Denver, Colorado

Saturday PM Update: Christmas Week Forecast

Posted by @mattmakens247wx and @brendansweather

Remember the opening sentence in Thursday night’s post: “A lot of folks jumping on the White Christmas bandwagon today, so I’ll begin this post with a word of caution”. This was inspired by a local TV station posting one model’s operational run snowfall forecast over eight days, leading many to believe there’d be a foot or more of snow on the ground by Christmas in Denver. Other media outlets followed the lead by promising a Christmas snow…really?! Did no one look at the models? Today, just two days after those forecasts, that same model that started the craze has cut its forecast snow totals by 84% through Thursday. It’s time that media outlets start analyzing the models and their ensembles, looking for the hows and whys, to prevent eating crow.

As the season snows, snow goes the snow?
We have a lot of moisture coming in over the western third of the country. Rain and snow cover much of California, Oregon, Washington, Idaho, Nevada, Utah, Wyoming and Montana. This moisture is now moving into Colorado for tonight night, Sunday, and Monday.

 

GFS precip forecast GFS 6 precip shows heavy mountain snow Sunday night

The direction of this "flow" is from the northwest, which is generally not a good direction for anyone east of the mountains to get snow and or rain. However, the central mountains to the western slope will have lots of moisture Sunday and Monday. Some will have feet of snowfall. Many of the resorts will be adding well over a foot of new powder in time for the holiday rush of skiers/riders. Those resorts that face west and north will come out ahead of the others.

It may surprise you that very little of all that moisture is going to get on the eastern half of the state, including the Denver area, although the entire state will see the clouds. A cloudy sky expected tonight through Monday for the metro areas. Denver and the Front Range has a slight chance for snow/rain by Sunday night and Monday. During this period, a stronger wind will be blowing overhead, 20,000 feet or so, and that may draw enough energy to give a snow band or two for the city areas. That is only If that wind can tear enough moisture away from the mountains, like pulling strands from a cotton ball.

While a few days ago it looked a bit more promising that we’d have enough energy to enhance a few of these snow bands, at this time this does not appear to be the case. In cases where some banding may occur, the troughs will be to the east of us, thus limiting the lift and limiting the extent and amount. A weak cold front Monday will not do too much to shift our winds and enhance upslope either. If we manage to ‘steal’ some moisture from the mountains, the result will be a few areas with a few inches of snow, but not for everyone -- just in small pockets.


Christmas
This first push of moisture will take a bit of a break Tuesday and Wednesday, which will be the best travel days within the state ahead of Christmas. Then, another push of moisture will move through around Christmas Day. We don't see nearly as much humidity with that next one though, so the chance for snow will be less than this first system, and again focusing on the central and northern mountains the most.

Being realistic we won't and can't say that Denver and the metro areas won't have snow falling on Christmas. It certainly remains a possibility. However, the chance for snow on the ground or actively falling isn't overly optimistic. The operational ECM which once showed several inches of snow Christmas night produced 0.00” QPF for DEN at 12z today, and its ensembles about an inch. The ECM has been underperforming the GFS this December anyway. The GFS has remained most consistent with this system producing little snow for the eastern half of the state.

The lack of snowfall for Christmas isn't much of a shock, as it fits with our history. Only 18 times since the late 1800s has snow fallen on Christmas Day in the city. Denver does not have snowfall on Christmas Day very often.
Click here to read more about historical our chances.

Still some hope for snow this week, maybe?
Maybe. Again, a few locations may “get lucky” on Monday and see quick hitting dusting to a few inches, but most others will likely stay dry, or even see some rain.

The best chance for things to turn in the other direction may still be with the Thursday/Friday storm, but for now this doesn’t appear likely. While the trend has very clearly been for less snowfall with this system, there are still enough differences between the models keep some uncertainty going here. As it stands, the upper level support just doesn’t appear to be there with this system to produce any meaningful snowfall for the Front Range, even with the more bullish models. Still time of this to change, but it would require a little help from the storm out in front of it, and a good kick to the current trend, which is hard to do.

Enjoy your weekend, and get to the mountains if you can!

A Wintry Christmas Week?

Posted by @brendansweather

A lot of folks jumping on the White Christmas bandwagon today, so I’ll begin this post with a word of caution before jumping into the details on what’s shaping up to be an active weather week across the country next week. Despite the excitement, there’s very little confidence in how much snow will fall along the Front Range and eastern Plains next week. It looks likely many across eastern Colorado will see at least two snow chances next week, but how much will fall remains very much in question.

That’s not to say we’re not pulling for a snow (yes, we’ve seen the EURO). In fact, nothing would validate what we’ve been saying for weeks better than the deterministic ECMWF: bitter cold and a good snow to end this otherwise quiet month.

