by Louis A. Giordano, NWS Pittsburgh Lead Forecaster (retired)
2. A FINGER TIP GUIDE TO KEY UPPER AIR INDEX VALUES USED IN EVALUATING SEVERE WEATHER AND FLASH FLOOD POTENTIAL
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In recent years, there has been a significant increase in the number of rawindsonde-derived upper air parameters and indices used in evaluating severe weather and flash flood potential. For example, operational forecasters can quickly access over 35 such diagnostic tools using the computer-based Skew-T Hodograph Analysis and Research Program (SHARP) Workstation. SHARP also provides users with an excellent glossary section which gives concise explanations of all these indices and parameters along with references. This report can give additional help in fast-breaking convective situations. Section 2 is a table of 19 key indices and general warm-season values that has been compiled to serve as a time-saving fingertip guide for quickly diagnosing moderate and high potentials for severe weather and flash flood events.
This report recognizes the arbitrary and somewhat artificial nature of making such exact value stipulations as regional, seasonal, and synoptic-type variations also exist. Consequently, specific references are given in Section 3 for the index values chosen. Users are encouraged to review these references to gain a more complete understanding of the indices including their applicability and limitations in describing complex atmospheric processes. Subsequent regional, seasonal, and synoptic-type adjustments to table values are also encouraged.
2. A FINGERTIP GUIDE TO KEY UPPER AIR INDEX VALUES USED IN
EVALUATING SEVERE WEATHER AND FLASH FLOOD POTENTIAL
by Louis Giordano NWS Pittsburgh, PA -1994
Temperature-Humidity Profile (Stability) Indices
|B+ (CAPE) Convective Available Potential Energy||>1500||>2500||J/kg|
|LI Lifted Index
SWI Showalter Index
|KI K-Index (Flash Flood)||>28||>38||none|
|TT Total Totals
CT Cross Totals
VT Vertical Totals
|LR75 Lapse Rate 700-500 mb||>8||=10||deg C/km|
|TEI Theta-E Index 2||>5||>10||deg C|
|CAP Lid Strength 3||1-3||deg C|
|B- (CIN) Convective Inhibition Energy 3||25-75||J/kg|
|PW Precipitable Water Anomaly (Flash Flood)||>130||>170||percent of normal|
|WBZ Wet Bulb Zero Height (Hail)||5-7, 9-11||7-9||kft AGL|
Wind Profile (Shear) Indices
|SR DIR SHEAR Storm Relative Directional Shear 4,5||>70||deg|
|POSITIVE SHEAR (Tornado) 4,5||>4 (F2)||>6 (F2)||10-3/sec|
|SR HELICITY (Supercell)
Combined Stability-Shear Indices
|EHI Energy Helicity Index (Tornado) 4,5,6||>2 (mesocyclone)||>4 (F4)||none|
|BRN Bulk Richardson Number (Supercell) 3,5,6||10-15, 35-45||15-35||none|
|SWEAT Severe Weather Threat Index 6||>300||>400||none|
1 Index values apply to both severe weather and flash flood potentials unless type specified within parentheses.
2 With cross-isentropic flow > 10 m/sec.
3 With B+ > 1500 J/kg.
4 Computed over 0-2 km or 0-3 km AGL.
5 With storm relative (0-3 km AGL) inflow > 10 m/sec.
6 With low and mid-level (1-6 km AGL) winds generally > 10 m/sec.
This section explains how moderate potential values (MPV) and high potential values (HPV) for severe weather and flash flood events were chosen for each index. These MPV and HPV are general warm season values to be applied to soundings modified to forecast conditions near the times events may occur. Reference sources for each index and its MPV and HPV have been provided. Users should review these references to better understand how these indices can be applied as quick approximations to complex convective processes. The references often explain the limitations of using such approximations and may include information regarding MPV and HPV adjustments based on regional, seasonal, and synoptic-type variations.
B+: Weisman and Klemp (1986, p. 340 and p. 354) was the main source for MPV (1500 J/kg) and HPV (2500 J/kg). Johns et al. (1990, p. 595) found 68% of warm season tornado cases with B+ exceeding 2500 J/kg.
LI and SWI: Miller (1972, p. 5-2, Table 1), NOAA (1978, p. 4), and NOAA (1984, pp. 2-3) were sources for MPV (-2ºC) and HPV (-6ºC). MPV was a compromise between NOAA (1978), a study of flash flood indicators, and Miller (1972) and NOAA (1984) which concentrated more on severe weather indicators.
KI: NOAA (1978, p. 4) and Funk (1991, p. 555) were sources for MPV (28) and HPV (38) when this index is used as a flash flood indicator. The MPV was a compromise between the NOAA (1978) recommendation of 24 and Funk (1991) recommendation of 30.
