Prior to February 1, 2007, the National Weather Service would issue warnings for entire counties, even if the severe storm was in the far southern tip, or north or east edge of a county, the entire county would be put under the warning.  You could literally be under a Severe T'Storm Warning with sunshine & a beautiful evening north of Battle Ground, while Romney was getting a severe t'storm.  It would still be a Severe T'Storm Warning for Tippecanoe County.

Eventually, this system was converted to warning polygons, which were a much more accurate way to warn of severe weather.  The warning could be placed for a small area to only apply to those in the path of the severe storm.

In this research, I examined numerous severe weather & flood events to determine what the largest area was under a particular warning at once in our viewing area.  Data was utilized from all of our local National Weather Service Forecast Offices (Indianapolis, Chicago & Northern Indiana).

I included both polygons & county warnings going all the way back to 1995.

These are the largest warning areas at once in the viewing area:

1.  The Largest Severe T'Storm Warning:

December 6, 1998 Serial Derecho

At one time, every county in the viewing area except two were under Severe T'Storm Warnings as line skyrocketed through in the 9:15-9:50 p.m. time frame.

Radar image below is just prior to warning issuance for basically every county east of US 231.  The black-outlined counties were under warnings from the Indianapolis NWS office at this radar snapshot courtesy of Iowa State University.

This serial derecho passed in 1 hour 45 minutes with widespread straight-line winds of 60-80 mph with cores of 90-100 mph gusts blasted through the area.

University Hall on the Purdue campus had major roof damage totaling $100,000. The wind blew over a 35-car freight train in Carroll County near Rockfield, while the third story of a Total Discount store in Logansport collapsed & fell into a restaurant. A grain bin was blown over near Walton & a barn was partially unroofed. At Peru, several buildings & homes were damaged in the city with the roof blown off a warehouse. A mobile home was destroyed near the city & another was overturned while trees fell on automobiles near Logansport. Widespread tree, powerline& barn/farm damage occurred area-wide. A wind gust of 82 mph was recorded at Grissom Air Reserve Base & 77 mph at West Lafayette. The only hail report was 0.75” at Wheatfield. Record warmth with highs of 68-74° preceded the storms & just prior to storm passage just after midnight, temperatures were still 65-70°.

Image during the evening of December 6, 1998 courtesy of Iowa State University Mesonet archives:

2. The Largest Tornado Warning:

October 26, 2010 QLCS

A "blanket" Tornado Warning, this massive Tornado Warning was issued due to the number of strong low-level meso-circulations in the line for every area east of US 421 at one point.

Such tornadoes are quick, brief & usually rain-wrapped in damaging winds.

Racing through the viewing area in the morning hours, the narrow squall line brought winds to 80 mph in northern Jasper County & an EF0 tornado to Howard County. An industrial ag building under construction was unroofed at Crawfordsville & a radio tower was toppled northeast of Frankfort.

13 tornadoes were confirmed from the line in the Northern Indiana National Weather Service forecast area.

Image of the QLCS courtesy of UCAR:

3.  The Largest Flash Flood Warning:

July 5-6, 2003 Multiple MCSs with Torrential Rainfall (Part of 4-day stretch of historic rainfall, which led to the historic Summer 2003 floods):

From what I glean from records all but two counties were under Flash Flood Warnings at once July 5-6.  I found that only Benton, Warren & Fountain counties were not under a Flash Flood Warning at one time on July 5-6.

Benton, Warren & Foutnain counties did end up with a Flash Flood Warning on July 9.

WGUS53 KIND 051315
FFWIND
INC015-023-067-157-159-051715-

BULLETIN - EAS ACTIVATION REQUESTED
FLASH FLOOD WARNING
NATIONAL WEATHER SERVICE INDIANAPOLIS IN
814 AM EST SAT JUL 5 2003

THE NATIONAL WEATHER SERVICE IN INDIANAPOLIS HAS ISSUED A

* FLASH FLOOD WARNING FOR...
CLINTON COUNTY IN CENTRAL INDIANA
THIS INCLUDES THE CITY OF FRANKFORT
HOWARD COUNTY IN CENTRAL INDIANA
THIS INCLUDES THE CITY OF KOKOMO
TIPTON COUNTY IN CENTRAL INDIANA
CARROLL COUNTY IN NORTH CENTRAL INDIANA
TIPPECANOE COUNTY IN WEST CENTRAL INDIANA
THIS INCLUDES THE CITY OF LAFAYETTE

* UNTIL 1215 PM EST

* AT 813 AM EST...WEATHER SERVICE DOPPLER RADAR INDICATED VERY HEAVY
RAIN FROM 17 MILES NORTHWEST OF BATTLE GROUND TO 12 MILES SOUTHWEST
OF SHADELAND...OR FROM 18 MILES SOUTH OF RENSSELAER TO 13 MILES
NORTHWEST OF CRAWFORDSVILLE...MOVING EAST AT 20 MPH.

RAINFALL OF OVER 5 INCHES HAS ALREADY OCCURRED IN SOME AREAS.
ANOTHER 1 TO 2 INCHES OF RAIN IS LIKELY THROUGH MID MORNING.

DO NOT DRIVE THROUGH FLOWING WATER. IF IT IS TWO FEET DEEP...IT CAN
CARRY MOST VEHICLES AWAY. VEHICLES WITH LARGER TIRES...SUCH AS
SUVS...ARE MORE BUOYANT AND AT GREATER RISK.

A FLASH FLOOD WARNING MEANS THAT FLOODING IS OCCURRING OR IS
IMMINENT. IF YOU ARE THREATENED BY RISING FLOOD WATERS MOVE TO HIGHER
GROUND IMMEDIATELY. DO NOT CROSS SWIFTLY FLOWING WATERS OR WATERS OF
UNKNOWN DEPTH.

TO REPORT FLOODING HAVE THE NEAREST LAW ENFORCEMENT AGENCY RELAY YOUR
REPORT TO THE NATIONAL WEATHER SERVICE FORECAST OFFICE IN
INDIANAPOLIS.

LAT...LON 4081 8708 4024 8709 4023 8588 4074 8591

Radar image for early morning of July 5 (image from UCAR):

Record levels on stream gauges in the early July flood (from USGS):