvil clouds are streaming intoanother storm, then that storm will NOT bean LP, but instead will turn into a classicor, more likely, and HP, since the ice fromthe interfering anvil will create more rainclose to the circulation center. Thus, any storm will be “wetter” if “seeded”

Low-Precipitation Supercell

How can you determine most likelystorm type for an area?A) First determine the entire region where anystorms could occur, concentrating on instability(in summer, this can cover much of USA)1. Look at some soundings2. Look for regions of fairly high temperature anddew point and sufficient moisture at 850 mb3. Eliminate areas too warm at 700 mb (cap)4. Check 500 mb temperatures and include areasthat seem “cool” compared to sfc temp/dewpoints. If 85/65 at surface, -10 C may beneeded at 500 mb. If 70/55 at surface, mayneed -16C or colder aloft.

5. If all of these factors look good, airmassthunderstorms could develop, evenwithout noticeable forcing mechanisms, aslong as there aren’t any strong downwardmotion forcing mechanisms, like NVA orbeing directly beneath a synoptic high.6. If not all parameters look good, be carefulin areas of STRONG FORCING, due tostrong PVA or along fronts, because theforcing can overcome some deficiencies.

B) Check winds over your thunderstorm potentialarea 1. IF winds are fairly strong at most levels ANDsome type of forcing exists to trigger storms,potential for MULTICELL or SUPERCELLincreases2. If wind shear is mostly due to speed changesand not direction AND speeds are typically 30knots, MULTICELLS are favored.3. If wind shear includes significant veering withheight, and winds 35 knots at least at one levelfrom 500 mb to the surface, SUPERCELLS arepossible

Single Cell (Ordinary Cell) A) Forecasting Hints 1. Generally occur with instability (CAPE 1000 J/kg) and a lack of other NORMAL severe weather parameters like shear or forcing 2. Weak wind shear/random hodographs 3. May occur with inverted-V soundings 4. Common