Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
36 Cards in this Set
- Front
- Back
|
flanking line - southwestern flank along the gust front of the supercell. rarely develop into new Tstorms. "hydrometeors" is used to describe types of precip like fog droplets, cloud droplets, ice crystals, wet haze, etc) |
flash floods are rapid rising surges of water that occur with little advance warning over a local area. Flooding is the #1 storm related killer in the US. Two feet of moving water can sweep cars downstream. Flash flooding is especially dangerous near mountain ranges and in deserts. typ either upper-level wind flow must be weak or the upper-level winds must blow parallel to tstorms. |
|
|
with weak upper-level steering winds, storms can stall or move very slowly focusing heavy precip over a single area for an extended period of time. Forecasters estimate steering winds for single cell tstorms by determining the average wind in the layer occupied by the cumulonimbus cloud (850-300mb) |
training - when storms erupt over the same area and then move along a fixed path by steering winds that persistently blow from one direction a series of individual storms can pass over the same location like railroad cars on a train track. |
|
|
FLASHFLOODING SIGNS: weak vertical wind shear between 850-300mb, overall low wind speeds. high dew points near the surface/high moisture level through a deep layer of the trop. a stationary or near stat mechanism to promote rising air. |
To gauge the moisture content of the troposphere, a quantity called precipitable water is used, which is the amount of liquid that would accumulate if all the water vapor in an air column were forced to condense and fall as rain. Used to predict heavy rainfall rates. |
|
|
Most dangerous recipe for flooding: stationary thunderstorms over mountains, persistent upslope flow of moist air that orographically lifts air parcels to their LFC, once storms erupt the presence of weak steering winds encourages them to stall. The steep terrain then conspires to channel runoff from heavy rain rapidly into the valleys below. |
low-level jet - narrow ribbon of fast winds (moisture pipeline) Weather patterns of low-level winds rapidly transporting moisture over a stationary front or a stationary outflow boundary. If the orientation of the steering winds relative to the stationary boundary is favorable for training tstorms then the risk of flash flooding increases a lot. |
|
|
Hail can occur with single cell (pulse), multiple, and supercell. Impact on agriculture is huge. Hailstones can begin as small frozen raindrops or small soft snow-like particles called graupel. Strong updrafts suspend these particles above the 0C (32F) level around supercooled droplets new layers of ice form on the baby hailstones developing an onion-like pattern of ice. |
In regions of the cloud with low water content, air gets trapped as water freezes onto the hail giving the layer a milky appearance. The level of wet-bulb zero - an adjustment to the altitude at which the melting point of ice is observed by the cooling effect that evap of precip has on the air. When precip evaps energy is drawn from air, lowering surrounding temp. |
|
|
When precip falls into dry air, evap cooling tends to lower the altitude of melting level which is the wet-bulb zero. Preferred range of altitudes for the WBZ lies between 2200-2800m (7K-9K ft). If WBZ is under 2200m, updrafts are not enough to form hail, if over 2800m the hail melts before reaching the ground. |
Cheyenne is one of the US most hail-prone cities, along the front range of the rockies high freq. b/c thin layer of ground-warmed air that typ overlies such high elevations. Kericho, Kenya is one of the most hail-prone cities in the world averaging 100+ days per year with hail. |
|
|
Size of hail depends on strong updrafts (relatively large instability) allowing growth in the area of supercooled water. Updraft speed of 10-15m/s (22-34mph). The suspense of great vol. of hail and raindrops high in a tstorm is a process called precipitation loading. Vert cross sections scanned by radar reveal high reflectivity's that indicate presence of large hail. |
Hail can become too massive to be supported by the updraft and fall, or hail can find its way into either the downdraft region of tstorm or be tossed into clear air ahead of the tstorm. Hail produced by potent storms tend to fall in narrow streaks often a few km wide and 10+ km long, parallel to the path of the storm. |
|
|
A downburst is a strong downdraft of air which upon reaching the earth's surface causes an outrush of damaging straight-line winds in the lowest one or two kilometers of the trop (sometimes exceeding 100mph). often occur w/ strong downdrafts w/heavy precip in storms. can further accelerate when they entrain dry air which increases evap cooling and enhances neg buoyancy/downward acceleration. |
