6. FORECASTING DAMAGING WINDS

The four primary forecasting keys for damaging winds with environments with weak shear (Hales 1996) are:

  • Amount of dry mid-level air
  • Strength of the updraft
  • Amount of low-level moisture
  • Downdraft instability.

Operational forecasters should look for Inverted-V soundings, steep low level Theta-E lapse rates, and the strength of the CAPE. Sometimes high CAPE and weak shear environments can lead to derecho development. When the environmental shear is weak, the thermodynamic profile is a primary signal for identifying when strong convectively induced winds are likely to occur. (Johns and Doswell 1992).

The four primary forecasting keys for damaging winds of air-masses with moderate to strong shear (Hales 1996) are:

  • Amount of low and mid level helicity
  • Degree of instability
  • Amount of dry mid level air
  • Rapid storm motion (> 40 knots)

Precipitation loading and negative buoyancy due to evaporative cooling are recognized factors in initiating and sustaining downdrafts (Johns and Doswell 1992). Once a downdraft is established, continued entrainment of unsaturated air in the mid levels aids in evaporation and consequently stronger downdrafts. Storms in moderate to strong shear can turn into supercells, bow echos, and derechos (Hales 1996).

 

 

 

Figure 2. Hail Size Prediction Methods (Miller 1972).

 

A. Bow Echoes and Derechos

1. Warm Season

  • Often form and travel along a quasi-stationary low level thermal boundary orientated parallel to the mean tropospheric flow (Johns 1993).
  • Almost always initiated in an area of low level warm air advection. Can also form with weak upper troughs with lifted indices < -8 and CAPE > or = 4500 J/kg.
  • Many times moisture values in the convergence zone are higher than in surrounding areas.
  • Numerical simulation experiments by Weisman (1990) suggest that a favorable environmental condition for bow echo development is moderate to strong shear between the surface and 2.5 km AGL (0-8200 ft). The wind field is unidirectional above.
  • Bow echoes can occur in areas where the mid level wind speeds are relatively weak (25-35 kts) (Johns 1993).

2. Cool Season

  • Usually a squall line develops ahead of a cold front and bow echo activity is embedded within this line
  • 500 mb wind speeds up to 75 to 85 kts
  • Dry, potentially cold layer in the downdraft entrainment region (3-7 km or 9800- 23,000ft)
  • Bow echoes often display hodographs that are relatively straight with storm motions a little to the right of the hodographs and very fast, even faster than the 0-6km AGL (0-19,700 ft) mean wind (Johns and Hart 1993).
  • Can occur in marginally unstable air-masses

There is a large number of excellent operational papers on bow echoes. Forecasters seeking additional information on these phenomena should refer to Przybylinski (1995), Johns (1993), Weisman (1993), and Johns and Hirt (1987).

3. Johns and Hirt Derecho Checklist

Derechos are essentially long-lived bow echoes. Johns and Hirt (1987) developed a checklist for warm season derechoes. They defined a derecho "to include any family of downburst clusters provided by an extratropical mesoscale convective systems (MCS)". The checklist was designed for the operational forecaster to assess the potential for warm season derechoes, which occur with stagnant weather patterns and weak synoptic scale features (Johns 1993). The checklist is used after convection has developed in the area of interest (Anderson 1994), and forecasters should look over a large area and not just a single point.

TABLE 3
Part A - Johns/Hirt Derecho Checklist

Parameter

Yes

No

500mb flow direction > or = 240°
Quasi-stationary boundary nearly parallel to 500 mb flow?
850 mb warm advection within 200 nm
700 mb warm advection within 200 nm
ELWS > or = 25 knots *
If all of the following five conditions are present in the area of interest, proceed to Part B. Otherwise, derecho development is not likely

*Johns and Hirt define ELWS as the Estimated Lower Mid Tropospheric (LMT) Wind Speed. The LMT is the layer approximately 8200 to18,000ft. A simple way to calculate this is to average the 700 and 500 mb winds.

TABLE 3
Part B - Johns/Hirt Derecho Checklist

Parameter

Yes No
500 mb 12 hour height falls > or = 60m?
SPC lifted index (using the mean temperature and mixing ratio of the lowest 100 mb) -6 or lower?
ELRH (relative humidity in the LMT) < 80 percent? **
If all of the following conditions are present in the area of interest, proceed to Part D. Otherwise, proceed to Part C.

**ELRH is the Estimated Lower Mid Tropospheric (LMT) Relative Humidity.

TABLE 3
Part C - Johns/Hirt Derecho Checklist

Parameter

Yes

No

SPC lifted index -8 or lower?
ELRH < 70% in initiation area (<80% downstream?) **
If both of the following conditions are present in the area of interest proceed to Part D. Otherwise, derecho development is not likely.

