After some morning sun, showers and downpours have popped up especially over eastern Massachusetts. Some of these showers could develop into small thunderstorms. It will depend on your exact location as to how much rain you receive. The radar map below shows the heavier rain south of Boston, but this will be moving through the area this morning.
Meteorology 101 (skip below to forecast)
When you have cold air at higher levels of the atmosphere and warm air below it creates something called a lapse rate. The rate is positive if it gets colder as you increase in height and negative if it got warmer as you went up higher. The latter situation is quite common and is called an inversion. The rate at which the temperature drops as one moves from the ground to higher levels fluctuates. This fluctuation can be dramatic over a period of day or even during a single day.
The simple image below shows several scenarios of temperature dropping as one rises higher in the atmosphere. The lines on the right represent less of a dramatic cooling as height increases. The lines on the left represent a much more volatile scenario with rapid cooling with height.
The average lapse rate on any given day is about 18.8 degrees for every mile you rise into the atmosphere. A steep lapse rate of say 30 degrees per mile would create a situation where the air was very unstable and showers and thunderstorms more likely. Conversely, a lapse rate of 15 degrees cooler for every mile you rise would yield a very stable situation and thunderstorms and heavy showers would be highly unlikely.
This morning our lapse rates are steep enough that thunder is possible and I just heard some while updating this blog!
Another way to think of this is to look at temperatures on Mount Washington over a period of days and compare them with temperatures here. Some days you will notice readings drop over 30 degrees from sea level, where we live to the top of the rock pile at 6,288 feet or 1917 meters.
Today we have a steeper lapse rate. As temperatures rise at the ground the lapse rate will increase. This is why most showers and storms normally occur in the afternoon because that is the time of the day when the difference between readings at the surface and those at cloud level are greatest. This is also the reason why you often hear ďif we get some sunshine today the chance of thunderstorms increases.Ē
Currently, some upper level energy is combining with the lapse rates to bring us the showers, downpours and thunder. The most likely time of storms should be in the afternoon, however, based on current radar trends you should expect showers and storms to be around periodically for much of the day. At times it will be dry and the sun will make an appearance, but it's not going to be anything like the past 4 days.
The image below shows the predicted radar at 5PM. It appears most of the showers should be moving east of Boston by that time which is good news for the Red Sox game this evening.
Tomorrow and Friday we return to conditions of higher stability and a lot of sunshine. Thursday winds will increase and temperatures will be cool. Winds may become strong enough for wind advisories to be issued. More on that tomorrow.
On Friday the wind will slacken and highs will reach the 60F mark or a bit better in the afternoon.
The weekend is looking like a 50/50 split. Saturday brings the chance of showers, but the day is not a washout. Baseball and soccer games could still be played in dry weather although the timing will have to be just right. Donít expect a lot of sunshine Saturday. Temperatures will reach the upper 50s and lower 60s.
Sunday is the better of the two weekend days with more sunshine and seasonable readings in the 50s. The weather may turn unsettled again during the first part of next week.
Iíll be updating the forecast on Twitter @growingwisdom.