Complex Forecast, But Don't Believe the Hype 3-6" Likely, *NOT* 5-10" That NWS Predicts

6:30pm Update: Switch to mostly snow has occurred earlier than expected. Also, if you have any specific questions regarding the forecast, leave a comment and I'll answer it!

Late winter/early spring storms in this area are often especially tough to forecast due to temperatures right around freezing (and not colder), more moisture rich systems than most that impact the area earlier in the winter, a higher March sun angle and warmer ground/pavement temperatures. All of these issues come into play for the storm system that will bring moderate amounts of snow to the area tonight through Monday.

After taking a look at all of these issues in concert along with the most recent computer model guidance, it appears to me that we are likely to see less snow than the National Weather Service's forecast of 5-10" in Boulder. I think we are looking at more of a 3-6" storm of wet snow. Note that while much of what falls overnight may stick to paved surfaces, I believe that the vast majority of snow that falls during the day Monday will not stick on paved surfaces. It is possible that by mid-morning Monday many roads in Boulder on Monday are wet as opposed to snowy or icy.

Note that higher snowfall amounts are possible in areas north and east of Boulder (which is somewhat atypical of most winter storms here which tend to bring higher amounts to the west of Boulder).

And now, the detailed forecast:

REST OF TODAY 3/16:

• A mix of rain and wet snow along will transition to more snow than rain by early evening with temperatures dropping to the low to mid 30s.
• By 9pm, precipitation will be primarily snow as temperatures drop below freezing.
  
• After midnight, moderate to heavy snow will fall as temperatures drop through the upper 20s.
  
• By 7am Monday, I expect that 2-4" of snow will have fallen.

MONDAY 3/17:

• Snow, sometimes moderate, will continue to fall during the first half of the day Monday.
• Due to a rather high late March sun angle, along with temperatures just at - or perhaps even a couple degrees above - freezing, most of what falls during the day will only stick to grassy surfaces.
  
• By afternoon, snow will taper to scattered light snow showers which may linger until the early evening.
• By 7pm Monday, I expect that an additional 1-2" of snow will have fallen.

TOTAL EXPECTED ACCUMULATION:
THREE to SIX INCHES*

* = I concede that there is an outside chance (<25%) of up to 8" of snow falling in some spots, especially in western Boulder County or areas well to the northeast of Boulder.

Mountain Meteorology & Ski Forecasting

I recently stumbled across an interesting article in The New York Times, called "A World of Data to Help Your Plans to Go Skiing". The article should appeal to anyone interested in snow, skiing, or mountains -- which is probably the vast majority of people who read this blog.

One of the more pertinent parts of the article discusses the pitfalls of forecasting weather near and around mountain ranges and goes a long way to explain why forecasting in Boulder is much more difficult than places not immediately adjacent to mountains. The article says:

"Meteorologists...rely on sophisticated computer models and technology like satellite imaging. But they are still wrong sometimes, precisely because there is so much to take into consideration. Mountains create their own weather systems and present a suite of phenomena like lake effect and orographic lift, which is the forcing of air to rise by rising terrain, like a mountain.

Mountain meteorology, like skiing, requires not just technical skill but intuitive understanding of movement and motion and flow. 'The atmosphere is a fluid,' said Larry Dunn, meteorologist in charge of the National Weather Service in Salt Lake City...it behaves just like any other fluid, and you can use physics and mathematics to describe the behavior of that fluid in equations. If you knew the exact state of the atmosphere over the whole planet right now, if you knew where the cold air was, the warm air, the moisture, the wind and its directions, everywhere at all levels of the atmosphere, then you could plug that initial condition into those equations and solve them out into the future and get the exact state of the atmosphere tomorrow or the day after.'”

I, for one, couldn't agree more with what the meteorologist says in this article about mountain forecasting. It also explains why sometimes downtown Boulder may only receive an inch of snow during the same event that brings 7 or 8" of snow to areas just 15-20 miles to our west (like Ward, CO).

When is the next chance of snow, you may ask? Some wet snow flakes may fall early Friday, though I don't expect much, if anything in the way of accumulation. And early indications suggest another shot at snow early next week, though I'm again skeptical about accumulations.

I'll keep you posted...

Decidedly Wintry Over Next 48 Hours

Despite the fact that the normal high temperature in early March in Boulder is well into the low 50s, the weather over the next couple days will feel decidedly more wintry than that. In fact, highs Wednesday and Thursday will be some 15-20 degrees below normal and mired in the mid 30s. A weak disturbance will push through Tuesday night into Wednesday which may provide scattered snow showers -- enough to whiten the ground in some spots. Temperatures will moderate considerably by the weekend, when high temperatures will return to the 50s.

Tonight 3/4
Mostly cloudy and cold with scattered snow showers, especially west of Boulder. A coating to an inch is possible in some spots, though others will remain dry. Expect overnight lows near 20.

Wednesday 3/5
Expect more clouds than sun on Wednesday with a few scattered snow showers or flurries, primarily during the first half of the day. Little or no accumulation is expected. High temperatures will top out around 34. Clearing and quite cold overnight, with lows in the mid teens.

Thursday 3/6:
Expect a blend of clouds and sun on Thursday with still-chilly highs in the mid to upper 30s and overnight lows near 20.

Extended Outlook:

• Dry and into the low 40s on Friday
• Sunny and milder -- into the mid 50s -- for the weekend

Most Abrupt Change of the Season Record Highs Give Way to Blowing Snow

In what promises to be the most dramatic turn-around in weather so far this winter, we'll go from a record high temperature in Boulder of 73 on Saturday (which broke the record set back in 1967!) to blowing snow on Sunday with wind chills in the mid teens at times. Welcome to Colorado!

A very strong cold front and associated area of low pressure will push through the area during the early part of the day Sunday. This will lead to high temperatures a whopping 40 degrees lower than Saturday's balmy highs and an evident NW wind from 15-30mph at times.

Unfortunately, however, it's a fairly low confidence forecast because -- due to the recent record warmth as well as the higher sun angle as we're now in March -- it is difficult to predict how much of the snow that falls will actually stick. I think snow will have a very difficult time sticking to paved surfaces until late afternoon; as such, most of the accumulations should be on grassy surfaces.

DETAILED FORECAST

Sunday 3/2:

• Expect drastically colder temperatures along with overcast skies.
• Early high temperatures will be near 35 but will drop through the day.
• NW winds 15-25mph will produce wind chills in the teens at times!
  
• Light snow is probable during most of the day, especially after mid AM.
• Expect 1-3" by late afternoon, mostly on grassy surfaces.

Sunday Night 3/2:

• Overcast and breezy with light to moderate snow.
• Temperatures will drop into the low 20s by late evening.
• Another 1-2" is possible before midnight.

TOTAL EXPECTED ACCUMULATION:
TWO to FIVE INCHES

Monday 3/3:

• Becoming mostly sunny and breezy with highs in the mid 40s.