Last Friday night, several rotating “supercell” thunderstorms developed unexpectedly from northern Virginia through southern Pennsylvania.
The storms formed as a cold front punched through the region, with strong turning of the winds with height (vertical wind shear). A small (EF-0) tornado even touched down north-northeast of Baltimore in Jarrettsville, Md. causing some minor damage. A larger tornado (EF-1) formed in Lancaster county, Pennyslvania (see Ryan Hanrahan’s great blog post “Pennsylvania Tornado Debris Signature”)
As I said, these storms were not well-forecast. At the time the storms were forming (around 5 p.m.), the National Weather Service’s official forecast for Washington, D.C. was for partly cloudy skies with no mention of rain. The forecast from the Capital Weather Gang (CWG), issued early in the morning, was only slightly better, mentioning a 20 percent chance of pre-sunset thundershowers.
The storms “overachieved” as a result of some afternoon sunshine and a weak feed of moisture from the south that destabilized the atmosphere. Given the strong turning in the atmosphere, this destabilization was sufficient for these storms to blossom and rotate.
CWG’s Ian Livingston, who closely followed the storms on radar, said he counted at least three mini-supercells (rotating storms) cross the District that evening.
Several CWG followers on Facebook and Twitter captured photographs of wall clouds. Wall clouds are large, lowering cloud formations that develop beneath the base of cumulonimbus clouds, typically those with strong vertical motions. Sometimes they are precursors to tornados. Tornadochaser.net adds some more detail:
Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm. When seen from within several miles, many wall clouds exhibit rapid upward motion and cyclonic rotation. However, not all wall clouds rotate. Rotating wall clouds usually develop before strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour.
Here is a gallery of wall cloud and supercell thunderstorms captured Friday evening:
Finally, here is the National Weather Service report on the small, EF-0 tornado that touched down near Jarrettsville, Md. It grew to a width of 100 yards and was on the ground for 1.2 miles.:
A TEAM FROM THE NATIONAL WEATHER SERVICE CONDUCTED A GROUND SURVEY OF DAMAGE JUST NORTHWEST OF JARRETTSVILLE. BASED ON THAT SURVEY ALONG WITH RADAR DATA THE TEAM DETERMINED THAT AN EF-0 TORNADO OCCURRED IN NORTHWESTERN HARFORD COUNTY MARYLAND.
DAMAGE OCCURRED NORTH ALONG THE 2100 BLOCK OF COX ROAD. NUMEROUS TREES AND LARGE TREE BRANCHES WERE EITHER UPROOTED OR SNAPPED HALF WAY UP THE BASE AND FOLDED ACROSS COX ROAD AND INTO THE FRONT YARD OF A RESIDENCE. LARGE TREES AND TREE BRANCHES WERE EITHER SNAPPED OR UPROOTED IN DIFFERENT DIRECTIONS BEHIND A RESIDENCE ON COX ROAD. THE TELEVISION ANTENNA WAS BLOWN OFF THE ROOF AND LANDED IN THE BACKYARD. ADDITIONAL MINOR ROOF AND GUTTER DAMAGE ALSO OCCURRED TO THE FRONT AND BACK OF THE RESIDENCE. A SECOND AREA OF DAMAGE WAS LOCATED ALONG THE 3800 BLOCK OF BREIDENBAUGH COURT LOCATED ACROSS FROM NORRISVILLE ROAD AND NEAR TO SALEM CHURCH ROAD. SHINGLES AND SIDING WERE STRIPPED ON SEVERAL HOMES ALONG WITH SNAPPED TREE BRANCHES. A GAZEBO WAS LIFTED OFF THE BACK PORCH OF A RESIDENCE ON THE 3800 BLOCK AND PLANTED ON THE ROOF. THE TWO DAMAGE SWATHS WERE NARROW. THIS TORNADO HAS BEEN RATED AN EF-0 WITH MAXIMUM WINDS ESTIMATED AT 80 MPH.