National Weather Service United States Department of Commerce

Critical to Extreme Fire Weather Threats Continues in Southern California; Strong Cold Front in the Eastern half of U.S.

Strong offshore winds in southern California will keep Fire Weather threats Critical-to-Extreme into mid-week. A strong cold front from the Southern Plains to the Great Lakes is slowly moving eastward. A broad area of snow will continue from the Rockies into the Middle Mississippi Valley. Heavy rain and some severe storms with brief tornadoes will shift to the eastern Carolinas late tonight. Read More >

 

A strong upper level system developed over the south central Plains on the afternoon of Tuesday, April 19th. This helped to intensify a strong surface low pressure system over Missouri that moved northeast into central Indiana during the evening hours.  A warm front extended east of the system center into Indiana which slowly moved north during the afternoon. However, the northward advance of the warm front was hindered by clouds and strong winds from earlier rain and thunderstorms and the front remained just south of the county warning area (CWA).  This caused a large difference in temperatures on either side of the boundary (30s and 40s to the north, 60s to the south). More moisture was also present south of the warm front.

Storms, in the form of supercells, began to fire near the surface low in the afternoon in western Illinois and quicky merged a large line known as a LEWP (line echo wave pattern) with bowing segments. Below is a composite radar image as the storms progressed across the Midwest.

 

One of these bowing segments moved into the southwest portion of the CWA just before 10 PM EDT on the 19th and traveled across the entire southern portion of the CWA by the early hours of Wednesday, April 20th. Strong winds were mixing down to the surface, which is rare given that the surface warm front was well to the south and temperatures were in the 30s and 40s. Winds, in excess of 70 mph, were able to penetrate the very shallow cold layer at the surface (a temperature inversion) and caused significant wind damage as the storms rolled through the area. A temperature inversion typically acts like a lid separating two layers in the atmosphere. This counteracts downward transport of air from aloft, including higher momentum air from thunderstorm downdrafts. Typically, when an inversion is present, fast flow from thunderstorms will “reflect” off the inversion, with little influence to the surface winds. An inversion also significantly decreases the chances for a tornado, to near zero in many cases. However, on the 19th the inversion was much shallower than anticipated by forecasters (around 2100 ft as indicated on an aircraft sounding from Fort Wayne International Airport).  As a matter of fact, while it was just 41 degrees F at the surface it was 67 degrees 2100 feet above the Fort Wayne Terminal. Hence, the very strong thunderstorm generated “jet cores” (in excess of 80 knots) were able to mechanically break through the inversion and produce damaging winds at the surface. Much of the damage was in the form of roofs that had blown off of sheds and barns destroyed. Damage surveys completed by IWX also found evidence of brief and isolated weak tornadoes.

Here is a radar reflectivity loop of the storms as they progressed across the IWX CWA.

Nearly all the tornadoes occurred very near or along the surface warm front, which lifted quickly north in response to a mesoscale area of low pressure associated with the thunderstorm complex moving through the area (see Warm Front Progression below). Cold NE low level flow (likely accentuated by the cold waters of the Great Lakes) was persistent through much of the day. Winds actually backed to the NE as strong surface pressure falls developed upstream over IL.  The bowing thunderstorm complex that progressed through the area intersected the lifting warm front and supported a local high shear environment that was ingested by the line of thunderstorms. This stretched the ambient local shear allowing for brief “spin ups” within the line of storms, a much different tornado formation mechanism from a storm that contains a strong and deep rotating updraft or mesocyclone. This process is known as non-mesocyclone tornado formation. Each one of the tornadoes was very quick to develop and dissipate, lasting one radar volume scan or less (very typical of non-mesocyclone tornadoes).  Non-mesocyclone tornadoes can be very hard to detect given the fact that they develop and occur very near the ground, below the radar beam in many cases. Hence, forecasters rely on mesoscale analysis and environmental conditions to anticipate the possibility of this type of tornado, which can be very challenging. Meteorologists will continue to study this event and similar events to help increase understanding of the environmental processes that lead to this type of tornado, all in an effort to increase detection and warning lead time in future events.

 

Warm frontal analysis. Low pressure symbol indicates approximate position of the thunderstorm complex generated mesoscale low pressure center.
Warm Front Progression


Below is an image of all the storm reports across the Midwest as the system moved east.

 


Damage Survey Results

 

...VAN WERT COUNTY OHIO...

 

...VAN WERT COUNTY OHIO INTO ALLEN COUNTY OHIO...

 

...JAY COUNTY INDIANA...

 

...CASS COUNTY INDIANA...

 

...GRANT COUNTY INDIANA...
 


 
 
Storm Damage Photos:
Cass County Indiana Grant County Indiana Jay County Indiana Van Wert County Ohio Van Wert County to Allen County Ohio Allen County Ohio

     (Click on an image for more photos from each county.) 


 Page compiled by OBERGFELL/LASHLEY/CHAMBERLAIN