A Messy Forecast

Late update here on a Saturday night.  There's been some big changes since yesterday's blog post, so I wanted to fill you in.  As expected, the rain has arrived tonight, and it looks like we're in for a soaking.  Through Sunday night, much of the region is expected to rack up 1-2" of additional rainfall.  Here's the latest rainfall forecast graphic (6PM Sat-6PM Sun):


Along with this rain, a strong cold front will move through by Sunday morning, causing winds to shift to the NNW and temperatures to drop as the day wears on.  We'll drop to the mid 40's overnight, then fall to near 40 by Sunday at dusk.

The big change since yesterday is that the "closed-low scenario" that forecast models had gone back and forth on appears to be happening after all, which will have a major impact on our Monday & Tuesday weather.  Instead of getting back to dry weather, things will stay cold and wet until Wednesday.  Before I go into more details on the forecast, let's looks at what I mean by a "closed low".

At some point you've probably heard us discuss troughs and ridges....ridges are associated with high pressure, troughs with low pressure.  Usually cold fronts and unsettled weather accompany a trough, which is certainly the case this weekend.  Here's a look at a forecast model showing a large trough across the central U.S. as of Sunday AM:

See the big dip with the bright colors that looks like a "V"....that's the trough, sometimes called an "open wave".  As this trough deepens and evolves, it will develop into a closed low by Monday morning. Here's what that looks like on the models:

Notice how the white lines have closed off?  There's your closed low.  These are also sometimes called "cold core lows".  The ultimate result of this low is that it will keep moisture flowing in from the east, and at least on the western side of the low, will pull in colder air from the north.  This means rain chances will continue into Monday and Tuesday.  It also means that some of us may see our first snowflakes of the year.  Let's check out another model chart...

I know they are a little tough to see, but look at the dotted red and blue lines on this chart.  Those are atmospheric thickness lines.  Without getting too technical, they basically give an indication of temperatures, and where you see the blue lines, that is sort of a "quick glance" check of where it is cold enough for frozen precip to form....something we call the "540 line".  Sure enough, right in the very same region as our closed low, we find the 540 line, and a chance for rain to mix with or change over to snow.  Based on the forecast position of the low, it looks like the coldest air will be found over southeast MO & northwest TN, and thus the best chance for some flakes Sunday night and Monday.  There may also be a little mixing or a changeover again Monday night and Tuesday morning.

As far as the snow portion of the forecast, this is really only a big deal simply because it's the first chance of snow this season.  With warm ground temperatures, there's no reason to think anything will stick, and any snow that does fall won't amount to much anyway.  So no reason to go buy bread and milk...unless you just need some!  The rainfall portion of the forecast is a different story.  By the time this system moves out Tuesday night, many of us will have added another 2-4" to rain gauges that have already measured a lot of rain this month.  Before the beginning of tonight's rain, we had 9.25" here at the station this month, and the NWS Paducah had 7.48".