Johnson: Regarding the snow, take a deep breath...
Posted February 26, 2015
Updated February 27, 2015
Raleigh, N.C. — And…exhale. It’s been a long week!
Now that the second round of Old Man Winter is behind us, let’s look back at how the week unfolded.
Tuesday’s “surprise” snow threw a wrench into the run-up to the main event Wednesday night. For starters, it resulted in an extra, unexpected day off for many students and employees in the area — and longer hours for road and emergency crews. It also complicated the forecast for how warm it would get Wednesday and what we would see going into Wednesday night’s storm.
We expected Tuesday’s storm to come mid-morning, drop a little bit of snow, and be gone well before the afternoon commute. Instead, it was a little bigger, a little stronger, and more long-lived than forecasted - a rare over-achieving snow-producer. Fortunately, impacts outside of travel were limited, with few power outages.
Wednesday Night into Thursday
The storm we expected would be the main event all week did not disappoint in this regard. A high-impact snow storm for the Triangle and areas north, and basically a cold rain for the Sandhills and the southern coastal plain, is what we forecasted. And that’s what we got, although, details were problematic.
This system was consistently forecasted to be a classic “Miller Type-A” storm, featuring a single low pressure area and a very narrow transition between snow to the north and rain to the south. We also expected it to be a very prolific precipitation producer, spreading plenty of moisture throughout the area.
Whatever we got, we were going to get a lot of it.
However, the availability of cold air was always the sticking point. For a good winter event, we look for a source for a feed of cold air from the north, usually from an area of high pressure in eastern Canada or New England. However, we didn’t have that. We’d need to manufacture some cold air by melting snow and evaporating rain into otherwise dry air to cool it down and counteract the warm air being pulled in ahead of the parent low pressure system.
The good news was we had plenty of dry air to start, and that’s why we the precipitation started, or changed over, to snow so quickly at the outset. We were able to dynamically cool the atmosphere by pulling heat energy out to melt the snow and evaporate raindrops. Many more areas saw snow at the outset of the event.
Unfortunately, that was also a curve ball that we swung at, upping our forecast Wednesday evening and anticipating that the cold air was a sign of things to come.
After an initial round of mostly snow for many in the area, the dynamical cooling gave way to the warm air coming with the main system. Unfortunately for snow lovers, the parent low pressure area tracked a little farther north than forecast, bringing that warmer air a little farther north. The result was the transition zone – a narrow band of sleet and rain – hanging around the Triangle, especially in Chatham, Wake, and Johnston counties, for much of the event. North of that zone, into Durham, Chapel Hill, and towards the Virginia line, it was all heavy, wet snow. To the south, it was mostly rain. Along that zone, we started as snow but changed over to sleet or rain, which not only prevented additional accumulations but ate into what we had already collected.
We forecast – and observed – a very tight gradient between high snowfall totals near half a foot and very little snow along and south of where the transition zone set up. It set up a bit farther north than we expected, however, so that shifted the higher snowfall totals from right across Wake County to northern Wake and into Durham, Chapel Hill, and points north.
The other issue was power outages. We always expect at least some power outages with winter storms, but the combination of the high water content of the snow (ratios were 5:1 to 8:1 in the area just north of the transition zone, as compared to the average of 10:1 to 12:1) and snow concentrated on populated but more heavily-wooded areas resulted in far more power outages than expected. While we addressed the potential for some power outages on Wednesday, we should have anticipated more widespread issues somewhere in the area given the expected snowfall ratios.
On one hand, the exact snowfall total forecasts were too snowy across the Triangle and east toward Rocky Mount, owing to an expectation of a more southerly low track, slightly higher snow to water ratios, and the early onset of snow egging us into bumping the numbers up more. However, the forecast was for a high-impact winter event for the Triangle and areas north, and mostly cold rain for points south, and that’s about what we got, albeit with more power outages than expected. Snow photos: Feb. 24-26, 2015
As of now, we don’t have any more wintry precipitation in the 7-day forecast, although, melting and refreezing of the snow and icy patches on our roads will likely be an issue for a couple more days.
But hang in there – we have a couple of days in the 60s next week with an outside shot at 70 degrees on Wednesday.
We appreciate the comments we’ve gotten leading up to and during both winter storms. Your reports, pictures, Facebook shares and tweets help us know what’s happening and the real-time intelligence about what was falling where and when it was changing over (and back) is invaluable in helping us help everyone. Thank you for watching and sharing what you’re seeing with us!