Notes: Forecasting Severe Storms

Severe Local Storms Forecasting

Robert H. Johns, Charles A. Doswell III

Weather and Forecasting V7 1992 Amer. Meteor. Soc.

-Initiation of deep convection-3 necessary ingredients;1) a moist layer of sufficient depth in the lower or middle troposphere, 2)a steep enough lapse rate above the moist layer to allow for a substantial "positive area", 3) sufficient lifting of a parcel from the moist layer to allow it to reach its level of free convection (LFC).

-usually limited to areas where mean RH is greater than 40-45 %. LI's are zero or less. Forecast 700 mb temperature is colder than 12 degrees C. 1000-500 mb thickness values are 5790 m or less. NGM 700 mb vertical motion fields indicate neutral (-3 to +3 x10-3 mb/s) or upward motion (-3 x 10-3 mb/s or greater)

NOTE NGM- sign is reversed on output

-Forecasting large hail- need a strong updraft (instability-large positive area). Numerical simulations suggest that vertical accelerations induced by perturbation pressure gradients created by the interaction of the updraft with environmental winds can contribute substantially to updraft speeds. Factors affecting hail size at ground; 1) the distance between the freezing level and the ground, 2) the mean temperature of the downdraft air between the hailstone's freezing level and the ground, 3) the size of the hailstone.

The wet bulb zero (WBZ) level approximates the height of the freezing level for the downdraft air, the higher the mean temperature between the WBZ level and the ground, the faster the melting proceeds. It is important to note that supercells associated with relatively weak instability often do not produce large hail.

In cold low patterns, the common occurrence of hail at the surface is aided by low WBZ heights and low mean temperatures below the WBZ height, in these cases, hail is usually smaller than 1 3/4 in.

-Forecasting convectively induced damaging winds - Precipitation loading and negative buoyancy due to evaporative cooling are involved with initiating and sustaining a downdraft. The greater the quantity of liquid water per unit volume, the greater the precipitation drag. Negative buoyancy is created when precipitation falls through a layer of unsaturated air. The strength of the downdraft is enhanced by the availability/entrainment of relatively dry air. Evaporative cooling (and downdraft strength) is also enhanced by 1) large liquid water content per unit volume in the falling precipitation, 2) small drop sizes, & 3) a steep lapse rate ( close to dry adiabatic). 1 & 2 relate to the amount of liquid water surface available for evaporation. Downward transfer of momentum - the stronger the environmental winds in the downdraft entrainment region, the greater the potential contribution to outflow strength. Updraft instability is associated with positive buoyancy, downdrafts are the result of negative banco - water loading has the effect of increasing negative banco, thereby enhancing downdrafts. Whereas virtually all unstable updrafts are saturated, unstable downdrafts may or may not be saturated.

-Weak shear environments - the thermodynamic profile (pattern) is the primary signal for identifying when strong convectively induced winds are likely to occur. 1) Inverted V, 2) Weakly capped "wet microburst" profile.

-Inverted V - has a deep dry-adiabatic layer from near the surface to the middle levels, a very dry lower layer, and a moist layer in mid-troposphere.

-Wet microburst - has moisture values in the boundary layer with the top of the moist layer sometimes extending into portions of the middle levels (up to 4.5 km). Relative humidities above 4.5 km (15,000 ft) are typically low. As diurnal heating occurs, a dry adiabatic layer often develops in the lower 1.5 km (5000 ft). The afternoon climate often displays a theta-e difference between the surface and the mid-levels equal to or greater than 20 C. On days when no microburst activity was observed, the theta-e difference was less than or equal to 13C.

-Strongly sheared environments - derecho - the bow echo complex often moves away from the stronger low-level warm advection, into areas of increasingly unidirectional shear. A "loaded gun" sounding is typically associated with the warm season pattern.

-Wind environments associated with supercell development. 3 important wind-related factors; 1) the nature of the wind profile in the storm inflow layer, 2) the strength of the storm-relative inflow, & 3) the strength of the wind in the middle levels.

-Rotational potential of the storm inflow layer - The depth of the inflow layer is roughly coincident with the height of the LFC. The veering often is associated with baroclinicity, but it is also affected by the friction in the boundary layer as well as low-level warm advection. 2 parameters are related to rotational potential; 1) positive mean shear, & 2) storm-relative helicity. Essentially, positive mean shear estimates the mean shear of the straight-line or right turning hodographs in the lowest 2 km. Helicity takes storm motion into account. Most strong and violent tornadoes are associated with helicity values greater than 300. These values exhibit a diurnal oscillation.

