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.