Microbursts are downbursts that typ last several minutes w/ outrush of winds that extends no more than 4km. Macrobursts are downbursts whose outrun exceeds 4km-10km typ last several-30 minutes, tend to occur when momentum from strong winds in the mid trop or higher up transferred downward tot he earth's surface. |
|
|
On 02/01/07 the Enhanced Fujita scale (EF-Scale) replaced the Fujita Scale (F-Scale) as the accepted standard for estimating a tornado's max wind gusts based on the degree of damage. A tornado is a violently rotating column of air in contact with the ground and the cloud base. |
Tstorms become severe when it produces: a tornado, hail at least one inch (2.5cm), straight-line winds exceeding 50 knots (57.5mph). Supercells account for almost all violent tornados (110+mph winds) & 2+in hail. Strong lifting favors individual tstorms to merge into mesoscale convective systems - organized group of tstorms spanning at least 100km. |
|
|
Recipe for supercells: thick layer of warm/moist air in lower trop, steep unstable lapse rate above LFC (cold enough air aloft ensuring rising air can ascend to high altitudes), strong vert wind shear, weak to modest lift (low-level convergence and/or upper-level divergence), layer of warm dry air near altitudes of 2km (6500ft) acting like lid. |
On sounding graph, the area between the moist adiabat & the temp sounding called "positive area" which represents the total energy available to the positively buoyant parcel - Convective Available Potential Energy (CAPE) - governs the upward accel. of the air parcel above the LFC. When CAPE is large, parcels accel explosively.. measure of potential for strong updrafts. |
|
|
CAPE is largest when low levels of trop are warm and moist (LFC occurs at low alt) and upper levels are cold. Stronger lift leads to organized tstorms instead of discrete. Dry lines form where maritime tropical air (very moist) meets continental trop air (very dry). Low-level convergence along dry line is modest with 500mb short-wave troughs sometimes adding upper-level divergence. |
In these situations dry line can initiate supercells that persist as discrete storms. This happens when winds through a deep layer of the trop blow at right angles to the dry line, upper-level winds from west (dryline NtoS) carry hydrometeors directly east. Rain from one supercell likely stay separate from neighboring supercells. |
|
|
Tornadoes are more likely when persistent discrete supercell develop, squall lines are more likely to produce damaging wind gusts. Squall line is narrow legion of tstorms that are often severe - type of mesoscale convective system (MCS) - generally solid area of precip spanning 100km (62mi). |
MCS can develop from isolated tstorms that merge. Pools of rain-cooled air associated with the gust fronts of single storms merge into a larger-scale cold pool that can initiate new tstorms to sustain the MCS or expand it. Otherwise, can develop immediately after storms erupt, with strong overrunning associated with night low-level jet. |
|
|
Moisture rapidly transported north of stationary front builds MCS. Nocturnal low-level jet (LLJ) is a narrow ribbon of fast winds at altitude between 500-1500m (1600-4800ft) - typ develops at night over midwest during warm season but occasionally over Eastern states. Development: day convective eddies stir lower trop to create well mixed boundary layer (uniform windspeeds) |
except near ground where friction is greatest, at night cooling near ground stabilizes lowest few hundred meters convective eddies dissipate and vert mixing ceases. the boundary layer "decouples" separating into a stable layer directly above the ground and another layer above it. b/c eddies are gone a wind layer accelerates paving way for nocturnal low-level jet (LLJ). |
|
|
Sloping terrain of Great Plains (downward from rockies) enhances LLJ. Air at given alt in lower prop cools more at night over western high plains b/c air is closer to Earth's surface. Pressure surfaces tend to slope downward from east to west creating pressure gradient force directed to the west at low levels. Coriolis force turns the flow to the right, setting stage for enhancing LLJ |
LLJ can blow at speeds of 60kt (70mph). Large MCS qualifies as a Mesoscale Convective Complex (MCC) when: spatial extent of cloud shield w/cloud-top temps < or = -32C (-26F) at least 100K km2 or diameter approx 350km. spatial extent of coldest cloud tops w/temp < or = -52C(-62F) must be at least 50k km2. MUST LAST at least 6hr. cloud shield circular @max extent |
|
|
MCC typ form in area by ridge of high pressure in mid to upper trop, synoptic-scale surface front, and LLJ. MCC tend to develop ahead of 500mb short-wave trough typ approaching from the SW as it tries to move through the ridge. LLJ rapidly imports moist air from gulf over stationary front and MCC (or large MCS) develops. Moves slowly/parallel to synoptic-scale front. |
New tstorms form in convective region near edge of complex that faces the LLJ. Gust fronts help lift inflowing warm/moist air (slope of gust front is small so overrunning of LLJ over synoptic-scale front helps lift parcels to their LFC). North of convective region there is a broad region of steady, less intense rain in stratiform region. |
