Major Storm: heavy mountain snow, heavy rain then snow lower elevations

12:45 PM, Wednesday, May 17, 2017

The coming storm may be one for the record books! The storm is expected to move across southern Colorado tonight through Friday. The current track suggests the heaviest impacts are likely from the mountains and plains around Denver north to Cheyenne. Lesser impacts are likely from Colorado Springs southward. The area between Colorado Springs and south Denver will probably be on the edge of the intense part and should probably prepare for a heavy storm. The rest of this post will be referring to the likely heavy precipitation areas from Denver north to Cheyenne.



Rain should develop this evening and there may even be some areas that get a heavy thunderstorm. Rain should change to snow above 8500 feet this evening, and the snowline is likely to descend to the lower foothills by morning, with some heavy snow during the night. It is unlikely that there will be snow below 6000 feet, but there could be some heavy rain and an occasional mix with snow.


As the storm makes its close approach late Thursday through early Friday the temperatures should drop a few more degrees and that may be enough to bring the snowline down to the urban corridor.  There is a lot of uncertainty for the lower elevations.


Amounts through Friday

1-4 inches of liquid (rain and melted snow) throughout the region

Above 7500 feet: 1.5 -3 feet of snow

6000-7500 feet: 6-20 inches of snow (very variable depending on elevation and convective snow bursts)

Below 6000 feet: most likely a trace-3 inches with some areas of 3-6 inches (we cannot rule out more widespread amounts of 6+ inches and will re-evaluate tomorrow)

**Some model guidance suggest there could be some localized high elevation areas of the Front Range with more than 4 inches of liquid and 3 feet of snow.



Difficult travel in the mountains and foothills

Although it’s less likely, if a 6-inch snowfall does occur in the lower elevations, that would result in serious tree damage and power disruptions. Be prepared for power outages and don’t park under trees.



A hard freeze is not likely in the lower elevations through Thursday afternoon. There is a good chance for temperatures near or below freezing Thursday night.

With clearing expected Friday night, areas of frosts and freezes are likely Saturday morning. If there is snow cover the chance for a freeze increases.


Winter’s last stand

12:15 PM, Tuesday, May 16, 2017

A slow moving storm system in the Southwest and an unseasonably cold air mass from the north will affect the Front Range region from Wednesday to Friday. This weather pattern is threatening to bring measurable snow, even in the lower elevations.


The first threat of snow will begin Wednesday night into Thursday morning. There will likely be some heavy wet snow for the upper foothills and mountains, with a chance for that snow reaching down the plains by Thursday morning. The storm system probably won’t move to the east of the Front Range until Friday, so there may be addition chances for snow Thursday evening.

At this time of year enough of the sun’s energy gets through the cloud cover to make it difficult (but not impossible) for accumulating snow during the day. The things to watch for are whether the storm center will pass just to the south of the Front Range and whether the heavier precipitation will occur during the evening through early morning hours.

Keep in mind that only a few inches of snow at this time of year can cause damage to trees and power lines. Although there is still uncertainty about the snow, it’s not a bad idea to check that your flashlights have good batteries.



Depending on the evolution of the storm, both Thursday and Friday mornings could see temperatures around freezing. If the skies clear out Friday night, Saturday morning may be at risk for areas of frosts and freezes as well.


Historical Context

Although it is unseasonably late for snow and freezes, it is not rare. In Boulder there have been at least two measurable snows recorded in June (the latest was 1.0 inch on June 12, 1947). On May 20-21, 1931 there was a 19-inch snowfall in Boulder (the trees must’ve loved that). Record lows below freezing extend into the early part of June.

More frosts yet to come?

5:50 AM, Monday, May 15, 2017

The next organized storm system to impact the Front Range is likely to move slowly through the Southwest Wednesday night through Friday night. It appears that it may drag down a polar air mass from the north which could result in temperatures of around 32F later this week or Saturday morning. For those who have tender annuals outside this is not a high-confidence forecast yet, but you should probably have a plan to cover vulnerable plants.

Precipitation this week

There is only a small chance for an afternoon shower/thundershower today and Tuesday, and most areas will likely remain dry. As the next storm system approaches the coverage of afternoon/evening showers is likely to increase a bit Wednesday. It’s too early for details, but sometime in the period Thursday thru late Friday we may experience a period of overcast and steady precipitation. If the storm system passes south of Colorado it may be associated with enough cold air to help drag the rain/snow line down into the lower foothills or plains. More on that later this week.

