Tuesday morning has the makings to be the coldest morning of the season

As cold as it has been lately, temperatures early Tuesday could even dip below those numbers.  The metro has seen -11° so far this month, but that number could be in jeopardy tonight.  A cold Canadian high pressure will be directly overhead which could allow for perfect radiational cooling.  Radiational cooling is the effect we see EVERY night that allows temperatures to cool.  The surface of the earth emits long wave radiation that sends heat off the surface and back into the atmosphere.  With clouds around, or even smog or fog, long wave radiation bounces off these objects and heads back to the surface which makes it a vastly less efficient and keeps temperatures warmer. 


Along with radiational cooling, we have talked a lot about how mixing (winds) in the atmosphere and how it is a warming process.  At night, wind prevents all of the cold dense air in the lower levels of the atmosphere to sink to the surface which also prevents temperatures from falling as quickly.  This will keep numbers warmer overnight as well.

When we talk about "perfect" radiational cooling, we mean that skies will be clear, winds will be calm, and the center of a large air mass is directly overhead.  THIS is what could happen for much of the night Monday night.  Look at futurecast below.  This shows the likely mean sea level pressure for the Upper Midwest Monday night.  I have added the center of the high pressure which is obviously the blue "H" along with lines of equal pressure to show you where the pressure change is the strongest (indicative of stronger winds) AND added a color to the map to give you a good idea of where the center of this cold air mass is located (often the same spot as where your high pressure center, the big "H", is located).




So Monday around noon, the center of the high is about over Fargo, but as the day and night progress, this high pressure is centered over the Twin Cities around Midnight.  This would give perfect radiational cooling for the Twin Cities metro area through the evening and early overnight.  The ONLY thing that may prevent temperatures from REALLY bottoming out to -15° or even -20° in some areas is that the center of the high pressure is on the move.  By 7am, it will likely be just southeast of the area.  This could prevent perfectradiationall cooling from happening the entire night as return flow (southwesterly winds) begins to develop before dawn Tuesday.  This would give the area about 8 hours or so of perfect cooling versus 12 full hours.  This may not sound like much, but that could mean the difference between hitting -12° and -20°. Stay warm!

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