|
|
Net radiative cooling near tops of storms also helps MCCs to thrive at night. This coupled w/warming in mid trop associated with release of latent heat in storms, destabilizes the mid/upper trop intensifying the MCC in wee hours of morning. Cyclonic circulation assoc. w/lower pressure in mid trop develops in response to latent heat of convection. |
This is called a mesoscale convective vortex (MCV) and explains why MCC is an organized system of storms (sometimes called land hurricanes). MCC typ weaken by dawn. Leftover outflow boundaries produced by MCC storms serve as catalyst for tstorm development during day. weak circulation in midtrop/soggy ground can rapidly re-moisten lower trop for re-ignition of MCC next night. |
|
|
MCC's waters crops, rainfall can exceed one trillion gallons over 12-16hr (can cause flash flooding). Squall lines can be solid or with breaks of weaker reflectivity. Sometimes referred to as a Quasi-linear Convective System (QLCS) Most favorable environments = large CAPE & strong lift (low-level convergence/sharp cold front/or surface trough b4 cold front) |
A surface trough ahead of a cold front is called a pre-frontal trough. Both favor squall lines esp. when strong upper-level divergence assoc. w/500mb short-wave trough works in tandem w/low-level convergence to create lift through a deep layer in trop. Occurs more freq. early-midspring in SE & S Plains, mid-spring-early summer in Central/N Plains. |
|
|
QLCS can evolve from storms initiated along outflow boundaries w/in warm sector of mid-lat cyclone. Ones affecting Great Plains can develop from storms initiated by high-level heat sources (mountains). Winds blowing from SW through deep layer of trop nearly parallel to initiating cold from/pre-frontal trough can contribute to linear structure of developing squall lines. |
High alt. winds carry hydrometeors produced by storms NE along the initiating boundary where they fall and help consolidate the pool of rain-cooled air. Squall lines tend to have single gust front & deep cold pool that helps initiate/maintain intense storms along leading edge. Their position may be marked by a shelf cloud - low wedge-shaped cloud associated w/ gust front. |
|
|
Shelf cloud is attached to parent tstorm, signals approaching gust front means business. Squall line that produces large-scale damaging windstorm is called a derecho. A rear-inflow jet develops in concert with an area of mesoscale low pressure (mesolow) in mid trop behind leading edge of squall line. This low forms in response to release of latent heat in backward-tilted updrafts along leading edge. |
Some rear-inflow jets remain elevated but some descend to the surface, setting stage for squall to bulge forward. This appears on images of radar reflectivity as a bow echo. A Bow echo is a crescent-shaped radar echo often associated with a portion of a QLCS. Indicating damaging straight-line winds. spans 40-120km (25-75mi). |
|
|
The strongest winds are usually observed at the E-arching apex of the surge region of the bow echo. Here, the rear-inflow jet which descended to the surface causes squall to surge forward in tandem w/some damaging straight-line winds. After several hours the Coriolis force kicks in the cyclonic vortex becomes dom & can spawn tornadoes "comma shaped pattern" |
Cyclonic & anticyc work to enhance the descended rear-inflow jet. Bow echos tend to form places w/large CAPE, strong vert wind shear (45kt/52mph+). Strong winds aloft intensify rear-inflow jet. N-end of squall forges out ahead of strong low-pressure system can evolve into a series of long-lived downbursts called derechos. Noticeable on radar as one+ bow echoes. |
|
|
Derecho Qualifications: must be numerous wind damage reports and/or gusts greater than 50t (57mph), major axis 400+km (250+mi) long, some of straight-line wind damage must have several reports of winds over 65kt(75mph). Line Echo Wave Pattern (LEWP) = connected series of bow echoes w/wave-like appearance. |
Another type of derecho can form over N states E of rockies during late spring/summer. Warm season derechos can evolve from a MCS that forms radar bow echo. Warm season derecho more common than cool. Mid-lat low-p systems are catalysts for warm-season derechos. Almost all form along stationary front from W to E. Steering winds in the mid-trop blow WtoNW nearly parallel to front. |
|
|
When weak low-p system farther W draws warm/moist air N over stationary front, warm advection sparks system of storms on cool side of front. high dew-point air tends to pool along portion of stationary front where warm-season derecho lives. Warm-season derechos move 40-50kt (45-55mph) faster than cool-season. |
A mix of discrete/semi-discrete/organized tstorms is called a mixed mode. Lark's Triangle is a sector where synoptic-scale meeting (of mid-lat cold/warm fronts & second closed isobar around low's center) can promote severe thunderstorms. Surface convergence around the low/cold front along w/upper-level divergence w/500mb trough sets stage for tstorms. |