There will be a series of disturbances affecting Colorado as we move into the weekend and next week. Timing and strength of each of these systems have been varying greatly run-to-run in large part due disagreements on what happens when they get east of Colorado. A system due to affect Colorado Sunday and Monday will eventually move into the Great Lakes and deepen. Where this system eventually closes off will greatly affect the eventual track of our end of the week Christmas storm. A lot to watch here.


Monday system
Models have largely been dismissing our first system due in Monday -- but this one seems like it could sneak up on us. If you’re to believe the 18z GFS, it believes this could be the case too. The image below represents the jet stream location and speed on Monday evening.

GFS Jet 300 hpaWeatherBell Analytics


As the strong jet stream energy moves over Colorado on Monday, the mountains will get some very heavy snow and wind. East of the mountains, the inevitable snow bands will set up in narrow, but very intense streaks. This far out, it is almost impossible to predict where these “streaks” are going to set up. However, those that do get in those heavier bands will see some good snow. While it’ll be a quick hitting storm, there’s quite a bit of energy with it. We’ll need to watch where models trend with the placement of the jet over the coming days, and also if we do get precipitation Monday, how much of that may at least begin as rain as per the GFS...

Christmas system
The next system will be moving in right on it’s heels Christmas Day. This is the system the EURO has latched onto for a good snow across eastern Colorado, though its ensembles continue to be far less gung-ho here as well. The 18z GFS, which really grabbed onto Monday’s system, is now tracking the surface low too far north of Colorado Christmas Day for much of any snow in Denver, a similar issue we’ve seen with the Canadian. I’m curious to see if the EURO trends to the northern track or stays the course tonight.

There does seem to be enough model consensus at this time to at least introduce a chance for snow Thursday and Friday next week, even with differences in tracks. The EURO has been pretty consistent over the last few days, and it’s ensembles have been showing a pretty good trough digging into the west from the 25-29 for sometime now. Not all models are showing snow on Christmas, however, so a good amount of uncertainty exists.


Mountain Snow
Where there’s greater confidence is in a good mountain snowfall event over the next week. As we head later into the weekend the flow aloft becomes very favorable for snowfall in the north central mountains. Totals from 1 - 2 feet will certainly be possible through Sunday through Monday, with more snow likely as we head into the middle of next week.

Here’s the current 5-day snowfall forecast from
FreshyMap:

FreshyMap Forecast

Cold, cold, cold?
Next week is forecast to get progressively colder as we go, and all indications are the cold sticks with us into the New Year. Here’s today’s 12z GEM 850 hpa mean temperature ensemble control run for 12z Dec 29, and it’s mean ensemble run below.

GEM Cold
GEM ensWeatherBell Analytics


Stay tuned!
 

A White Christmas in Denver is Surprisingly Hard to Come By

Posted by @MattMakens247wx

Snow has fallen on Christmas Day just 18 times in 132 years of record keeping
With such a warm and dry to start the month of December, a white Christmas has been feeling more and more out of reach with each passing day. As we discussed earlier this week in the State of the Atmosphere, the last part of December certainly looks colder than the start, but what snow may accompany that cold remains trickier to pin down. We see two systems over the next week which will have at least a chance to bring some measurable snow to northeast Colorado by Christmas Day.

In a typical year, odds are about 38% for Denver to have at least 1” of snow on the ground for Christmas, with just a 14% chance we see snow fall on the day. With only spotty snow on the ground currently across much of the metro area, we'll be dependent on the two upcoming systems to deliver if we're going to have a white Christmas this year. 

A quick check on the season so far is a dismal one. Denver has only recorded
4 inches at Denver International Airport where official records are kept, whereas “normal” is greater than a foot by now. This slow of a snow season (to date with 4” or less) has only happened 23 other times for Denver since 1882. In only one of those years (1982/83) did the season bring above average snowfall by spring, just a handful had near average, and the rest were significantly snow-less. Years like this one average 15” less than average by the end of the snow season, where 57.5” is considered our average. While last weekend’s snow brought several inches of snow to some on the eastern Plains, Denver International Airport (where official records are kept) only recorded a Trace.

Here's more from the folks at the
Boulder office of the National Weather Service:

On the other hand, if a white Christmas means having measurable snowfall (0.1 inch or more) on Christmas Day, then the odds drop to about 14 percent (18 days in 132 years) since 1882.

Taking a closer look at recent statistics for the last 30 years, Denver has received measurable snowfall on 5 Christmas Days, or 17 percent. Meanwhile, if we look at Christmas Days with 1 inch or more of snow on the ground, then 15 of the last 30 years (and 6 out of the last 8) have been a white Christmas, or 50 percent.

