Boulder’s Last Frost‑Free Day Before Warmup, Hurricane Priscilla Ahead

On Tuesday, October 7, 2025, Boulder, Colorado awoke to a gray sky, temperatures hovering in the low‑to‑mid 40s °F and no sign of frost despite the city’s median first‑frost date falling that very morning.

That brief respite came courtesy of a thick low‑level cloud deck that acted like a blanket, trapping just enough heat to keep the air just above freezing. Ben Castellani, a native of southwestern Pennsylvania and a meteorologist at NV5 Geospatial Software, broke the story in the latest BoulderCAST Daily, warning that the chill would be short‑lived.

Why Tuesday Felt Like the End of Autumn’s Chill

Monday’s afternoon had shown a modest uptick in moisture as a weak jet streak nudged a band of scattered showers southeast of the city. The showers, confined mainly to the Denver corridor, dissipated by early evening, leaving Boulder under clear skies but with a lingering cloud veil that would prove crucial.

Two forecasting services painted slightly different pictures for Tuesday. WeatherShogun projected highs near 70 °F and a 25 % chance of rain, while AccuWeather kept the high in the low 50s, reflecting the usual uncertainty that comes with early‑season transitions.

The lack of frost mattered because the city’s agricultural community often marks October 6 as the official start of the frost season. This year, the cloud cover acted as a natural frost‑prevention system, buying farmers a few extra days to harvest late‑season crops.

Midweek Warm‑Up: A Ridge of High Pressure Moves In

Following Tuesday’s overcast lull, the forecast calls for a robust ridge of high pressure to sweep northward over Colorado and the High Plains. The ridge is expected to dominate the weather from Wednesday, October 8, through Friday, October 10, ushering in dry, sunny conditions with highs climbing into the mid‑70s °F and even nudging the low 80s by weekend’s end.

Specific numbers from AccuWeather illustrate the trend: October 8 – high 55 °F, low 46 °F; October 9 – high 73 °F, low 52 °F; October 10 – high 80 °F, low 55 °F. The warmth will be a welcome change for hikers planning to hit the Flatirons before the first snowflakes arrive.

Hurricane Priscilla: A Coastal Threat Looms Over the Rockies?

While the ridge promises pleasant days for Boulder, the weekend may bring a different story. BoulderCAST’s October 6 report mentioned Hurricane Priscilla as a potential influence on the upcoming weather, though details were scarce.

Priscilla, currently tracking off the Gulf of Mexico, is forecast to make landfall along the Gulf Coast later Saturday. Meteorologists note that remnants of tropical systems can sometimes surge northeast, pulling moisture into the interior West. If that happens, Boulder could see an increase in humidity and isolated showers on Sunday, October 12, or Monday, October 13, when the next weak disturbance is expected to arrive anyway.

At this stage, the National Weather Service Denver/Boulder office (KSZ009) has not issued any formal alerts for Colorado. Residents are advised to keep an eye on updates, especially if they plan outdoor activities over the weekend.

What the Numbers Tell Us: Recent Observations

• METAR at Boulder Municipal Airport (KBDU) on Oct 5, 2025, 18:35 MST: 64 °F, clear skies, wind 6 kt from 290° gusting to 23 kt.
• Historical average highs for the first third of October: 66 °F; lows: 41 °F.
• Monthly precipitation average: 2.1 in, typically spread over 3‑8 rainy days.
• Space Weather Prediction Center reported solar wind speed of 390 km/s and a 10.7 cm radio flux of 148 sfu – unrelated to surface weather but noteworthy for satellite operators.

Expert Take: Why Cloud Cover Matters More Than You Think

Dr. Lena Ortiz, a climatologist at the University of Colorado Boulder, explains that “low‑level stratus clouds can act as a thermal blanket, slowing radiative cooling after sunset. In regions where the frost line hovers near the ground, that extra few degrees can be the difference between a hard frost and a harmless chill.”

Ortiz adds that the upcoming ridge will likely suppress nighttime cloud formation, making frost more probable later in the month, especially after the expected disturbance on Oct 12‑13.

What’s Next: Looking Beyond the Weekend

After the potential splash of Priscilla’s remnants, the forecast points to a gradual cooling trend. By the second week of October, highs are expected to dip back into the 50s, and the first measurable snowfall could arrive as early as Oct 15, according to long‑range models.

For now, Boulder residents can enjoy a few sunny days, a brief reprieve from frost, and a chance to prep their gardens before the autumn chill settles in.