Light snows arrive tonight/Winter Weather Advisory

Light snow is expected tonight — but it will be fighting dry air near the surface.  Some areas may see a couple of inches by this time tomorrow.

Outlook:

Tonight…Clouds return east and continue west. Snow won’t develop (beyond flurries) until late evening in KC and toward morning in STL/SEMO/SW IL.  Temperatures will slowly ease to around freezing, cooling below when snow begins. Greatest potential for accumulations in the KC area 12-6AM, accumulating snow potential really begins for STL 4-6AM & 3-6AM SEMO/SW IL.

Thursday…Light snow winds down as drizzle by 9 AM in KC and by noon SEMO/SW IL/STL.  Total accumulations 1-3″ for KC, a dusting to 1″ for STL and a dusting to 2″ for SEMO/SW IL.

Friday through Sunday…A cloudy period with slightly above average temperatures.  Highs in the 30 Friday -mid 40s over the weekend. Lows in the 30s.  Weak southern track system (discussed yesterday) may brush SEMO/SW IL with some scattered flurries or light snow.  We’ll discuss this system in more detail tomorrow.

Christmas Week…Little confidence can be placed in numerical weather prediction models eight days out; in a changing pattern. Two models now bring a windy storm to the Great Lakes… mild here to start.. windy colder finish possible. We’ll have to watch if future model runs maintain this system or change it’s placement.

Winter Weather Advisory for much of the area into Thursday:

A light snow event is expected to unfold tonight across the district.  This is not going to be a major event, but the timing will be an issue for the morning commute, especially in the metro areas. For some areas, it will be the first accumulating snowfall of the season and the out of practice driving factor will be present.  The areas in purple are under Winter Weather Advisories. The times on the graphic show how long into Thursday those advisories run.

2014-12-17-1513-WWAEAXLSXEDD

The “storm” itself has no defined surface low, with only a front banked up on the Front Range and through the southern Plains.  This yet another mid level storm system centered 5-10,000 feet above us.

2014-12-17-1434-WPC12Z1218F

Here’s the noon run of the standard resolution NAM model of the total snow..based on a 10:1 ratio.  It focuses particularly on three areas, SC-NE KS, EC KS to N MO and central MO to SEMO/S IL. The model expects a general inch east to 2-4″ far west.

2014-12-17-1429-18ZNAMSNWH39

The higher resolution version fills in a little more detail with finer bands.  These fine bands have still been changing:

2014-12-17-1429-18ZNHRSNWH45

It forecasts a dusting to 1″ east and 1-3″ west.

One thing that is working against this snow is a layer of very dry air aloft, from just off the surface to just over 7,000 feet above us.  That layer shows up well on the morning weather balloon sounding.  The green line traces the dew point (an indicator of the moisture in the air) and that sharp jag to the left indicates much drier air.

2014-12-17-1434-SGF12ZUAPLOT

With snow events we always have to deal with dry air at the onset of the event, which means some of the snow that falls will evaporate into the dry air before reaching the ground.  This will happen until enough snow evaporates to moisten the layer and allow the snow to reach the ground.  In short-duration events like this, that means we “waste” some of the snowfall time moistening up the air which cuts down the snowfall totals.  How fast that dry layer goes away and how much it has changed (shrunk/expanded) since this sensor reading was taken at 6AM will go a long way toward determining snow accumulation.  The next weather balloon launch won’t be until 6PM tonight.  If tonight’s readings suggest the dry layer has expanded, you might expect snow totals to come in under what is forecast right now.

Here’s the latest maximum snowfall accumulation forecast:

2014-12-17-1547-EDD72HSNWACC

We’ll see how this all plays out in the next 24 hours and recap totals tomorrow.  We’ll also take a look at the weaker late week/weekend system. Monitor local media for updates and plan for extra travel time in the morning – just in case.

 

 

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