RES 2025054 - Laserfiche WebLink

The Current Fire Environment Wildfire behavior is governed by terrain, weather, and vegetation. Within the WUI/FHSZ areas in the City, the fire environment can be broadly summarized into two types: the WUI (Area 1) on the west side and the WUI (Area 2) on the south side (see Figure 2). A different make-up of terrain and vegetation characterizes each area. Both areas experience similar weather conditions regarding temperature and relative humidity, but each area has unique wind patterns. Terrain Terrain influences wildfire in two ways. First, fires burning uphill on a slope have higher rates of spread and higher intensities because the slope brings the flames closer to vegetation above it, preheating them and allowing them to ignite more readily. Second, the aspect influences how much sun exposure is received. South and west aspects receive the greatest exposure, which makes them the hottest and driest slopes and, therefore, the most readily available to burn. The WUI area's terrain on the City's west side (WUI Area 1 on Figure 2) is characterized by a long, east -facing slope that extends from Foothill Road to Pleasanton Ridge in the east -west direction and stretches from 1-580 to the southern boundary of the City in the north -south direction. While the slope aspect of this area is mostly east -facing, there is some minor variation in aspect due to the numerous drainages that run along the hillside. There is approximately 1,200 -feet of elevation difference between Foothill Road and the top of the ridge and the slopes average between 25% and 65% along the upper third of the slope (Google Earth, 2025). Numerous hills characterize the terrain in the WUI areas along the City's south and southeast sides (WUI Area 2 on Figure 2). These hillsides are generally composed of north -south running ridgelines (closer to 1-680) or east -west running ridgelines (closer to State Route 84), consisting of all aspects in this WUI area. These hills are considerably shorter than the Pleasanton Ridge, with only a 200 to 300 -foot elevation difference between the ridgeline and the adjacent valley. Slopes are similar to WUI Area 1, averaging about 25% with a maximum of 65% (Google Earth, 2025). Weather The City of Pleasanton's fire season typically begins in May when grasses and annual plants have dried and ends in October when significant precipitation usually occurs. The actual beginning and end of fire season can vary from year to year. The beginning of the fire season is dependent on how much precipitation occurs during the winter and spring and when the last wetting rain (at least a tenth of an inch of rain in or more) occurs. The end of the fire season is dependent on when the first wetting rains of the fall arrive. Precipitation is generally absent during fire season. The hottest and driest conditions occur July through September with a mean maximum temperature over 80'F and with mean minimum relative humidities below 10%. Winds are predominantly out of the west and west-northwest (WRCC 2024). Wind speeds vary depending on the location in the City. The WUI areas on the City's west and south sides experience pronounced diurnal winds during the fire season. On the City's west side, particularly in the hillsides below Pleasanton Ridge, a significant increase in wind speeds can occur in the late afternoon and persist until sunset. On the south side of the City, winds are primarily from the southwest, and also typically increase in speed during the early afternoon up until sunset (Altamont and Las Trampas RAWS, 2024.) 11 of 219