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Major Storm Pounding California; Winter Weather Threats Through This Weekend From the Central to Eastern U.S.

The last and strongest in a week-long parade of Pacific storms is pounding California with a variety of hazards, including heavy rainfall, mountain snow and flooding near recent wildfire burn scars. Meanwhile, a weak, progressive system may produce light snow from the Ohio Valley into the northern Mid-Atlantic region through early Friday. Then a major winter storm will develop for the weekend. Read More >

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