Precipitation

As discussed previously, precipitation in the White Mountain area results primarily from the passage of cyclones with associated fronts during fall, winter, and spring; from closed cyclones in late winter and spring; and from the flow of moist tropical air from the southeast to the southwest quadrant in the summer. Annual amounts vary from 5–6 in (125–150 mm) on the valley floors to 20 in (508 mm) or a little more at the highest elevations. Totals appear to increase right up to the crest of the range. The rate of increase averages about 1.5 to 2.5 in per 1,000 ft (120–205 mm per km) rise. However, this average is difficult to apply to any one portion of the range, and the increase is not linear, being higher at upper elevations. Table 1.3 gives average monthly and annual precipitation amounts for stations within the region.

From west to east in the White Mountain area, there are important differences in the seasonal distribution of precipitation. Bishop, on the west, has the typical regime of most California stations: winter wet and summer dry, with January the wettest month. White Mountain I and White Mountain II have precipitation much more evenly distributed throughout the year. At both mountain stations January is the wettest month, but only by a slight margin. There is no pronounced dry season; June is the driest month, reflecting the gap between the cyclones of winter and spring and the thunderstorms of July and August. Early fall is relatively dry, with a gradual buildup of precipitation to the winter months. Deep Springs, at the southeast edge of the White Mountains, has maximum precipitation amounts in January and February, with a minor peak in July and August, and minimum amounts in June and October. On the east margin of the White Mountains, Dyer has a slight maximum in spring and mid-summer and a minor minimum in December and January. At lower elevations, the western slope of the White Mountains is relatively open to cyclones from the west in winter, partially subject to closed cyclones from the north in the spring, and somewhat protected from thunderstorms in the summer. The eastern slope, in the double rain shadow of the Sierra Nevada and the White Mountains, is protected from winter cyclones but is more open to closed cyclones in spring and thunderstorms in summer. Upper elevations in the White Mountains are relatively open to all three types of storms and show a trimodal maximum of precipitation. Thus, the White Mountain range is truly transitional in seasonal distribution of precipitation between the winter maximum of California and the Sierra Nevada and the more even annual distribution of the eastern Great Basin and Rocky Mountains.

Yearly precipitation totals not only increase with higher elevation in the mountains but very likely are larger in the northern part of the range. There are no station

records to substantiate this assertion, but the experience of many long-time residents of the area and of both writers suggests that the portion of the range from White Mountain Peak north to Boundary Peak receives more precipitation from cyclones and thunderstorms than the region south of the main peak. Occasional measurements made with a standard snow sampler at comparable elevations show greater depth and water content in the snowpack in the northern part of the range. There is also higher streamflow, more extensive former glaciation, and a less xerophytic vegetation north of White Mountain Peak. In the nearby Sierra Nevada, snow survey records show a general decrease in precipitation from north to south, reflecting a lower frequency of passing cyclones. This could also affect the White Mountains. Moreover, the crest of the Sierra Nevada is lower opposite the northern half of the White Mountains than opposite the southern half, and this may allow more moisture to reach across to the northern segment of the White Mountain Range. Still another possible effect of the Sierra Nevada is that, as previously mentioned, fronts may be retarded in crossing the massive barrier of that range, bringing in cooler air from the north and northeast, which may strengthen the fronts and increase precipitation in the northern portion of the White Mountains.

Empirical observation also indicates that the buildup of cumulonimbus clouds in summer thunderstorms is more likely to occur over specific portions of the summit upland than at random. Topographic influence on air moving into the area from characteristic directions is the probable cause. This could add a checkerboard pattern of precipitation distribution independent of more general patterns, such as the increase with elevation and from south to north. Four areas of cloud concentration are noticeable. From south to north, these are Sheep Mountain-Piute (or Paiute) Mountain, the plateau just south of White Mountain Peak, Chiatovich Flats and the area just north of the Cabin Creek-Birch Creek saddle, and the northern portion of Pellisier Flats at the head of Chiatovich Creek. Common features of the four areas are rises in elevation from south to north and broad lateral extent from west to east. Cumulonimbus clouds may form over any part of the range on any summer day, and during extensive storms all or most of the higher elevations may be cloud-covered, but initial formation and greater subsequent development more commonly occur over these four areas.

