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MESOSCALE WEATHER I

Severe Storms. MSC 243 Lecture #10 5 November 2009. MESOSCALE WEATHER I. Mesoscale Weather. Mesoscale: Pertaining to atmospheric phenomena having horizontal scales ranging from a few to several hundred kilometers. Thunderstorms Squall lines Fronts

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MESOSCALE WEATHER I

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  1. Severe Storms MSC 243 Lecture #10 5 November 2009 MESOSCALE WEATHER I

  2. Mesoscale Weather • Mesoscale: Pertaining to atmospheric phenomena having horizontal scales ranging from a few to several hundred kilometers. • Thunderstorms • Squall lines • Fronts • Precipitation bands in tropical and extratropical cyclones • Lake effect rain and snow • Topographically generated weather systems • Sea and land breezes

  3. Mesoscale Weather • Mesoscale prediction is the new wave of weather forecasting. • Prior to the mid-1990’s, numerical model resolution was too coarse to capture many mesoscale phenomena. • New mesoscale model: Weather Research and Forecasting (WRF) model

  4. Thunderstorms develop where moist air is forced aloft rapidly • Occurs frequently in the tropics, nearly daily in some locations • In the U.S., most frequent region is the Gulf South • Absolute peak in Florida as the land protrudes into warm waters • Secondary peak along front range of Colorado

  5. Components of a Thunderstorm Overshooting top tropopause Downdraft Updraft core Shelf cloud (Hail)

  6. Cirrus Anvil Overshooting Top Overshooting Tops Outflow direction Fig. 11.2a

  7. Updraft Core

  8. Mammatus clouds at bottom of anvil

  9. 2) MULTICELL T-Storms a) Squall line 1) Ordinary (Air Mass) T-Storms SINGLE CELL b) Mesoscale Convective Complex 3) Supercell T-Storms 3 Types of T-Storms

  10. Lifecycle of Single Cell T-Storms Ordinary “Air Mass” Thunderstorms • localized, short lived phenomena • generally not severe • self-extinguishing • most common type

  11. Life Cycle: a. Cumulus Stage • Clouds form • Only updrafts present • Little rain.

  12. Life Cycle: b. Mature Stage • Anvil cloud forms • Both updrafts and downdrafts • Heaviest rainfall • Most lightning

  13. Life Cycle: c. Dissipating Stage • Downdrafts dominate -> suppress inflow • Decreased rainfall • Cloud base evaporates, leaving cirrus remnants behind

  14. Multicell Thunderstorms • Differ from single cell T-Storms in that up- and downdrafts support each other to intensify the storm. • Generally are more severe. • Usually occur in groups over fairly large areas. • Downdrafts from Cell 1 form a cold pool and a propagating gust front. Warm, moist air is forced to rise over the gust front, thereby reaching the LFC and triggering a new cell. • This process can repeat itself multiple times.

  15. Gust Front With Shelf Cloud Warm, moist air forced over forward edge gust front Cold outflow from downdraft Formed by the lifting of warm moist air over the forward edge of the colder outflow gust front.

  16. Multicell Thunderstorms: Self-Propagating Downdraft Updraft

  17. Mesoscale Convective Complex • MCCs account for the greatest amount of severe weather in the U.S. and Canada • Roughly circular clusters of thunderstorms which are self propagating in that individual cells create downdrafts which interact to form new cells • Colder, denser downdrafts spread across the surface and help force warm, moist surface air aloft • This outflow boundary initiates a new cell by converging with the stream of warm, moist surface air

  18. Squall Line • Convection triggered along a pre-existing convergent boundary (e.g. cold front, dry line) • Differ from MCCs in that thunderstorms are quasi-linear • Squall lines often propagate in the same direction of motion as the cells. • New cells form ahead of the squall line as old cells dissipate. • (Structure and evolution depends on the orientation of vertical wind shear)

  19. Squall Line

  20. Cross-Section of a Squall Line • Dry (unsaturated) air is entrained near the cloud top. • The evaporation of cloud and rain water cools the air. • The cooler, denser air sinks as a downdraft. • When it reaches the surface, the downdraft spreads out horizontally as a turbulent gust front.

  21. Shelf cloud: often the visualization of an outflow boundary

  22. Supercell T-Storms

  23. One type of precursor disturbance for supercells: the dryline mT: Maritime Tropical (moist, warm) cT: Continental Tropical (dry, warm) cP: Continental Polar (dry, cool) • Abrupt geographic changes from moist to dry air, called drylines, form in western TX, OK, and KS in the spring and summer. • The diagram illustrates how cool “cP” air pushes hot and dry “cT” air over the warm moist “mT” air. • This mixing of different air masses can create supercell storms.

