3. SURFACE, UPPER AIR, AND COMPOSITE CHARTS

A. Surface

Hourly surface mesoscale analysis is critical in severe forecasting. Detailed analysis can uncover features such as boundaries, Mesolows, bubble highs, strong pressure falls, and moisture pooling. Here is a list of the optimum surface features to key in on for severe weather:

  1. Dew Points > or = 65°F
  2. Theta-E ridge and positive Theta-E advection
  3. Low-level moisture flux convergence
  4. Thermal ridge over or west of the moisture axis
  5. Areas experiencing strong temperature and dew point rises
  6. Rapidly developing cumulus congestus within areas
  7. Areas reaching convective temperature
  8. Focusing mechanisms (fronts, troughs, gust fronts, dry lines, outflow boundaries, etc.)
  9. Surface pressure < or = 1005 mb
  10. Areas with concentrated pressure falls of > or = 5 mb over 12 hours. Pressure falls can sometimes give clue to where Mesolow's form. Mesolows may develop from intersections of discontinuity lines. Winds back by 10 to 30 degrees near the northeast quadrant due to enhanced convergence. Some prime locations for Mesolow's development include a squall line intersecting a front or dry line and a low level jet intersecting a warm front.

B. Upper Air Conditions

The rawinsonde sounding is the principle tool for diagnosing the stability of the troposphere quantitatively (Kula 1996). Despite, only two upper air soundings a day, a forecast should start with a 4-dimensional mental picture of the atmosphere (Doswell 1982). Upper air and surface maps can and should be enhanced to emphasize features of importance to convective storm forecasting (Maddox 1979b). Listed below are the optimum upper air features for severe weather.

  1. 925 mb
    * Areas under and just west of the low level jet (winds > or = 25 kts)
    * Thermal ridge west of moisture axis
    * Significant warm air advection
    * Strong moisture flux convergence
    * Focusing mechanisms (fronts, troughs, and dry lines)
  2. 850 mb
    * Areas under and just west of the low level jet (winds > or = 35 kts) or on the nose of the jet
    * Thermal ridge west of moisture axis
    * Dew Point > or = 8°C
    * Significant warm air advection
    * The greater the angle of the winds from dry to moist air, the greater the instability.
    * Strong moisture flux convergence
    * Focusing mechanisms (fronts and troughs)
    * Moisture transport axis
  3. 700 mb
    * Wind veering > or = 30 degrees between surface and 700 mb
    * Dry air intrudes at a 40 degree angle and speeds of at least 25 knots. Look at Skew-T's, model soundings, and gridded data for significant entrainment.
    * Dew point depression > 6°C. Significant dry air in mid levels may signal possibility of strong downdrafts.
    * Winds cross 12 hr temperature no change line at > 40 degrees
    * Focusing mechanisms (fronts and troughs)
    * Significant upward vertical velocity (UVV)
  4. 500 mb
    * Wind speeds of > or = 50 knots
    * Short Waves, especially negatively tilted rapidly moving short waves
    * PIVA with contours crossing vorticity pattern > 30 degrees
    * Significant cold pool aloft (-16°C Dec-Feb, -14°C Mar-Apr and Oct-Nov, -12°C May-Jun, -10°C Jul-Sep)
    * Horizontal shear over 90 miles is > or = 30 kts
  5. 300 and 200 mb
    * Wind speeds > or = 85 kts
    * Diffluent areas
    * Left exit region and right entrance region of straight jet streaks; left exit region of cyclonically curved jet streak; right entrance region in anticyclonic jet streak
    * Most severe weather occurs south of the polar jet, and north of the subtropical jet (coupled jet) or in the left exit region of a jet
    * Severe weather outbreaks often occur in the diffluent zone between the polar and subtropical jet streams.
    * Long wave troughs and strong synoptic scale lift
    * Significant height falls and or a deepening of an upper level low

C. Composite Chart

Many offices use a composite chart methodology which Miller (1972) advocated. The composite chart provides a structured way of combining the crucial features on one map. Including all the surface and upper air features on a central map in different colors will allow the operational forecaster to better access the potential for severe convection. However, the relevant parameters on any given day may or may not be useful. The weather situation should, in part, dictate the parameter choices (Doswell 1982).

In tornado forecasting, SPC uses composite charts (Doswell et al. 1993) that emphasize:

4. MESOSCALE ANALYSIS, HODOGRAPHS, PROFILERS AND SATELLITE

 


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