Cupertino, CA 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

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.

Aviation

(06Z TAFS) Issued at 955 PM PDT Tue Oct 20 2026

VFR with increasing high clouds and light winds through the next few hours. A low pressure system offshore will bring showers to the area on Wednesday with a slightly delayed onset time compared to the previous forecast. As before expecting a main rain band to come up from the Central Coast through Wednesday with convective activity behind the band. Low confidence (20-30% probability) for thunderstorm activity along the coast south of MRY. Confidence in the wind speeds beyond the next few hours remains low, as hinted at by the large differences in the wind field output from subsequent runs of the high resolution models. With the low pressure system so close to shore, the wind pattern may very well depend on how the low travels as it meanders off the coast and how it interacts with local terrain. Any thunderstorm activity on Wednesday is also liable to bring localized strong gusts and visibility reductions to the immediate area.

Vicinity of SFO... VFR with high clouds and light winds continues into the overnight hours. A band of showers arrives Wednesday morning and afternoon followed by a period of more scattered showers. . Low confidence in the winds on Wednesday as high resolution models are diverging on the direction of the flow, which could depend critically on the evolution of the low pressure system.

SFO Bridge Approach... Similar to SFO.

Monterey Bay Terminals... VFR with high clouds and light drainage winds through the overnight hours. Isolated showers will start to build early Wednesday morning before an organized band arrives later that morning, bringing the chance for MVFR ceilings along with it. Low confidence in the winds throughout Wednesday, and for a isolated thunderstorm (10-20% probability) to impact the terminals Wednesday.