1/17/12 Why did we get more snow last night?

Late last night, if you were awake after 10 PM, you noticed that it started to snow, and at a pretty good rate.

The overall forecast was for a warm front to push through the region, and bring a light mix of rain, snow and sleet and quickly change over to rain as temperatures rose to the lower 30s. No significant accumulations.

Well, by 11PM and midnight, I fielded 2 comments about the amount and intensity of the the snowfall. So I thought I would give a reason why this happened.

Temperature and Dew Point graph from 4PM 1/16 to 2:15AM 1/17. Recorded at  KMALYNN7

A . The arctic air mass was still in place at 5PM. Temperatures rose to a seasonal high, but as the sun set the temperature dropped to around 28F. The dew point was very dry at only 5F!

B.  As the warm front approached, the temperature slowly began to rise, but notice the dew point or the moisture in the atmosphere, it began to rise rapidly. The arctic air mass was being pushed out.

 At roughly 9:15 the temperature rose above freezing. But the dew point was still below freezing at 21F.


C. Precipitation begins to fall at around 10 PM with a temperature around 34F and dew point of 24, and relative humidity of 66%. (Meaning that the air still has to 34% more moisture to hold).

However, with the air still VERY dry the temperature fell to meet the dew point to become saturated. This is called evaporational cooling!   To best explain this think of swimming in the summer. Take 2 days. One hot and humid and one mild and comfy.  You notice that when you get out of the water on the hot and humid day it does not feel that bad and it takes along time to dry off.  However, when you get out of the water on a dry day with a breeze, you feel so much colder as you begin to dry off. The water can easily evaporate into the air and make you feel cooler. This is evaporational cooling, and it works the same in the atmosphere.


D. The temperature cooled, due to evaporational cooling to be closer to the dew point, at which point the air temperature fell below freezing to allow for a greater amount of snow to fall.




Forecast:

Today - Expect today to be mild as warm air moves in from the south and ahead of a low pressure system over the Great Lakes. Rainfall will be light and it will be mild overnight with temperatures not dropping below freezing.

This evenings weather map showing a storm over the Great Lakes with warm air over southern New England.

Wednesday - The cold front pushes through tomorrow morning and our highs only reach the upper 30s and it will become windy!

The next chance for snow showers comes Thursday afternoon as a weak clipper moves through.