TABLE 3
Part D - Johns/Hirt Derecho Checklist

Parameter

Yes

No

Do the parameter values satisfying the criteria for the SPC lifted index and ELWS extend downwind along the quasi-stationary boundary for a distance of at least 250 nm from the convective system?
If yes, be alert for derecho development for areas downstream for at least 250 nm. Otherwise, the potential for wind damage will probably be too localized to meet the areal criterion for a derecho.

In addition to a surface boundary parallel to the mid level flow and strong instability, Hales (1996) points to two other considerations for possible derecho development. He also looks for:

  • High surface Theta-E airmass
  • West to northwest flow aloft

B. Dynamic Squall Lines Imy (1998) and Sturtevant (1995)

Squall lines may form on some surface or low level discontinuity line and may or may not be parallel to a front. Squall lines produce a variety of severe weather with strong damaging winds the main threat.

1. Key Parameters

  • Strong directional shear from the surface to 700 mb with little or no shear aloft
  • 500 mb winds > 50 kts
  • Well defined westerly jet stream
  • Great range of instability, from marginally unstable to extreme
  • Active southeast low level jet transporting deep low level moisture into the threat area
  • Warm moist air is usually overrunning cooler air
  • Evaporative cooling in the upper levels
  • Cold air advection at 500 mb and layer of dry potentially cold air 3-7km AGL (9800-23,000 ft)
  • Essential ingredient that sustains a long lived line of time dependent cells is the amount of low-level wind shear which encounters the circulation induced by the cold thunderstorm outflow (Rotunno et al. 1988).

2. 850/500 mb Thickness Chart

  • Squall lines often develop about 100 miles upstream of the 850/500 thickness ridge
  • Squall lines often develop in the area of the maximum horizontal anticyclonic shear zone of the thickness ridge

3. Initial Outbreak Area

  • Along and just ahead of a cold front or along an advancing dry line
  • Along a low level trough, within or just above the moist layer
  • Bounded by 700 mb cold front and east from there to a point where air is too stable to produce severe weather
  • Where the dry air at 700 mb meets the overrunning warm moist air

4. Steering and Timing

  • Steering of squall lines is generally the 500 mb wind direction at 40 % of the wind speed
  • Maximum threat time is from peak of afternoon heating to shortly after sunset

C. Four Types of Squall Lines (Bluestein and Jain 1985)

1. Broken Line

  • Forms along cold front
  • Relatively weak wind shear
  • Large CAPE and large BRN
  • Low relative helicity

2. Back Building

  • Can occur with or along a number of different types of surface boundaries
  • Environments with large CAPE, small BRN
  • Usually unidirectional flow through a deep layer with minimal shear

3. Broken Areal

  • Formation appears to result from the interaction of outflow boundaries
  • Low CAPE

4. Embedded Areal

  • Convective line appears within a larger area of weaker stratiform precipitation

D. AFGWC Method For Determining Maximum Convective Wind Gusts

If severe thunderstorms are forecast, use Figure 3 to calculate maximum wind gusts. This method is over 25 years old, but still has merit today.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  1. Determine wet bulb zero from sounding.
  2. From wet bulb zero, project downward moist adiabatically to the surface. This results in a "downrush" or downdraft temperature.
  3. Subtract the downrush temperature from the surface temperature (or predicted surface temperature) to determine T2.
  4. From the graph, determine the range of maximum gustiness with thunderstorms.

Figure 3. Wind Gust Graph (Miller 1972)

E. McDonald Method For Gust Potential Forecast Procedure (1976)

Figure 4 is another useful method to determine maximum wind gusts (McDonald 1976). Figure 4 contains this technique, which has been successfully used in the western U.S. for many years.

F. Maximum Vertical Velocity Using CAPE

A maximum vertical velocity (w max) at the equilibrium level for a parcel can be computed using the CAPE. The following equation (Doswell et al. 1991) gives the maximum vertical speeds of about 95 to 135 knots for CAPEs in the range from 1500 to 2500 J/kg. Owing to water loading and mixing effects, the vertical velocity in real storms is usually about half this value.


 

 

Step 1. Multiply the 500 mb dew point depression by three and subtract from it the 700 mb dew point depression. If this number is negative, proceed; if it is positive, there is no gust potential. This step is optional.

Step 2. Compute the upper level instability index (UI) by lifting a parcel from 500 mb to it's LCL and then up the moist adiabat through 400 mb to 300 mb.

Step 3. Locate the point on the graph above using 700 mb depression and UI.

Area 1. Too moist for strong convective gusts even though thunderstorms may occur.
Area 2. Too stable for upper level thunderstorms.
Area 3. Thunderstorm gusts above 30 knots possible.
Area 4. Thunderstorm gusts above 40 knots possible.

 


Figure 4. Convective Gust Potential Forecast Procedure.

 

G. Downbursts and Microbursts

This section will address dry and wet microburst forecasting, and will not focus on radar techniques associated with either. If you desire additional information, two excellent operational papers regarding microburst techniques on radar are by Vasiloff (1997) and Ellis and Oakland (1989).

