Summary:

• Snow showers Thursday afternoon through Friday afternoon
• Heaviest showers likely on Thursday evening
• Some lucky areas could see 2-4+ inches
• Saturday through next Tuesday should be dry
• Next chance of snow will be next Wed --> Fri, March 18-20th
• Odds of cooler air and snow might increase starting around March 25th
• Possible explanation for this season and last season's weather pattern

Details:

We are starting off the day on Thursday with partly to mostly cloudy skies. The weak and warm storm that we've been talking about for a week will finally move into Colorado today. The main thing it has going for it is lots of moisture, but without a lot of cold air or storm energy (vorticity) to lift the air, this storm will not be efficient at converting the moisture to snow.

The radar image from Thursday morning shows some showers to our west pushing toward Colorado. 

Source: Weathertap.com

The Utah resorts reported 1-3 inches of snow as the storm moved past them on Wednesday night, and they could see more showers today on Thursday. These numbers look to be in line with what I'm forecasting for most mountains here in Colorado. As the initial surge of moisture pushes through on Thursday, an inch or two of snow could fall once the atmosphere overcomes the dry air remaining at the surface. Then by later Thursday afternoon and evening, the showers could become more convective (like summer thunderstorm which derive their energy from the sun heating the earth), and the strongest cells could produce a lightning strike and rumble of thunder. If one of these stronger cells moves over your mountain, you could see a quick 1-3 inches of snow and graupel. The snow level on Thursday and Thursday night will hover around 9,000ft; higher in lighter showers, and lower in heavier showers.

The higher resolution models are showing that the best chance for accumulating snow Thursday evening and overnight will be along and east of the divide as a weak wind blows from the northeast. Also, the San Juans could see a bit deeper accumulations due to additional moisture in this region, plus they are also favored for stronger showers on Friday afternoon while most other regions have lower odds of accumulating snowfall from showers during this time.

Amounts in the "bullseyes" are likely overdone, but the place of the bullseyes do a good job of showing areas with higher odds of precipitation. Multiply by about 10 to estimate snowfall. Source: Weatherbell.com

This storm will move away from Colorado on Friday night. Almost all mountains will be dry on Saturday with only a few weak showers left over in the San Juans. Then we'll probably see dry weather on Sunday, Monday, and Tuesday with partly to mostly cloudy skies (high clouds that will filter but not block the sun). Sunday and Monday could be very warm with on-mountain temperatures pushing near or above 50F, especially if we can get a bit more sun. 

Our chances for precipitation will increase next Wednesday through Friday (March 18-20th), and once again this will likely be a warmer and weaker storm, so I'm keeping my expectations low. It is possible that next week's storm is a bit stronger and cooler than I've led you to believe, though, so stay tuned. Also, an initial piece of this storm will affect the northern Rockies on Monday and Tuesday, and there is a slight chance that some precipitation will make it south to Colorado on those days. The odds of this happening are low, so I'm keeping the early-week forecast dry, but I did want to mention the chance that this could change.

When I peek out to "fantasy land", that is, the 10-15 day forecast, I do see hints that a slightly better pattern could develop starting around March 25th. That's a long way out, but something to watch.

Ok, now, as promised, here is a possible explanation for the cause of the weather pattern during this season and last season.

I showed yesterday (http://opensnow.com/dailysnow/colorado/post/3728) how this season’s weather pattern was very similar to last season’s pattern, with just a slight shift to the east. This slight shift is the difference between more consistent snow last season and less consistent snow this season. But still, the overall weather pattern is similar between the two season.

The big question is, then, why have we seen this pattern persist for two seasons in a row?

I recently read a guest post by Professor Dennis Hartmann on the blog at http://climate.gov/, and he addressed this exact topic.

First, he showed how last season’s pattern and this season’s pattern are similar.

