While the April 27 evening storms in our area were, by comparison, much weaker than those to the south, they packed more punch than we typically see around here with thunderstorms. Multiple rounds of activity moved through portions of the region, starting the evening of April 27 and continuing through the morning of April 28. When all was said and done, an astounding 38 tornado warnings were issued by NWS Baltimore/Washington for the broader region.
Keep reading to learn more about severe weather in the D.C. area on April 27 and 28, 2011.
The story for many days had been that a wild week of weather was at hand, and we were preparing for severe activity including tornadoes on CWG ourselves. However, the evening activity on the 27th “over performed” in some ways. Something we’ve now seen several times so far into this year’s storm season.
After a morning of cloudiness, breaks in the cover began to appear heading into afternoon. Temperatures rising into the low 80s most spots were just enough to heat the surface to a point where convection would initiate. Dew points, a factor in available energy, also crept from muggy morning readings in the mid-60s to summer-like afternoon numbers near 70. Add in a spoke of energy moving through, and a whole lot of spin to the atmosphere — thanks to an upper-level gyre that created the Southern low — and you’ve got a recipe for supercell tornadoes in the area.
The first of four overlapping tornado watches was issued around 2:00 p.m., running through the evening, as an isolated storm formed well off to the southwest. This initial round ultimately entered into the area by 5:00 p.m., when tornado warnings began to fly in places like Spotsylvania, Prince William and Stafford county in Va. and Charles county in Md.
An apparent tornado near Andrews Air Force Base Wednesday.
What was initially one individual storm eventually grew into a group of individual storms. These storms, referred to by their meteorological name of supercells, are more conducive to producing tornadoes than their counterparts the squall line because they have the ability to be more fully influenced by their surrounding environments than a solid line of storms. In this case, ample turning of winds with height — south, southeast at the surface to southwest and west moving upward produced strongly rotating updrafts even in relatively small (areal coverage) storms.
Outside this narrow corridor, just to the south and east of D.C., most activity was lesser in the evening, but one storm north of the city briefly showed signs of rotation and spit out some hail, while a few that sped by in between round one and round two triggered severe thunderstorm warnings for places like Loudoun County.
Round two worked its way into portions of the region after delivering numerous tornado warnings to our south in Southern Virginia, including one that spawned an incredibly photogenic tornado near Richmond. It ultimately arrived in a similar area as Round one, but this time moved a little more northward than northeastward. Tornado warnings moved from Spotsylvania county toward Fauquier County and Prince William County, before weakening a bit and passing west of D.C. toward Montgomery County.
During the two rounds of activity Wednesday, the late-evening one a little less widespread locally (compared to the early evening), 14 tornado warnings were issued between 5:00 p.m. and 10:30 p.m., and at least a dozen reports of a tornado or funnel cloud came in. In addition to the tornado warnings, 12 severe thunderstorms warnings were issued, with more to the south in southern Virginia and North Carolina, and more to come.
Round three, the most prolonged assault -- as the low which wreaked havoc in the South began to pass by -- mainly focused its fury on the north and west parts of the area. Early morning heavy training rains across Loudoun, Montgomery and Frederick counties prompted flash flood warnings as tornado watches were extended yet again, this time into the early afternoon. Almost right on cue, new tornado warnings began to fly in the immediate area.
On approach to D.C., storms became more disorganized but produced some brief heavy rain and rather gusty winds. As they began to depart to the east later in the morning, influenced by increasing instability thanks to a rising sun, storms again strengthened and prompted severe thunderstorm warnings for several counties to the east of I-95. Additional tornado warnings ended up being issued for southeastern Virginia and into the Carolinas.
By the time this batch was done, an additional 24 tornado warnings — mainly north and west of the immediate area — were issued by NWS Baltimore/Washington, bringing the total for the event to 38 tornado warnings.
Why did this happen here?
Outside the obvious of a near perfect storm in the South, one major reason locally for the outbreak on April 27 is the fact that there were two surface lows that impacted us rather than just one in the South. The one with the most powerful energy aloft (a jet stream maximum) was the one over the South that impacted us Thursday morning - but the first leading one late Wednesday packed the most punch around here.
As you can see in the image below, an hour 6 forecast from the midday NAM model on April 27, the additional (first or leading) surface low pressure system was headed into and through southern Canada by the afternoon and evening. Historically, this is a fairly typical position for D.C. area tornado events. A re-analysis of many of the days like those in Kevin’s recent post would show an 850mb (basically the surface center) low near the southeast Great Lakes late in the day. The fact that the upper-level low creating it was so anomalously intense added fuel to the fire.
Another major feature, and one that helped severe weather and a tornado risk last into the night, was a strong southerly flow at the surface, the ground level portion of the wind veering (turning clockwise) with height process. This feature is known as the low level jet, and it often strengthens headed into the first hours of the night. This is partly why many tornado outbreaks peak around or just after dark before slowly diminishing in intensity through the night.
Of course, this one still had more up its sleeves after the evening activity of April 27.
As seen in the image below, by 8 a.m. on April 28 (as viewed by the 6z NAM of the same day), the main trough (bright and brighter oranges/reds) that helped create the gigantic tornadoes to the south was working toward its closest “dangerous” path to the area. At the same time, the southern surface low pressure had positioned itself in a pretty similar area as the predecessor during the evening, an ideal location for tornado formation in the region, even in the morning in this case.
What’s the end result?
NWS Baltimore/Washington should conclude at least preliminary assessments of the tornadic activity across the area in the coming days. For the most part it seems that these storms ended up perhaps slightly too weak to produce more than numerous scary (or beautiful?) scenes and a few brief tornadoes which did unfortunately cause some light to moderate damage.
We will follow up with official word from the National Weather Service as information becomes available.
Warning map sources..