Denver, 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:

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
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.

Sunday’s System on Schedule

Posted by @MattMakens247wx and @coloradowx

Here is a quick update for you regarding this weekend’s storm system. In short, timing and strength remain relatively unchanged to what we laid out in great deal yesterday and Thursday. That said, there are two distinct “camps” regarding the snowfall forecast which are worthy of discussing further.

The most popular camp -- to which the global, and some of the mesoscale models belong -- still indicates a very “minor” snow for the Denver area. This includes our typically preferred ECMWF model, as well as the GFS, GEM, and additionally the NAM. In the other camp is the short range modeling of the SREF, which prefers much higher totals -- by about a factor of 3.

Here’s our latest analysis of snowfall amounts in the various models.

There is a key factor that is a dominant physical process to this storm system: downslope.  There is a pronounced “donut hole” within the snow accumulation forecasts in all models. This snowfall gap appears along and east of I-25 over the northern plains in varying degrees of size/position. This is the area of downslope, snow-limiting flow off the Rockies and the Cheyenne Ridge. The extent of that downslope is what varies within individual models and creates the two “camps”.

This downslope will develop as the system passes over Sunday, and will likely quash the potential for any major snowfall amounts. There simply isn’t enough development in the surface pressure pattern to develop a significant offset (upslope), so the mid-level system will be the only factor. As the mid-level low pressure area slides along the New Mexico border and reaches I-25 over Trinidad, Denver will have its greatest snowfall potential. This will be during the daylight hours Sunday. However, by late day into Monday morning, that low pressure area will be off into Kansas creating downslope for the greater metro area. The low pressure area will be deepening during this time, which could favor a surprise spot or two of higher snowfall totals near Denver, but again it will be brief given the speed of the system and quick development of the downslope flow.

We believe this development process is what SREF is grasping onto more than all other models, and it cannot be totally ignored. Dynamically, the mid-levels of this system could --
in theory --  produce Denver’s best snowfall of the season-to-date of perhaps several inches. However, the speed of atmospheric processes outlined above is puts Weather5280 into the lower snow amounts camp (which is supported by a majority of the models).

The only way for this greater snowfall depicted by the SREF to happen is if the system can slow down for just a few more hours before the downslope eats away the chance at decent precipitation. There is little to believe the system will slow, however, as there isn’t anything significant to block its north and eastward progress. In theory, lee-side troughing could be stronger than expected, giving us more snow, but the diffluence pattern in the mid-levels as indicated in ALL models doesn’t support that type of development.

To account for this last bit of uncertainty, the bust index is higher in a few areas to support any change in the strength or speed of the system.  You’ll note our forecast totals map is in very reasonable agreement with the model consensus, and has remained in line with the past few blog posts.

It should also be noted, however, that recent warm days don’t
necessarily imply that the ground is too warm for snow accumulation. Routine overnight lows below freezing, low sun angle, and short daylight periods have not allowed the ground to re-warm nearly as much as what may be believed. So, despite the fact that temperatures may be a few degrees above freezing during the prime period of possible sno Sunday, light accumulations -- particularly on non-paved surfaces -- are still possible.

You can find our latest snowfall accumulation map above, additionally, here’s a snowfall probability chart for Denver.

snowfall probabilities Denver

Friday Update: Changes Arrive this Weekend, Snow Possible

Posted by @brendansweather

In yesterday’s update we outlined in great detail everything that could go wrong with this approaching storm system. Somewhere around midnight last night it appeared as though the whole thing was falling apart. Today we’ve seen some rebounding from the models with regard to snow in Denver, and for reasons outlined below, we still think this storm deserves to be watched.

What we know
We know big changes are on the way beginning Saturday across the state. Highs Friday soared into the 60s for Denver, and while highs may climb back into the 50s Saturday, it looks like the real warmth is gone for awhile come Sunday. Sundays highs will be in the 30s.

Snow is probable for much of eastern Colorado late Saturday night through Sunday, though not everyone will see accumulating snowfall from this system. While models are all over the place with
how much snow, mean forecast numbers for Denver would put us in the 0.5 - 3” range, with snow-favored locales doing slightly better. In other words, not a big storm at all. There are scenarios where this range could go up (or down), which I’ll address below.

This is a warm system for December, and with that, what precipitation may begin with or mix with rain at least initially. It also means snowfall ratios will be low for this time of year, likely 12:1 or lower, meaning light QPF can’t be made up for with fluffy snow for higher totals.

While models absolutely tanked last night with regard to QPF, the overall track of this storm remains favorable for snow in Denver, which is why we can’t dismiss this thing completely yet. If someone were to ask you to draw a picture of ideal 500 heights for a Denver snowstorm, you’d likely track an upper level low across the Four Corners, and come up with something that likely looks something like the forecast for Sunday from the GFS:

GFS 500 mbUnfortunately for snow lovers, in this case this may not tell the whole story.