Then, to answer the question “why have we seen this pattern persist for two seasons”, Professor Hartman offers this (emphasis mine):

People have speculated that this weather ‘weirding’ might have to do with global warming, either through the Arctic Sea Ice decline (Francis and Vavrus 2012), or through warming of the tropical oceans (Palmer 2014; Wang et al. 2014).  I will argue here that these anomalies can be seen in past patterns of natural variability, and are likely caused by sea surface temperature changes in the tropics.  I will not address the question of whether the probability or intensity of this pattern might be influenced by global warming.

In other words, he is saying that a specific ocean temperature pattern in the tropics correlates to the weather pattern of warmer / dry in the western US and cooler / snowy in the eastern US. This ocean temperature pattern in the tropics is called the North Pacific Mode.

Here is the “ideal” positive state of the North Pacific Mode, where red colors show above-average ocean temperatures:

And below are the ocean temperatures right now, with red again showing above-average readings:

The current ocean temperature pattern in the Pacific Ocean (above) looks very similar to the ocean temperature pattern in the positive state of the North Pacific Mode (second graphic above).

When Professor Hartmann ran a computer model with the ocean temperatures in the Pacific set to look as they do now (the positive state of the North Pacific Mode), the models produced a weather pattern thats looks similar to the one that we’ve seen this season and last season, with a warm ridge over the west and a cool trough over the east. This indicates some causality, or in other words, that the ocean temperatures in the Pacific Ocean are controlling the weather pattern over the US. There is a nice graphic within his blog post comparing the computer-model simulations of this weather pattern to the actual observations.

All of this leads to the question: Is our changing climate the cause of the persistence of the positive state of the North Pacific Mode (i.e. is this temperature pattern in the Pacific Ocean more likely now than in the past)? Dr. Hartmann answers this question by saying, “So while we are fairly sure that tropical SSTs [sea-surface temperatures] are the apparent cause of the unusual nature of our past couple of winters, we do not know for sure whether this is just part of the natural variability of climate, or whether climate change is favoring the positive phase of the North Pacific Mode of SST variability.”

In short, while the activities from 7 billion humans on earth are impacting our environment in many ways, we do not know if one of these impacts is to increase the odds of our current weather pattern. And I think it’s OK to say that "we don't know" and to keep working hard to offer hypotheses around this question, to test those hypotheses, and then attempt to replicate the results of these tests.

While we have greatly improved our ability to predict weather 1-7 days in advance due to research over the last 50 years, our knowledge of the climate is still somewhat elementary. In another 10-20 years, I think we’ll figure out much more about how our climate varies from season to season and from year to year.

The full article from Dr. Hartmann is here, and it's worth the read:
http://www.climate.gov/news-features/blogs/enso/tropics-prime-suspect-behind-warm-cold-split-over-north-america-during

Additional at 12:15pm Thursday: Lots of great comments so far. Because of the popularity of this post and also yesterday's writing, I'm going to post additional summaries from other studies, including those related to climate change in Colorado (http://cwcb.state.co.us/environment/climate-change/Pages/main.aspx) and temperature trends and how they impact the California drought (http://news.stanford.edu/news/2015/march/temperatures-california-drought-030215.html). Stay tuned ... plenty to talk about even if there isn't a ton of snow in the immediate forecast!

Thanks for stopping by today, and please leave comments about the current snow conditions, your thoughts on the article by Dr. Hartmann, and what other topics you'd like for me to cover.

JOEL GRATZ

Geography Key:

Northern Mountains
Steamboat, Granby, Beaver Creek, Vail, Ski Cooper, Copper, Breckenridge, Keystone, Loveland, Abasin, Winter Park, Berthoud Pass, Eldora, Rocky Mountain National Park, Cameron Pass

Along the Divide
Loveland, Abasin, Winter Park, Berthoud Pass

East of the Divide
Eldora, Rocky Mountain National Park, Cameron Pass

Central Mountains
Aspen, Sunlight, Monarch, Crested Butte, Irwin, Powderhorn

Southern Mountains
Telluride, Silverton, Durango, Wolf Creek (Telluride and Silverton are on the northern side of the southern mountains)