Just because it is April, doesn’t mean it is going to act like Spring this weekend….this weather just goes to show, out of all the things to forecast, precipitation is the hardest!
Let’s start with my last post (you can read it here). Last Sunday, my attention was called to a Facebook post from a meteorologist who posted an image from the ECMWF model stating that there could be up to 40 inches of snow on Friday. Not only is it not good to post from a model you need licensing permission from, it also is not good to post something from just a model WAY in advance.
As I said in my last post, models change a lot and it is beneficial for a meteorologist to hold off on forecasting until you get closer to the actual time something is supposed to happen.
So, now I can say what is going to happen….
IN BRIEF FORECAST:
Expect snow to start on Saturday. Unfortunately, model guidance is really sketchy about when it is supposed to happen so I would say just expect it to start snowing sometime tomorrow.
And then, it will continue to snow it’s little heart out over the weekend until Monday. I’m going to go with snow totals anywhere from 3-8 inches and it is going to be pretty sloppy and dense.
Temperatures will be in the low to mid 30s the whole weekend. Because of how dense and wet the snow will be this weekend, expect the roads to get pretty slick at night and into Sunday and possibly Monday.
I will have more on the start of the week’s forecast on Sunday evening. I will most likely be live blogging this weekend during the snow event. Keep it here for the latest on the snowstorm.
IN DEPTH FORECAST (FROM MY FORECAST DISCUSSION FOR CLASS):
Yesterday, a low pressure system set itself up in the Pacific Northwest. You can see it here in the satellite imagery. Notice the swirl over Washington, Oregon and Idaho. When we see a swirl like that, we want to know which way it is spinning. Since it is spinning cyclonically (counter-clockwise), it is a low. If it were anti-cyclonic (clockwise), it would be a high.
Plotting current pressure and wind speed at 700 mb on the HRRR model to locate shortwaves, jet streams and pressure systems, there is currently a deep trough over the western part of the United States. This pattern is indicative of storms forming over the Western part of the United States. Also, there are high wind speed at the 700 mb over the Pacific Northwest. Putting the low pressure information from the satellite imagery together with the high wind speeds in below, this indicates that the low pressure system in the northwest is moving southward.
The solutions from the NAM (Figure 5), ECMWF (Figure 6) and GFS (Figure 7) for the next 84 hours are much different from each other, especially in pressure and precipitation. In terms of pressure, all three differ in how close together the isobars are and where a pressure system is located. In Figure 5, the GFS depicts very tight isobars in the center of Colorado while the center of the low that travels through the state over the weekend ends up in the southeastern corner of Wyoming. The GFS shows more evenly spaced out isobars with the low in Central Wyoming by Monday. The solution for the ECMWF shows the center of the low sitting over the southwesterly corner of Wyoming with the bottom corner still over most of Colorado and Utah. All three solutions indicate storms over Colorado, but vary in location.
The NAM and GFS precipitation models in Figure 5 and 6 show varying degrees of precipitation over Denver on Monday. The NAM model shows 0.03 inches of liquid equivalent while the GFS shows .3 inches. The GFS shows that precipitation will be mostly throughout Colorado while the NAM shows the more heavier snow will be in Wyoming. Either way, both indicate there will be some sort of precipitation on Monday.
(Figure 5. NAM MSLP and Precipitation)
(Figure 6. GFS MSLP and Precipitation)
(Figure 7. ECMWF MSLP)
Using the GFS40 Model to plot precipitation accumulation values (Figure 8), the model shows 0.3 liquid water equivalent falling over Saturday evening. Plotting temperature (Figure 9), the temperatures at that time will be in the low to mid 30s which is indicative of snow or a rain snow-mixture falling.
(Figure 8. GFS40 Precipitation Accumulation Sunday 5:00 z)
(Figure 9. GFS40 Temperature Map Sunday 5:00 z)
Conversely, the NAM Precipitation Model (Figure 10) shows a little less than 1 inch of liquid water equivalent falling and temperatures (Figure 11) in low to mid 30s as well which indicates more snow or a possible rain-snow mixture. Using a 10:1 ratio rule when estimating snow accumulation from liquid water equivalent, the GFS shows 3 inches of accumulation while the NAM shows 8 to 9 inches. By looking at both model runs, the Denver-Metro area can expect a moderate amount of snow over the weekend.
(Figure 10. NAM Precipitation Map Sunday 5:00 z)
(Figure 11. NAM Temperature Map Sunday 5:00 z)