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Wintry mix arriving tonight

RAD_MOS_REG_SE_L2WINTER

The radar at 8:40 this evening shows lots of light precipitation spreading our way. It will take the atmosphere  a while to moisten up, so it will take some time for precip to reach the ground. Some key points to remember include:

  1. Precip will be light and this is NOT an ice storm scenario.
  2. Travel will be impacted more than anything else. Please be careful if you must travel Monday morning.
  3. Temps will warm above freezing by mid-morning, with isolated valley locations possibly remaining below freezing until noon.
  4. The ground is very cold and precip that falls on it has the potential to create slick conditions.

Some basic meteorology

Currently, our dew point is 7 degrees and the air temperature is 33 degrees. The dewpoint and air temperature must get within 5-10 degrees of each other in order for wintry precipitation to occur here at the ground.  The dewpoint is the temperature at which precipitation can occur. If our temperature is 30 and our dewpoint is 7, we are a LONG way from getting precipitation. For instance, if you have a dewpoint of 30 and an air temperature of 30, the relative humidity will be 100% and it will be precipitating outside, or you will have extremely dense fog. With an air temp of 33 and a dewpoint of 7, our humidity is a very dry 33%. Any precip that tries to fall right now will evaporate in this dry surface air. Remember, earlier today I blogged about how arctic air is very dense and heavy. Warm air is very light (warm air rises, right?) and will ride over the top of this cold air here at the surface. Warmer air is more moist than cold, dry air and so moisture can occur up there in the warmer air. But, with our surface humidity at 33% any precip that falls into this air will evaporate until we can get our humidity higher.

As we go through the night, our dewpoint will quickly rise as our air temperature slowly falls. At this time, we expect them to become close enough together for us to get precip at about the 29 degree mark. This will give us several hours of light freezing rain/drizzle and will be more than enough for hazardous road conditions to develop. A moist air mass changes temperature more slowly than dry air, so when the atmosphere gets good and moistened up the temp will rise slowly. Therefore, our winter wx advisory is in effect until 9:00 a.m.

Another caveat we have to keep in mind is that as precip falls into this drier air, it evaporates. Evaporation is a cooling process and this will cause our temp to fall a few degrees when precip starts. Again, we think the magic number where precip begins will be 29 degrees. Heavier precip results in more evaporation and that results in temperatures falling even lower. However, we do not expect heavy precip, so evaporation will have a minimal effect on our temp for tonight.

That’s probably more than you wanted to know but that’s the gist of it. Now you know why we meteorologists keep aspirin handy (haha).

You all have a great evening and be careful if you have to get out in the morning!

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