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Published on October 3, 2007

Author: Tarzen

Source: authorstream.com

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Skywarn Basics II:  Skywarn Basics II National Weather Service Baltimore-Washington Forecast Office Basics II Outline:  Basics II Outline Part I - Ingredients for the development of Severe Thunderstorms (40 mins) Part II - Radar Basics (30 mins) How it works and its products Part III - The Thunderstorm Spectrum (60 mins) Visual and Radar Part IV – Review and Questions Severe Ingredients:  Severe Ingredients Instability = heat and moisture Shear = Increasing winds with height and veering winds with height Lift = a lifting mechanism to focus the energy How to Make a Thunderstorm:  How to Make a Thunderstorm In Basics I Class, we showed this slide Instability:  Instability Cold air parcel packs in a lot of molecules. There is less movement. With more molecules per area, this air is heavier and denser. Warm air molecules are actively moving around limiting the number of molecules that an air parcel can hold. With less molecules per area, it is lighter. Same size air parcels Cold air versus Warm air Instability :  Instability Cold air is heavier than warm air. If the air is colder than its surroundings, it will sink and stay close to the ground. Warm air is lighter than cold air. If air is warmer than its surroundings, it will rise. Sun heats ground. Ground heats the air above. Daytime heating is one way to warm up the lowest layer of the atmosphere. Cold air versus Warm air Instability:  Instability Water vapor is lighter than air Dry air versus Moist air Instability:  Instability Moist air is lighter than dry air. Therefore if a parcel of air is more moist than its surroundings, it will rise. Dry air is heavier. If air is drier than that around it, it will sink. Evaporation is one way to increase the amount of water vapor and moisten the air. Dry air versus Moist air Instability:  Instability Sounding = A vertical profile of temperature and moisture Temperature Moisture Dulles sounding the evening of the Frostburg Tornado Instability:  Instability CAPE = Convective Potential Energy Dulles sounding the evening of the Frostburg Tornado Instability Indices:  Instability Indices BEST Instability Indices:  Instability Indices Shear:  Shear Increasing winds with height Veering winds with height (wind turning in a clockwise direction) Shear:  Shear With light and variable wind or uniform winds, the thunderstorm cloud grows erect. With increasing wind with height, the growing tower begins to tilt. Increasing Wind with Height Shear:  Shear Increasing Wind with Height Wind Shear:  Wind Shear Induces rotation Veering Wind with Height Wind Shear:  Wind Shear Wind Shear creates horizontal rolls A horizontal roll gets sucked up into the updraft of a storm turning the roll into the vertical Wind Shear:  Wind Shear Increasing and veering winds with height is bad news Increasing and Veering Wind with Height Wind Shear:  Wind Shear Increasing and Veering Wind with Height Wind Shear Indices:  Wind Shear Indices BEST Wind Shear Indices:  Wind Shear Indices BEST Instability and Wind Shear:  Instability and Wind Shear Increasing and veering winds with height Hot and Humid air mass Combination Indices:  Combination Indices Lifting Mechanisms:  Lifting Mechanisms Cold Front – cold air moving into warm Warm Front – warm air moving into cold Stationary Front – a stalled weather front Bay Breeze – moist cool air moving inland Low Pressure Areas – low pressure draws air in T.Storm Outflow – cold air blowing out of a thunderstorm Hills / Mountains Cold Front:  Cold Front Colder air is denser than the warm air ahead of the front. The warmer air is forced to rise up. If the air is unstable, it will keep rising. Cold fronts often initiate lines of showers and thunderstorms. Warm Front:  Warm Front Again, the colder air is denser than the warm air. As the warm air encounters the cold air, it is forced to rise up and over. If the air is unstable, showers and thunderstorms can form. Bay Breeze Front:  Bay Breeze Front Acts similar to a cold front as the cooler air off the water moves into to replace the warm air over the land. Low Pressure Area:  Low Pressure Area As air moves into the low pressure center or trough, it converges and is forced to rise. Thunderstorm Outflow:  Thunderstorm Outflow Cool air flowing out of a thunderstorm acts like a cold front. The rain cooled air displaces the warm air beginning the development of a new storm. Mountains:  Mountains Lift Mountains area a barrier Heat up more than air over valley Basics II Outline:  Basics II Outline Part I - Ingredients for the development of Severe Thunderstorms (40 mins) Part II - Radar Basics (30 mins) How it works and its products Part III - The Thunderstorm Spectrum (60 mins) Visual and Radar Part IV – Review and Questions Radar Principles:  Radar Principles How the radar operates What is “Reflectivity” Hail Indications Cross-sections What are “Doppler Velocities” Storm Relative Motion Precipitation Estimates Radar Principles:  Radar Principles 70 Ft Tower 30 Ft Dome 25 Ft Radar Dish 750 KW Power 0.8° Beam Basic Radar Operation Basic Radar:  Basic Radar Basic Radar :  Basic Radar Cone of Silence Basic Radar :  Basic Radar Causes a storm to appear to be weakening when approaching the radar Storms moving away seem to get bigger Harder to detect severe inside 10 nm Cone of Silence Radar Principles:  Radar Principles Reflectivity Composite Reflectivity:  Composite Reflectivity Hail Indicators:  Hail Indicators Radar Principles:  Radar Principles BWER Hail core Allows us to see a slice through the storm (This is where the tornado develops) This storm was producing a weak tornado near