-Strength of storm inflow is a critical factor in the development of a strongly rotating updraft.

-Strength of shear through the middle troposphere - important for 2 reasons; 1) it removes precipitation from the updrafts by increasing storm-relative flow, and 2) the interaction of this deeper shear with the storm updrafts can induce vertical perturbation pressure gradients that enhance updraft intensity. A "spike" seen on the hodographs in the low levels indicate mid level winds are too low while a low level jet is indicated can alert the forecaster that the wind environment is not conducive to the development of strong rotation.

-Instability associated with supercell development - Supercells occur in environments with an extremely wide range of CAPE. During cooler months, wind environments that favor mesocyclone development are common and widespread, however, instability is usually infrequent and values are relatively low. In the warmer months, moderate to high values of instability occur while sufficient wind environments are infrequent. There is a tenancy for the very low instability cases to be associated with strong helicity and strong wind shear. Some of the high instability/weak shear cases are associated with bow echoes and isolated supercells that are either moving faster than the 0-6 km AGL mean wind or are extremely right moving supercells, in both cases, the storm-relative helicity and inflow are enhanced by the deviant storm motion.

-supercell-induced tornado development - It appears that the development of sufficiently strong outflow from the storm updrafts is required for tornado generation. The evaporatively cooled downdraft outflow results in a baroclinicity generated, low-level contribution to vorticity that is necessary for tornadogenesis.

-Climatology - Warm advection pattern - tornado activity occurs in the vicinity of a quasistationary or warm frontal boundary orientated roughly parallel to the mid-level flow. Cold low/occluded front - tornadoes may occur with thunderstorms developing near the upper low, these are often non-supercell. There are also localized tornado patterns. Tropical cyclones - the hodograph often displays a very large loop that approaches being a circle, the wind profile seems highly supportive of supercell development, but there is moist air through the mid-levels and has relatively little CAPE. Will the instability be sufficient & will there be sufficient dry air in the downdraft entrainment region to develop low-level baroclinic vorticity for tornado development.

 

 

Forecasting Severe Thunderstorms: A Brief Evaluation of Accepted Techniques

Robert A. Maddox, Charles A. Doswell III

Preprints 12th Conf on SVR LCL Storms 1982 Amer. Meteor. Soc.

-It should be quite obvious that most forecasting rules key upon highly baroclinic synoptic settings. However, it is well documented that significant severe thunderstorm episodes often occur within relatively weak large-scale meteorological settings. Thus the forecaster must be extremely cautious if general guidelines are used to evaluate the severe thunderstorm threat.

-Case of 22/23 Sep 1990 - Classic case - During the afternoon of 22 Sep. a strong cold front pushed through the mid-Mississippi Valley and lower Great Lakes Region. The use of composite charts and tables (shown in paper) showed this to be a classic severe storm episode.

-Cases of 3/4 June 1980 and 20 April 1981 were not classic cases with both composite charts and tables not helping the forecaster to determine the outbreak area.

-Both of the weak settings illustrated here were similar in that very pronounced lower-tropospheric warm air advection was apparently the dominant mechanism that triggered release of conditionally instability.

-If other guidelines are used...Such criteria should focus upon important physical mechanisms leading to strong storms within such an environment. In particular, the importance of favorable surface patterns and pronounced east/west thermal boundaries, the degree of conditional instability and vertical motion forced by lower-tropospheric warm advection need to be strongly emphasized.


Forecasting Violent Tornadoes

Preston W. Leftwich, William R. Sammler

Preprints 11th Conf on Weather Forecasting Analysis 1986 Amer. Meteor. Soc.


-Violent tornadoes represent about 2% of the tornadoes that occur in the U.S., they cause over 2/3rds of the tornado related fatalities.


Tornado Forecasting: A Review

Charles A. Doswell III, Steven J. Weiss, Robert H. Johns

Tornado Symposium III 1993

-Scientific connections between parameters and physical processes can be made at SELS by use of composite charts that emphasize:

1) synoptic and mesoscale upward motion

2)Sufficient moisture and lapse rate for a parcel to be positively buoyant

3) Vertical wind shear structure

-Recent research has suggested that the vertical wind shear structure is the most crucial element in supercells..