Current model breakdown for snow at DEN looks like this:

12z GFS: 0” (though has 0.19” as rain, exactly 0.18” wetter than its previous four runs)

18z GFS: (appears slightly colder, but also down to 0.12” QPF)

18z NAM: 0.5 - 3” (highest south and west of Denver)

12z ECMWF (deterministic): 2.7”

12z ECMWF (ensemble): 3”

15z SREF (mean): 4”

18z GFS Upgrade: 2 - 6”

12z GEM: 4 - 8”

GEM is the clear outlier, though the ECM is close for southern/western Denver suburbs, and produces big time snowfall across extreme eastern Colorado into Kansas and Nebraska. The ECM ensemble forecast has also been pretty steady with 3 - 4” for Denver over last day or two. The operational GFS is finally producing moisture but not as snow, while the upgrade produces several inches for Denver. The NAM was snowless for all of eastern Colorado this morning, but now produces a bit for Denver metro at 18z. Mahem.

What we’re watching for
Better model agreement on snowfall.

The big thing we’ll be watching for over the next 24 hours is if models latch on to a higher snowfall forecast trend, or keep us somewhere between dry and a few inches. Was the slight uptick seen by some models today just a fluke? Or, are they starting to come around to a higher end solution?

If not, we’ll likely lock in a low end forecast for Denver tomorrow, with higher totals possible for the foothills, Palmer Divide, and portions of the eastern Plains into Kansas and Nebraska. While current
NWP forecasts would mean a no-biggie storm for Denver, we think it’s prudent to watch this thing over the next 24 hours and see if anything changes.

More updates will follow as needed, so

Oh, and if you’re holding out for next week’s storm, get ready for more of the crazy. Last night’s EURO dumped snow on Denver, today it’s moved the snow band about 400 miles south. The GFS? Virtually dry.

Thursday PM Update: Weekend System Slams California, Models Back Down on Snow

Posted by @brendansweather

There is not a lot of change from our update last night, but in effort to keep you all abreast as to what we’re watching, here’s another update.

Our storm system is currently slamming California with heavy rain, wind, and eventually snow. Many California ski resorts could see up to 2 feet of snow before all is said and done Friday. With rainfall totals in the Bay area already at 1 - 3”, with locally higher amounts, the NWS continues numerous Flash Flood Warnings, Flood Warnings, Coastal Flood Warnings, and Flood Watches for north central California until 9:30pm this evening, as well as wind, blizzard, and winter storm warnings.

GOES 16km Water Vapor - IR3, RAMMB

By Friday the system digs into the southwestern US and begins its trek east. This is where we start to watch it carefully for its eventual impacts on Colorado.

Rather than reiterate everything we’ve been saying for days now, here’s a rundown of what concerns we have for snowfall potential along the Front Range, and what we continue to like about this setup. With the system coming onshore today and tonight, we expect the models to slowly get a better handle on things going forward as well.

The good, the bad, the ugly
The models, nearly unanimously, have backed off snowfall across eastern Colorado with this system since yesterday. Ironically, as of the 12z suite, the upgraded GFS was leading the way with highest totals for Denver, while the EURO and GEM had backed off snowfall totals considerably from yesterday’s 12z run. The NAM has been fastest, but also pretty wet at 18z, and the operational GFS continues to suggest basically no snow for the city.

Here's a look at the current model spread for snowfall this system (at 10:1):

model spread snowfall DenverIASTATE

Now, there could be number of reasons for the models backing off on QPF. The first, and simplest, is that the models are on to something and that this trend may well turn out being correct.
If this were the case, we would not expect much snow for most of the I-25 urban corridor. Another possibility is that the models are struggling with this system, and we’ll see them come back around in the next few runs.

One potential issue is that the models are putting too much stock in convection out in front of this the system, which can be detrimental to our precipitation chances along the Front Range historically. Convection can rob the incoming system of energy and available moisture, leaving the I-25 corridor lacking in both. We’re putting less weight on this for now, which means if this is indeed what the models are trying to do, we should see them correct wetter for the FR going forward. The other issue we’re seeing, especially in today’s runs, is a trailing low to the northwest of Colorado Sunday. If this low trails too close to the primary feature, its winds can also act to shut off precipitation along the Front Range more quickly, and effectively split the energy of our main system. Lastly, models with the lowest QPF for Denver are trying to introduce the dreaded downslope off the Cheyenne Ridge, with lots of lee side troughing; this is certainly a concern, and would hinder moisture transport to the Front Range.

ECMWF 500 forecastAccuWeather

Of course, then there’s the eventual track. Despite being all in the same ballpark, there understandably is quite a bit of fluctuation run-to-run with the exact positioning of the low. While it may seem frivolous to worry about a few miles here, a few miles there, the fact of the matter is those few miles can shift the heaviest precipitation dramatically, from a bullseye over Denver, to a bullseye across the northeast Plains, to bullseye over southern Colorado. The greatest risk for Denver with respect to current projections is that this system ends up digging too far south for a good snowfall event in the city.

All this considered, there’s still a lot to like about this system. It’s got *almost* everything you’d look for in a good upslope storm for Denver. Almost. Overall we still like many aspects of this storm: the strength, the track, while varying, is pretty good for this far out, and tilt as it moves across the region Saturday night through Sunday. We believe this leaves the door open for models to flip once again in how much snow they produce for Denver.

It’s been a long wait for snow across eastern Colorado, and it looks like we’ll need to be patient yet before calling this one a “snow globe” or “s
now go” for Denver.

There are several more days to go, so we’ll continue to send along updates as this system progresses. For what it’s worth, models continue to suggest there will be another system on this one’s heels by the middle to later part of next week, so buckle up!