An ensemble look at Wednesday's winter weather

You've probably heard us mention ensembles as a forecasting tool, one that utilizes multiple runs of the same or a small group of computer models starting from the same basic input conditions, but with a series of small "perturbations" to the initial conditions that are on the same order as we would expect from measurement errors from instruments at weather stations, on balloons and aircraft, and the uncertainties in initial analyses that result from the fact that we're only able to measure the state of the atmosphere at certain locations, especially aloft, where there are great distances between the points where balloons are launched. In addition, those launches only occur on a routine basis every twelve hours, occasionally every six in critical weather situations.

All that said, it turns out that for longer range forecasts, on average a mean or median of ensemble forecasts will prove more accurate than a single run of the same model. In the short term, like a couple of days, this is not as true, but the spread (or lack therof) in how the different runs progress gives us some idea of how big a range of possible outcomes exists for the weather pattern we're starting from.

Whet I'm showing here is a couple of screens that show a time series of all 21 "members" of a system called Short Range Ensemble Forecast, or SREF. These are all valid for the Raleigh-Durham airport and cover the period through Friday afternoon. The first shows the precipitation probability for each type that is produced by the the various models as the system moves through. It is based simply on the fraction of the 21 models that produces rain, snow, sleet or freezing rain at a given time. As you'll see, among these models the chance of snow (blue line) dominates as the system moves in on Wednesday, with the chance ramping up to between 50 and 70% in the afternoon (18Z on the 12th is 1PM our time, 00Z is 7PM and so on). During that same time, the chance we have sleet or freezing rain instead is about 10-15%. Later tomorrow evening and overnight, things change more with the chance of freezing rain (red line) climbing to around 40%, while the chance of snow drops to about 20-25% at the same time as the chance of a switch to rain climbs to about 20-25% as well due to warmer air potentially edging in from the east. This raises concerns about icing that could lead to power outages. By Thursday, as low pressure pulls off to the northeast and colder air wraps back around it, precipitation amounts drop off but the chance of rain or freezing rain decreases and the chance of snow as the precipitation type climbs back up, to about 30-40% in the early afternoon.

The second image shows the amount of liquid equivalent precipitation generated for RDU by the 21 members over the course of the entire event. You'll see those amounts ramping up in the early to mid afternoon hours Wednesday, and then really building through the night before tapering off on Thursday. The totals range from around 1 inch to just under 2.5, with the biggest consensus of multiple runs somewhere around an inch and a quarter. How this ends up partitioned between snow, sleet and freezing rain (and maybe some rain) will greatly influence the amounts of any of those we see. Based on what we've seen of this group and several other projections, it remains possible that we'll see a very significant snowfall, but the chance is not insignificant that several tenths of an inch of icing could occur, and the outside chance remains that snow and ice amounts will be notably limited by a changeover to rain. Generally, these possibilities will favor rain and freezing rain more toward the east and southeast, and more/longer snow toward the west from Raleigh.

Based on all of the possibilities, though, we should all be prepared to deal with the road hazards associated with significant snow and sleet, but also to get through a period of power losses that could come in the event freezing rain dominates. As a reminder, more than about a tenth of an inch of freezing rain can add to slick roads and bridges, make sidewalks, decks and stairwell treacherous for walking, and weigh down some branches enough for isolated power interruptions. As we top .25 inch of ice, trees bending and branches breaking, along with drooping power lines, can add more numerous power outages to the mix, and when ice thickness tops .5 inch (at least a small possibility with this system) larger trees can topple and some power lines can actually be snapped.