3 PM MDT, Monday, May 27, 2019

As if May didn’t have enough cold and wet days already, we are about to get another. After showers and thunderstorms this afternoon/evening (some severe, especially up toward Wyoming and Nebraska), we are in for a 36 hours period of much below average temperature with mountain snow and low elevation rain. It may be Thursday or Friday before temperatures return to average late May levels.

There is likely to be some long intervals of clouds and rain (maybe some heavy thundershowers tonight and Tuesday) along with some drier periods. The snow level will probably inch its way down the foothills to at least 7000 feet. It is not expected to snow in the lower elevations, but it may get close. Widespread temperatures in the 30s are likely Wednesday morning, and if it clears out, some of the normally colder spots along the Front Range may see light frosts. The record low of 30 in Boulder is probably going to hold firm.

5:30 AM MDT, Sunday, May 26, 2019

In my blog on Friday I mentioned the very small chance for a late day thunderstorm today. Although the chance is still small (about 1-in-3) for the Front Range region, it looks more likely than it had appeared. A few of the thunderstorms may produce brief high wind and damaging hail. The chance for severe thunderstorms (and tornadoes) increases as you go east from Denver toward the Kansas border. Until those develop later it should be party cloudy (there are some areas of low clouds as moisture moves in from the east).

Memorial Day should bring another chance for thunderstorms, although it looks like the main threat will shift north and east into southeastern Wyoming and Nebraska. Some severe thunderstorm activity is possible in those areas.

The work week will likely start out cooler and maybe a bit showery on Tuesday, then drier on Wednesday.

5:30 AM MDT, Friday, May 24, 2019

Sunny weather and warmer temperatures return today after an unusually persistent cool spell for this time of year. The weekend looks springlike too. I can’t completely rule out a stray shower Saturday or Sunday, but most areas are likely to stay dry with mainly sunny skies and low elevation temperatures in the 70s.

Monday will probably continue the mainly sunny and dry weather, but that day is a little more questionable. The next change to cooler and more showery weather will probably arrive Monday evening, but the timing is still a bit uncertain.

Where the recent cool snap ranks historically

The following statistics are for the Boulder climate station. There was one record low maximum (43 on May 21st), and we tied one record low minimum (32 on May 22nd). But the most notable aspect of the cool snap was the number of consecutive days that failed to reach 50 degrees. Since 1897 there have been four times when a cool snap in May kept the temperatures below 50 for 4 consecutive days: 1897, 1907, 1917, and 1920 (in 1907 it was 5 consecutive days). This year we had 4 consecutive days below 50 from May 20-23rd. It was the first time for such an occurrence in 99 years and the only time at the current climate station location (established in 1990). But what is even more notable is that the previous four May cool snaps ended on or before May 16th. So this year had the latest occurrence of such a stretch of cold days.

5:15 AM MDT, Wednesday, May 22, 2019

Although the clouds might thin a little today, another storm form the southwest is likely to make late today into Thursday cloudy with some rain (snow in the mountains).

This is the latest in the series of storms coming out of a cold atmospheric trough in the western United States. The storm this evening is not quite as strong or as cold as the last one, but it will still remain much colder than average with snow levels descending to about 6500 feet. Below 6500 the precipitation is likely to be rain or drizzle, but I would not be too surprised if some flakes mixed in by Thursday morning.

The weekend Friday-Monday should see a return to more spring-like temperatures with dry periods dominant over the periods with clouds and showers. For the areas that might see  a shower or thundershower, those would likely be late in the day. Monday might see an increase in the chance for clouds and rain as the next storm approaches, but at this point the timing of that change is still uncertain.

1 PM MDT, Sunday, May 19, 2019

A powerful storm will move from the 4-corners region, across southeastern Colorado, and into Kansas on Sunday evening through Tuesday. The headlines from the storm Monday and Tuesday could be from a severe weather outbreak with tornadoes in West Texas, Oklahoma, and Kansas. Closer to home we will see a variety of weather in the Front Range region, with some thunder here too. Much cooler than average temperatures will continue into midweek.

The dry slot

With these kinds of storms there is often a dry sector that moves up from the south on the east side of the storm and is known as the dry slot. That will likely affect south central parts of Colorado Monday and Tuesday, so areas along the Front Range from Colorado Springs southward have a lower chance of persistent clouds and wet weather.

