Data Strongly Suggesting A Frost In Our Area
As we close in on the weekend, we are now starting to get the higher resolution models looking at how cold Sunday and Monday morning will be. The forecast is rather interesting because if we go clear skies with calm winds, then I think almost all of us could see a frost by Monday morning. I can't believe we are talking about this just days after this brutal summer will end!
There are key features that we look for when forecasting how cold night time lows will be. Clear skies and light winds are absolutely critical to realize the full "cold" potential of any air mass. In a situation like this, we would need the high pressure to anchor itself over us to allow for maximum cooling during the nights this weekend. Let's look at where the high pressure is sitting on Sunday and Monday morning.
Surface Pressure Sunday Morning
Notice the high pressure on Sunday morning is centered over Missouri, so we know this is not the "perfect" setup for atmospheric cooling.
Surface Pressure Monday Morning
Notice that Monday the high drops anchor right on top of us. This means the atmosphere will be perfectly set up to realized the cold potential of the air mass.
As we start to look at the raw data from the computer models, keep in mind the name of the model and timestamp is on the top right of each image. Remember the NAM and AdvanceTrak have much better resolution of the atmosphere, in particular the lower part of the atmosphere where we live. That means they tend to be more reliable when looking at low temperatures.
Based on our discussion above, we would expect the coldest morning to be Monday. The GFS has stayed firm on it's forecast low and the EURO continues to trend toward the GFS.
My Thoughts On A Potential Frost
From my perspective, I see a lot of things coming together for a potential frost on Monday. High pressure sits right on top of us assuring an area of clear skies and light winds. When this happens, the heat at the surface easily escapes to space in a process we call "radiational cooling". With a unseasonably cold air mass in place, I think a frost is likely for a large portion of us outside of the city and scattered frost in the metro. I actually think a frost is almost a lock for our northern counties. I also think it would be wise to prepare to cover you tender vegetation Saturday and Sunday night to protect things.
Records in Louisville are 39 degrees on both Saturday night and Sunday night, so it will be tough to break these. The best chance we have at that record will be Sunday night / Monday morning. Right now I am forecasting a low of 41 by Monday morning. Just remember, you could need this by Monday morning...
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