Flying to Aspen is not like flying to most U.S. airports. The runway sits in a steep mountain valley at 7,820 feet, and the weather changes fast. Some months the airport runs near perfectly. Other months, almost one flight in ten gets canceled or diverted.
This guide ranks every month from safest to riskiest, using flight completion data from the U.S. Bureau of Transportation Statistics (BTS) and from the airport's consultant Air Planners (as reported in the local press). If you have any flexibility on your travel dates, this calendar tells you when to book and when to keep a backup plan ready.
The Quick Verdict: A Tier List by Month
Here is the year at a glance. Completion rate means the share of scheduled flights that actually landed, instead of being canceled or diverted somewhere else like Eagle (EGE), Grand Junction (GJT), or Denver (DEN).
| Month | Typical Completion | Delay Risk | One-Line Reason |
|---|---|---|---|
| January | 90–94% | High | Heavy snow, low clouds, holiday traffic spillover |
| February | 89–92% | Highest | Peak ski season; storms stack up |
| March | 92–96% | High | Spring blizzards, busy weeks around spring break |
| April | 95–98% | Moderate | Shoulder season; weather still mixed |
| May | 97–99% | Low | Mud season; light schedule, airport closures |
| June | 94–97% | Moderate | Afternoon storms start; valley winds pick up |
| July | 94–97% | Moderate | Thunderstorms in the afternoon |
| August | 94–97% | Moderate | Continued storms; gusty afternoon winds |
| September | 99–100% | Lowest | Clearest, calmest stretch of the year |
| October | 99–100% | Lowest | Quiet schedule, dry weather pattern |
| November | 96–99% | Low | Pre-season; early storms occasionally hit |
| December | 88–96% | Volatile | Wildcard month; one bad storm can meet peak traffic |
Sources: Bureau of Transportation Statistics; Aspen/Pitkin County Airport operational reports; Air Planners / Fly Aspen Snowmass data, as reported in the Aspen Times and Aspen Daily News (2022–2025). Ranges reflect year-to-year variation.
The Best Months: September, October, and May
If your only goal is to actually land in Aspen, the answer is simple. Fly in September or October. The Aspen Times reported that the airport posted a 99.8% completion rate in September 2025 and a full 100% in October 2025, after a bumpy summer. A perfect month is rare anywhere in commercial aviation. At a mountain airport, it is almost unheard of.
Two things line up in the fall. The afternoon thunderstorm season has ended, but the winter storm track has not arrived yet. And the flight schedule is light, because the ski season has not started and summer events are over. Fewer flights also means fewer chances for one delay to cascade into the next.
May is the quiet shoulder choice. Locals call it mud season. The mountain is closed, the town is empty, and the air is calm. Hotels and flights are cheap. If you can travel during mud season, you may never see a delay.
Why September and October Are So Reliable
Three factors line up. The North American Monsoon, which pushes moisture into Colorado from July through early September, is over. Winter storms typically do not reach the Roaring Fork Valley until mid to late November. And the daily temperature swings that drive valley winds are smaller in fall than in summer. The result is long stretches of clear skies and light winds. Pilots get the easy flying conditions they prefer, and dispatchers do not have to scrub flights.
The Worst Months: February, December, January, March
Now the hard part. February is, on average, the toughest month of the year. The Aspen Times reported that February 2023 came in at just an 88.9% completion rate, with February 2024 improving to about 92%. So in a typical February, roughly 8 to 11 flights out of every 100 do not arrive as scheduled. That is roughly one bad day per week.
Why February? It is the heart of ski season, which means peak passenger volume. It is also when Colorado gets its heaviest snowfall, and ASE sits in a narrow valley where snow squalls can drop visibility below the legal minimum in minutes. Add holiday weekends like Presidents' Day, and the schedule cannot absorb even one weather closure without delays piling up.
December is the wildcard. The Aspen Times reported that December 2022 finished at 88.7%, while December 2023 jumped to 95.8%. December 2024 landed in between at 91% for the full month, although the core holiday window of December 17 through 29 ran cleaner at 96%, with 466 of 487 flights completed. The lesson: a single multi-day storm during Christmas week can swing the whole month. If you have a non-negotiable arrival date in December, fly in at least a day early.
January and March are not as bad as February, but they are close. January carries the holiday traffic crush into early month, then settles into pure winter weather. March still gets heavy snow, sometimes the wettest of the year, plus spring-break crowds. Spring storms can also bring strong westerly winds, which trigger the airport's wind limits more often than dry winter cold does.
