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Forecast

Another chance of rain doesn’t materialize

The Boy and Bird Fountain in the Boston Public Garden might be the only source of water in the city today after expected rain missed the region.Jonathan Wiggs/Globe Staff

Every Tuesday data for the drought monitor are collected and assimilated and a report is issued two days later. I was thinking yesterday that when the drought monitor comes out Thursday it might not be accurate because we could see some rain Wednesday. I should have known better.

Here, in a pattern where much of the region hasn’t even received an inch of rain since July 1, significant help from Mother Nature was just too much to hope for. Check out the two charts below to see how much our rain deficit is growing. Notice that when it has rained, it’s been very little. The grey shaded area under the line of average precipitation is our deficit.

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Total accumulation of precipitation is well below normal.NOAA
Typical rainfall over the past 90 days would be over 10 inches or about 250 millimeters.NOAA

I think it’s worth explaining a little bit about why the rainfall today didn’t happen and why the forecast was all over the place in the past week.

In winter, nor’easters feed on a big temperature contrast. A major storm can have readings in the 60s on the warm side and teens on the cold side. This contrast in temperature gives us a lot of what meteorologists refer to as baroclinicity. When you have a lot of it, it’s easier for the atmosphere to produce more precipitation.

A summer nor’easter, which is technically what missed us today, doesn’t have much temperature contrast to work with.

A coastal storm was bringing rain to eastern Maine Wednesday morning. The red lines represent isotherms or lines of equal temperatures.NOAA

In this case it’s in the 60s on the coldest side and near 80 on the warm side for limited amounts of baroclinicity. This means whatever precipitation forms is going to be in a much smaller area. The models have a tough time discerning exactly where the precipitation is going to be and ended up overplaying this parameter.

Additionally, dry air in the middle of the atmosphere evaporated the rain as it was trying to come down out of the clouds.

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Thick rain producing clouds can be seen across eastern Maine Wednesday morning.COD Weather

Since we’re missing this rain it’s going to be back to the sunshine tomorrow with pleasant temperatures between 75 and 80. We start to warm up Friday with readings of 85 to 90 and that’s where it’s going to remain for much of the next 10 days. There will also be more humidity throughout the weekend. There’s no opportunity for any rain until sometime early next week; again, it’s only a chance.

Eventually, the drought will break but this one’s definitely going into the upper tier of the record books.