A Word About The Forecast
Last night, we wrote that the HRRR model predicted a “rain/snow mix” that was to end around 8 AM (this will turn out to be off by 2 hours, and off because it was all snow, no rain). The NAM3 model predicted precip will hold off until mid morning (it had the timing a bit better, but was way off on the precip type). Several models did not have this AM snow at all.
As early as 317 AM, there was no reliable indication of an accumulating morning snow event. A few hours later, though, the morning weather balloon went up and showed a slug of dry air at the surface, but a saturated column aloft. Dry air was going to eat a lot of that snow. But, then, the radar returns out west (NWS called this an “unexpectedly robust band of snow”) showed snow coming, and coming, etc. The dry air was going to be quickly overwhelmed. You know the rest.
Our atmosphere is incredibly complex. I’m willing to bet it is way more complex than you realize. The tools we have to sample it do not keep up with the complexity. For example, weather balloons are only launched twice a day. Why? Money. Radar sites are scattered across the country. They aren’t close enough together. Why? Money. Weather models are very helpful, but they are not perfect, and they do not provide the certainty the public demands. Why aren’t they better? Money. Commit more resources for more balloons, more radar sites, more computing power, and you’ll get better (but not perfect) forecasts.
After all, forecasting is future-prediction. It’s hard even with better tools.
Then you have the weather apps, aka crap apps, passing off iffy data as forecasts as if there is no uncertainty. No wonder the weather community gets called out for “being wrong.” The apps are convenient, but they are binary (yes/no) forecasts with a percentage and a number that suggests to you, the user, the forecast is without nuance and does not need an explanation. So, I don’t blame anyone who uses the collective “they” when saying “they missed it.” We’ve all been raised to think clip art and a number is all we need to know the weather. That’s OK for many days, but when impactful weather is on the way, a lightning bolt or snowflake and a temp isn’t enough. Not even close.
That’s why 2, 4, 5, and 17 are on the air, with living, breathing, intelligent people expressing uncertainty and explaining the forecast. It’s why we spend all this time on this website. It’s why NWS-Nashville repeats this phrase “Please continue to stay informed of the weather forecast this weekend and be ready to adjust your travel plans as needed” over and over again.
A good Middle TN snow forecast goes beyond a piece of clip art and an accumulation total. Read https://t.co/54RXqOj0oH. Watch 2, 4, 5, 17.
— NashSevereWx (@NashSevereWx) March 11, 2017
Your app isn’t giving you what you need. So, yeah, we are going to miss things. We will be wrong! A forecast is only as good as its tools, and sometimes, sure, we are going to just miss it. God knows I’m not perfect! Again, future prediction is difficult, whether it’s “who’s going to win this game,” or “will this stock rise,” or “who will win that election.” So, please don’t get your “forecast” from your apps and then switch off when impactful weather is on the way. Watch local TV broadcasters. Read the blog! Believe us when we discuss forecast uncertainty.
With apologies to Senator Vinick: If you demand expressions of certainty in weather forecasting, you’re just begging to be lied to.
Rest of Saturday
NAM3 and HRRR models think the snow will stop falling well before lunch. Sure, yes, these are the same models that were kinda right and mostly wrong about this morning’s event, but they are the best we have at correctly “initializing” this morning’s conditions.
As for the rest of today, the HRRR model appears to be model handling the snowfall best.
The afternoon looks uneventful. The big issue is snowmelt. High temps were forecast to be in the low 40°s this afternoon, but because of the snowpack we are running much, much colder than that. I’ll bet temps won’t even make it to 40°, and we’ll end up in the mid 30°s most of the day. Snow will melt, very slowly.
The HRRR model predicts more snow to form to our west, then arrive along and south of I-40 sometime after dark. The time stamp is in the upper right.
Let me zoom in on that:
If this model is off by 30-40 miles, or by a few hours, we could alot more, or alot less, than this, depending on which way it’s off.
The NAM3 model, for what it’s worth, thinks there will be a little more rain/mix during the afternoon (very light), and, like the HRRR model, a line of rain/snow forming along and south of I-40, then moving south after dark tonight.
If we do see this snow tonight, NWS says “it looks like a generally slushy snowfall, but with late night temps expected to drop well below freezing, we could have some travel problems by daybreak Sunday, especially on elevated surfaces.”
We will update this on Twitter @NashSevereWx all day. We will be watching the radar very closely to see whether this evening snow “along and south of I-40” tonight will materialize.
Don’t forget to spring your clocks forward one hour! This also means on any model images (tomorrow until November), the local time will be UTC -5 hours…i.e., 1200UTC = 7AM CDT.
A freeze warning will be in effect from midnight tonight through 7AM Sunday. Animals and tender vegetation will need extra care during this period due to the unseasonably warm conditions we have had leading up to this cold snap.
Early morning risers will likely have to contend with some slick spots; however, we think the sun will be out and temps will soar through the 40°s, helping with snow melt. The high Sunday is 50°.
Monday and Beyond
A wet start to the week looks to be in the cards. Rain will move in late morning to early afternoon on Monday, as shown below by the latest GFS model:
Below normal temperatures will be centered over the eastern U.S., which includes us, along with one or two opportunities for light rain/snow Monday night through Tuesday night. Travel impacts look minimal right now, but we’ll keep our eyes peeled for any mid-week shenanigans.
Near-normal, warmer temperatures will make a return by Friday, in addition to some rain.
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Categories: Forecast Blogs