TT, CT, and VT: Miller (1972, p. 8-3, Table 2, and p. 5-2, Table 1) was the source for TT MPV (50) and HPV (55), CT MPV (24) and HPV (29), and VT MPV (26). NOAA (1984, pp. 3-5) and NOAA (1988, Appendix C) also gave values for specific regions like the Rocky Mountains and southeastern U.S. coastal waters.
LR75: Doswell et al. (1985, pp. 398-400, Figs. 2, 3, 6, and 8) and Davies (1993a, p. 53) were sources for MPV (8ºC/km). Davies preferred using the LCL-500 mb lapse rate with MPV of 7.5º/km. HPV (10ºC/km) is the dry-adiabatic lapse rate.
TEI: Moore (1992, p. 81) was the source for MPV (5ºC) and HPV (10ºC) assuming a cross-isentropic flow of 10 m/sec or greater. Elson (1991) has developed a program to determine TEI values in real-time using a personal computer.
CAP: Graziano and Carlson (1987, pp. 132-137) was the source for MPV (1-3ºC). A sounding with substantial instability, for example, B+ exceeding 1500 J/kg, capped by a small mid-level inversion can be favorable for an explosive energy release. However, Fig. 4 (p. 132) shows less than 15% of intense storms (radar reflectivity exceeding 50 dBZ) have a CAP exceeding 1.5ºC. The upper limit for severe storms was around 3ºC.
B-: MPV (25-75 J/kg) was inferred from Smith and Benjamin (1993, pp. 71-73) and Graziano and Carlson (1987, p. 128, Fig. 1). B- is the energy needed by parcels to reach the Level of Free Convection (LFC). Like CAP, B- is relevant only if instability is substantial, for example, B+ exceeds 1500 J/kg.
Smith and Benjamin (1993) examined a similar parameter called CIN (Convective Inhibition) for model-produced soundings near severe weather events. CIN was found to average 44 to 74 J/kg for tornadoes and 13 to 73 J/kg for large hail and damaging wind events. Calculations of B- for unstable soundings with mid-level capping inversions similar in shape to Graziano and Carlson (1987) Fig. 1 with a CAP strength of 2ºC yielded a typical B- of 20 to 40 J/kg.
PW: NOAA (1978, p. 4) was the source for MPV (130 percent of normal) and HPV (170 percent of normal). In addition, the National Meteorological Center Forecast Branch has found the precipitable water value of 1 inch (25 mm) or greater to be a useful criteria for diagnosing potential flash flood situations (Funk, 1991, p. 555).
WBZ: Miller (1972, p. 5-2, Table 1, and Chapter 7) was the source for MPV (5-7 kft or 9-11 kft) and HPV (7-9 kft).
SR DIR SHEAR: Lazarus and Droegemeier (1990, p. 274) was the source for MPV (70 deg) for severe thunderstorms, assuming an inflow of 10 m/sec or greater. Maddox et al. (1979, pp. 119-122, Tables 1-4) examined ground-relative directional shear and implied a similar MPV for flash-flood producing storms.
POSITIVE SHEAR: Davies (1989, p. 220) was the source for MPV (4 x 10-3/sec) and HPV (6 x 10-3/sec). Storm inflow had to be 10 m/sec or greater.
SR HELICITY: Lazarus and Droegemeier (1990, p. 272, Fig. 2) was the main source for MPV (150 m2/sec2) and HPV (250 m2/sec2) to identify supercell situations. Davies-Jones et al. (1990, p. 590) was another source for MPV for identifying supercell situations and the main source for MPV and HPV for strong tornadoes. Later research (Johns and Doswell, 1992, p. 233) have corroborated these values. MPV (300 m2/sec2) identified F2 tornado intensity situations while HPV (450 m2/sec2) identified F4 tornado intensity situations. The storm inflow had to be 10 m/sec or greater.
EHI: Davies (1993b, p. 111, Table 4) and Hart and Korotky (1991, p. IV-3) were the sources for MPV (2) and HPV (4) for identifying mesocyclone and violent (F4-F5) tornado situations. This assumed a storm inflow of 10 m/sec or greater and a mid-level flow of 12 m/sec or greater.
BRN: Weisman and Klemp (1986, pp. 353-354, Fig. 15.18) and Lazarus and Droegemeier (1990, p. 270, Tables 3 and 4) were the sources for MPV (10-15 or 35-45) and HPV (15-35) to identify supercell situations from multicell ones (BRN greater than 45).
Pulse-type thunderstorms have been generally associated with BRN less than 10. These BRN criteria work best when the B+ is between 1500 and 3500 J/kg (Weisman and Klemp, 1986) and the low and mid level environmental winds and storm relative inflow is 10 m/s or greater (Lazarus and Droegemeier, 1990).
