A. Mesoscale Analysis
There are a number of useful tools to diagnose the mesoscale meteorological environment. Evolving mesoscale analysis tools on AWIPS workstations (MSAS and LAPS) allow forecasters to utilize hourly surface data to identify mesoscale lifting mechanisms needed to force boundary layer air beyond the LFC. Hourly parameters include lifted indices, pressure change, and moisture flux convergence, just to name a few. The ADAP decision tree section on pages 28 to 33 (Bothwell 1988) can be very useful in severe weather forecasting and can be adapted to the new mesoscale analysis programs. Figure 1 is just one portion of this decision tree. In addition to Bothwell's paper, three other operational papers on mesoscale analysis are from Byrd (1994), Vallee (1991), and Waldstreicher (1988).
Figure 1. ADAP/LAPS Decision Tree (Bothwell 1988)
1. Will the CAP Break?
One of the most challenging parts of severe weather forecasting is determining when, and if, the cap will break. Most forecasters have experienced a busted forecast where they had a very unstable environment, with a cap in the low levels, and a convective temperature that was never reached. Conversely, situations such as the Plainfield, IL 1990 tornado, can occur when you have a strong cap with prodigious CAPEs and helicities. Temperatures > or = 12°C at 700 mb and/or CIN < -50 j/kg normally inhibits thunderstorms, however, severe storms can develop regardless of CAP strength, if low level forcing exists. Typically, the cap will be eroded by surface heating and/or large scale lifting.
Emlaw (1991) has the one of the best operational paper we've come across on this subject to date. He offers two techniques designed to be used 4 to 8 hours in advance of late afternoon convection to determine the probability of strong convection in a capped environment. Forecasters should also utilize data from special and modified soundings, pireps, mesonet hourly data (where available), LAPS and MSAS based skew-t's , and GOES soundings.
2. Isallobaric Analysis
Hales (1996) states, that many severe weather outbreaks, such as the Tulsa tornado of 1993, are preceded by 1-2 hours of strong pressure falls. The potential is further enhanced if there is a corresponding pressure rise moving towards the fall area. Additional information on isallobaric analysis can be found in Togstad (1994).
Hodographs indicate vertical wind shear from an upper air sounding wind profile or model derived wind profile. Environmental storm motion and shear vectors (both general and storm relative) can be determined by hodographs. These parameters are a useful tool in determining storm organization.
A hodograph's length and shape over a given depth are the best means for determining a potential storm environment's vertical wind shear and provides implications for anticipating convective storm structure and evolution. Hodographs are an extremely helpful forecasting tool, especially in recognizing the potential for supercell thunderstorms. Severe weather hodographs can be broken down into four main groups (Sturtevant 1995):
1. Long Straight Line Implies storm splitting into left and right movers and some supercells 2. Small Closed Loop Implies short lived multicells 3. Small Open Loop Implies squall lines 4. Large Loop Implies supercells (can be open or closed)
Additional information can also be found in Bunkers et al. (1998) and Johns and Hart (1993).
There are a number of operational uses for profiler data. Wind profilers can be extremely beneficial to identify features for general and severe thunderstorms. An excellent operational paper on profilers is by Rich (1992). Some of the operational convective uses for profiler data include:
- Identification of increasing or decreasing vertical wind shear.
- Identify the presence and location of upper level diffluence.
- Location of jet streaks, descending jets, and mid level rear-inflow jets.
- Onset, cessation and location of low level jets.
- Areas of possible differential temperature advection leading to the destablization of the air mass.
- Locate troughs, ridges, and fronts.
- Identify areas of upward vertical motion that will enhance convection. In addition, observed profiler winds along with isentropic surface analysis can indicate areas of isentropic lift
The sounding's density layers and areas of low and mid level wind shear can also be modified. Modified data can be used to create a modified hodograph and recalculated helicity.
Satellite data contain a cornucopia of invaluable severe weather information. New satellite tools such as NSAT, AWIPS, and internet sites with special satellite features, have improved severe weather forecasting. Goetsch (1987) is the best paper we have come across addressing the satellite aspects of thunderstorms. His paper addresses such features as low level boundaries, intersections, arc clouds, low level moisture axis and wind flow, upper level short waves, and cirrus streaks.
Reports of altocumulus castellanus (ACCAS) offer the first clue to an unstable atmosphere. Water vapor imagery can be very beneficial at locating such features as mid level drying (dark areas), upper level jet maxes, and tropical moisture connections. For additional satellite information, refer to the NESDIS web site (http://www.nesdis.noaa.gov/).