Most of the ingredients appeared to be in place for a severe weather outbreak on Friday, July 20 in the afternoon and evening. The combination of strong instability and strong winds aloft were two of the key ingredients needed for such an outbreak. However, much of our region escaped the storms completely. There were several reports of severe weather near the Tennessee and Arkansas borders, as well as Spencer County, IN.
Looking back at what happened, here are a few takeaways.
1. A strong "cap" was in place through the morning. A "cap" is an elevated layer of warm air that acts as a lid to stop updrafts from forming into thunderstorms. The cap weakened during the day, but never completely eroded in some areas. The cap can actually aid severe weather development if it holds back thunderstorms until the hottest and most unstable time of day, when storms are surrounded by plenty of "fuel." However, it is possible in this case that the cap inhibited thunderstorm formation by not completely dissipating.
2. A surface front or "outflow boundary" usually serves as a focus for storms to develop along. An outflow boundary is a kind of "small-scale surface front" formed by the leading edge of cooler air from prior thunderstorms. On Friday, the actual cold front remained well to our northwest most of the day. There were no organized thunderstorm complexes the previous night to generate an outflow boundary that would serve to generate thunderstorms on Friday. The lack of an organized focus for storms was likely a contributing factor.
3. The timing of disturbances in the upper levels of the atmosphere is another factor. Even relatively weak disturbances aloft can serve to initiate thunderstorms. On Friday, storms developed mainly to the east of our area, across central Kentucky and Tennessee. It appears one such weak disturbance moved across our region during the morning, when the cap was still in place. This apparent disturbance later generated a round of severe storms in central Kentucky during the afternoon.
The following is a list of severe storm reports received from the Paducah NWS county warning area on Friday:
PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE PADUCAH KY
1129 PM CDT FRI JUL 20 2018
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0519 PM TSTM WND GST 2 SE VAN BUREN 36.99N 90.99W
07/20/2018 E60.00 MPH CARTER MO LAW ENFORCEMENT
0520 PM TSTM WND DMG VAN BUREN 37.01N 91.01W
07/20/2018 CARTER MO EMERGENCY MNGR
TREE DAMAGE AND POWER OUTAGES REPORTED THROUGHOUT THE
COMMUNITY. CORRENTION FOR TIME.
0520 PM HAIL VAN BUREN 37.01N 91.01W
07/20/2018 M0.75 INCH CARTER MO EMERGENCY MNGR
REPORTED ALONG HIGHWAY 60. CORRENTION FOR TIME
0540 PM TSTM WND GST 5 SE ELLSINORE 36.88N 90.68W
07/20/2018 M50.00 MPH CARTER MO EMERGENCY MNGR
0605 PM HAIL 9 SSE ESSEX 36.69N 89.80W
07/20/2018 M1.00 INCH STODDARD MO BROADCAST MEDIA
QUARTER SIZE HAIL REPORTED VIA MEDIA. CORRECTION FOR
LOCATION.
0608 PM TSTM WND GST POPLAR BLUFF 36.76N 90.41W
07/20/2018 M55.00 MPH BUTLER MO ASOS
0615 PM HAIL 5 N QULIN 36.67N 90.25W
07/20/2018 M1.00 INCH BUTLER MO TRAINED SPOTTER
QUARTER SIZE HAIL. RELAYED THROUGH EM.
0716 PM HAIL 2 E CAYCE 36.55N 89.00W
07/20/2018 E1.00 INCH FULTON KY TRAINED SPOTTER
0716 PM HAIL 5 NW FULTON 36.56N 88.94W
07/20/2018 E1.00 INCH FULTON KY BROADCAST MEDIA
1006 PM TSTM WND DMG HATFIELD 37.90N 87.22W
07/20/2018 SPENCER IN EMERGENCY MNGR
SEVERAL TREES AND POWERLINES DOWN. A TREE FELL ON A
MOBILE HOME
1006 PM HAIL ROCKPORT 37.88N 87.05W
07/20/2018 E0.88 INCH SPENCER IN TRAINED SPOTTER
1011 PM TSTM WND GST ROCKPORT 37.88N 87.05W
07/20/2018 E51.00 MPH SPENCER IN TRAINED SPOTTER
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