The Middle Months: Summer Is More Hit or Miss Than You Think
Summer at Aspen looks safe on paper. The runway is clear. Days are long. But summer 2024 was a wake-up call. The Aspen Times reported the airport ran a 94.4% completion rate from June through August in 2024, well below the 97.5% average for those months over the prior four years. Two things drove the dip. The summer monsoon brought stronger than usual afternoon storms. And the FAA control tower switched from instantaneous to averaged wind readings, a change that the National Business Aviation Association confirmed went live at ASE and which caused more flights to be held when gusty conditions hit.
The practical takeaway for summer: book the earliest flight of the day you can. Morning air is calm and storm cells have not built yet. By 2 or 3 p.m. on a typical July or August afternoon, towering cumulus clouds form over the surrounding peaks. That is when diversions stack up.
April and November sit in the middle of the pack. They are shoulder months. The schedule is lighter than ski season, so any one delay hurts less. But weather is variable. April can bring late-winter snow, and November can bring the season's first real storm. Both months are usually fine. Just do not assume they will be.
Why the Calendar Looks This Way: Three Forces in Play
Three things drive the monthly pattern. Understanding them will help you read forecasts and pick smarter dates.
Weather: Aspen sees three distinct weather seasons. Winter storms from late November through April bring snow, low clouds, and reduced visibility. The summer monsoon from mid-July through early September brings afternoon thunderstorms with strong gusty winds. And the dry shoulder periods, late April through June and mid-September through mid-November, bring mostly calm air. You can dig deeper into the specific weather factors in our guide to why Aspen flights get delayed.
Schedule density: ASE has one runway and no spare aircraft on the ground. When the schedule is full, every storm forces hard choices. SkyWest, which operates 100% of commercial flights to ASE, runs busy schedules in ski season (December through March) and a second peak in summer (June through August). The light fall schedule absorbs disruptions better.
Aircraft limits: Only two aircraft types are certified to serve Aspen routinely: the Embraer E-175 and a single Bombardier CRJ-700 variant. Both have specific wind and visibility minimums that are tighter than at most airports. You can read the full story behind that restriction in why Aspen only flies E-175s. The short version: when the wind shifts or the clouds drop, these aircraft simply cannot land.
How to Use This Calendar When You Book
- Flexible travelers: Pick September, October, or late May. You will get the cheapest fares and the most reliable flights. If you ski, the first half of December and the last week of March are the most overlooked ski windows with better odds than February.
- Ski-season travelers: If you must fly in February or over Christmas, make sure you know your contingency plans. Arrive at least one day before any event you cannot miss. And consider a backup ticket into Denver or Eagle that you can switch to if conditions go sideways. Our diversion playbook explains the rebooking math.
- Summer travelers: Book the first flight out and the first flight in. By mid-afternoon, the odds drop sharply. If you are flying in for a Friday or Saturday wedding, fly in Thursday.
Real-Time Risk Beats the Calendar
The monthly averages are useful for picking dates. But on the day you fly, only the actual weather matters. Even September has bad days, while February has stretches of perfect blue sky. The KASE Weather dashboard shows live visibility, ceiling, and wind readings scored against the actual operating limits of the E-175 and CRJ-700. Green means the aircraft can land. Red means it probably cannot.
If you want a heads-up before your phone buzzes with a cancellation alert, watch for our email and SMS notifications launching with KASE Weather Premium. We push the alert when conditions cross from yellow into red, so you can rebook before everyone else hits the same delay.
Data Sources and Caveats
The numbers in this article come from:
- U.S. Bureau of Transportation Statistics (BTS): Airlines with at least 0.5% of domestic scheduled passenger revenue report on-time performance, cancellations, and diversions monthly. Data is available at transtats.bts.gov and the DOT delay-cause tool.
- Aspen/Pitkin County Airport reports: Annual passenger counts, schedules, and operational summaries on the airport's reports page.
- Air Planners / Fly Aspen Snowmass: The airport's flight-completion consultant. Their figures are quoted in the Aspen Times and Aspen Daily News.
- NOAA Climate Normals: Long-term monthly precipitation, snowfall, and temperature averages for Aspen Sardy Field are available through the NOAA NCEI Climate Normals portal.
A note on the ranges: completion rates shift year to year because mountain weather is variable. The percentages listed here reflect 2022 through 2025 observations. For a deeper look at the underlying numbers, see our full Aspen delay and diversion statistics post.