Dry and warm weekend

5:50 AM, Friday, May 12, 2017

We are in for a stretch of warm and dry weather with temperatures in the lower elevations reaching up to around the 80-degree mark. This is the time of year when afternoon thundershowers become common, but for the next few days the chance for that is nearly zero. Even with some small chances for afternoon thundershowers next week, many areas along the Front Range could remain dry through mid week. The next chance for an organized storm system is around Thursday and at this point it appears that could impact the mountains more than the eastern plains.

Showers/T-storms and maybe a period of steady rain Monday-Wednesday

3:55 PM, Sunday, May 7, 2017

Gusty showers and thunderstorms are rolling through parts of the Front Range this Sunday afternoon, and we will likely see more of that Monday and Tuesday afternoons. There is even the chance for a period of steady rain, especially Monday evening and again Wednesday.


The culprit is a storm system in the Southwest U.S. that is moving so slowly that it will likely be Thursday before it is east of Colorado. Until that storm is east of us it is drawing moisture into the Front Range from the Great Plains and even a little from the tropical Pacific (in the middle levels of the atmosphere).



Showers and thunderstorms are likely again Monday afternoon and evening, and maybe some steady rain or drizzle in the evening. Almost everyone should get some rain, and a few areas may experience a severe thunderstorm with briefly intense rain and hail.



Some showers and thunderstorms may be around during the afternoon and evening.


Wednesday-Wednesday night

As the storm system passes slowly to the south of Colorado the Front Range region may have more organized easterly (upslope) flow. The result may be an extended period of cloud cover with a multi-hour stretch of rain and cool temperatures either Wednesday or Wednesday night. Snow levels will drop, but will likely stay above 9000 feet.

Snow is coming this evening

11:40 AM, Friday, April 28, 2017

The sunshine and low humidity should change rapidly from north to south this afternoon and evening, probably during the early afternoon in Cheyenne, the late afternoon and evening from Boulder County southward. Although it’s well above freezing in many areas now, the dry air should allow for strong evaporative cooling when the precipitation starts, so rain, if any, won’t last long.



4-10 inches mountains, foothills, Palmer Divide area (locally  more than a foot in some mountain areas, especially south of I-70)

3-7 inches Colorado Springs

2-5 inches Boulder, Denver

1-3 inches Longmont, Fort Collins, Cheyenne

8-15 inches and windy for southeastern plains (along and south of I-70)

** mainly Friday evening through Saturday morning, longer in southeastern Colorado


What can boost amounts?

The storm is a rather classic setup for big Spring snows but the air over the Front Range Friday morning is remarkably dry. The storm will try its hardest to draw in moisture from the east starting this afternoon, but the really rich moisture may not be able to make it all the way to the Front Range before the storm system begins pulling east on Saturday.


That said, there is a small chance that the richer moisture will make it this far west and boost amounts along the Front Range, but that is probably about a 10% chance. For southeastern Colorado and the Front Range south of Colorado Springs the chance is greater for big snow.


Hard freezes

Milder weather should return after a widespread hard Freeze Sunday morning. But current indications are that we may have another period of frosts and freezes around midweek, so be patient with the tender annuals.



Snow late Friday

3:30PM, Thursday, April 27, 2017

A storm system late Friday through Saturday morning is likely to bring measurable snow to most of the Front Range region Friday night and Saturday morning.


Factors in favor of snow

The storm system, a 4-corners low, will be in a favorable position to cause widespread deep upslope flow throughout the eastern slopes of the Front Range. Temperatures should be cold enough for precipitation to be mostly or all snow.


Factors against really big snow

Despite the favorable conditions, atmospheric moisture is not likely to be very impressive by April-May standards. Although the storm may be very efficient at converting available moisture into precipitation, there could be a limit on how heavy it gets.


The result

Clouds and intervals of snow beginning Friday mid to late afternoon (possibly a bit a rain at first in the lower elevations) continuing through Saturday morning. Snow may begin by midday up toward Wyoming and in the Cheyenne area.

4-10 inches mountains, foothills, Palmer Divide

1-4 inches lower elevations



After a widespread hard freeze, mild Spring weather should return.