A southwest flow between an upper-level low over the Great Lakes and high pressure over the western Atlantic ushered in plenty of moisture from the Gulf of Mexico. The moisture combined with strong surface heating to produce an unstable atmosphere. Showers and thunderstorms developed due to the unstable atmosphere and some thunderstorms were severe.

Thunderstorm Wind in FREDERICK, MARYLAND
From 21-AUG-09 17:45:00 to 21-AUG-09 17:45:00 (EST-5)
Location: DOUBS
Magnitude: 52.00 EG
Reported by: Fire Department/Rescue

A tree was down at Route 464 and Route 15.

Thunderstorm Wind in FREDERICK, MARYLAND
From 21-AUG-09 17:50:00 to 21-AUG-09 17:50:00 (EST-5)
Location: PLEASANT VIEW
Magnitude: 52.00 EG
Reported by: NWS Employee

Several one to two inch branches were down along Tuscarora Road.

Thunderstorm Wind in FREDERICK, MARYLAND
From 21-AUG-09 17:56:00 to 21-AUG-09 17:56:00 (EST-5)
Location: TUSCARORA
Magnitude: 52.00 EG
Reported by: NWS Employee

Several six to eight inch trees were down over Park Mills Road and numerous one to two inch branches were snapped as well.

Thunderstorm Wind in FREDERICK, MARYLAND
From 21-AUG-09 18:02:00 to 21-AUG-09 18:02:00 (EST-5)
Location: URBANA
Magnitude: 52.00 EG
Reported by: NWS Employee

Trees and power lines were down along Thurston Road from Dixon Road to Burnt Hickory Circle.

Thunderstorm Wind in FREDERICK, MARYLAND
From 21-AUG-09 18:02:00 to 21-AUG-09 18:02:00 (EST-5)
Location: PARK MILLS
Magnitude: 61.00 EG
Reported by: NWS Employee

A six to eight inch diameter tree was snapped at the base. Several four to six inch branches were also down near Barnesville.

Thunderstorm Wind in HOWARD, MARYLAND
From 21-AUG-09 18:30:00 to 21-AUG-09 18:30:00 (EST-5)
Location: DAISY
Magnitude: 50.00 EG
Reported by: Fire Department/Rescue

A tree was down near Daisy.

Thunderstorm Wind in MONTGOMERY, MARYLAND
From 21-AUG-09 18:00:00 to 21-AUG-09 18:00:00 (EST-5)
Location: DICKERSON
Magnitude: 56.00 EG
Reported by: NWS Employee

One eight inch tree was snapped off about 30 feet above the ground along Mount Ephraim Road. Twig and leaf debris were also along the road.

Thunderstorm Wind in HOWARD, MARYLAND
From 21-AUG-09 18:32:00 to 21-AUG-09 18:32:00 (EST-5)
Location: WATERSVILLE JCT
Magnitude: 50.00 EG
Reported by: Fire Department/Rescue

A tree was down along Watersville Road.

Thunderstorm Wind in HOWARD, MARYLAND
From 21-AUG-09 18:35:00 to 21-AUG-09 18:35:00 (EST-5)
Location: DAISY
Magnitude: 50.00 EG
Reported by: Fire Department/Rescue

A tree was down along Cars Mill Road.

Thunderstorm Wind in CALVERT, MARYLAND
From 21-AUG-09 14:00:00 to 21-AUG-09 14:00:00 (EST-5)
Location: ISLAND CREEK
Magnitude: 56.00 EG
Reported by: Trained Spotter

Numerous limbs were down and several large branches struck a boathouse causing significant damage.

Thunderstorm Wind in ST. MARY'S, MARYLAND
From 21-AUG-09 17:44:00 to 21-AUG-09 17:44:00 (EST-5)
Location: MECHANICSVILLE
Magnitude: 61.00 EG
Reported by: Fire Department/Rescue

Two trees were down over Old Village Road. A chimney was also blown off a roof.

Hail in BALTIMORE, MARYLAND
From 21-AUG-09 19:13:00 to 21-AUG-09 19:13:00 (EST-5)
Location: GLYNDON
Magnitude: 0.75 
Reported by: Amateur Radio

Penny size hail was reported in Reisterstown.

Hail in BALTIMORE, MARYLAND
From 21-AUG-09 20:45:00 to 21-AUG-09 20:45:00 (EST-5)
Location: BOWLEYS QUARTERS
Magnitude: 0.75 
Reported by: Storm Chaser

Penny size hail was reported near Bowleys Quarters.

Hail in FREDERICK, MARYLAND
From 21-AUG-09 18:20:00 to 21-AUG-09 18:20:00 (EST-5)
Location: FOUNTAIN MILLS
Magnitude: 0.75 
Reported by: NWS Employee

Penny size hail was reported in Green Valley.