  24. Rotation of Supercell Storms • Supercells are more intense than multicell t-storms; possess a rotating updraft (mesocyclone) • Horizontal vortex tubes created by vertical wind shear may be tilted and forced in a vertical path by updrafts. • This rising, rotating air may then turn into a mesocyclone. • Wall clouds, a precursor to tornados, often form at the base of a mesocyclone.

  25. Rotating Wall Cloud • The first sign that a supercell may form a tornado is the sight of rotating clouds at the base of the storm, which may lower and form a wall cloud.

  26. Tornadoes! Miami: May 12th, 1997

  27. Mean number of days with one or more events within 25 miles of point

  28. Enhanced Fujita Tornado Scale (updated February 2007) The Enhanced F-scale is a set of wind estimates (not measurements) based on damage. Its uses three-second gusts estimated at the point of damage based on a judgment of 8 levels of damage to 28 indicators. The 3 second gust is not the same wind as in standard surface observations. Standard measurements are taken by weather stations in open exposures, using a directly measured, "one minute mile" speed. http://www.spc.noaa.gov/efscale/

  29. Storm Prediction Center http://www.spc.noaa.gov/

  30. Tornado Watches and Warnings • Nobody knows for certain how tornadoes form, but they do know when conditions are most likely for them to form. • When the conditions are conducive, the Storm Prediction Center issues a tornado watch. (lasting 4-6 hours) • When a tornado watch is issued, local storm spotter networks activate. If a spotter sees a funnel cloud or tornado, the local Weather Service office will issue a tornado warning. They can also issue warnings based on radar. • Most cities have tornado sirens to alert people to the danger. • Also, using a new network called the Emergency Alert System, people can hear about warnings right away: http://www.fcc.gov/pshs/services/eas/

  31. February 10 2009 tornadoes

  32. SPC Convective Outlook

  33. Dewpoint(shaded)

  34. CAPE (red contours)

  35. The moisture return continued through the morning hours, eventually setting up a broad area of upper 50s and lower 60s surface dewpoints generally along and west of Interstate 44, and along and south of Interstate 40. The low-level moisture return, combined with relatively strong surface heating and cooling air aloft, created a very unstable atmosphere, especially for early February.

  36. 4 hours later…

  37. HOOK ECHO

  38. BULLETIN - EAS ACTIVATION REQUESTED TORNADO WARNING NATIONAL WEATHER SERVICE NORMAN OK 217 PM CST TUE FEB 10 2009 THE NATIONAL WEATHER SERVICE IN NORMAN HAS ISSUED A * TORNADO WARNING FOR... SOUTHEASTERN CANADIAN COUNTY IN CENTRAL OKLAHOMA... NORTHWESTERN OKLAHOMA COUNTY IN CENTRAL OKLAHOMA... * UNTIL 245 PM CST * AT 217 PM CST...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A SEVERE THUNDERSTORM CAPABLE OF PRODUCING A TORNADO OVER WESTERN OKLAHOMA CITY...4 MILES NORTHWEST OF MUSTANG...MOVING NORTHEAST AT 40 MPH. * LOCATIONS IN THE WARNING INCLUDE BETHANY...MUSTANG...NICHOLS HILLS...NORTHWESTERN OKLAHOMA CITY...THE VILLAGE...WARR ACRES... WESTERN EDMOND AND YUKON. THIS INCLUDES INTERSTATE 40 BETWEEN MILE MARKERS 132 AND 145. THIS INCLUDES INTERSTATE 44 BETWEEN MILE MARKERS 122 AND 127. IN ADDITION TO A TORNADO THREAT... THIS STORM LIKELY CONTAINS DAMAGING HAIL TO THE SIZE OF BASEBALLS OR LARGER. A TORNADO MAY FORM AT ANYTIME. TAKE COVER NOW. ABANDON MOBILE HOMES AND VEHICLES. IF POSSIBLE... MOVE TO A BASEMENT OR STORM SHELTER. OTHERWISE... MOVE TO AN INTERIOR ROOM OR HALLWAY ON THE LOWEST FLOOR. STAY AWAY FROM WINDOWS. LAT...LON 3563 9739 3537 9773 3537 9776 3544 9791 3573 9756 TIME...MOT...LOC 2017Z 223DEG 35KT 3545 9775 $$ WR

  39. Products Issued For Oklahoma County Tornadoes 212 PM Significant Weather Advisory Oklahoma County 217 PM Tornado Warning SE Canadian and NW Oklahoma 226 PM Warning Continues for Canadian/Oklahoma Counties 235 PM Warning Continues for Canadian/Oklahoma Counties 244 PM New Tornado Warning NW Oklahoma County and Srn Logan County 251 PM Warning Continues for Oklahoma/Logan Counties 259 PM Warning Continues for Oklahoma/Logan Counties 301 PM Tornado Reported; Warning Continues for Oklahoma/Logan Counties

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