A Much Colder Weekend ... Will The Precipitation Be All Rain?
This weekend will be an enormous change from the brilliant temperatures we are seeing this afternoon. We will see waves of precipitation moving across the area, so it is important to time it out. Beyond the weekend, the very cold temperatures could mean the first hard freeze of the year! With tons to talk about let's dive in...
Waves Of Precipitation
The data right now suggests the front will be blasting into our area late tomorrow. That will start showers late tomorrow north of the river and mainly overnight for Louisville / areas south of the river. It is possible that the rain could affect some of our southern Indiana football games on Friday night.
By Saturday morning, our first wave of rain will be ending and we will wait for a second mid level disturbance that will come through Saturday night and Sunday. Notice the disturbance is moving across our area by around 8 am Sunday.
One question people are asking is if "any of this air will be cold enough to produce any snow"? Let's dive into that discussion and start with Saturday. Saturday is WAY to warm to support snow, so we move to Saturday night and early Sunday. I want to show you the height of the freezing line by Sunday morning first.
Freezing Line Height Sunday Morning
Notice freezing line is not just a little above the ground, but WAY above the ground at 2,500 - 4,000 feet.
One thing you may not know is snow falls at about 2 mph. That means snow will fall through about 10,000 feet in an hour. Now, when we look at the above freezing line we know that even if snow was generated in the clouds it would fall through above freezing temperatures for 15 - 25 minutes. We are not talking about temperatures that are a little above freezing early Sunday, but 8 degrees above freezing! That means this is generally a rain showers only storm system. I really see no chance for snow as things stand. The NAM and GFS agree. Notice they both show ALL rain for our area.
GFS Precipitation Type Early Sunday
Simple... all rain.
NAM Precipitation Type Early Sunday
Could We See A Hard Freeze?
From there we have to turn toward Monday and how cold it can get. Two things are critically important to seeing a freeze in parts of our are on Monday morning... the high pressure needs to sit right on top of us and dewpoints must be in the 20s. Let's look at both!
GFS Dewpoints Early Monday
Notice the dewpoints drop into the 20s along and north of the river early Monday. This is exactly what would need to happen to allow the atmosphere to cool at the max rate.
GFS Surface Pressure Early Monday
Notice the high pressure has dumped itself right over our area early Monday.
This means both criteria are met for us to have a maximum cooling setup early Monday. The computer model data is showing some VERY cold stuff early Monday. Keep in mind, the name of the computer model and timestamp are on the top right part of each image.
My Thoughts On This Major Fall Cold Push
In my opinion, there isn't any evidence to support anything other than rain for the weekend. Upper 30s can support snow flakes, but it has to be really cold above the ground. Sunday just doesn't look like it has cold enough air to allow this to happen. Even if snow flakes formed a few miles above the ground, they would have to fall 15 - 25 minutes through above freezing temperatures and and that will effectively melt it all to rain at the ground where we live.
The bigger concern comes Monday. We have ideal conditions for radiational cooling Sunday night and Monday morning. This means the atmosphere can realize the full "cold potential" of the air mass. That means temperatures outside the city will very likely fall to 32 degrees and a frost is almost guaranteed. North of the river, the cold air is a bit deeper and temperatures may drop to or below freezing for 4 hours which by definition is a hard freeze. Please make preparations to protect any of your plants or vegetables that are growing outside by early Monday morning or they might not survive. Monday morning looks like the coldest morning of the season and I have the official metro low at 34 degrees!
If you ever have any questions, please remember I can be reached on facebook easily! Just follow the link below to my facebook page and click "LIKE"!
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Marc, any chance for any sleet pellets mixing in?
Posted by: Danny | 10/04/2012 at 08:51 PM
Danny, for sleet, you normally need a deep layer of cold air above the warmer ground temps and this doesn't meet that criteria. The other way is simply by a strong shower that pulls down the colder air from aloft and this can never be ruled out. The truth of the matter is we are splitting hairs here since neither would be any consequence.
Posted by: Marc Weinberg | 10/04/2012 at 10:03 PM
Marc is the best in the business.
No other Meteorologists take the time and effort to explain to us like Marc does.
Awesome job-hoping for snow :)
Posted by: Dax | 10/05/2012 at 07:52 AM
Dax, thanks for reading through my long winded blog. :)
Posted by: Marc Weinberg | 10/05/2012 at 03:33 PM