Wednesday, January 10, 2018

Friday's winter weather event - our first call at the details

The best chance of wintry precipitation so far this year arrives this Friday. While (most) everyone looks forward to "snow days," if you end up with a change of plans from a normal day this time, it will most likely be due to ice, not snow. Snowball fights are currently not in the cards!

First a reminder that, as of this writing Friday evening, we're still about 36 hours away from the onset of winter precipitation, and while the computer models are generally coming into some alignment, we also see some "wild hairs" out there that lead to reduced confidence in some areas (which is not atypical for a southern winter event). Exact details are yet to be resolved and the situation remains fluid.

I'll start with the bullet points, based on the latest data and sound meteorological reasoning, unaffected by the wishes of any particular lobbying group. :-)

A Winter Storm Watch is in effect Friday for the blue counties, which are areas that have the greatest potential for impact to the general population. The counties under a watch, and later a warning or advisory, could change with new information.

What will fall: Besides temperatures, rain, then freezing rain, then a mix with sleet and snow, ending as light snow.
Start time: Rain transitions to freezing rain (ice) early Friday morning, probably between 3-5am
End time: Perhaps the lowest confidence portion of the forecast, but most likely by mid-day Friday
How much: Freezing rain: up to 1/4"; sleet and snow: 1" or less

Now for some details...

Despite very warm temperatures today and Thursday, a major cold front will escort our next Arctic air mass into the Mid-South Thursday late evening. This front will cause temperatures to drop rapidly towards freezing by the early morning hours on Friday on a strong and gusty north wind. Rain could occasionally be heavy along and just behind the front and a rumble of thunder can't be ruled out. Precipitation intensity should let up a bit as the temperature nears freezing. Once it drops to 32°, rain continues to fall (because the near-surface layer is cooling more rapidly than temperatures aloft) and starts freezing to objects that also drop below freezing - power lines, trees, and other exposed objects.

Forecast temperatures at midnight Friday just after the cold front passes through Memphis. Northeast MS is in the lower 60s, while north-central AR is in the upper 20s. (WxBell)

The European model from Wednesday morning shows the primary precipitation type between 12-6am Friday to be freezing rain, with some trailing snow to our west. Note that the precipitation does not extend very far to the west, indicating that it likely won't last long after 6am. This solution is one that favors an early departure to the precipitation and little snow. THis is just one of many models that are available to the meteorologist however. (WxBell)

Initially, roads should still be fine as they will be warm, but they will also cool quickly, particularly those with cold air blowing under them (think about an overpass). It won't take long for those to start feeling the effects of the freezing rain. As the air just aloft also cools, the rain (that freezes on contact) should start to change over to some sleet (frozen raindrops), and then probably light snow as precipitation rates continue to fall. This entire process will probably take 3-6 hours. So, from when the surface temperature first reaches freezing (right now, I'm saying 3-5 am) to when it's cold enough to snow will take 3-6 hours, or sometime during the morning hours. During that time, depending on how much sleet mixes in, we could see upwards of 1/4" of freezing rain.

During the course of Friday's event, we expect to see all of these precipitation types, moving from the left to right as warm air is overtaken by cold air, starting at the surface and moving up.
Models all agree on the details to this point fairly well. It's after this time that they start to diverge. There are multiple indications that once it gets cold enough to snow, the moisture moves out. Others keep a little around for a short time. Either way, the probability of more than a dusting to an inch of snow is not high. Then there is the occasional "outlier model" that keeps snow falling well into the afternoon. You can hope for that, but my hard-earned money is not on it!

Confidence Levels

Given we are still a little ways out, the models aren't in as good of agreement as I'd like, and we're in the south, confidence levels are worth a short discussion. Here is my confidence in the following events:

Onset time: Medium-High. If I'm wrong, it could be a bit later, but not by much.
End time: Medium-low. This could go either of two ways. A couple of solutions point to the precip ending not long after sunrise. An outlier from this afternoon (I'm looking at you 18Z NAM) says it'll snow all day. I'd pick the under if I were you.
Precip amounts: Medium. If I'm wrong, I am aiming high on the freezing rain total. I wouldn't be surprised to see it stop at 0.10" or so. That's still a real problem, so based on impact, it's not a big difference. I'm fairly convinced we won't see much snow. Less than an inch seems a solid (educated) guess right now.
Schools Cancelled: You thought I was going to bite on that didn't you...

What is the bust potential? There are valid model solutions that show precipitation ending not long after it gets cold enough to start causing problems. This is GOOD because nobody wants an ice storm, but it could result in minimal accumulations and no snow. If the precipitation ends early, snow will be the first sacrifice.

Impacts to Business and Commerce

Roads - Contrary to popular opinion, Memphis and surrounding jurisdictions do have the ability to salt/sand roadways, as does TDOT, which handles the interstates. However, given that these events are not frequent and the number of lane-miles in the metro, every road (even every major road) cannot be treated with current assets. Priority is given to the most critical and accident-prone areas, especially including elevated roads, such as Bill Morris Parkway and various interstate flyovers and ramps. In this event, rain will be falling prior to the arrival of the cold air, so pre-treating with salt brine would be a waste of money. It'll wash right off. Expect roads to be treated once freezing begins, and avoid known trouble spots altogether, even if you leave early for work Friday morning. If even light amounts of freezing rain occur, the elevated roadways will be slick and could be closed for treatment at times.

Airport - Believe it or not, the Airport Authority and FedEx are probably better equipped to handle winter weather events than the road crews are around here. They have plenty of equipment, and there's just not a ton of traffic. Cancellations into relatively minor events like this are lower than you think. The runways will be cleared, even if one at a time (that's all you really need), airplanes will be deiced, etc. The main concerns are: A) if freezing rain is more than forecast, more cancellations would occur, and it would be on flights heading into Memphis (which then means there is no plane to take you somewhere else), and B) weather elsewhere. This storm system will affect a lot of places on Friday. Cancellations elsewhere will ripple through the system like a boulder thrown into a pond. As always, check with your airline or your airline's website or app for the latest before your cross multiple iced up bridges and overpasses to get to the airport and find out your flight is cancelled.

Schools - Fortunately, I don't have to make this decision! However, the fact that (currently) the event is supposed to start very early in the morning COULD make their decision a bit easier. By 5am when many school decisions have to be made, we should know what we're dealing with. From experience, districts don't tend to mess around with ice. Kids - do your homework, then stick the spoon under your pillow. Hopefully you get to stay home and enjoy a four-day weekend (Monday is MLK Day)!

Bottom line it for me, Erik

Things are lining up for a potentially impactful winter weather event, particularly from the perspective of ice. At this time, I don't expect to see massive and widespread power outages or trees downed, but travel could definitely be hazardous to treacherous, especially Friday morning, and there could definitely be an impact to business and commerce. Don't be surprised at little to no snow though. Plan ahead now and exercise caution later. If not necessary to get out Friday morning and ice is on the ground, don't. I'm planning another formal update Thursday evening, hopefully via video. Stay tuned to our social channels listed below for updates prior to then, and of course during the event.

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Erik Proseus
MWN Meteorologist

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