10.2	20th AMS Conference on Severe Local Storms
11-15 September, Orlando, Florida

1. INTRODUCTION
The effectiveness of National Weather Service (NWS) severe thunderstorm and tornado warnings is the subject of much debate. One of the points often put forth in this ongoing discussion is that a tornado warning conveys the same level of threat whether the warned tornado is violent or weak. As a result, weak tornadoes tend to be overemphasized by the warning process. Since most tornadoes are weak, tornado warnings, in general, are overemphasized in the current operational system. The focus on tornado warnings can have unintended negative results. Very strong, non-tornadic thunderstorms may not receive adequate response from the public if covered by standard severe thunderstorm warnings. Since both severe and tornadic thunderstorms are defined by the phenomena they produce and damage they cause, forecasters are effectively required to warn for something they do not observe. This paper describes a proposed 3-tier categorical severe thunderstorm and tornado warning system that would mitigate the negative aspects of the current operational system. The proposed scheme allows forecasters to indicate the degree of threat posed by a particular thunderstorm without unduly complicating the warning interpretation. The new system provides objectively defined thunderstorm categories which can be directly monitored by NWS forecasters on the Advanced Weather Interactive Processing System (AWIPS).

2. PROPOSED THUNDERSTORM CATEGORIES
Thunderstorms and thunderstorm intensity can be objectively defined in a variety of ways by using remotely-sensed observations derived from lightning detection sensors, radar, and satellites. Kitzmiller and Breidenbach (1995) developed an algorithm to compute the probability of severe weather for radar-detected thunderstorm cells. Both radar and storm environment predictors are used to compute this probability (Cell Prob. Svr Wx). Using this as a measure of thunderstorm intensity, we can define a 3-tiered classification as follows:

CAT	DEFINITION
3	Cell Prob. Svr Wx > 70 %
2	70% > Cell Prob. Svr Wx > 30%
1	Cell Prob. Svr Wx < 30 %

Since this probability is calculated automatically for each thunderstorm cell which is detected, each cell could be assigned a category and displayed on the AWIPS principle graphical user interface, the D2D (Biere 1998; Fig. 1). Forecasters could use the AWIPS WARNGEN application to issue warnings based on thunderstorm categories.

3. PROPOSED WARNING PRODUCTS There are currently two operational warning products for thunderstorms, the severe thunderstorm warning (SVR) and the tornado warning (TOR). In the proposed system there would be 4 warning products:

-Category 3 Thunderstorm (CAT3)
-Category 2 Thunderstorm (CAT2)
-Tornado and Category 3 Thunderstorm (TORCAT3)
-Tornado and Category 2 Thunderstorm (TORCAT2)

The CAT3 and CAT2 (see Fig. 2) would replace the SVR. TORCAT3 (see Fig. 3) and TORCAT2 would replace the TOR. Although they would be tracked and monitored on AWIPS, no warnings would be required for Category 1 Thunderstorms. They could be mentioned in other official NWS products such as the Nowcast (NOW). The TORCATs would be the same as the current TOR except they would also contain the intensity category of the parent thunderstorm. This provides no information on the intensity of the tornado itself. Although, weak tornadoes can be generated from very strong thunderstorms, it is rare that violent tornadoes, capable of producing F4-F5 damage, are produced from anything but the strongest supercell thunderstorms. Thus it is hoped that these new tornado warnings can provide some additional information as to the nature and strength of the warned threat. The CAT3 and CAT2 could be verified objectively without the need for storm reports. The TORCAT3 and TORCAT2 would still require tornado storm reports for verification.

Operational risk reduction, along with societal impacts studies, would be essential in order to validate the utility of the proposed changes to these critical warning products. Some case studies using the proposed system will be presented at the conference.

4. ACKNOWLEDGMENTS
The author would like to thank Mike Churma and Tom Filiaggi for stimulating discussions on the subject of this paper.

5. REFERENCES
Biere, M., 1998: The WFO-Advanced Two-Dimensional display software design. Preprints, 14^th International Conf. on Interactive Information and Processing Systems. Phoenix, Amer. Meteor. Soc., 376-379.

Kitzmiller, D. H., and J. P. Breidenbach, 1995: Detection of severe local storm phenomena by automated interpretation of radar and storm environment data. NOAA Technical Memorandum NWS TDL 82, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, 33 pp.

Figure 1: Example of Thunderstorm Categories on the AWIPS D2D. Large thick dark circles are Category 3 Thunderstorms. Smaller thick bright circles are Category 2 Thunderstorms. Smallest thin, bright circles are Category 1 Thunderstorms.

CATEGORY 2 THUNDERSTORM WARNING NATIONAL WEATHER SERVICE AMARILLO TX 815 PM CDT TUE JUN 27 2000 THE NATIONAL WEATHER SERVICE IN AMARILLO HAS ISSUED A * CATEGORY 2 THUNDERSTORM WARNING FOR... DALLAM COUNTY IN THE TEXAS PANHANDLE NORTHERN HARTLEY COUNTY IN THE TEXAS PANHANDLE * UNTIL 915 PM CDT * AT 815 PM CDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A CATEGORY 2 THUNDERSTORM 5 MILES SOUTHWEST OF TEXLINE...OR ABOUT 36 MILES NORTHWEST OF DALHART...MOVING EAST AT 40 MPH. * SOME LOCATIONS AFFECTED BY THE STORM INCLUDE... DALHART...TEXLINE AND WARE DESTRUCTIVE WINDS AND LARGE HAIL CAN BE EXPECTED. IF YOU CANNOT GET UNDER GROUND...GO TO AN INTERIOR ROOM ON THE LOWEST FLOOR OF A STURDY BUILDING. AVOID WINDOWS!

Figure 2: Sample of proposed Category 2 Thunderstorm Warning (CAT2).

TORNADO AND CATEGORY 3 THUNDERSTORM WARNING NATIONAL WEATHER SERVICE DENVER CO 558 PM MDT TUE JUN 27 2000 THE NATIONAL WEATHER SERVICE IN DENVER HAS ISSUED A * TORNADO AND CATEGORY 3 THUNDERSTORM WARNING FOR... EASTERN ADAMS COUNTY IN NORTHEAST COLORADO * UNTIL 645 PM MDT * AT 558 PM MDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A CATEGORY 3 THUNDERSTORM WITH A POSSIBLE TORNADO 12 MILES NORTHWEST OF SHAMROCK... OR ABOUT 19 MILES SOUTHWEST OF FORT MORGAN...MOVING SOUTHEAST AT 20 MPH. LOCATIONS IN THE WARNING INCLUDE...BUT ARE NOT LIMITED TO...SHAMROCK THE SAFEST PLACE TO BE DURING A TORNADO IS IN A BASEMENT. GET UNDER A WORK BENCH OR OTHER PIECE OF STURDY FURNITURE. IF NO BASEMENT IS AVAILABLE...SEEK SHELTER ON THE LOWEST FLOOR OFTHE BUILDING IN AN INTERIOR HALLWAY OR ROOM SUCH AS A CLOSET. USE BLANKETS OR PILLOWS TO COVER YOUR BODY AND ALWAYS STAY AWAY FROM WINDOWS.

Figure 3: Sample of proposed Tornado and Category 3 Thunderstorm Warning (TORCAT3)

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