The most snow ever recorded on the ground in Denver on Christmas Day was 24 inches which was measured after the Christmas Eve blizzard of 1982. Other significant Christmas snow depths include the following:

5 Greatest Snow Depths on Christmas since 1900

1982: 24"

1913: 19"

2006: 15"

1973: 12"

1918: 11"

The heaviest snowfall on Christmas Day was 7.8 inches which occurred in 2007 and the second most was 6.4 inches recorded in 1894.

***Do keep in mind that the official site for Denver snowfall has changed over the years.
Denver International Airport (2008-Present), Denver Stapleton Airport (1950-2007), 19th & Stout (1916-1949), 16th & Larimer (1882-1915).


In recent years we've done well with snowfall around Christmas. While in 2013 Denver recorded no snowfall or snow on the ground on the 25th, 2012 featured 2" and 2011 featured 3" of snow on the ground. The last time we had significant snowfall on Christmas day as in 2007, when 7.8" of snow was recorded. 

With many of you traveling, and many more hoping for a white Christmas next week, we’ll be offering many updates in the days to come for what looks like a relatively active weather week across Colorado, and the United States.

We'll leave you with a snowy picture from Snowmass over this last weekend, more mountain snow on the way through the coming week! 


snowmass snow image Colorado
 

The State of the Atmosphere: Tuesday, December 16, 2014

Posted by @BrianBledsoe and @brendansweather

Our weekend storm is now well east of Colorado, and our weather, while cooler than last week, has calmed down. Our snowfall forecast, for the most part, worked out pretty well with what was a complex system. The biggest issue was not coming far enough west with the heaviest band of snowfall across the eastern plains. Around the metro area, snowfall totals from just a trace (DIA) to 4” (southeast Aurora) were common, with very little snow reported across the northern I-25 urban corridor. Denver proper saw about 0.5” of snow, with a Bust Index of 5.

Here’s a look at our final snowfall map Saturday night, with CoCoRaHS reports below. Overall not a bad forecast, and a great snow for northeast Colorado.

weather5280 snowfall forecast
CoCoRaHS snowfall totalsCoCoRaHS

Another interesting aspect of this system was the incredibly low Snow Water Equivalent (SWE), at 12:1 and lower across much of the metro. This is very uncharacteristic of a December snow, and even on the low end for a Spring snow. This,
you’ll recall, was another concern we outlined last week for snow accumulation potential in Denver.

Remainder of the week
The remainder of the week will feature more December-like temperatures than what we experienced over much of the first half of the month. Highs will be in the 30s and 40s through the weekend. There will be another chance for snow Thursday, but the threat for accumulating snow looks very low at this point. Plan for chilly temperatures and about a 20% chance for snow late Wednesday night through Thursday.

Second half of December
Beyond this week our pattern remains interesting enough to keep an eye on, but nothing convincing with regard to snow. The 12z EURO yesterday showed a good Christmas Eve storm, while other medium range models are dry. While the storm track continues, the latest trend is to keep much of the energy too far south over the next 10 days to bring any appreciable snowfall to northeast Colorado. However, the pattern shift that is to take place for the last part of the month is certainly a lot colder than what we’ve had for most of the month. It has to do with a ridge of high pressure strengthening in the North Pacific and over Alaska. When that happens, cold air usually gets sent southward. Per the latest WMO ECMWF model, you can see that trend happening. Reds equal higher pressure and quieter/warmer weather, while the blues and greens equal lower pressure and general storminess and colder weather:

ECM HPA 500
Notice that all of that orange, red, and white near Alaska? That is a classic signal for cold air to dump into most of the US -- and similar to the pattern that brought record cold to our neck of the woods in mid-November.

The various teleconnections support this trend too.

ECM AO teleconnection
GFS EPO
ECM PNAWeatherBell Analytics


While the Arctic Oscillation isn’t projected to be terribly negative, the EPO and PNA support much colder air and general storminess impacting parts of the western US. This is a similar pattern to what happened with our big cold shot in early November. It isn’t as potent looking at this time, but would be significant nonetheless.

Snowfall is always the hardest piece of the puzzle. There will be storms moving through, and there will be cold air in place. However, the devil is always in the details -- which are quite sketchy this far out. That is why it is so important to stay
current with our forecast. Lots of folks are traveling this time of year. If you don’t pay attention to the forecast, it can jump up and bite you. For those that love winter weather (cold and snow), we would say that the most exciting times of the winter are yet to come. Remember when we said that winter would be a “late show”? Well, we believe that mother nature is simply just getting warmed up.