Pool season · Colorado
Longmont Pool Season: the Year on One Bar
Last updated: July 25, 2026 · dates · water temp
This is the one-page view of Longmont's pool year: both model windows, the estimated water-temperature curve, and today's live position on the season bar. The detailed checklists live on the opening and closing guides; this page is the calendar you check between them.
Longmont, CO — 105 swim days a yearEach dot on the bar is one week of Longmont's typical year, colored by what the model says that week is for: gray weeks are off-season, hollow teal marks the opening window (May 6–May 27), solid teal is swim season, and hollow amber is the closing window ending September 28. The black marker is today — its position against the colors is the entire executive summary.
The Longmont pool calendar
Opening — target May 13
Open by May 13 — a two-week head start on the May 27 crossing keeps startup chemistry small and beats the local service rush.
Closing — deadline September 28
Hold the cover until September 21, then winterize by September 28, a week ahead of the October 5 freeze normal. Late and cold beats early and warm.
Estimated water temperature by month
Monthly averages of the site's water model run on Longmont's own normals — the shape of a typical year, not a forecast. The widget above shows where the current year actually sits.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 29° | 30° | 37° | 45° | 52° | 63° | 71° | 72° | 68° | 56° | 44° | 33° |
The curve peaks near 73°F — comfortable-water months here usually involve a heater or a solar cover.
Every station tells its own story: compare this curve with Lakewood's pool season and Loveland's — neighboring normals stations can shift the windows by a week or more. The full ranking lives on the Colorado season index.
Longmont pool season FAQ
When should I open my pool in Longmont, CO?
The site model recommends opening by May 13 — about two weeks before Longmont's 7-day mean temperature crosses 61°F, while the water is still too cold for algae. The step-by-step sequence is in the Longmont opening guide.
When should I close my pool in Longmont?
Winterize by September 28. The model holds the deadline a week ahead of Longmont's NOAA 50% first-freeze date, so the plumbing is protected before the first hard night. The full sequence is in the Longmont closing guide.
How long is the swim season in Longmont?
Longmont averages 105 days a year with normal highs of 80°F or better — right on the statewide median. The Colorado season index ranks all 19 covered cities by the same metric.
How warm does pool water get in Longmont?
An unheated pool near Longmont typically peaks around 73°F in high summer and bottoms out near 29°F in winter, never quite reaching the 78°F comfort line without a heater or solar cover. The month-by-month curve above shows the whole cycle; the methodology page explains the ±5°F error bars.
Email me when Longmont hits the closing window
Data: NOAA 1991–2020 climate normals via Longmont 2 ESE (1.2 mi); live outlook by Open-Meteo. Attribution: sources · assumptions: methodology.