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Colorado Climate

I like to think in terms of a base climate about which typical variations occur. Colorado usually has dry, fair weather, and it's not unusual for a given location to go many weeks or months without any real precipitation. These long stretches are punctuated at intervals of varying length by periods when the weather seems "broken". This generally happens when we're stuck under the storm track, or when it's wiggling back and forth over us (which is most likely to occur near, or in the month following, the equinox). The chief symptom of this is an over-abundance of storms (duh) and things changing, sometimes wildly, from day to day, or sometimes hour to hour. Both the amplitude and the duration of variations can themselves vary.

• Mid-winter: I think of this period as being the one from about Christmas-time to Valentine's Day. It's fair and pleasant considering the time of year. The largely stationary high pressure system responsible for this can cover much of the western US. Highs at lower elevations can easily reach almost to 70 F, but in the mountain valleys cold air can accumulate and keep highs from going much above 0 F. When the storm track shifts to being overhead, there can be a succession of storms taking a day or two to pass thru, with a break of several days between episodes. These usually bring only light to moderate amounts of precipitation because the air is cold and can't hold much moisture.

• Late-winter/early spring: This period starts in the latter half of February and can continue until anywhere from the end of March to almost Memorial Day. It's one of the more reliably unsettled times of the year, corresponding to when the jet stream/storm track is moving back northward over the region. Very heavy snows can occur because the atmosphere is warmer and can hold more moisture. The Front Range is not infrequently buried by large, slow-moving low pressure systems a few hundred miles to the south, creating textbook upslope conditions; the western slope can be largely unaffected in these circumstances, unless the low pressure heads more directly across the state.

• Spring: I don't think of Colorado as having this season. It seems like we usually go straight from late-winter into summer. Nice summery weather can occur as early as mid-March, but usually isn't prevalent until May -- I always think of April as the month when spring doesn't arrive. By June it seems almost too summery to call it spring. Maybe the biggest transition is at the lower elevations, where first the snow that comes with the storms doesn't last very long, and then later comes down as rain rather than snow. In between storms it just might as easily be coolish and blustery as nice and spring-like.

• Early-summer: This period consists of the nice warm weather that we start to see more of from mid-April into May and continues until the summer monsoon season starts any time after the 4th of July. The period around the end of June or the beginning of July can see the warmest weather of the year. Warm moist air backing in from the midwest and colliding with cold fronts coming down from the north and northwest can spawn violent thunderstorms any time in May or June. There can still be enough cold air aloft to cause there to be a lot of hail with these storms.

• Mid-summer: The mid-summer period consists of the monsoon season and is characterized by sunny warm mornings with afternoon and evening thunderstorms. The main variable is in how early in the day the storms pop up and whether the rain is spotty or widespread, and whether it's light or heavy. This period starts any time after the 4th of July, usually about mid-July, and lasts until about Labor Day. It's principle signature is a large high pressure system parked over the western end of the Gulf of Mexico pumping moisture up thru Mexico and into the US southwest; a diurnal thermal low over approximately Death Valley also helps pump the moisture into Colorado. I've seen summers when this pattern appeared before the solstice or was weak or almost entirely absent. At it's worse, it can be cold and rainy for days on end, which hardly seems summery. The end of the monsoon season is typically marked by the breakdown of the high pressure system mentioned above, which also opens the door for the beginning of the hurricane season in the Gulf and Caribbean.

• Late-summer/early-fall: Sometime towards the end of August the monsoonal moisture flow diminishes and we start to have fewer, later, and/or less intense PM thunderstorms. It can still seem quite warm in the mid-to-late afternoons, but it's nowhere near as hot as nearer the solstice. With the drying out of the atmosphere and the longer nights it can start to seem downright nippy at bedtime and in the mornings. Many people consider this the nicest time of the year, and September can certainly see some of those beautiful, calm, "golden days of summer", the ones without much threat of serious weather. This period can easily last until Halloween, with diminishing temperatures and an increasing chance of a Pacific autumn storm coming in from California or the Northwest. The large high pressure system over the western US can make this a nice time of year over a large region.

• Fall/early-winter: Just as it's debatable whether we really have Spring or not, we frequently go from summer to winter without much of an autumn. So it's common to think of Fall occurring when the first winter storms appear. This corresponds to the jet stream/storm track dropping back down south, bringing in storms as it's overhead; these storms form out in the Pacific and usually hit the Pacific coast somewhere between northern California and southern British Columbia on their way in. This pattern typically starts to develop around mid-October, but it can be seen as early as a month (or more) sooner. It's really a winter-like pattern which we get tastes of varying degrees of, depending on whether the jet stream just dips down or parks overhead, or stays north.

• Winter: I think of this period as lasting from about mid-October or Halloween to about Christmas-time. It's when the storm track is most likely to be of concern. We can get either cold air coming down from the north or a lot of moist air coming in from the Pacific, and sometimes both together. I tend to think of November and most of December as being the time when, after a deterioration, the weather only improves to coolish and blustery. Good weather is cold (but not frigid) but fair and calm, caused by high pressure descending from up near the arctic.