Review of Basic Severe Thunderstorm & Tornado Spotting Concepts Jim Allsopp National Weather Service Chicago/Romeoville, IL Why Do We Need Spotters? • Radar has limitations, only spotters can provide view into what’s really happening • Spotter reports are used to verify warnings, which helps forecasters improve warning skills • Reports are disseminated to media, EMs, other spotters, public • Spotter reports can help elicit proper response • Spotter reports are a vital ingredient in the Warning Decision Making Process Warning Decision Making Process NWS Warning Forecaster NWS Doppler Radar Data Spotter Reports Warning Issued Weather Observations Before you go out spotting… Have An Idea What To Expect • Utilize NWS products (AFD, HWO, radar) to brief yourself on expected hazards and magnitude of threat • Hazardous Weather Outlook (HWO): one stop shopping for all forecasted hazardous weather for next 7 days. • HWO is updated at approximately: ‐ 430 AM, 1100 AM, 430 PM ‐ when forecast changes from previous HWO Have An Idea What To Expect Under “Current Hazards” menu “Outlooks” Have An Idea What To Expect Latest “HWO” from NWS Chicago National Severe Weather Forecasts from SPC National Rainfall Forecasts from HPC Thunderstorm Basics Thunderstorm Lifecycle Cumulus Stage Mature Stage Dissipating Stage Updrafts & Downdrafts Updrafts Downdrafts • area where warm, moist • area where rain‐cooled air feeds in and rises up air is moving down and into the storm out from storm • identified by dark, rain • area often identifiable by free base of storm heavy rain (possible hail) falling • wall cloud (if storm has one) located in this area • heavy rain/hail have murky appearance in distance Updrafts & Downdrafts Updraft & Rain Free Base Wall Cloud Updrafts & Downdrafts Downdraft & Heavy Rain/Hail Updrafts With Rain Free Base Audience Review Question Is that area highlighted updraft or downdraft? Thunderstorm Types Single Cell (Pulse) Storms • most common during summer • occur when wind shear is weak • typically last 30‐60 mins • minimal severe threat – very localized wind damage or hail, if anything Thunderstorm Types Multicell Storms • most common spring through early autumn • usually occur with moderate wind shear • can lasts hours to more than a day Thunderstorm Types Multicell Storms • includes squall lines • can lasts hours to more than a day • most common threats: – damaging winds – marginally severe hail – heavy rainfall Thunderstorm Types Squall Lines • most common multicell type storm • many squall lines have shelf clouds along leading edge • worst is first with squall lines, wind then heavy rain Squall Lines The shelf cloud is the visual clue to squall lines Movement Outflow and downdraft Squall Line Tornadoes • Rare, usually brief, narrow • May be embedded in rain or may form just ahead of, and then get overtaken by downburst hookHook • Challenging to spot Rotational couplet August 4, 2008 near Griffith, IN Thunderstorm Types Supercell Storms • rarest type, potentially most dangerous • have a rotating updraft (mesocyclone) • most common spring & early summer Thunderstorm Types Supercell Storms • can last for many hours • threats include: – tornadoes – large hail – damaging winds • cause most very large hail & significant tornadoes Thunderstorm Types Supercell Storms • discrete storms • many have a distinctive radar signature • “hook echo” often present • mesocyclone, possible tornado located in hook Supercell Structure on Radar Light Rain Forward‐flank Moderate/Heavy downdraft (FFD) Rain & Hail Gust Front Rear‐flank downdraft (RFD) Anvil Edge “Hook echo” Supercell Thunderstorm (top view) Nautical miles 0 5 10 Spotter Positioning “Right Hand Rule” When spotting a supercell, try to position yourself with heavy rain, hail to your right and updraft to your left Supercell Structure x Best location for viewing Overshooting Supercell Structure top Same storm from last slide, Anvil panoramic view Main storm Mammatus tower clouds Flanking line Downdraft Updraft Heavy rain Wall cloud Rain-free base & Hail You should be here X Storm motion Supercell Structure Identifying Cloud Features Shelf Clouds vs. Wall Clouds Shelf Clouds • associated w/squall lines • stretches far across skyline (“shelf stretches”) • along leading edge of storm • strong winds possible as it moves overhead • vertical rotation (or rolling motion) sometimes visible • marks edge of cool outflow air from storm Shelf Cloud Shelf Clouds vs. Wall Clouds Wall Clouds • associated w/supercells • local lowering in cloud base • usually located back of storm • favored location for tornado development • warm inflow air rushes into most wall clouds • horizontal rotation (spinning) sometimes visible Wall Cloud Shelf Clouds vs. Wall Clouds Shelf Cloud A B Wall Cloud Name that