Berryville, VA Cross-sections Radar Principles:  Radar Principles Particles moving away from the radar appear red Particles moving toward the radar appear green Doppler Velocities Storm Relative Motion:  Storm Relative Motion We subtract the storm motion to better see the storm’s rotation SRM versus Base Velocity:  SRM versus Base Velocity Radar Principles:  Radar Principles Precipitation Estimates:  Precipitation Estimates We use spotter reports and automated rain gauges to verify the radar’s rainfall estimates We use these reports to calibrate the radar Basics II Outline:  Basics II Outline Part I - Ingredients for the development of Severe Thunderstorms (40 mins) Part II - Radar Basics (30 mins) How it works and its products Part III - The Thunderstorm Spectrum (60 mins) Visual and Radar Part IV – Review and Questions Thunderstorm Spectrum:  Thunderstorm Spectrum Instability = heat and moisture Shear = Increasing winds with height and veering winds with height Lift = lifting mechanism to focus the energy Instability, the wind profile and the lifting mechanism define what type of thunderstorms will occur Weak Shear / High Instability:  Weak Shear / High Instability Classic peak summer pattern Heat and humidity combine for high instability Light winds give you weak shear Pulse Severe Storms Microbursts Flash Floods Pulse Storms:  Pulse Storms High instability Weak shear Storm shoots up and then collapses Wet microburst or hail may occur with the collapse Short lead-time on warnings Pulse Severe Storms:  Pulse Severe Storms Can be a single cell storm as shown on previous slide Can occur in multicell clusters as shown here Microbursts:  Microbursts Drier air is entrained into the storm in the mid-levels Evaporation causes a cold ball of air which then descends and accelerates to the ground Flash Flood Storms:  Flash Flood Storms High humidity leads to heavy rainfall Weak winds means little storm movement, hence storm sits over a single location longer dropping more rain Also, storms often develop along boundaries in this environment and may repeatedly fire and move over the same location Madison County, June 1995 Squall Lines or Multicell Lines:  Squall Lines or Multicell Lines Lift mechanism (such as a cold front) is usually the initiator Often have winds greater than 50 mph up at 5000 feet pushing the storms Bow Echoes Derechoes Squall Lines or Multi-cell Lines:  Squall Lines or Multi-cell Lines Severe Weather occurs on the leading edge Bow Echoes:  Bow Echoes A downburst causes a portion of the storm to accelerate ahead Bow Echoes - Radar:  Bow Echoes - Radar On radar, the leading edge bows out Notches or indents appear on the rear side July 2003 – out the window at NWS Sterling Supercells / Mesocyclones:  Supercells / Mesocyclones High shear High instability Produce 90% of thunderstorm damage, deaths and injuries Supercells / Mesocyclones:  Supercells / Mesocyclones BWER (“Bee-weir”):  BWER (“Bee-weir”) Bounded Weak Echo Region (BWER) can be a tornadic indicator BWER:  BWER June 2, 1998 Was producing a F1 tornado near Berryville, VA July 9, 2003 – Out the window at NWS Sterling, a tornado is trying to form and drop down Hook Echoes:  Hook Echoes Hook Echo / Appendage:  Hook Echo / Appendage Hook echo is an appendage near the inflow area to the storm Hook Echo / Appendage:  Hook Echo / Appendage Hook echo is another tornado indicator La Plata Tornado, April 2002 La Plata Tornado:  La Plata Tornado La Plata Tornado:  La Plata Tornado 7 am EDT : Rain and fog over the area cancels the Chesapeake Bay Bridge Walk. 7 pm EDT : Tornado approaches La Plata, MD. College Park Tornado:  College Park Tornado Warm Sector – warm front had moved north in the morning; cold front approaching from the west Jet dynamics; lots of shear Good moisture (surface dew points 70°) Southeast surface winds = high storm relative helicity (favorable low-level shear) CAPE ~1600 J/kg, SRH~200 m2s-2, EHI~2.0, LI ~ -5 Weather Pattern (pre-storm environment) 1200UTC 24 Sep 01 Raob: IAD (modified to 2100 UTC conditions):  1200UTC 24 Sep 01 Raob: IAD (modified to 2100 UTC conditions) 1200 UTC 24 Sep 01 Raob: IAD (observed/modified to 2100 UTC):  1200 UTC 24 Sep 01 Raob: IAD (observed/modified to 2100 UTC) KLWX (Sterling) Radar VWP observed wind profile at 2100 UTC:  KLWX (Sterling) Radar VWP observed wind profile at 2100 UTC Univ. of MD:  Univ. of MD Top Photo published in the Washington Post. Upper left photo by Associated Press. Lower right photo from terpidiots.com and lower left photo from blahaway.com/yearbook MD Fire & Rescue Institute Temporary buildings destroyed. September 24, 2001 Radar over College Park:  Radar over College Park Base Reflectivity at 5:21 pm EDT. Storm Relative Velocity Map at 5:21 pm. Warning issued at 5:10 pm Crossing the Beltway:  Crossing the Beltway Base Velocity at 5:26 pm EDT Storm Relative Velocity Map at 5:26 pm Base Reflectivity at 5:26 pm Between I-95 and US Rte 1:  Between I-95 and US Rte 1 Base Reflectivity at 5:31 pm EDT Basics II Outline:  Basics II Outline Part I - Ingredients for the development of Severe Thunderstorms (40 mins) Part II - Radar Basics (30 mins) How it works and its products Part III - The Thunderstorm Spectrum (60 mins) Visual and Radar Part IV – Review and Questions Credits:  Credits Radar graphics from Oklahoma Climate Survey, 2000 Contact Information:  Contact Information Please send SKYWARN related administrative inquiries to LWX-Skywarn@noaa.gov Please continue to contact our office with reports via telephone (1-800-253-7091 ext #1), HAM radio (147.300 MHz WX4LWX), or e-mail (LWX-Report@noaa.gov) http://www.weather.gov/baltimore or http://www.weather.gov/washington Thank you for Supporting Skywarn !

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