418 BULLETIN AMERICAN METEOROLOGICAL SOCIETY Forecasting Summertime Shower Activity at Grand Junction, Colorado WOODROW W. DICKEY U. S. Weather Bureau ABSTRACT Due to the lack of a dense network of reporting stations in the Grand Junction, Colorado area and the inadequate representation of shower activity by observations of rain at a single rain gage, certain arbitrary criteria were set up to define a "shower" day at Grand Junction. Relationships of a number of meteorological variables to shower activity were determined and the four variables which showed the strongest relationships were combined graphically to form an objective forecast aid for determining the probability of shower activity at Grand Junction during the 18-hour period 1130MST to 0530MST. Tests on independent data confirm the relationship found in the developmental data. INTRODUCTION accuracy equal to those that have been issued in the past. HIS study was initiated with the objec- tive of providing one of a number of DEFINITION OF A SHOWER DAY AT objective aids for forecasting various T GRAND JUNCTION weather elements in the Grand Junction, Colorado area. Specifically it was aimed at forecasting Summertime shower activity even in a rela- July and August shower activity during the pe- tively small area is not adequately represented by riod 1130MST to 0530MST the following morn- observation of rain at a single rain gage. Lack- ing, the forecast to be based on data available no ing a dense network of reporting stations in the later than 0530MST of the forecast morning. Grand Junction area, an attempt was made to Since the aid was to be used for the generalized obtain a more representative picture of shower weather forecast which is normally released to activity by inspecting the remarks entered on the the public, no attempt was made to pinpoint the WBAN Form 1130's and setting up certain arbi- trary criteria for deciding if the day were a forecast with respect to time, and a deliberate "shower day" or not. The criteria for a shower attempt was made to keep the procedure as sim- day decided upon are as follows: ple as possible. As it turned out the least com- plicated variables tested showed the strongest 1. Observed precipitation at the station, in- relationship to shower activity. cluding traces. The graphical techniques used to relate the 2. Thunder heard at the station, but no pre- variables to the element to be forecast have been cipitation recorded. explained in great detail in numerous papers on 3. Showers in sight, but no precipitation re- objective forecast aids in recent years, so will not corded at the station (for example: RW SE be dwelt upon here. One advantage of the graph- Quad, or RW west, etc.). ical techniques which might well be stressed again, Remarks of towering cumulus or cumulonimbus however, is that they usually not only result in in certain quadrants or all quadrants, and obser- a fairly accurate yes or no forecast, but in addition vations of virga or distant lightning were not indicate the reliability of the forecast. Or in other considered as shower activity in the immediate words a probability statement can, if desired, be vicinity of Grand Junction. These criteria re- attached to the categorical forecast. An addi- sulted in 133 days being designated as shower tional value of objective aids in a local forecast days during the 248 days of the 8 months of office is that if for some unforeseen reason a new July and August 1947-1950 inclusive, which com- forecaster must be sent to the station on short prised the developmental data upon which the notice, forecasts of this particular weather ele- study was based. The shower days were dis- ment can be made by the new forecaster with an tributed according to the above criteria as follows: Unauthenticated | Downloaded 09/26/21 01:53 PM UTC VOL. 37, No. 8, OCTOBER, 1956 419 1. Precipitation observed at the station: Measurable precipitation (.01 inch or more) 33 days Trace of precipitation 50 days 2. Thunder heard at the station but no pre- cipitation 14 days 3. Showers in sight 36 days Total 133 days VARIABLES INVESTIGATED A number of meteorological variables which have been found useful in the problem of shower and thunderstorm forecasting were investigated to determine their relationships to shower activity in the Grand Junction area. These variables included: 1. Upper air moisture, both at the station and "upstream" 2. Surface moisture, both at the station and "upstream" 3. Measures of instability 4. Height of convective condensation level and FIG. 1. Graph for determining "upstream" radiosonde height of freezing level station. Upper Air Moisture labeled with the "upstream" station to be used when the point determined by the 700 mb height In a similar study to this one for Salt Lake differences falls in that sector. At the reference City, Williams [ 1 ] found that the minimum station thus determined, the 2000M sounding was spread between the temperature curve and the inspected and the minimum spread between the dew point curve in the 700-500 mb layer at an temperature and dew point curves in the 700-500 upstream radiosonde station showed a useful cor- mb layer tabulated. TABLE 1 shows the relation- relation with showers at Salt Lake City. Wil- ship of this minimum spread to the frequency of liams picked the upstream station by noting the occurrence of subsequent shower activity at Grand wind direction at 12,000 feet over Salt Lake City Junction. (For brevity the minimum difference and then going directly upstream and selecting between the temperature and dew point tempera- the closest raob station. A slightly more gen- ture has been called "Minimum Dew Point eralized method of selection was used in this study Spread.") The minimum spread in the 700-500 partly because it was felt a single wind over the mb layer at Grand Junction itself was also tabu- station would not necessarily indicate the general upper air flow and partly because sufficient wind TABLE 1 data at the 12,000 foot level were not readily RELATIONSHIP OF MINIMUM DEW POINT SPREAD IN THE available. This method consisted of determining LAYER 700-500 MB AT 2000M AT AN UPSTREAM REFER- a direction of flow by obtaining a measure of the ENCE STATION TO THE FREQUENCY OF OCCURRENCE E-W and N-S components of the height gra- OF SHOWERS AT GRAND JUNCTION. dient at 700 mbs over the plateau area from the Class intervals of minimum No. of Frequency of 2000M (0300Z) chart. Height differences were Total cases dew point spread shower cases occurrence of obtained between Ely, Nevada and Denver, Colo- °C showers % rado (ELY minus DEN) for a measure of the E-W component, and between Lander, Wyoming 0-2 20 16 80 3-4 54 42 78 and the average of Phoenix and Albuquerque, 5-6 36 21 58 New Mexico (LND minus AVG) for the N-S 7-8 20 11 55 component. These differences were used as co- 9-10 22 9 41 11-12 25 ordinates of a graph as shown in FIGURE 1, where 9 36 >12 71 25 35 the graph is divided into sectors with each sector Unauthenticated | Downloaded 09/26/21 01:53 PM UTC 420 BULLETIN AMERICAN METEOROLOGICAL SOCIETY TABLE 8 TABLE 3 RELATIONSHIP OF MINIMUM DEW POINT SPREAD IN THE RELATIONSHIP OF 0530M DEW POINT AT GRAND JUNCTION LAYER 700-500 MB AT 2000M AT GRAND JUNCTION TO TO FREQUENCY OF OCCURRENCE OF SHOWERS AT GRAND FREQUENCY OF OCCURRENCE OF SHOWERS AT GRAND JUNCTION. JUNCTION. Class intervals of Frequency of Total casse No. of Class intervals dew point °F occurrence of Frequency of shower cases of minimum No. of showers % Total cases occurrence of dew point spread shower cases showers % °c >55 21 19 90 51-55 46 35 76 0-2 40 35 88 46-50 49 31 63 3-4 72 45 63 41-45 44 25 57 5-6 54 30 56 36-40 49 16 33 7-8 34 15 44 31-35 21 5 24 9-10 22 4 18 <31 18 2 11 11-12 18 2 11 >12 18 2 11 activity in this region is associated with high surface dew points and since the dew point tem- lated and the relationship to subsequent shower perature is more representative of the moisture activity is shown in TABLE 2. Surprisingly, the content of the air than the random occurrence relationship of the Grand Junction spread appears of showers, the 0530M dew points at three sta- to be stronger than the spread "upstream." tions to the southwest of Grand Junction were It might be argued that the method of selecting tabulated. These stations were Milford and the reference station does not depict the right Blanding, Utah, and Grand Canyon, Arizona. source of moisture in many cases, which is un- The average of these three dew point tempera- doubtedly true. However, due to the wide sepa- tures was then related to subsequent shower ac- ration of the reference stations geographically, tivity at Grand Junction as shown in TABLE 4. there would be very few differences in the refer- This variable showTs the strongest relationship to ence stations selected regardless of the method shower activity of any tested. used to select them. A limited check on two months data using the 700 mb wind at Grand Measures of Instability Junction, the 500 mb wind, and following the 500 mb contour through Grand Junction up- In a study of September thunderstorms at stream to select the reference station showed that Denver, Colorado, Tillotson [2] found a fairly all methods used picked the same reference sta- strong relationship between thunderstorms at tion around 90 percent of the time.