SWEAT: Miller (1972, pp. F-2 and F-3) was the source for MPV (300) and HPV (400). MPV identified severe thunderstorm situations and HPV identified tornado situations. The 850 mb and 500 mb winds had to be 7 m/sec or greater.
Davies, J. M., 1989: On the use of shear magnitudes and hodographs in tornado forecasting. Preprints, 12th Conf. Wea. Forecasting and Analysis, Monterey CA, Amer. Meteor. Soc., 219-224.
_____, 1993a: Wind and instability parameters associated with supercell and non-supercell tornado events in the southern high plains. Preprints, 17th Conf. on Severe Local Storms, St. Louis, MO, Amer. Meteor. Soc., 51-55.
_____, 1993b: Hourly helicity, instability, and EHI in forecasting supercell tornadoes. Preprints, 17th Conf. on Severe Local Storms, St. Louis, MO, Amer. Meteor. Soc., 107-111.
Davies-Jones, R. P., D. Burgess, and M. Foster, 1990: Test of helicity as a tornado forecast parameter. Preprints, 16th Conf. on Severe Local Storms, Kananaskis Park, Alberta, Amer. Meteor. Soc., 588-592.
Doswell, C. A., F. Caracena, and M. Magnano, 1985: Temporal evolution of 700-500 mb lapse rate as a forecasting tool--a case study. Preprints, 14th Conf. on Severe Local Storms, Indianapolis, Amer. Meteor. Soc., 398-401.
Elson, D. B., 1991: Computing the Theta-E Index (TEI). NOAA Nat. Wea. Serv. East. Reg. Comp. Prog. NWS ERCP-12MC, Bohemia, NY, 13 pp.
Funk, T. W., 1991: Forecasting techniques utilized by the forecast branch of the National Meteorological Center during a major convective rainfall event. Wea. Forecasting, 6, 548-564.
Graziano, T. M., and T. N. Carlson, 1987: A Statistical Evaluation of Lid Strength on Deep Convection. Wea. Forecasting, 2, 127-139.
Hart, J. A., and J. Korotky, 1991: The SHARP Workstation - v1.50. A skew-T/hodograph analysis and research program for the IBM and compatible PC. User's manual. NOAA/NWS Forecast Office, Charleston, WV, 62 pp.
Johns, R. H., J. M. Davies, and P. W. Leftwich, 1990: An examination of the relationship of 0-2 km AGL "positive" wind shear to potential buoyant energy in strong and violent tornado situations. Preprints, 16th Conf. on Severe Local Storms, Kananaskis Park, Alberta, Canada, Amer. Meteor. Soc., 593-598.
_____, and C. A. Doswell III, 1992: Severe local storm forecasting. Symposium on Wea. Forecasting, Atlanta GA, Amer. Meteor. Soc., 225-236.
Lazarus, S. M., and K. K. Droegemeier, 1990: The influence of helicity on the stability and morphology of numerically simulated storms. Preprints, 16th Conf. on Severe Local Storms, Kananaskis Park, Alberta, Amer. Meteor. Soc., 269-274.
Maddox, R. A., C. F. Chappell, and L. R. Hoxit, 1979: Synoptic and meso-alpha scale aspects of flash flood events. Bull. Amer. Meteor. Soc., 60, 115-123.
Miller, R. C., 1972: Notes on Analysis of Severe-Storm Forecasting Procedures of the Air Force Global Weather Central. AFGWC Tech. Rep. 200 (Rev.), Air Wea. Serv., Scott AFB, IL, 181 pp.
Moore, J. T., 1992: Isentropic Analysis and Interpretation: Operational Applications to Synoptic and Mesoscale Forecast Problems. St. Louis Univ., Dept. of Earth and Atmos. Sci., St. Louis, MO, 88 pp.
NOAA, 1978: How not to issue a flash flood watch. Nat. Wea. Serv. East. Reg. Tech. Attach. No. 78-10, Garden City, NY, 4 pp.
_____, 1984: Convective stability indices. Nat. Wea. Serv. West. Reg. Tech. Attach. No. 84-14, Salt Lake City, UT, 8 pp.
_____, 1988: The Skew-T, Log P diagram. Nat. Wea. Serv. Tech. Train. Cent. MMFDC230 (1/88), Kansas City, MO, 85 pp.
Smith, T. L., and S. G. Benjamin, 1993: Evaluation of CAPE/CIN fields derived from model soundings. Preprints, 17th Conf. on Severe Local Storms, St. Louis, MO, Amer. Meteor. Soc., 70-73.
Weisman, M. L., and J. B. Klemp, 1986: Characteristics of Isolated Convective Storms. Mesoscale Meteorology and Forecasting, P. S. Ray, Ed., Amer. Meteor. Soc., 331-357.