Rain and thunderstorms

Showers and a few thunderstorms are likely to develop in some areas this afternoon and evening despite the low clouds and cold temperatures. Tomorrow and early Tuesday will likely see some intervals of rain or drizzle, with thunderstorms developing in some areas in the afternoon. The most intense thunderstorms are likely near the Kansas or Oklahoma borders, but a few strong thunderstorms (with hail) may occur along the Front Range.

Snow, again?

Heavy snow (over a foot) is likely above treeline and mainly north of the Pikes Peak region. The snowline is likely to work its way to lower elevations Monday night and early Tuesday. Some snow accumulation appears likely in most areas above 7000 feet (and mainly north of I-70). A couple inches appears likely for Nederland and Estes, with a small chance of more than 6.

Some mesoscale models bring the snow all the way down to the urban corridor late Monday night with a bit of slushy accumulation by Tuesday morning. It appears these models don’t really have temperatures cold enough, but it’s close, and nighttime is the right time if it’s going to happen. I’d say that snow in Boulder, Denver, and Fort Collins is unlikely but not out of the question (about a 20% chance).

Low elevations freezes

A widespread hard freeze is unlikely, but areas of local frosts are possible, especially if there is some clearing on Monday, or Tuesday nights (Tue or Wed morning). It’s probably good to protect sensitive plants just in case.

Whew, and I didn’t even get into the latter part of the week yet…

5:30 AM MDT, Friday, May 17, 2019

The 80 degree days are over for at least 5-7 days. Two major storm systems will move through the Colorado/Wyoming region, one later today and tonight, and another early next week.

Today and tonight

It will still be near or a little above average in temperature today, but with a better chance for thunderstorms this afternoon and evening. The best chance for strong or severe thunderstorms will be on the far northeastern plains of Colorado, southeastern Wyoming, and in Nebraska. The lowest chance for thunderstorms is south of Denver.

Saturday and Sunday

Cooler weather with some showery periods possible, but for the foothills and plains it should be dry most of the time. In the mountains, the chance for showery periods is a little greater, and there may be some snow above treeline. The potential for clouds and precipitation increases a little late Sunday as the next storm approaches.

Monday-Tuesday

This could end up being a very cool and occasionally wet period, with substantial snow above 8000 feet and cold rain in the lower elevations. I’ll have to update on this later in the weekend. There is a small chance for damaging nighttime temperatures for tender plants, but it’s too early to tell yet.

6:00 AM MDT, Tuesday, May 7, 2019

We are in for several days of generally cloudy, damp weather with some freezing temperatures in the lower elevations possible Thursday and Friday mornings.

The clouds may thin a bit at times today, but even if that happens, expect some more showers or even some thunderstorms this afternoon, and cool temperatures. The coldest stretch of this pattern will move in on Wednesday. Several intervals of steady rain (snow in the higher elevations) are likely Wednesday afternoon into Thursday, It is now looking more likely the rain will mix with or change to snow in parts of the urban corridor Wednesday night or Thursday morning. I would not be surprised to see a couple inches, mainly in grassy areas. The amount of snow will be greater as you go up in elevation, with 5-12″ above 7500 feet.

Temperatures are likely to be within a few degrees of freezing on Thursday and Friday mornings, so prepare to protect tender annuals that are outside.

11:30 AM MDT, Sunday, May 5, 2019

This coming week is likely to bring a multi-day stretch of below-average temperatures and a good chance of rain (snow in the mountains).

Although Monday may be a little cooler than today, it will likely still be a mild day with only a slight chance of afternoon showers or a thunderstorm. Tuesday might be a bit cooler with another chance for afternoon showers or a thunderstorm.

The peak of the cold snap appears to be on track for Wednesday and Thursday. During that time there may be long cloudy stretches and a period or two of steady rain (snow in the mountains). Although this spell is not likely to be quite as cold as last week’s cold snap, I can’t completely rule out the chance for some wet snow in the lower elevations, but the chance appears to be small. Gardeners even in the low elevations should prepare for the risk of damaging frosts (for tender annuals) on one or more mornings from Wednesday to Friday.