cloud! Shelf Clouds vs. Wall Clouds Shelf Cloud Name that A cloud! B Wall Cloud National Weather Service Valley, NE Shelf Cloud or Wall Cloud? More On Wall Clouds • Most storms don’t have wall clouds • Most wall clouds don’t produce tornadoes • Wall clouds that are likely to produce tornadoes tend to… – Exhibit rapid rotation – Have strong inflow – Often have “clear slot” wrapping around wall cloud Rapid Rotation Strong Inflow Rear Flank Downdraft (clear slot) Wall Cloud RFD Undercutting Wall Cloud Once a storm becomes dominated by outflow, a tornado is highly unlikely SLC’s ‐ Scary Looking Clouds (Scud Clouds) SLC’s ‐ Scary Looking Clouds (Scud Clouds) • form as a result of high humidity near storm • can extend very low to the ground • not an indication of severe weather (can be found in both severe & non‐severe t‐storms) SLC’s ‐ Scary Looking Clouds (Scud Clouds) • often fragmented and/or not connected to base of parent cloud • not rotating, though sometimes rising into parent cloud near updrafts • pose no hazard SLC’s ‐ Scary Looking Clouds (Scud Clouds) Tornado Impersonators • look for visual clues before reporting a tornado… – is the feature rotating? – is it in the part of the storm you’d expect a tornado? – Is it in contact with the ground? Common Tornado Impersonators Scud Clouds Common Tornado Impersonators Rainshafts Common Tornado Impersonators Virga Common Tornado Impersonators Smoke What Do I Report? What Do I Report? • tornado impersonators, scud, poor visibility due to trees & buildings make spotting very difficult • even seasoned meteorologists don’t always agree on what particular cloud features are • important to express uncertainties you have, if you’re not sure what you’re looking at, say so! Would This Be A Good Report? “I’m a trained spotter on I‐88 near mile marker 96 about 4 miles east of DeKalb right now. I’m looking north and about 5 miles from me I see a lowering that could be a tornado. I’m too far to see whether it’s rotating and I can’t tell if it’s in contact with the ground, but if it’s not, it’s close.” Would This Be A Good Report? • Good report because it gives all the important information… – Who (“I’m a trained spotter…”) – When (“…right now…”) – Where (“...on I‐88 near mile marker 96 about 4 miles east of DeKalb…I’m looking north and about 5 miles…”) – What (“…I see a lowering that could be a tornado…”) Would This Be A Good Report? • Good report because it gives all the important information… – Uncertainty (“I’m too far to see whether it’s rotating and I can’t tell if it’s in contact with the ground…”) • uncertainty builds credibility in your report Would This Be A Good Report? • uncertainty is better than overstating your certainty and ending up being wrong • report very useful still, correlated to other spotter reports and radar data What Do I Report? • hail (largest hailstone observed) – if it’s measured, say so and give exact measurement – if estimated, related to size of coin or ball – please don’t report “marble sized hail” because we don’t know how big your marbles are! What Do I Report? What Do I Report? • winds over 50 mph and/or wind damage – if reporting damage, report as many specifics as possible and location – if reporting measured wind gusts, please specifically state that is was measured – if reporting estimated wind gust, please stress that it was estimated – wind strength is often overestimated – estimating wind strength is difficult Estimating wind speed • 40 to 55 mph ‐ Non severe...Trees swaying, rain coming down horizontally, twigs and small limbs break, loose lightweight objects (trash cans, lawn chairs) blown around. Estimating wind speed • 60 to 80 mph...medium to large tree limbs downed, sheds, barns and weak structures damaged, truck pushed off the highway. Estimating wind speed • 80 to 100 mph+...numerous large tree limbs downed, shallow rooted trees pushed over, buildings partially unroofed, farm buildings, weak structures severely damaged. What Do I Report? • wall clouds, funnel clouds, tornadoes – always report where you are then approximately how far and in what direction it is from you – is rotation visible with wall cloud? – how far does funnel extend downward? – any damage or debris noted with tornado? What Do I Report? • flooding and/or measured very heavy rainfall – fast flowing water over roadways, estimate depth of water if possible – deep, impassable standing water blocking roads – rainfall >2” in less than an hour – streams or creeks out of their banks – flood waters threatening homes or businesses How To Report • Phone: (800) 681‐2972 (Put # in your cell phone now, if it’s not already) • eSpotter: http://espotter.crh.noaa.gov • Amateur Radio: http://weather.gov/lot/?n=am_radio [email protected] 815‐834‐0600, ext 726 © Amy Pavlik 2008.