Wednesday, January 8, 2014

Winter Weather Advisory: light icing expected Thursday AM

It seems we can't catch a break from wintry weather, and yet no good snowfall!

Before we ended our 67-hour streak of below-freezing temperatures this morning at 10am, the National Weather Service had issued a Winter Weather Advisory for the metro, in effect tonight until noon Thursday.  An upper-level disturbance will move across the area Thursday, bringing with it a good chance of precipitation.  Unfortunately, the remnants of the Arctic outbreak linger and temperatures will be near freezing for at least a portion of the precipitation period.

Winter Weather Advisory area in purple

Without going into much detail on model data, we'll just say it's not been cooperating and confidence in the event has been, and still continues to be to an extent, somewhat low. However, I'm convinced of a couple of things: precipitation will fall and temperatures will (as usual) be tricky, floating near the freezing point for perhaps a good part of the time precip is falling. The main concern for this event will be ice and the potential for hazardous travel conditions, mainly Thursday morning.

Light precipitation has already broken out across portions of AR and this will shift east this evening. Since our starting point for temperatures will be the lower 40s this evening, I expect intermittent periods of mainly light rain or sprinkles from the evening hours into the early overnight period as the lower atmosphere moistens up (the air is very dry in the lowest 3,000'). As it moistens, surface temperatures will be falling into the 30s. Some sleet could mix with the light rain after midnight through about 6am. This will not be a steady period of precipitation through 6am, but rather intermittent "showery" type of precip.

After 6am, and especially after 8am, temperatures will be hovering near the freezing mark as steadier precipitation arrives. The steady, but overall light, precip is expected to last throughout the morning.  The big question - and one that models disagree on - is temps during the morning hours.  One camp says they warm up during the precip resulting in very minor ice accumulations.  The other (and one we put more faith in for METEOROLOGICAL reasons, not HYPE), keeps temps near freezing for much of the morning.  The reason: with steady precip and cloud cover and no movement of warm air into the area from a warmer source nearby (advection), we don't see temps rising much on their own until precip tapers off some. If precip were to be more spotty than we are expecting, then temperatures likely would be a touch warmer and we'd see more rain than ice.

Below you'll see two graphics from the winter weather desk at the Weather Prediction Center at NOAA: the probability of accumulating (at least 0.01") freezing rain and the "worst case" freezing rain accumulation, defined here by the 90% threshold.  In other words, there is only a 10% chance of exceeding the values shown in the second image.  Our chances of accumulating freezing rain are near 100%.  The "worst case," according to WPC, is near 0.10".  (However it should be noted that one model that has been consistent for a couple of days says we get nearly 4 times that during the morning hours. It's an outlier, but it's also a consistent one.)

Probability of freezing rain accumulating at least 0.01" by noon Thursday

"Worst case" map showing the 90th percentile freezing rain accumulation, meaning there is only a 10% chance an area will see more than indicated.

The bottom line

So our forecast is for spotty light rain this evening through midnight, mixing with a little sleet after midnight.  The rain will become light freezing rain mixed with sleet by dawn as temps hit 32, then change back to rain showers by late morning as temps warm above freezing.  No (or VERY little) snow is expected due to above-freezing temperatures aloft. Total freezing rain/sleet accumulations could be up to 0.20" though will more likely be closer to 0.10".  The low will be 31 and tomorrow's afternoon high 40, though we'll be in the 31-34 range throughout the morning.

One-tenth of an inch of ice is enough to cause bridges/overpasses to freeze.  I understand TDOT is already on the roads pre-treating.  This is a wise move.  Plan ahead for tomorrow morning's commute.  By afternoon, I expect very few problems with rising temps, but the morning could be dicey.  I don't know about schools closing, but I do know you working parents want them in school after an extended winter break! LOL  Follow our social media accounts listed below for updates and more detail as they become available.

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