SnowSTORM RECAP
WOW! What a storm. We haven't had one like that anywhere near the metro in a while. It reminded me a lot of the storm of December 10th-11th storm of 2010 (11 years to the DAY: kind of creepy). Remember that one? It collapsed the roof of the metronome! I was working weekends on TV at the time & had to walk to highway 55: which was the nearest drivable road to be picked up by a photographer because my truck was stuck in drifts at home! In Minneapolis you usually are waiting a good day before plows come by and the alleys can get REAL bad (as I discovered).
Below: A wide view of snow totals (by color-coded total)
A closer, Metro view:
The big story is the explosive, narrow band of heavy snow that fell from about 5pm to 8pm in the south & eastern suburbs into parts of western Wisconsin.
Here's a short clip from 5:36 PM Friday to 6:15 PM Friday as the band REALLY got going:
What's incredible, is that there was so much moisture the radar was able to pick up on the rate of precipitation (usually hard to do in most snow events because we're talking hundredth of an inch per hour instead of tenths of inches like in this case):
In the clip below you can watch the precipitation pile up. This is liquid equivalent remember, not snow (roughly multiply by 10 to 12 in this situation). In 1 hour & 15 minutes the cursor I marked goes from 0.76" to 1.16"! Basically they picked up 3"-5" of snow in that period (roughly the Eagan/Inver Grove Heights area).
I discussed how we could get higher ratio snow, despite the mild temps (were between 28° & 32° the whole time) in my video forecasts leading up to & during the storm, but it's worth a recap. Normally at those temps you'd expect the snow to water ratio to be anywhere from 9:1 to 11:1, and during the afternoon and then again late at night, the ratios were in that range. What was key, was to get that big burst of energy in that late afternoon-early evening time frame to produce the faster accumulating snowflakes: DENDRITES!
All the factors yielding higher ratio (fast accumulating) snow:
In the upper left image, vertical velocity is shown at about 8,000 feet up (yellow & red being the strongest). This is key because we had a big push of upward moving air due to the power of this storm system. This pushed air into the zone necessary to produce the big snow flakes (dendrites: accumulating at 12:1 to 15:1 ratios). With more modest upward air in the afternoon & late night, we got the smaller dendrites & sectored plates again which accumulate more at that 10:1 ratio. Temperatures of -12° C to -18°C (single digits in °F) are the ideal range to produce dendrites. Now, on a day like last Tuesday, we got dendrites to form no problem because the whole lower atmosphere was that cold, in a storm like Friday's we needed to push air up to 8,000 to 12,000 ft above the ground and BOOM.
Now the question everyone always asks. Which model did better? Well, they all did a decent job at identifying a very narrow, very heavy band of snow, with higher than normal ratios given the temperatures. BUT, they were all off on the location. On average, they were about 20 miles too far south with the heavy band. That 20 miles makes a BIG difference in this situation.
Below: Several models on their last runs early Friday morning before the start of the snow. The dashed yellow line is the model's heaviest axis of snow. In dashed red, is where the heavy axis of snow really was.
ABOVE: I took many of the snow totals & broke it down into the heavy swath (middle red line) & then the southeast & northwest totals on each side. Listed is the median snowfall for each group and the distances between them to decipher the snowfall gradient.
On average, the gradient was 1" of snow every 1.7 miles! In other words, let's say you were in downtown Minneapolis (like me) and picked up about 8": 5 miles (as the crow flies) southeast of me would have 11" and 5 miles northwest of me would have 5". 10 miles southeast of me had 14"! For some perspective: the models generally had a gradient of about 1" every 7 miles: tight, but not nearly as tight as reality. This is normal for the computers: they have a hard time with such a crazy contrast in such a small area. So, technically, it appears the NAM (North American Model) did slightly better: but they all did a decent job at setting up this storm. They all under-estimated snow totals in the heavy band, but did a pretty good job for most of us who were not in the 9 mile wide swath.
And now.... Will this snow stick around...?
That's a good question. It depends how much snow pack you have! While those heavy totals were well over a foot: it was a lot of fluff in the heavy accumulations that will compact a lot over the next day or two. In other words, if you had one of those 20" snowfalls, it'll really become 16" quickly, then further compact and then melt this week somewhat.
Temperatures Wednesday still look to be in record territory (the record is 51° set in 2014). We stand to melt off about 4" to 5" of snow next week, possibly more. To add insult to injury, most models give us RAIN Wednesday night too, which will really melt snow. Rain is like acid on snow: either way it'll get sloppy & icy as it refreezes late next week. Enjoy the powder, while we have it...
Below: Forecast rainfall Wednesday night:
-Meteorologist, Sven Sundgaard
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