Evening Storms Had RotationMay 23rd, 2014 at 9:44 am by Jeremy Wheeler under Weather
Yesterday’s storms were impressive. Parts of the region saw heavy rain, large hail, and even isolated tornadoes. The storms weren’t a surprise (see yesterday’s blog). However, the rotation in the storms was a bit surprising. We typically don’t see a lot of rotation in storms that move from northwest to southeast. Once in a while though the low level winds setup to create some rotation. Here is what the storms looked like at 7:30pm:
Notice that there were several areas of heavy rain (red colors). There was definitely rotation in 2 of the storms. The first one was over Gloucester county. That storm moved ESE at about 30mph. There was some wind damage reported near Gloucester courthouse, but so far it has been declared as wind damage, not torando. This may get surveyed later today to see if there was indeed tornadic damage. The second area was from near Petersburg down to Suffolk. The storm prompted numerous reports of funnel clouds. There was one official report of a tornado east of Petersburg, and many reports of hail of 1″ or larger.
Notice the line of green dots (hail reports) that extends from north of Richmond down to Sussex county. The storms then weakened as they headed into the metro area. Some folks didn’t even see rain last night. That goes back to the scattered nature of the storms.
So the question is…Where did the rotation come from? Overall the winds were from the same direction at all levels (from the northwest). The key word there is (Overall). There was an exception at the lower levels. Looking at the surface reports the wind there were out of the west and possibly even southwest. Here is an atmospheric sounding from Wallops Island. It is from the Storm Prediction Center from last night:
Notice the winds at the surface were actually out of the southeast. Then up through about 1 mile the winds were out of the west. Finally, they were more out of the northwest aloft. So the winds basically turned clockwise with height. We call that veering, and it can contribute to cyclonic spin in the atmosphere. This example isn’t ideal. It is very subtle and is mostly in the lower levels. Course that is where the tornadoes and funnel clouds exist. I looked at the surface winds in the region. Most of them were out of the west. That would still contribute to some low level veering. So the while the models had the overall winds right and showed mostly straight lined winds, there was some rotation present. Here is the mesocscale discussion from the SPC:
SUMMARY...LARGE HAIL/DAMAGING WINDS REMAIN POSSIBLE WITHIN SEVERE TSTM WATCH 174...BUT AN ISOLATED/BRIEF TORNADO MAY BE POSSIBLE PARTICULARLY ACROSS SOUTHEAST VA/CHESAPEAKE BAY VICINITY...AND PERHAPS INTO NORTHEAST NC /WHICH MAY WARRANT A SPATIAL EXTENSION OF WW 174/. SEVERE TSTM WATCH 174 CONTINUES UNTIL 02Z. DISCUSSION...SCATTERED SUPERCELLS CONTINUE TO STEADILY SPREAD GENERALLY SOUTHEASTWARD ACROSS SOUTHEAST VA/CHESAPEAKE BAY VICINITY. THE INLAND AMBIENT AIR MASS REMAINS MODERATELY UNSTABLE WITH MLCAPE IN EXCESS OF 1000 J/KG. REGIONAL WSR-88D VWP DATA FROM THE LIKES OF WAKEFIELD VA SAMPLE THE RELATIVELY STRONG QUASI-UNIDIRECTIONAL MID/HIGH-LEVEL NORTHWESTERLY FLOW WHICH IS CONTRIBUTING TO AS MUCH AS 45-50 KT OF EFFECTIVE BULK SHEAR...WHICH WILL CONTINUE TO REMAIN FAVORABLE FOR SPLITTING SUPERCELLS AND FAST SOUTHEASTWARD-MOVING LINE SEGMENTS. LARGE HAIL/DAMAGING WINDS WILL REMAIN THE PRIMARY RISK. AN ISOLATED TORNADO MAY BE POSSIBLE WITH ANY OF THE PERSISTENT RIGHT-MOVING SUPERCELLS...BUT SOMEWHAT MARGINAL MOISTURE/HIGH TEMP-DEWPOINT SPREADS AND MODESTLY STRONG LOW-LEVEL SHEAR/SRH SHOULD TEMPER THE OVERALL TORNADO RISK.
If I remember right, then I think their tornado index was either a 0.5 or a 1. I don’t know the range, but this was indicative of the rotation based upon previous events. Luckily there weren’t too many tornadoes in the region despite several warnings. There was definitely a lot of instability (potential for air to rise and create a thunderstorm) . The CAPE was about 2,500 J/Kg west of the above sounding. 500-1,000 J/Kg is about the minimum for strong thunderstorm development. So the storms definitely had some energy as they formed to our northwest. They did weaken as they moved into Hampton Roads.
While there was only one official report of a tornado (thus far), there were numerous reports of funnel clouds. Here was one photo of a funnel cloud that may have briefly touched down in Carrsville, VA:
There was no official report of a tornado there, but it looks like it reached the ground to me. It’s possible that some of the tornadoes touched down without doing any damage. Here is another example of a funnel cloud from Angie Harp:
This cloud definitely had some shape. Hard to be sure if it had rotation, but the solid column shape is a good indicator of rotation. There were many reports out there like this.
Going forward today is looking pretty good. High pressure is building in behind the cold front. Skies have cleared up a bit this morning.
We will see more clouds this afternoon. Highs will rise to the upper 70s to near 80. Winds will be northwest at 10-15mph. We’ll steadily dry out through the afternoon. Notice in the above graphic that there are a few showers over eastern Tennessee. The cold front will stall out to our south today, and a weak area of low pressure will form along that. So some of those showers will try to push east into our region. This will be just a few scattered showers, but a thunderstorm or two may form over northeast North Carolina. We will dry out though after midnight. Hopefully, there are some breaks in the clouds tonight because there is a possible meteor shower expected. It is debris from an old comet that passed the orbit of the earth back in the late 1800s. It could be good viewing, but I can’t promise. Look to the north. Probably after midnight. Here is a link with more info: NASA talks about meteor shower.
Anyway, then we are looking good for tomorrow. Highs will be in the low/mid 70s with partly sunny skies. Winds will be from the northwest at about 10mph. Should be good for boating, but a tad cool at the beach. If you like warmer weather, then we’ll see highs near 80 on Sunday, and mid 80s on Monday. Dry for both days. It will be great weather for a lot of the Memorial Day weekend events. Have a good, but safe weekend!
Meteorologist: Jeremy Wheeler