Tuesday, May 15, 2018 featured one of the largest severe weather outbreaks in CT's recorded history. 4 confirmed tornadoes and hundreds of large hail and damaging wind reports made for a wild Tuesday afternoon. While severe t'storms are not uncommon across the state, significant severe and widespread significant severe weather are a bit more uncommon, especially during the month of May. What lead to such a large and destructive event? We will explore the science and more below.
The catalyst behind the potential for this event was that of the elevated mixed-layer. The elevated mixed-layer (EML) is the biggest contributor in severe weather outbreaks which occur across the southern Plains. Without the EML, the degree and severity of severe weather events in the south are likely lessened.
What is the elevated mixed-layer? In the United States, the EML can originate across the desert Southwest, Rocky Mountains, or across Mexico. Across these regions air can become quite warm and quite dry. Certain weather conditions (such as surface winds) can displace this region of warm and dry air farther east. Keep in mind that east of the above mentioned areas, the elevation becomes much lower. This area of warm and dry air becomes elevated above the surface. We will show this process by looking at two various forecast soundings. The forecast sounding on the left is from Albuquerque, NM valid 6:00 PM MDT Sunday, May 12, 2019. The station elevation is about 5,300 feet above sea-level. The red line denotes the temperature with increasing height while the green line denotes the dewpoint temperature with increasing height. Note how far apart the two lines are. This indicates extremely dry air. the surface temperature (which correlates to about 850mb or 5,000 feet above the ground) here is just shy of +20°C. The forecast sounding on the right is for Corpus Christi, TX from 7:00 AM CDT Monday, May 13, 2019. The station elevation is only 43 feet above sea-level. Notice at just above 850mb the temperature line makes a sharp turn to the right (indicating rapid warming) while the dewpoint line makes a sharp turn left (indicating rapid drying). This is the EML:
The significance (or role) of the EML and severe weather is the EML creates a "lid" or atmospheric cap. Below the EML (depending on the type of season) the airmass can become quite warm/hot and very humid which can lead to extreme instability. The cap, however, prevents convection (showers/t'storms) from developing which increases the degree of instability that can develop. If there is a strong enough source of atmospheric lift, the cap can be broken and violent t'storms can quickly develop.
Now that we have gone over a bit of what the EML is let's discuss the connection between the May 15th, 2018 severe weather outbreak.
The morning of May 15th featured an ominous look and it was become quite apparent we would be dealing with a very active severe weather day. The observed forecast sounding from 8:00 AM EDT at Pittsburgh, PA showed a well in tact EML which would continue to advect into New England through the morning:
The presence of the EML combined with a highly sheared environment, moderate instability, and a potent shortwave/cold front yielded all the necessary ingredients for a severe weather outbreak. A rather potent upper-level jet (300mb or ~30,000 feet above the ground), characterized by an west-southwesterly 80+ knot jet streak passing through northern New England helped to increase large-scale lift/upper-level divergence while a west-southwesterly 50-60+ knot mid-level jet (500mb or ~18,000 feet above the ground) helped with strong values of vertical shear and potential for updraft rotation.
Embedded within the jet stream was a strong piece of vorticity digging southeast from Canada. The timing of the shortwave energy coincident with the most unstable part of the day and best wind shear:
Perhaps the more impressive aspect of the sheared environment was how winds were changing between what was mentioned above and down at the surface. Winds around 5,000 feet were more out of the southwest with winds near the surface more from the south. This turning of winds with height (along with increasing wind speeds with height) helped to create very large amounts of helicity - a measure of the rotational potential within a t'storm. The higher the values of helicity, the greater the likelihood for a t'storm to acquire rotation, and the higher the potential for tornadoes. Below is a forecast sounding for Windsor Locks, CT from the 12z run of the NAM forecast model the morning of May 15th valid for 4:00 PM that afternoon. Note the rapid changing of wind direction with height and increase in wind speed with height (highlighted with the red box). The blue box shows helicity values just over 400 m2s2 (typically values approaching 125-150 can be enough to start thinking of the potential for an isolated tornado) and the orange box shows the hodograph which is just a plot showing how winds are changing with height. The long and curved look indicate that winds were increasing in speed with height and changing direction with height:
Forecast sounding from the RAP forecast model from 2:00 PM valid for 3:00 PM at Poughkeepsie, NY shows a very volatile environment...more than sufficient for violent t'storms:
The combination of temperatures well into the 80's, dewpoints well into the 60's, and mid-level lapse rates approaching 7.5 C/KM (temperatures decreasing on average of 7.5 C/KM between about 10,000 feet and 18,000 feet above the ground) thanks to the EML helped to create high amounts of instability characterized by mixed-layer CAPE values approaching 2000 J/KG:
With these parameters in place the SPC issued severe weather probabilities you almost never see in this area (particularly with regards to the damaging wind and tornado probabilities). Their afternoon update contained an upgrade to a moderate risk of severe weather (the second highest risk level issued by the SPC) followed by 10% tornado probabilities, 60% damaging wind probabilities, and 30% large hail probabilities. These probabilities the percent likelihood of witnessing such event within 25-miles of any point within the outlined area:
These damaging wind probabilities are the highest percent issued for all of the above severe weather probabilities. In fact, if that area had been "hatched" (this would indicate 10% probability for damaging wind gusts greater than 65 knots within 25-miles of a point) the moderate risk would have been a high risk...the highest risk level issued by the SPC. The only other time a high risk has been issued in the Northeast is May 31, 1998.
When all said and done numerous severe t'storms develop just west of CT and pushed into the state causing significant damage. Below are some radar snap shots:
As the severe t'storms pushed through the state they caused extensive damage. Very large hail (up to the size of baseball's!) damaged cars, windows, and siding in NW CT, destructive winds in the form of microbusts/macrobursts leveled trees across several cities, and a total of 4 tornadoes were confirmed. Some of the hardest hit areas included; Newtown, CT, Bethel, CT, New Fairfield, CT, Danbury, CT, Naugatuck, CT, Oxford, CT, and Hamden, CT.
And finally the map of storm reports:
CT is certainly no stronger to severe t'storms or even tornadoes, however, severe weather outbreaks of this magnitude are rare and we don't see them often. Anytime the presence of an EML may coincide with high amounts of instability, wind shear, and approaching shortwave energy/cold front high-end severe weather is not only possible, but likely.
