Cupertino, CA 95015 Weather

Key Messages

Updated at 801 PM PDT Tue Oct 20 2026

- Cooling trend continues through the end of the week

- Beneficial light rain showers expected Wednesday through Thursday

- Thunderstorms possible Wednesday afternoon into Thursday morning across Central Coast

- Warm and dry next weekend

Update

Issued at 939 AM PDT Wed Oct 21 2026

We are starting to see the some of the first rain bands moving into the north Central Coast this morning as the core of the cut-off upper level low approaches the region. There aren't currently any lighting strikes as of yet across the region this morning. However, we are expecting that to change as we head into late morning and moreso this afternoon and evening. This is as instability increases with up to 400-800 J/kg of MUCAPE (Most Unstable Convective Available Potential Energy). The greatest instability is forecast to remain over the north Central Coast and points southward. All eyes remain on the burn scars of the Timber and Plaskett fires given the threat for debris flows. Rain rates across these regions matter and the scattered nature of these showers and/or thunderstorms gives us less confidence of exactly how much rainfall will occur over any given one area. Please see the AFD for the Short Term for additional details.

RGass

Short Term

Issued at 1230 AM PDT Wed Oct 21 2026 (Today and tonight)

The center of the 500 mb cut-low is inching closer to the coast as the sub-tropical jet stream supports the left portion of the system, bringing enhanced momentum and moisture flux. This is where the most instability and lift can be found. As of 11:30 PM PDT, KMUX radar is picking up on a few cells offshore exhibiting a ~10-25% chance of lightning with a few lightning strikes observed along the occluded front (~250-300 miles SW from Monterey). The current offshore activity is foreshadowing what is to come early Wednesday morning.

The first few rain bands are expected to make their way over the Central Coast just before sunrise. Going into the afternoon, precipitation gets wrapped around the system's center and eventually spreads through the rest of the Bay Area. Precipitable water is expected to rise above the 90th percentile for this time of year, which prompted WPC to include the southern Central Coast in a marginal risk of excessive rainfall for Wednesday.

To further assess the potential for severe thunderstorms, three high- resolution model soundings were analyzed: the 3km NAM, 10km RDPS, and the RRFS. Keep in mind that the NAM tends to be the most pessimistic model out of the three in terms of moisture. Upon evaluation of the near-storm environment for each, there are a few inconsistencies across the models in terms of the severe thunderstorm threat and whether the environment is conducive to tornado spin-ups.

Before getting into what *is* consistent across the models, let's run through a quick overview of a couple notable stability indices: the Total-Totals and K index. The Total-Totals index measures atmospheric instability by taking the thermal difference between 850 to 500 mb to identify the potential for severe thunderstorms or tornadoes. The K index is used to assess atmospheric buoyancy by measuring the low to mid-level moisture as well as the 850-500 mb thermal difference. The greater the K index value, the greater the potential is for widespread thunderstorms (K > 20 = scattered thunderstorms, K > 35 = numerous thunderstorms). Similarly, higher Total-Totals (TT) index values indicate an enhanced potential for severe thunderstorms and/or tornadoes (TT > 48 = severe thunderstorms, TT > 50 = tornadoes).

Why am I talking about stability indices? Well, according to these indices and other environmental parameters, the three high-res model soundings that were analyzed all indicate a heightened risk for at least scattered, severe thunderstorms along the Big Sur coastline. The NAM estimates TT ~ 56 and a K index ~ 36, RDPS estimates TT ~ 54 and a K index ~ 30, and RRFS estimates TT ~ 50 and K index ~ 31.

Long Term

Issued at 1230 AM PDT Wed Oct 21 2026 (Thursday through Tuesday)

The rain and thunderstorm threat are gone by Friday. With the trough to our east, building high pressure brings warm and dry conditions into early next week. Euro and GFS ensemble members suggest warming to near seasonal averages meanwhile precipitable water values are forecast to dip below the 25th percentile. Monday onward, there is a great deal of uncertainty on when the weak ridge breaks down. An energetic Central Pacific plus the most recent 6-10 day Climate Prediction Center precipitation outlook suggests another trough coming our way with the potential for more rain.