Thursday, November 13, 2014

Lessons learned from the first snow of the season and their application to the next event (this weekend)

A surprise snow to start the season

Weather and traffic folks had their hands full early this morning when "surprise" snow showers passed over the metro around dawn. I quote "surprise" because there were clues last evening that some precipitation could occur this morning. In fact, I updated the official MWN Forecast shortly after 10pm last night to add the mention of flurries and a 10% chance of precipitation. In other words, the evidence wasn't particularly convincing, but it was there.

The MWN Forecast following an update at 10:15pm Wednesday

Lessons Learned

My biggest surprise, and it shouldn't have been, was the impact. Most every weather event presents learning experience(s) for meteorologists if we're paying attention. This one was no exception. Here's what I learned:

  • Sometimes you have to ignore the calendar. Today's 0.1" of snow was the earliest on record at the Agricenter (records date back to 1987) and only 11 days past the earliest snow on record at Memphis International. But the atmosphere doesn't care what the calendar says. It was cold enough to snow, so when precipitation fell, that's what it did.
  • Be wary of the "first of the season." Whether it's winter weather in the fall, severe storms in the spring, or 100 degree temps in the summer, forecasters tend to underestimate the first of those events in a particular season. We need to not be shy about forecasting what we expect just because it's the first one of the season. So what if it hasn't snowed yet this year? That doesn't mean the first time it does it'll be nuisance flurries.
  • At the risk of crying wolf, winter weather impacts in the south cannot be underestimated. After all, we don't handle winter weather well in this part of the country. The clues I personally missed that should have resulted in a more strongly worded advisory included: roads not pre-treated, temps at or just below freezing, a hint from high-res models last night that precipitation could be more than just flurries, and the fact that it was the first event of the year. These points, taken together, resulted in bridges shut down, a plethora of accidents, and overall a pretty awful commute if you had to leave secondary roadways.

So, this event was a learning experience for all of us. I believe that if this exact event were to happen again sometime soon, I would handle it much better and be able to better forecast the impacts (because that is what really matters). Overall, those impacts would likely be reduced as well because now everyone won't think it can't happen because it's "too early" and be better prepared. I'm just glad that, in the end, we got to "re-learn" winter weather with a relatively weak event. (UPDATE: After posting this, I read that a young man passed away in a single car accident on Highway 385 after sliding on ice and rolling his vehicle. The previous statement was not intended to diminish the impacts to those who were most directly affected by the conditions. Our condolences to the young man's family.)

A second chance?

So about that next opportunity! We may not have long to try out our new-found knowledge! We're carefully watching this weekend's weather pattern. Model data has been hinting at frozen precipitation of some sort Saturday night into Sunday morning and perhaps again Sunday night. A low pressure system will move along the Gulf coast with precipitation spreading to its north, falling into cold air over the Mid-South. The latest is presented below in the form of a couple of those model solutions. Right now we favor the GFS/European solution over the NAM, but their solutions are starting to converge fairly closely.

GFS forecast total precipitation through Sunday afternoon. It forecasts most precipitation to remain south of the metro with very light amounts along and south of I-40.
GFS forecast low temperatures Sunday morning. Lows in the mid 30s are borderline for snowfall, In fact, where most precipitation is forecast to fall, lows are in the upper 30s. There is a slight chance we could see light snow or sleet in this scenario. 
NAM forecast precipitation through Sunday evening. There is a bit more very light precip in the metro, but the 0.10" (green) amounts are in almost exactly the same place in north MS as the GFS.
Forecast low temperatures Sunday morning from the NAM model are nearly the same as the GFS, perhaps a degree or two warmer in fact. The NAM presents a similar outcome to the GFS.

Bottom line on the weekend

So, for now, we are forecasting a chance of a wintry mix of cold rain, sleet, or snow early Sunday morning, becoming rain showers during the day. Sunday night may end up dry, but if precipitation lingers, it could change back into light snow before ending as temperatures fall. The parameters that are favorable compared to this morning include slightly warmer near-surface temperatures, drier air ahead of this system (reducing precip amounts), and a bit more "forecast-able" event since we have a track record of many model runs worth of data to spot trends in.

We'll be sure to let you know what we expect as the event draws closer, achieving a balance between under- and over-forecasting and stating the knowns and unknowns as best as we are able. For now, don't change any plans! What we do expect is very cold air to remain in place over the area into early next week at least. A hard freeze is expected tonight and tomorrow night, so take any precautions necessary to protect pipes, plants, and pets and click here for the complete MWN Forecast.

Erik Proseus,
MWN Meteorologist

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