Sunday night to Monday night
The warm sector dominates with increasing southerly wind and cloud cover. While a few spotty showers are possible, not a lot of rain is forecast. This is a bit of a change, particularly related to Monday. I don't expect much precip through sunset Monday, though can't rule out a shower or two. Temperatures will hold steady or climb a bit tonight, remaining above 50 degrees and possibly near 60 by sunrise Monday. Gusty south wind is offset a bit by cloud cover, but still expect mid 60s during the day. Fallen leaves will be blowing around in wind gusts to 25 mph. |
| The surface map for 6pm Monday shows the cold front on our doorstep with plenty of moisture to our southwest that will move this way behind the front. (NWS/WPC) |
Late evening is the current ETA for the arrival of the blast of cold air. Wind will shift quickly to the northwest and temperatures drop overnight with rain likely, but still most likely not heavy, other than perhaps briefly along the front. By sunrise Tuesday, most areas should be in the mid 30s with rain falling and wind chills in the 20s.
Tuesday - what form of precipitation?
This is when it gets tricky. Cold air will continue to rush in behind the front so the daytime high will likely be at sunrise (mid 30s) with temperatures steady in the mid 30s in the morning, falling to near freezing by sunset. Most precipitation actually occurs behind the front in this scenario as a strong jet stream overrides cold and moist air below. Therein lies the issue. |
| NWS Weather Prediction Center forecast for (liquid) precipitation on Tuesday is about 1/4" in the metro. Given that some of that is likely to fall as winter precipitation, accumulation looks entirely possible, but not a lot. |
For veteran readers, particularly those who have read wintertime posts, you know that the surface temperature is not the only driver of precipitation type however. It can snow in the upper 30s and rain at 33 degrees. Temperatures aloft are also a significant factor. And in this case, they also cool Tuesday with mostly below freezing air above a thousand feet or so by mid-day. As that air cools, we also see the potential for dry air aloft to factor in. The cooling rate and amount of dry air is where the models diverge right now and makes the forecast tricky. I believe the hours between rush hour and noontime could present a variety of precipitation from cold rain to sleet to some snowflakes. The dreaded "wintry mix"!
After lunchtime, there will still be some moisture lingering and temperatures aloft continue to cool. I think this is our best chance of straight snow until precip comes to an end by late afternoon. The biggest question mark in the afternoon, and one that models also have varying perspectives on, is how much precipitation falls when the air is cold enough to support snow. By sunset, all precipitation should be wrapping up as abundant dry air moves in with precip following the entire system to our east. Clearing takes place overnight as temperatures drop into the upper 20s.
Factors that affect precipitation amounts
Today's computer models present a wide range of snow accumulation amounts, but few account for all factors that can (mostly) reduce the ultimate amount we observe. First, most do give the Memphis area at least a light accumulation, while a few are just plain drunk. I won't show them, but if their solutions pan out, you won't be at school for a couple days. That's not my expectation. Why?- Warm ground: we'll be in the mid 60s Monday and it's been fairly warm for several days leading up to the event. Unless the snow falls fast, ground temperatures don't support high accumulations.
- Dry air: the presence of dry air aloft "eats up" the falling snow, as my friend David at NashSevereWx likes to say, and it's true. Not only can it result in a bit of sleet (which reduces snow totals), but it "evaporates" the falling precip as well.
- Wet ground and falling rain: Hypothetically, if an inch of snow falls and is mixed with rain, it washes off. The snow fell, but it melted and went into the gutter. An inch of falling snow doesn't mean an inch on the ground when it quits. Any mixture of snow with rain basically doesn't count.
All of these factors are not the friend of snow lovers.
So bottom line it for me
So here's my best (educated) guess as of 6:00pm Sunday evening.- Rain transitions briefly to a mix of rain/snow/sleet Tuesday by mid-morning, then to mostly snow by the lunch hour, ending by the evening rush hour.
- Temperatures remain above freezing, barely, until most precipitation ends.
- Snow amounts range from a coating (maybe 1/4-1/2") up to an inch, along with a small amount of sleet. Where the highest potential for an inch lies is unknown. Models vary on where the main "snow axis" lies, but agree it will stretch from southwest to northeast somewhere near the I-40 corridor.
 |
| NWS Weather Prediction Center currently offers the probabilities above for accumulation of at least 1" of snow on Tuesday. It's not high, but it doesn't mean it won't snow, and it is a lower probability than many model solutions. We currently believe a half inch to an inch is the "sweet spot." |
What are the impacts?
As far as impacts, well traveled roads should have little issue on Tuesday during the day as long as temperatures stay just above freezing. I don't foresee anything too heavy for any length of time. Grassy or earthen areas and outdoor objects have the best chance of seeing a coating of white by Tuesday afternoon.
Tuesday night, as skies clear, wind dies down, and temperatures drop below freezing, could pose minor issues. It will depend on the state of the streets when the sun sets. We'll want to keep a close eye on that, but for now, expect there might be some patchy areas of ice Wednesday morning as water freezes, especially on elevated surfaces. Wednesday will be dry and sunny with highs in the mid 40s. Any lingering issues should clear up quickly.
A couple final notes
1. Roads will NOT be treated ahead of the event due to rainfall leading up to the transition. All response from local government agencies will be reactive by plan.2. Also, as described, this will not be a case where school officials will be able to "see what's on the ground" at 5am Tuesday morning to make their decisions. It'll just be wet. I also can't predict the chance of a snow day, and honestly it could be hard even Tuesday morning to know what it will be like in the afternoon when buses roll to bring kids home. God bless the school administrators - I wouldn't want your job!
Of course as computer models start to (hopefully) iron out the details, the forecast is subject to some modification. Please stay tuned to our social media channels listed below, as well as our forecast online or in your MWN app, for the most up-to-date information.
Erik Proseus
MWN Meteorologist
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