Capabilities of Thermodynamic and Kinematic Severe Weather Parameters

III. How To Use the Parameters in Severe Weather Forecasting and Warning Operations

	The process of analyzing thermodynamic and kinematic parameters from observed and model upper air soundings is part of routine synoptic scale assessment. Parameter evaluation in conjunction with large scale pattern recognition and climatology helps define potential severe weather threat areas (Johns and Doswell, 1992). Evaluating variables such as instability and shear from local and surrounding sounding locations are important components in determining severe weather potential in the County Warning Area (CWA). Once deep convection is determined to be possible (moisture, conditional instability, and a source of lift are available), then the next step typically involves an assessment of the short term potential for severe thunderstorm development (type, intensity, and timing). Anticipating the most likely mode of severe convection is very challenging in operational forecasting because of limitations of the observations of mesoscale and storm scale processes which effect eventual storm and system evolutions. Model analyses of soundings (such as the RUC-2) can be examined hourly to provide a suitable basis for objective guidance to operational forecasters since the data retain the signals from the previous and ongoing observed soundings (see Thompson et al, 2002). Pattern recognition plays an important role in this process as well. For instance, often used is recognition of the "Inverted-V " microburst profile (Brown et al. 1982) in the forecasting of convectively-induced strong winds. Assuming the model forecasts are fairly good, model soundings allow the forecaster to see how key elements of the model atmosphere change with time and provides for evaluation of derived parameters such as CAPE, CIN and shear. For model forecast soundings, whether you use AWIPS, BUFKIT, or SHARP, it is important for the forecaster to know the various ways the sounding parameters are calculated and any systematic biases in the models (See Section II and the AWIPS validation web site). Even the slightest difference such as the layer a mean parcel is lifted can affect the resulting derived parameters. Once we understand how the parameters are derived, the question then becomes how to use them. For the severe weather outlook process where forecasters are analyzing the potential for severe thunderstorms in the next 12 hour time period, sounding parameters from observed data should be analyzed first to get an idea of the thermodynamic and kinematic profile. This involves analyzing the observed local rawinsonde and any other proximity soundings upstream. To assess the potential for deep, moist convection, the forecaster must then be able to analyze the forecast changes resulting from thermal and moisture advection and vertical motion fields. Modification of 1200 UTC, 1800 UTC , and 0000 UTC soundings with observed surface and/or upper air data (using expected changes or real-time data) and then comparing it with model forecast soundings is always a good way to use soundings in the forecast process. Always try to confirm model soundings with observed data. Once the forecaster has decided that thunderstorms are possible in the CWA, then the next step is defining the risk. Determining specific severe weather types possible in the CWA involves much more than sounding analysis; it involves diagnosing the 4 -dimensional aspects of the atmosphere on the mesoscale and then analyzing convective storm structure The warning decision is made ideally using an integrated, holistic process which results from synthesizing all relevant data inputs. Thus, applying any environmental sounding parameters (such as CAPE and shear) helps to define the threat area, identifies potential severe weather types (hail, tornado, high wind, flash flooding) and is certainly a part of mesoscale assessment. Properly assessing the mesoscale (a tornado proximity sounding for example) can help the warning forecaster by narrowing the range of possibilities (or heightening awareness of the threat) and areas of most concern with storms that develop across the CWA. Moreover, analyzed environmental parameters, whether favorable or unfavorable in terms of severe weather potential, can give added confidence (and potential lead time) to warning decisions based on radar signatures and other input. One of the biggest problems with using sounding parameters in forecasting or warning is that it is difficult to apply a binary or deterministic result from the evaluation. For instance, if one analyzes 30 kts of shear from 0 to 1 km in a model sounding, and then tries to translate that information into tornado warning, it can often fail. It is much more sensible to realize that these values are guidance to predicting severe weather likelihoods (such as significant tornadoes) and should be used as such. Using environmental parameters helps support other inputs like radar or spotters in the integrated warning decision making process. For example, much of the recent research in proximity soundings has stratified the parameter distributions into various categories, such as "general thunderstorm" (so severe), "severe" (3/4 in. hail or 50kt winds), "significant severe" (usually this means hail to 1.75" and/or winds 50-64 kts, F0-F1 tornadoes), and "significant tornadoes" (F2 damage or greater). Results from studies such as these (see Craven et al, 2002 for example) display some overlap in just about every statistical category. This means there are no magic numbers that will work in every weather situation . There are some parameter thresholds however, that deserve attention because, based on the statistics, they show the most promise from helping to delineate and discriminate significant tornado threats in the environment. The following is a list from various research of the "best" parameters and their associated threshold values in forecasting supercell tornadoes: 0 to 6 km Shear (or bulk shear): 20 m/s ------- Use : potential supercell development 0 to 1 km Shear : 10 m/s --------- Use: lower threshold for significant (F2 or greater) tornadoes LCL height (using a mean 100 mb deep layer): 1500 m AGL -------- Use: Above this height, significant tornadoes (F2 or greater) are not likely CIN (again using a mixed 100 mb layer): 150 J/kg ------- Use :If a sounding displays more CIN than this, a significant tornado is unlikely. Note: Again, keep in mind that the sounding that displays these values might not be representative of the actual storm environment. Note from the list above that low-level CAPE is not on the list. By itself, 0-3 km CAPE is not the most useful discriminating parameter for tornadoes. However, if you combine total CAPE and 1 km SRH (see section on EHI), 1 km EHI becomes a very good predictor for significant tornadoes. Results from Davies (2002) (see graph below) indicate some parameter values can be successfully categorized into ranges indicating relative likelihoods for expecting significant supercell tornadoes (F2 or larger). These results are based on 321 cases using RUC-2 soundings but keep in mind, the specific criteria numbers are intended for guidance only . Note: The SPC maintains a mesoscale hourly objective analysis web site (see http://www.spc.noaa.gov/exper/mesoanalysis/) that incorporates many of the parameters shown on WDTB's parameter web page and more. Their analysis is based on the latest RUC model data and we encourage all forecasters to use this web site for real-time analysis of thermodynamic and kinematic parameters. In summary, environmental parameter values are very useful in determining the potential threat for specific severe weather types. Some values which can be helpful in determining significant tornadoes were shown. It is hoped that this guidance helps to improve forecaster proficiency and effectiveness in issuing convective warnings.
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