I'll use the atmospheric sounding profile below to explain. If you want "just the facts," skip down below the graphic for today's bottom line. As mentioned in previous blog posts, the atmosphere has some recovering to do with very dry air in place behind Friday night's Arctic front. That will delay the onset of precip until after lunch, and maybe mid-afternoon, on Monday. The changes in the models have to do with the temps in the lower (non-surface) levels of the atmosphere. The graphic below shows the GFS model's interpretation of the temperature and moisture profiles at 6pm Monday. This is after precipitation has been falling for a couple of hours.
Quick orientation of the graphic: this is a slice of the atmosphere at Memphis with the top of the picture at 17,000' and the bottom at the surface. The red plot is temperature, green plot is dewpoint, and dotted pink line is freezing. Where red/green are right next to each other, the humidity is very high (near 100%).
From this image, we see that the temperature between 3,000-8,000' is above freezing (red line right of the dotted pink line), while from the surface to 3,000' it is below freezing. Actually at the surface in this image, it is right at 32. The max temperature in the warm layer is about 37 degrees, while the minimum temperature in the cold layer near the surface is about 28 degrees. As precipitation falls from above (as snow), most of it will melt between 3-8,000', then some of it will re-freeze below 3,000'. Hence IF this model is correct, we will see sleet, possibly mixed with a little snow, and rain. And if the surface temperature is indeed 32 or less, the rain will freeze on contact - freezing rain. It's complicated... I know! While not shown, the other model we typically look at, the NAM, is warmer than this interpretation and depicts more rain/sleet with above freezing temperatures at the surface. It's our job to figure out which we think is right - or perhaps a combination of them - and make the forecast. So, onto the bottom line (jump below):
|GFS model sounding valid 6pm Monday showing a setup for mixed precipitation types|
Timing & temperatures: Both have changed very little. Precipitation is expected to start after lunchtime Monday, perhaps as late as 3pm, and continue through the evening, with most ending (except for light rain/drizzle) around 9pm or so. At onset, temperatures will likely get to near 40, then fall as precipitation starts, reaching the lower 30s (perhaps freezing) by rush hour. They will then recover slightly to the mid 30s overnight Monday night.
Precip type: At onset (early afternoon), I expect a mix of snow, sleet, and possibly rain. That should fairly quickly (by late afternoon) change to a rain/sleet mix with some flakes possible as well. By early evening (6-7pm), it'll probably be mostly rain. The heaviest precipitation should be between 3-7pm, when the predominant precip type will likely be rain/sleet. I also cannot rule out some freezing rain during this period if the surface temperature gets down to 32 (some bridges and overpasses could get to 32 even if the air temp is 33-34. Watch for slick streets during the afternoon commute home! Overnight, temperatures should stay above freezing in most locations, though outlying areas could see 32 degrees. Roads should be in decent shape overnight into Tuesday morning, though drizzle/light rain will be possible until dawn Tuesday.
Accumulation: In the immediate metro, the most likely scenario brings up to a half-inch of snow/sleet by rush hour, perhaps a light coating of freezing rain, topped off by light rain and rising temps that will likely melt anything that falls during the night. To the north (Tipton Co and points north), higher amounts of snow/sleet are more likely with a slightly higher chance of freezing rain in the evening. Over north Mississippi, it's possible that sleet/rain will be the predominant precip type, with little accumulation, washed off in the evening by rain.
Much of the finer details may not become known until the event begins. Be prepared for anything mentioned above, anywhere from mostly rain to some minor ice accumulations, to snow/ice mix with rain on top of that.
We'll continue to monitor the situation very closely and bring you updates via the MWN Forecast and on our social media feeds, including Facebook and Twitter . In addition, our mobile apps for Android and iPhone include StormView Radar with precipitation type (which shows you rain/snow/ice), the forecast, current conditions, and our Twitter feed (without having to sign up for Twitter). Finally, within the apps, you can upgrade to StormWatch+ which will push you notifications of any Winter Storm Watches, Warnings, or Advisories as soon as they are issued by the NWS.
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