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Mid-winter cold tonight, but milder and wetter weather coming

Posted by David Epstein  November 24, 2013 03:29 PM

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What a cold and blustery day. I walked the dogs at 6:30 this morning because as the day progresses temperatures arenít going to warm much and the winds will actually increase. Wind chills are going to remain in the single digits and teens all day and at times they may even fall below zero. While this isnít record breaking cold, it is somewhat unusual to have such a powerful arctic blast this early.

I'll be updating the forecast for the week on Twitter at @growingwisdom If you are traveling this week and need a specific forecast let me know.

You can get a sense of how strong the cold is by looking at temperatures across the country. The cold air has penetrated into Dallas, Texas and the panhandle of Florida. This is more typical of late December. Here in the northeast the high temperature today was reached at 12:01 this morning as no town will make it above the readings back at midnight.
cold air.gif
Patriots Game
The game is going to be played in very cold conditions. Temperatures will be falling back into the teens by the end of the game with wind chill readings in the single numbers to just below zero so bundle up!.

The air is also exceedingly dry. When you get air coming in from the arctic regions it contains no moisture. As you bring that air inside your home and warm it up, you drop the relative humidity to under 10%.

Humidity lesson
Relative humidity tells us how close the air, at any given temperature, is to being full of water or saturated. Today temperatures are in the 20s and there is very low amounts of moisture are present. In this case, the relative humidity outside is about 40%. As you take that exact same amount of moisture and bring it into your home, then warm the air some 40 degrees the relative humidity drops profusely. If you donít have a humidifier to add moisture back into the air, then itís going to feel very dry the next two days.

As air cools the level it becomes saturated falls. In other words, cold air can contain or ďholdĒ less moisture. When I teach relative humidity to my students I illustrate the point by asking them to think of a swimming pool as warm air and a small cup as cold air. When the swimming pool is full there is a lot of water, a full pool is saturated. Relative humidity tells you how full the air is at a given temperature. So, the pool at 50% full or relative humidity still has a lot of water, thatís warm air. It can feel very humid even if the relative humidity is 50%.
relative humidity lesson.gif
The cup can be 100% full or saturated, but not have very much water. Thatís why even if the relative humidity is 100% on a cold day, you donít feel uncomfortable, there just isnít enough moisture in the air.

The dew point is another term you often here. Dew point is actually a temperature and represents the temperature the air would need to be in order to be saturated. If the temperature is very high and the dew point is very low, the spread is very large and the relative humidity is low. The term dew point makes sense because when the temperature and dew point are the same, dew forms.

On a clear morning with a temperature of 40F and a dew point the same, the grass would be wet and the relative humidity would be 100%. On an afternoon with the dew point at 68F and the temperature at 86F the relative humidity would be a seemingly comfortable 55%, but you would feel very uncomfortable from all the moisture in the air. Conversely, if you were in Phoenix, Arizona and the temperature was a whopping 100F and the dew point 20F, the relative humidity would be only about 5%.

Upcoming rain
Our cold dry air is going to stick around for a couple of days before warmer and more moist air returns to the region. Tomorrow morning will be the coldest one since January or February earlier this year. It will feel a bit better Monday afternoon with less wind, but it's going to be very cold. Highs will approach 32F around 2PM.

As a storm moves up the coastline later Tuesday and Wednesday a period of rain, some of it heavy will cross the area. Right now it appears enough warm air will move into the region so all the precipitation will be liquid, not frozen. This is great news for folks traveling on Tuesday night and Wednesday.

This is going to be the first major rain we have had in a while. I expect most areas to receive close to an inch of water. Some of us could even see closer to two inches of rain before this is over later Wednesday. This is great news as the ground really needs to be saturated before we go into winter. As cold as it will be Tuesday, you might want to be sure your gutters are not clogged before the storm begins later in the day and overnight.
Thanksgiving looks sunny, but cold with highs just around freezing.

The map below shows how much rain is going to fall over the next 5 days across the east. Some of this might be snow well inland, but you can clearly see the path the upcoming storm takes as it moves from the Gulf coast states north along the Atlantic seaboard.
gfs_total_precip_east_20.png

Around the House This Week
I know some of you are already thinking about Christmas trees. Did you ever consider there are great trees you can add to the landscape that make great Christmas trees? Even if you don't decorate the tree, it will still be a wonderful addition in the landscape.


I'll be updating the forecast for the week on Twitter at @growingwisdom Please follow me there. Feel free to comment or ask questions too.

This blog is not written or edited by Boston.com or the Boston Globe.
The author is solely responsible for the content.

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About the author

David Epstein has been a professional meteorologist and horticulturalist for three decades. David spent 16 years at WCVB in Boston and currently freelances for WGME in Portland, ME. In 2006, More »
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