MODULE 4.2D SHORT RANGE FORECASTING OF CLOUD, PRECIPITATION AND RESTRICTIONS TO VISIBILITY Convective Cloud and Precipitation Table of Contents 1. INTRODUCTION ...................................................................................................................................................1 2. WARM SEASON CONVECTIVE WEATHER...................................................................................................1 ANALYSIS OF REAL TIME DATA ................................................................................................................................1 MODIFYING THE TEPHIGRAM.....................................................................................................................................1 DIURNAL AND SEASONAL TRENDS ............................................................................................................................2 SYNOPTIC CORRELATIONS TECHNIQUES....................................................................................................................3 PHYSICAL PROCESSES APPROACH .............................................................................................................................4 THUNDERSTORMS: WHAT TO DO WITH THEM ONCE YOU KNOW WHERE THEY ARE..............................................8 3. WINTERTIME CONVECTION - SNOWSQUALLS .........................................................................................9 AIR MODIFICATION TABLE................................................................................................................................9 SNOWSQUALL FORECAST PROCEDURE ....................................................................................................................11 NEW SNOWFALL TO ESTIMATED MELTWATER CONVERSION TABLE .................................................13 MOD_042D-2001-10-16.doc 1. INTRODUCTION Convective weather comes in a wide range of forms and sizes. The result of summer convection can be a variety of clouds and weather ranging anywhere from relatively innocent cumulus to cumulonimbus thunderstorms to mesoscale convective complexes and tornadoes. In winter, convective activity is considerably subdued over the continental regions. Any that occurs is mostly constrained to areas lying over and to the lee of open water bodies or else is embedded in synoptic cloud associated with developing lows. Wintertime convective activity frequently results in snowsqualls and enhanced snowfalls. The major portion of the material in the first section will cover warm season convective weather, which is generally applicable for the spring, summer and autumn seasons when low level heating is of significance. Wintertime convection, in the form of snowsqualls and enhanced snowfalls, will be covered in the second section. 2. WARM SEASON CONVECTIVE WEATHER A number of techniques for forecasting summertime convective weather are described below. These techniques include the careful analysis of real time data, selection and modification of a tephigram, a consideration of diurnal trends, use of synoptic correlations, and the physical processes approach. Following will be a discussion on short range techniques for forecasting thunderstorms once they have formed and once you know where they are. Analysis of Real Time Data Convective weather is a mesoscale event which develops and dissipates rapidly. That makes it an ideal problem for short range forecasting techniques. Careful analysis and diagnosis of real time data is crucial in forecasting all short range events including convective events. Of particular value is examining the hourly weather data to determine dynamic and thermodynamic forcings. Examples are dewpoint analysis, which highlights areas of available low level moisture and trough lines, which highlight lines of low level convergence. The forecaster can use both traditional surface analyses and areal displays of weather elements using workstation display tools to analyze the data. Satellite, radar, and lightning are also short range analysis tools which give real time clues to the state of the atmosphere. Modifying the Tephigram The basic tool for stability analysis and consequently, for shower and thunderstorm forecasting, is the tephigram. Convective weather of all types is associated with an unstable sounding or can be expected to occur after dynamic and thermodynamic processes act to produce an unstable sounding. One of the most common techniques for first guessing the intensity of convective activity is selection and modification of a representative sounding. When a morning sounding is selected that could represent the airmass over a site of interest at forecast time, the 12Z profile must be Meteorologist Operational Internship Program 1 MOD_042D-2001-10-16.doc modified in the low levels for daytime heating. When a 00Z representative sounding is used, on the other hand, the observed profile can be considered roughly representative of daytime heating. In general, the 12Z representative sounding is commonly used as a forecast tool while the 00Z sounding is more useful as a diagnostic tool. To have greatest utility, the representative sounding (whether 12Z or 00Z) should be modified for processes other than daytime heating. When time permits, the most satisfying approach is the production of a prognostic tephigram. An alternative approach that has some utility is the estimation of stability changes at a site. It is a quick assessment of the thermal advections based on the current hodograph. By examining the hodograph, the forecaster can quickly assess stabilization and destabilization trends at the site and can adjust (qualitatively or quantitatively) the current sounding for these changes as well as for daytime heating effects. Diurnal and Seasonal Trends Diurnal trends must be considered when forecasting convective clouds and precipitation. As already mentioned, the most obvious diurnal trend is a maximum in convective activity near the time of greatest daytime heating and, in some locales, a secondary maximum near sunrise after greatest nocturnal cooling. In general, airmass thunderstorms tend to occur in the period from early afternoon to early evening with thunderstorms developing a bit later from about mid afternoon onwards. Frontal showers or thundershowers, on the other hand, may occur at any time although they may be more violent and widespread in the late afternoon to early evening due to the added contribution of daytime heating. Given an airmass situation, Figure 1 is a fairly common summertime sequence of convection on a day when the air is rather convectively unstable and diurnal heating is of significance. Figure 1. The intensity of the convection, the rate at which it develops, and the areal coverage or frequency (i.e. scattered or broken, isolated, widespread) all depend upon the degree of instability and the amount of moisture available. When the airmass is relatively dry, the timing of the convection will be delayed considerably and the convection will be much decreased in intensity and frequency. When daytime heating wanes, as in the autumn months, airmass convective activity tends to be less intense and less widespread as well as delayed to the point of perhaps not reaching the shower or thundershower stages shown in the above sequence. In late spring, however, airmass convection can sometimes develop surprisingly rapidly and become intense and widespread. Forecasters sometimes forget the potential for airmass convection in Meteorologist Operational Internship Program 2 MOD_042D-2001-10-16.doc spring; at this time, solar insulation increases rapidly and low level moisture often is abundant while the airmass aloft may be quite cool and convectively unstable. In many sections of the country, a significant secondary maximum in airmass convective activity occurs near sunrise. This secondary maximum reflects radiative cooling at cloud top levels (usually mid levels). The ACC and TCU associated usually develop a couple of hours before or near sunrise and dissipate by midmorning. In climatologically susceptible regions, early morning ACC can be suspected if scattered to broken AC was present overnight or if significant convection the previous day dissipated during the evening or overnight. Sometimes, AC and ACC can appear to suddenly form near sunrise, even though little cloud was observed previously (a phenomenon which also could be related to the observation). Synoptic Correlations Techniques Synoptic correlation techniques have limited value in forecasting the development of convection. However, once the convection has formed and can be tied to a synoptic feature, the activity often can be forecast in the short term through association with the correlated features. When the convective activity has not yet formed, synoptic correlation techniques cannot really be applied unless, of course, it is assumed that yesterday's correlated convective patterns will also apply later today. Considering the limitations inherent in synoptic correlation techniques (i.e. that the correlators are significant and that the correlations will persist), and considering the complexity of the thermal and dynamic influences that produce convective activity, much caution is advised if synoptic correlation is used to forecast development. A few situations, however, can lend themselves to prediction by synoptic correlation techniques with reasonable success. Most of these situations involve dynamically strong synoptic features. An active cold front, for example,