1. Forecasting Dry Microbursts

Wakimoto (1984) wrote an excellent paper on forecasting dry microbursts. The study used 155 dry microburst days out of 186 total. The highest number of microbursts occurred in mid July, although late May had several active days. Forecasters should look for the following features:

  • Shallow radiation inversion on 1200 UTC sounding at the surface, approximately 40–50 mb deep
  • Dry adiabatic layer must extend to approximately 500 mb
  • Mean sub-cloud mixing ratio is approximately 3-5 g/kg, with moisture present around 500 mb
  • Convective temperature must be reached during the day.

2. Forecasting Wet Microbursts

Atkins and Wakimoto (1990, 1991) wrote two excellent papers on wet microburst activity in the southeastern United States. All of the events studied were accompanied by heavy precipitation and a temperature drop. There was a peak of microburst activity at 2000 UTC, and a secondary peak at 1200 UTC. Forecasters should look for the following:

  • Sounding is close to the moist adiabatic lapse rate and has warm cloud bases.
  • Thermal environment is statically stable.
  • Shallow radiation inversion on 1200UTC sounding at the surface, approximately 50 mb deep.
  • Low level moisture extends from the surface to about 500 mb and is capped by dry mid level layer above 500 mb.
  • Equivalent potential temperature profiles are potentially quite unstable with a minimum at about 650 mb.
  • Larger (negative) lifted indices, CAPE, and BRN for both the morning and the afternoon.
  • Afternoon Theta-E from surface to mid levels (650-500mb) > or = 20K. For days with thunderstorms, with no microburst (null days), Theta-E was observed to be < 13K. Figure 5 depicts typical Theta-E profiles for downburst and non-downburst days.

 

 

Figure 5. Vertical profile of equivalent potential temperature for downburst and non-downburst days (Adkins and Wakimoto 1991).

3. An Approach to Forecasting Downbursts (Ladd 1991).

Ladd (1991) broke down forecasting downbursts into two phases:

  • Assessing the potential for downburst development
  • Determining the most likely location for downburst occurrence

You can further break down the first phase into two sub-phases: a thermodynamic assessment of the atmosphere and a kinematic assessment of evolving features. Forecasters should:

  • Narrow the region of suspicion based on upper level flow regimes and expected advection of properties. Therefore, an analysis of upper level charts is a must.
  • Look for subtle features by carrying out height analysis at a 10-20 meter interval and temperature and dew point analysis at 2-3°C intervals.
  • At 850 mb and 700 mb, look for convergence zones, temperature ridges and moisture axes.
  • Look for advection of dry mid-level air over low-level moist air.
  • Determine the CCL and note the degree of positive energy available on modified soundings. Eblen (1990) looks for the CCL > or = 5000 feet.
  • Look at subcloud lapse rates. High subcloud lapse rates (700-500 mb) have been positively correlated with downburst potential over the High Plains (4°F/1000ft or 8°C/km). Across north and east Texas, the subcloud layers are more aptly reflected by the 850-700 mb layer.
  • Analyze and track surface boundaries, development of temperature ridges, and increased moisture pooling (or convergence) into an area.
  • Look for "second generation" convection. In other words, outflow from a parent storm complex eventually triggered the downburst-producing thunderstorm. An hourly 1-2 mb analysis is necessary, as well as closely monitoring of radar and satellite to detect and track these outflows.
  • Use LAPS, MSAS, MESONET and MDCARS (aircraft) data to track instability, temperature and moisture convergence into an area.
4. WINDEX Equation To Forecast Microburst Wind Gust Strength
(McCann 1994)

WINDEX is specifically designed to help forecast microburst potential and is based on studies of observed and modeled microbursts. It can be computed from environmental conditions either based on upper air soundings or numerical model predictions. Data indicates a good correlation between WINDEX and maximum microburst wind gusts in knots.

WINDEX is better at assessing microburst potential versus the traditional stability indices (such as the Lifted Index) which only measure updraft potential. WINDEX not only considers environmental lapse rates, but also considers low level moisture availability for wet microbursts.

The radicand in the WINDEX equation is multiplied by 5 to estimate the maximum potential wind gust at the surface in knots. When the lapse rate is low, that is, the lapse rate squared is less than 30, the radicand may become less than zero. When this happens, WI is set to zero.

Studies indicate that outflow boundaries play an important role in the development of microburst producing thunderstorms. Commonly, strong microbursts develop where outflow boundaries move into the WINDEX maximum (in an analyzed WINDEX field). Large areas of high WINDEX often exist on an analysis in which, because of no significant outflow boundaries, no damaging microbursts develop. However, weaker microbursts can still develop without the presence of significant outflow boundaries.

7. SUPERCELLS AND TORNADOES

Figure 4. Convective Gust Potential Forecast Procedure.

 

Figure 4. Convective Gust Potential Forecast Procedure.

 

Figure 4. Convective Gust Potential Forecast Procedure.

 


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