Thursday, January 15, 2015

Snow Day Forecasts: 1/7/15 - 1/15/15

Back in November, I said that I would try my hand at predicting when Triangle-area schools would delay or close due to the weather, with a focus on the following public school systems: Wake County, Durham County, Orange County, and Chapel Hill-Carrboro. The last couple of weeks gave me some opportunities, so let's see how we did:

Thursday, January 8, 2015

Synopsis: It's cold! The forecast low was 11°F with wind chills below 0°F, and a Wind Chill Advisory was issued for the entire area. But is that enough to prompt schools to delay? ... Well, maybe. Last winter, there were Wind Chill Advisories on two separate days: one delayed all schools two hours (1/7/14), and one did not (1/24/14). I thought this would be cold enough to prompt delays.

What actually happened: Many rural school districts delayed schools by two hours, but Wake County and Durham County did not delay. Wind chills never actually got below zero in the Triangle, but they got close.

Bottom line: That 1/7/14 cold snap was especially unique, giving the Triangle its coldest temperatures / wind chills in over a decade. Plus, everyone was panicking about the "polar vortex". (1/7/14 was basically "peak polar vortex panic".) So, maybe that was an exceptional case, and that it takes the kind of cold snap that only happens once every few years to close the largest area school systems (Wake and Durham counties). Most everyone else in the area, though, including Orange County and Chapel Hill-Carrboro, did post a two hour delay. Haven't yet figured out exactly what amount of "cold" it takes to delay schools, but it's clear that a Wind Chill Advisory is a necessary, but not sufficient, condition.

(By the way...I'm not taking it upon myself to judge whether the schools' decisions are "right" or "wrong", or whether schools should have been delayed or closed or whatever. All I'm trying to do is analyze and predict what they do, not whether that's what they should have done.)

Wednesday, January 14, 2015

Synopsis: Freezing rain was a near certainty, but...how much, and where? Throughout the week the forecasts went everywhere from "nuisance event affecting only bridges and elevated surfaces" to "possible 1/4-inch of ice resulting in scattered power outages". While we were under a Winter Storm Watch for a time, the final forecast was closer to the "nuisance event" side of things: Winter Weather Advisory for around 0.10" of ice, with higher amounts east of the Triangle along I-95. I figured school delays were all but certain, and that closings would depend on the impacts, with closings more likely east.

What actually happened: Tuesday night, all schools announced a two-hour delay. Ice totals underperformed the forecast region-wide (I think), and RDU only reported 0.02" of freezing rain. But that was still enough to cause icy roads and several accidents Wednesday morning, and that prompted all local schools to close for the entire day...except Wake County, which kept the two-hour delay intact. Because ice amounts were light everywhere, roads were actually worse north/west (Durham / Orange) than they were east (Wake), due to colder temperatures.

Bottom line: It doesn't take much ice to close schools for the day. Basically, the threat of freezing rain - any freezing rain worthy of a Winter Weather Advisory or more - will, at minimum, delay schools. Then, schools usually wait until the morning to decide whether or not to close for the day. Generally speaking, lots of weather-related accidents = closed for the day, although larger school districts like Wake have a larger threshold for that. Although, being a large school district, Wake County also has to consider conditions in the entire county, which can vary greatly. See how complicated this is? Still, I thought I had a pretty good feel for what would happen here.

Thursday, January 15, 2015

Synopsis: So, the "ice storm" is over. We're out of the woods now, right? ... Nope! Temperatures will drop back below freezing overnight, so any remaining puddles / wet spots will refreeze, resulting in a new black ice threat. Although, the NWS technical discussion (my primary source for weather forecast info) only said the threat would be "isolated" or "patchy" - I forget the exact word they used, but the black ice threat wasn't enough to prompt a new Winter Weather Advisory, only a less severe "Special Weather Statement".

What actually happened: Most schools delayed two hours (Chapel Hill-Carrboro a notable exception), citing conditions on untreated secondary roads. Temperatures largely stayed at or above freezing across the Triangle, and roads were mostly fine - certainly nothing like the day before.

Bottom line: Even after a small ice or snow event, it sometimes takes longer than it seems it should for schools to return to normal schedule. That's because secondary roads are low on NCDOT's winter maintenance priority list, meaning they won't return to "normal" until they melt/dry on their own. That can sometimes take a couple of days, especially in January. Thus, depending on how quickly temperatures moderate, public school delays/closures tend to linger after ice/snow events, even if everything looks fine at your house. This is more true in county-wide schools districts than it is in a city school district such as Chapel Hill-Carrboro, which saw fit to open on schedule Thursday morning. These kinds of "residual" storm-related delays are very hard to predict more than a day in advance, I'm finding. I honestly thought we'd all be back to normal schedule by today, but "no Winter Weather Advisory" does not mean "all clear".

Snow day totals for the season

Wake County: 0 closed, 2 delayed, 0 closed early
Durham County: 1 closed, 1 delayed, 0 closed early
Orange County: 1 closed, 2 delayed, 0 closed early
Chapel Hill-Carrboro: 1 closed, 1 delayed, 0 closed early

Still have a long way to go to match last season's totals.

The week ahead

No more weather-related delays/closures are expected for at least the next week. But, the medium and long range folks are saying that the end of January / early February could be quite interesting. If so, hopefully it will be more "snow" than "ice".

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