Importance of “real world” storm evidence – Hail & how it forms

On June 9th 2009, The Shenandoah Valley…specifically for this post…Frederick Co. Virginia was under the following warning by the National Weather Service in Sterling, VA:

.A SEVERE THUNDERSTORM WARNING REMAINS IN EFFECT UNTIL 430 PM EDT
FOR FREDERICK...BERKELEY...HAMPSHIRE...JEFFERSON...CITY OF WINCHESTER
AND WASHINGTON COUNTIES...

AT 408 PM EDT...NATIONAL WEATHER SERVICE DOPPLER RADAR CONTINUED TO
INDICATE A LINE OF SEVERE THUNDERSTORMS CAPABLE OF PRODUCING PENNY
SIZE HAIL...AND DAMAGING WINDS IN EXCESS OF 60 MPH. HOWEVER...HAIL
UP TO THE SIZE OF HALF DOLLARS IS EXPECTED NEAR CEDAR HILL AND
RIDGEWAY. THESE STORMS WERE LOCATED ALONG A LINE EXTENDING FROM 7
MILES NORTH OF SHEPHERDSTOWN TO GERRARDSTOWN TO ROCK ENON
SPRINGS...OR ALONG A LINE EXTENDING FROM 7 MILES NORTH OF
SHEPHERDSTOWN TO 12 MILES SOUTHWEST OF MARTINSBURG TO 15 MILES NORTH
OF STRASBURG...MOVING EAST AT 25 MPH.

OTHER LOCATIONS IN THE WARNING INCLUDE BUT ARE NOT LIMITED TO
INWOOD...HAYFIELD...BUNKER HILL...CEDAR GROVE...VANVILLE...
WINEBRENNERS CROSSROAD...HIGH VIEW MANOR...RIDGEWAY...CEDAR HILL...
ALBIN...KEARNEYSVILLE...CLEAR BROOK...MIDDLEWAY...STEPHENSON...
BRUCETOWN...ANTIETAM AND SHENANDOAH JUNCTION

The National Weather Service considers a thunderstorm severe only if it produces ......Damaging Wind Gusts- 58 miles per hour (50 knots) or higher –OR -Large Hail 3/4 inch in diameter (penny size) or larger –Or- Tornado

• A SEVERE THUNDERSTORM WATCH means conditions are favorable for thunderstorms to become severe...or severe thunderstorms to move into the watch area. Watches are intended to heighten public awareness of the possible severe weather threat. Keep an eye on the sky and stay tuned to NOAA Weather Radio or local radio, television, or cable to know when severe weather warnings are issued for your area.

• A SEVERE THUNDERSTORM WARNING means a severe thunderstorms poses an imminent danger to life and property to those in the path of the storm. When severe weather is indicated by weather radar, or is reported by trained SKYWARN Severe Weather Spotters or law enforcement officials a warning is issued immediately.

Given our unique location…in a valley, surrounded by mountains…poses a very difficult problem for NWS Forecasters. The Doppler radar is located in Sterling, sweeps the sky approx. every 4.5 minutes (in rapid scan mode) and this data is interpreted by NWS computers and forecasters. However, the Blue Ridge Mountains creates blind spots for the radar and forecasters. So in addition to the images coming in from the Doppler, forecasters also rely on reports from Skywarn Radio, Fire, police and emergency services in the warned area for feedback. This is also considered (sometimes even more so than the images) in tweaking and updating severe weather warnings.

Having “real” world evidence to confirm the warnings that have been issued, along with an approx. time & location to compare radar data is immensely helpful. It helps validate the warning and enables forecasters to go back and compare radar loops to watch the cell as it progresses and moves on. This helps to tweak computer models and simulations as well as severe weather predictions. Overlaying real world data onto archived radar data helps “paint the picture” of what was going on out there.

The NWS got this warning pretty much spot on. Hail was indeed present. This should qualify as a confirmed warning.

How does hail form?

• Inside of a thunderstorm are strong updrafts of warm air and downdrafts of cold air.
• If a water droplet is picked up by the updrafts...it can be carried well above the freezing level. With temperatures below 32F...our water droplet freezes.
• As the frozen droplet begins to fall...carried by cold downdrafts...it may thaw as it moves into warmer air toward the bottom of the thunderstorm
• But...our little half-frozen droplet may also get picked up again by another updraft...carrying it back into very cold air and re-freezing it. With each trip above and below the freezing level our frozen droplet adds another layer of ice.
• Finally...our frozen water droplet...with many layers of ice - much like the rings in a tree...falls to the ground - as hail!

Should I be concerned about tornadoes when hail is observed?

The presence of large hail indicates very strong updrafts and downdrafts within the thunderstorm. These are also possible indicators of tornadic activity. Often large hail is observed immediately north of a tornado track - but the presence of hail doesn't always mean a tornado and the absence of hail doesn't always mean there isn't a risk of tornadoes

(NOAA Image)

 

 

Is there a way to simply look at a thunderstorm and tell if it will produce hail at the ground?

• There is no positive way to look at a thunderstorm in the distance and tell if it will produce hail reaching the ground.
• Meteorologists use weather radar to "look" inside a thunderstorm. Since hail reflects more energy back to the radar than raindrops it often shows up in red shades.
• The WSR-88D Doppler Radar can also estimate size of the hail based on the amount of energy reflected back.

 

 

 

Hail in Clear Brook, Virginia on June 9th 2009 just after 4pm (Severe Thunderstorm Warning issued minutes before-Thanks Natalie!)

 

 

 

 

 

 

 

 

 

 

 

 

 

NWS-Sterling WSR-88D Doppler radar (courtesy Flickr Weather Stream)