|
|
Meteorologists also look at mesoscale boundaries such as gust fronts, sea-breeze fronts, high-level heat sources, or dry lines. Storm Prediction Center (SPC) in Norman, OK, monitors country for conditions that promote severe tstorms. If conditions favor severe weather, Severe TStorm Watch is issued (typ over 50K-75K km2/20K-30K mi area. |
Tornado Watch issued when one+ tornadoes favorable, watch areas approximated by parallelograms issued by counties. Typ watch is in effect for 6-8hr, can be cancelled, replaced, reissued. The area directly affected would typ amount to only a fraction of one percent of the original watch area. Exception being a derecho 20% of watch area can be affected. |
|
|
Top windspeed in violent tornadoes rarely exceed 135m/s (300mph). A funnel cloud lowers toward the ground from the base of a severe thunderstorm, rotates but has no contact w/ground. If circulation reaches surface tornado is born. Tornadoes can be invisible when weak ascent inside twister can't cool the air to produce net condensation and/or when air below cloud base is too dry. |
Tornadoes have average diameter of 100m (330ft), width of damage path can exceed 2 km (1.3 mi), strongest wind speed typ lie just inside the edge of the dust cloud that outlines the lower part of twister and typ move SW to NE at speeds as high as 27m/s (60mph). Tornado Alley is a broad region of the central US where frequency of strong tornadoes is higher. |
|
|
Texas is #1 w/150 tornadoes/year, Kansas #2 w/97/year. Qualities of tornado-active regions: high convective available potential energy (CAPE), strong vert wind shear, weak-modest synoptic-scale lift, breakable capping inversion. Second unofficial tornado alley = AK, LA, MI, AB, GA, Jan-May prime season. Reg Tornado Alley season Mar-May, & NE states from May-Jul. |
May most active month, record in '03 of 543 tornadoes, peak hours 4-6PM. 20% form from midnight-noon. (Storm relative means subtract storms forward motion from wind speed) low-level winds blow from SE relative to supercell, while high-level winds blow from SW relative to storm carrying hydrometeors to NE away from updraft. forward region: drag on air by falling raindrops produces forward-flank downdraft. |
|
|
This is a consequence of vertical wind shear and promotes longevity, increasing chances of severe weather. Stage is set for mesocyclone (rotating updraft). Vert wind shear produces horizontal roll (vorticity) in lower trop. Developing supercell ingests spinning inflow of low-level warm/humid air, tilting the spinning tube upward w/updraft on S flank, forming mesocyclone. |
This process called stretching gives rotating air faster spin, mesocyclone contracts inward, it stretches vertically. Develops at altitude of 5-6km(3-4mi), descends as spinning air near its outer edge, hitching on the rear-flank downdraft, if some of the downward air is drawn back into the stream of humid air flowing toward the center of the mesocyclone... |
|
|
its angular momentum will further increase as the ring of inflowing air contracts creating faster cyclonic spin creating a tornado. Some tornadoes begin at ground and work way up, this involves low-level convergence near ground beneath tstorm & resulting vert stretching/spin-up of local air column producing landspouts - small/weak tornados in non-supercell tstorms. |
These often form in High Plains E of Rockies (continental cousin of waterspouts) Mesocyclone=counterclockwise. Coiling pattern of precip appearing on radar is called hook echo. Here, tornado forms in precip-free area within hook. Strong signal in Doppler velocities in area of hook echo confirms presence of corresponding mesocyclone called velocity couplet... |
|
|
relatively fast inbound (toward radar) and outbound Doppler velocities. if horizontal shear across the velocity couplet reaches threshold (NWS uses 90kt/140mph for couplet w/in 55km/35mi of radar) it is likely tornado formed. Velocity couplet then becomes Tornado Vortex Signature (TVS). Optimal range for detecting TVS is 60km/36mi. |
The Verification of the Origins of Rotation in Tornadoes Experiment 2 (VORTEX 2) involved almost 100 scientists/students, 10 mobile radars, used to deploy/create relatively dense observational network around potentially tornado storms. It documented complete life cycle of tornado spawned by supercell in SE Wyoming on 6/5/09. |
|
|
False alarm Tornado Warning account for 75%, the fraction of supercell that spawn tornadoes is between 5-30%. Weak tornados may not register TVS. The precip-free chamber of a tornado tstorm is called a vault. The rear-flank downdraft maintains a steady supply of evap cooled air behind gust front S of storm. If relatively warm may favor tornadogenesis. |
