Anniversary of the July 10th, 1989 Tornado Outbreak

One of the more infamous tornado outbreaks to occur across southern New England happened on July 10th, 1989.  On this day 10-tornadoes were confirmed across the states of MA (7) and CT (3) and if you expand to include the states of NY and NJ, a total of 17-tornadoes were confirmed (4-in NY and 3-in NJ).  Of the 17-confirmed tornadoes across the four states, two of them were rated as F4 tornadoes, one in NY and the other in CT.  (While this tornado was rated as an F4 during its peak intensity, there are some questions to whether the tornado was really that strong but that's a story for another time).

Atmospheric conditions on Monday, July 10th, 1989 were prime not only for the development of severe weather in the afternoon but for the development of tornadoes as well.  In fact, the airmass across a chunk of NY and into southern New England was something more typical for the southern Plains than for the Northeast or New England.  The biggest contributor to the tornado outbreak was the presence of an elevated mixed-layer (EML) which advected into the region.  EML's develop/originate across the inter-mountain states across the southwestern United States and are a very critical player in major severe weather/tornado outbreaks and are one of the primary reasons as to why the southern Plains sees the severe weather they do.  For a little more info on EML's check out a previous blog post here:

http://weatherwiz.blogspot.com/2015/06/what-is-elevated-mixed-layer.html

Looking at an observed skew-t forecast sounding from Albany, NY  from 8:00 AM on the 10th we see several eye opening features present:

1) EML:  You can see the presence of the EML as clear as day on the sounding.  This is a critical feature as it is associated with very steep lapse rates (change of temperature with height) and when placed over a warm/hot and humid airmass will help contribute to an extremely unstable airmass and with the passage of a morning warm front that's exactly the type of environment that would develop during the late morning and afternoon hours.

2) Winds:  Notice how towards the surface you have winds from the SE but just above the surface winds shift to the SW...this is an indicator that the warm front was in this vicinity and this also yields some degree of directional wind shear (change of wind direction with height).  This is another ingredient for the development of supercell thunderstorms and tornadoes.  Also, as you increase with altitude through the troposphere notice how the winds even back more towards the west and northwest.  This showed that there was a great deal of directional shear throughout the entire atmosphere.  Winds aloft too were strong and would actually even strengthen as the morning and afternoon progressed which provided some great speed shear (change of wind speed with height) as well.  This is key for storm organization and to further enhance the effect of the directional shear.
If we actually looked at the sounding for Buffalo, NY for the same time, you can actually see shear just off to the west much much stronger and this is what would work in for the afternoon hours:

Looking at the 500mb and 700mb patterns on that day, we see a very favorable pattern for the advection of an EML right into the Northeast and southern New England.  Established ridging across the southeastern United States places New England on the prosperity of the ridge (provides the more westerly flow in the mid-levels of the troposphere) and with a digging trough in the west coast of the United States, this allows for some EML air to get entrained in the wind flow around the ridge and this pattern configuration favors the advection of the EML right into portions of the Northeast and southern New England.  What we also see is a vigorous piece of shortwave energy right near the United States/Canadian border associated with a shortwave trough pushing a strong cold front southeast.  This would be the focus for the lift needed in order to develop the severe thunderstorms:

 The result...

As the shortwave trough continued to dig and the vigorous piece of shortwave energy continued moving east through northern New England, this allowed for a strong cold front to advance southeastward into an extremely unstable airmass.  As the cold front approached, and heights began to rapidly fall in response to the approaching shortwave trough and the atmospheric cap associated with the EML began to erode, explosive thunderstorm development occurred across New York state and then this activity progressed towards MA/CT/NJ where further activity would develop as well.  Given the atmospheric conditions of extreme instability and very strong speed and directional shear, the thunderstorms quickly became supercell thunderstorms (thunderstorms which have strong rotation known as a mesocyclone) with several spawning tornadoes, including the two violent tornadoes in NY and CT.  Another aspect which helped the development of tornadoes were the very low lifted condensation levels (LCL's) thanks to the strong presence of low-level moisture present with high dewpoints.

Major severe weather and especially tornado outbreaks are quite rare in this portion of the country, however, they do happen.  In order for them to happen you need a special combination of atmospheric ingredients such as; high instability, presence of an elevated mixed-layer, and strong directional/speed shear all to coincide with the passage of a weather system such as a cold front.  On this day in 1989, we saw this perfect combination of ingredients to coincide with the passage of a cold front.