-It is also important that the combination of vertical wind shear and storm motion produce enough storm

-relative helicity to allow the mesocyclone to reach down to the surface.



A Preliminary Synoptic Climatology of Violent Tornado Outbreaks Utilizing Radiosonde Standard Level Data

Robert H. Johns & William R. Sammler 1989

Preprints, 12th Conf. Wea. Analysis and Forecasting

Monterey, CA, Amer. Meteor. Soc. , 196-201.


- Leftwich and Sammler defined violent tornado outbreaks as 10 tornado events in which at least 2 are F4 or greater. This study expanded the definition to a) ten tornado events with one F4 having a path of 30 miles or more, and b) six or more tornado events with one or more F4 tornadoes having a combined path length of 60 miles or more. 77 Outbreaks were used in this study.

-Positive buoyancy - the data set reveals that the 700 mb and 500 mb temperatures usually remain constant or rise slightly prior to an outbreak. The 850 mb rise is due in part to diurnal heating. In almost every case the 850 mb dew points rise, frequently by 5 degrees C or more. Therefore it appears that destabilization of the air mass prior to an outbreak is a result of increases in low level temperature and moisture rather than from cooling aloft. In all outbreaks studied the low level moist layer extends above the 850 mb level.

-jets - Most outbreaks are associated with a double jet structure with the centerpoint usually between jets. When only one jet is evident (at 500 mb) the outbreak centerpoint is usually beneath the axis of the jet. The subtropical jet appears to be an effective limiting factor for the southward extent. Outbreaks are often associated with a rapidly moving 500 mb shortwave trough. The movement and location of the trough at 500 mb is more easily identified by height changes rather than a wind maximum. The associated 700 mb wind maxima are usually located within 200 miles of the centerpoint. The average wind speeds at all 3 standard levels are 10 to 20 kkts stronger in the weak instability cases (SI 0 to -3) than in the strong instability cases (SI -7 to -10). However, the 850 to 500 mb directional shear values are smallest with the weak instability cases and largest with the strong instability cases.

-Discussion - Directional shear appears to be a major contributor to the shear magnitude associated with violent tornado outbreaks in the Plains states during the warm season, while speed shear appears to be the primary contributor to shear magnitude in the area east of the Plains in the cool season. Davies (1989)

- He found that in strong tornado situations there are fundamental differences in the nature of the 0-2 km wind fields between situations that occur in the warm season in the Plains region and ones that occur in the eastern U.S. in the cool season. He also noted that the magnitude of the low level vertical shear in the east during the cool season is much greater that in the Plains in the warm season.



Differentiating Between Types of Severe Thunderstorm Outbreaks: A Preliminary Investigation

Robert H. Johns & John Hart 1993

Preprints, 17th Conf. Severe Local Storms, St. Louis,

MO., Amer. Meteor. Soc.


-Six outbreaks were used in this study, 3 cases exhibited primarily right-moving supercells and 3 cases exhibited primarily bow echo storm structures.

-Supercells - 26 April 1991 was the most prototypical major tornado outbreak case. The 16 June 1992 case was characterized by an environment where low level moisture and the resultant enhanced instability extended well north of the warm front and the most intense tornado activity occurred north of the front. The

3rd case was in the winter in the southeastern U.S. and was characterized by relatively weak instability and very strong wind fields. The soundings showed a large degree of curvature in the lower 2 to 3 km. The 0-3 km helicity values were above 400 m2/s2. Storm relative inflow at 1 km AGL is relatively strong in all 3 cases, about 35 kts from the east to southeast. Storm relative flow in the mid levels (2.5 to 6 km AGL), which aids in the removal of precipitation from the updraft region, is about 20 kts form the southeast to south in all 3 cases.

-Bow Echoes (BES) - 9 April 1991 with the other 2 in the late spring and summer. These events were generally associated with greater instability. The soundings display hodographs that are relatively straight with storm motions a little to the right of the hodographs and very fast, even faster than the 0-6 km AGL mean wind. These rapid storm motions result in strong inflow (ranging from 30 to 42 kts) on the right front flank of the BES. Helicity values for the 0-3 km layer were 120 m2/s2 or less. Numerical simulation experiments by Weisman (1990) suggest that a favorable environmental condition for bow echo development is one where there is strong shear between the surface and 2.5 km AGL with little shear in the layer between 2.5 and 5 km AGL.


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