There are significant departures from normal in amounts of precipitation from month to month and year to year at all elevations in the White Mountains. Most weather stations in the United States use the calendar year in calculating annual amounts. This causes problems in much of California, with its winter-wet, summer-dry regime, and in high-mountain regions, where much of the significant precipitation falls as snow. Thus, the annual snowpack begins in the fall of one calendar year and builds to a maximum in late winter or spring of the next calendar year. Most California stations use a 1 July–30 June precipitation year to avoid this problem. An even better breakdown for the White Mountain Range is to use the water year employed by many hydrologists — 1 October–30 September. This has the advantage of including snow buildup and important July and August precipitation in one annual total, thus giving a more accurate figure of the water available for streamflow and plant growth, much of which occurs from July to September.

The following discussion uses the 1 October–30 September year to show extremes and variation from normal (see Table 1.4). Thus, the year mentioned in the table ends on 30 September and includes the precipitation from October through December of the previous year. Bishop and, by inference, the lower western slope of the range show the largest departures from normal, with the wettest year (17.28 in or 43.9 cm) in 1969 and the driest (1.68 in or 42.5 mm) in 1960. This is a range of 308% to 30% of average. Fish Lake Valley and Deep Springs Valley to the east and southeast show less variation, with both Dyer and Deep Springs ranging from about 190% to 40%. White Mountain I had maximum precipitation in 1967 (26.59 in, or 67.55 cm) and minimum in 1960 (5.57 in, or 14.15 mm), a range of 206% to 45%. At White Mountain II the high total was 33.56 in (85.35 cm) in 1967, and the low was 9.51 in (24.15 cm) in 1960, a range of 187% to 53%. It is to be expected that Bishop, with its low annual average, would have a greater variation from normal than the mountain stations, with higher averages. But Bishop also varies more than the other lowland stations. Bishop and the lower western slope of the range receive most of their rain and snow from winter cyclones, and precipitation totals reflect seasons of frequent or sporadic passage of such storms. The eastern valleys and lower slopes get relatively more moisture from spring and summer storms, and the upper elevations are more open to precipitation in all three seasons. At all stations 1967 and 1969 were very wet, and 1960 was the driest year. Unfortunately, records for the obviously wet years of 1982 and 1983 are incomplete or missing at the mountain stations; both years brought high totals to lowland stations. It is noteworthy that neither White Mountain I nor White Mountain II was very dry in 1976 and 1977, critical drought years in central California. At both stations spring and summer precipitation partially

made up for winter deficiencies. It seems probable that higher elevations in the White Mountains are less subject to either very dry or very wet years than the neighboring Sierra Nevada and Owens Valley; there is less chance that all three types of storms will be common or rare in any one precipitation year.

Extreme monthly totals at Bishop vary from 8.93 in (22.7 cm) in January 1969 to 0.00 for all months of the year. At Dyer, the extremes are 3.44 in (8.75 cm) in August 1983 and 0.00 for all months; at Deep Springs, totals vary from 4.86 in (12.35 cm) in August 1983 to 0.00 for all months. At higher elevation, White Mountain I shows a maximum of 7.53 in (19.1 cm) in December 1966 and a minimum of no rainfall or a trace for all months but January; White Mountain II shows a high at 8.55 in (21.7 cm) for December 1966 and a low of no rainfall or a trace for all months but February. However, summer thunderstorms at high elevation have certainly exceeded these monthly totals. In part of July 1955, one of the authors (D. R. Powell) measured over 11 in (28.0 cm) in a standard rain gauge at Chiatovich Flats, between 10,000 ft (3,050 m) and 11,000 ft (3,350 m), 8.48 in (21.55 cm) of which fell in 2 1/2 hours on July 23. This is the greatest 24-hour total yet recorded in the White Mountains, although it probably has been approached or exceeded during other summer thunderstorms in the range. At the two mountain stations, maximum summer 24-hour totals are about 2 in (50 mm). It is evident that neither station has yet been in the direct path of the most intense thunderstorms. White Mountain II has a winter high 24-hour sum of 4.40 in (11.2 cm) on 6 December 1966, and White Mountain I recorded 3.80 in (9.65 cm) on the same date. Dyer and Deep Springs each show about 2 in (50 mm) for the maximum daily total for any season; Bishop has received more than 3 in (7.6 cm) in each of the three winter months, from Pacific cyclones.