This is b/c warm air lifts easier. When alt of lifting condensation level (LCL) is low, rear-flank downdraft tends to be warm when it reaches ground. This limits production of cold air there b/c limited vert space that raindrops can evap & cool air. When LCL is high, rear-flank downdraft tends to be relatively cold. Temp diff. between RFD & enviro. often less than 3C (5F). |
|
|
Tornadoes tend to form when LCL below 1200m (4Kft). Avg. height of strong tornadoes LCL is 700m (2300ft). When temp diff range 5-12C (9-22F) LCL above 1200m, tornadoes become increasingly unlikely. Gust fronts marking leading edge of supercell's outflows of rain-cooled air sometimes spawn whirlwinds called gustnadoes. (not counted in tornado tally) |
Typ weak, minimal debris shows their circulation, occur in dry/dusty places."Tornado Intercept Vehicle" takes measurements/video near twisters. Near mid of develop. severe tstorm, mesocyclone lowers from alt 5-6km(3-4mi). As pressure lowers w/in, air drawn toward center is cooled causing Wvapor in layer directly below base to condense, called wall cloud. |
|
|
As process deepens, vert stretching begins, cyclonic spin causes funnel cloud to descend from center of wall cloud. Streamers are formed when drops in pressure/temp in core of mesocyclone cause moisture in surface air being drawn toward tornado condenses into inward-moving clouds. Tornados can live from seconds-hours. Avg = 10 mins. When stretched into rope, ends. |
TOTO (Totable Tornado Observatory) early 80s, 400kg (880lbs) met tornado once (Apr 1985) measuring winds near 30m/s (67mph) & small drop in air pressure. Now, "turtle" 35cm/14in armored sensor pack is used. Similar probe called HITPR (Hardened In-situ Tornado Pressure Recorder) measured pressure drop of 100mb in period of 15 seconds as tornado passed overhead in Manchester SD 06/24/03 |
|
|
Some of most violent tornadoes contain smaller whirls orbiting around venter of parent tornado - multi-vortex tornadoes have smaller offspring called suction vortices - path of these not circular, parent tornado moves in straight path, combo of circular/straight-line movements by suction vortex creates a looping path for offspring. Cycloid pattern. w/in loop damage is extreme. |
Suction vortices = extreme damage b/c winds of parent tornado & suct. vert combine at some points w/in cycloid path. Forward speed of tornado adds to speed of tornado winds at places where they blow the same direction to boost max wind speeds. |
|
|
Movie photogrammetry/dop radar/forensic technique used for estimating wind speeds provide indirect measurements. In Movie photogrammetry, frame-by-frame analyses of flying debris in a tornado yield distances objects move over time. In 1971, Dr. T Theodore Fujita developed scale intended to relate degree of damage in aftermath to speed of winds - Fujita Scale (FScale). |
This was used for 3 decades to rate tornado intensity. F0-F5. Not accurate, used intuition. Enhanced Fujita Scale (EF-Scale) implemented 02/07, assignment begins w/ damage indicators that represent diff kinds of structures/objects (farm buildings/flagpoles/homes/trees/etc) specific guidelines link damage to estimated range of windspeed. (measured at any given point) |
|
|
EF-Scale rating is assigned by greatest damage at any point along its path. 1989 to 2008, US averaged 1230 tornadoes per year, nearly 80% relatively weak, 95% below EF3 intensity. Less than 1% of all tornadoes are EF4-5. Between 1950-2008 only 50 tornadoes rated EF5. Most deadly occurred 3/18/25, 353km/219mi path SE Missouri-SW Indiana killing nearly 700 people. |
After 1999, OK promoted safe room construction through rebate program resulting in 6K safe rooms built. Basement/underground safest place to take cover under something sturdy, away from windows like interior bathroom. Drive out of path or hide in ditch protecting head. |
|
|
Dust devil - rotating column of air that forms near ground kicking up dust and dirt. Typ small whirls that spin up on hot, sunny days, over dry landscape, short-lived/harmless, wind speed can top 20m/s (45mph). Formed when strong heating by sun promotes fast rising currents of air near ground, any horizontal wind shear can create them, clockwise or counterclockwise. |
Waterspout - tronado over water, first type forms w/ severe thunderstorm, begin as land tornado & move offshore or vice versa, second type is non-supercellular variety simply maritime cousin of landspout commonly called tornadic or fair-weather waterspouts - these develop at water surface and build skyward within humid enviro of lowest few hundred meters of trop. |
|
|
Typ require very warm water to initiate updrafts. Several hundred occur each year in the Keys w/ Aug/Sept peak season, typ along lines where surface air converges providing rising, moist air to generate lines of cumulus and cumulus congests clouds beneath which waterspouts can spin up. wind speeds rarely exceed 32 m/s (70mph). Lifespan of 10s of mins. |
Tornadoes of fire - develop b/c small-scale contrasts in air temp becoming very large, temp gradients create pressure gradients with several "mini" low-pressure areas developing over hot spots w/in the fire. |
