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Weather Wisdom

Period of heavy downpours and thunderstorms prompt flood watches

Cold weather in the Midwest this week is part of the reason why we have so much rain in the forecast for the next 48 hours here in New England. The jet stream is taking quite the dip across the center of the United States with the return flow up the coastline. This pattern is similar to what we saw in January, but in July when cities have a temperature 20 degrees below normal; itís still in the mid 60s, not 10 below zero.

During a more typical week in July the contrast in moisture and temperature across the country isnít very dramatic. This is one of the reasons why the jet stream is normally so weak in the summer. Temperature differences help build a strong jet stream and without the contrast the summer jet stream is quite feeble in this area.

The map below shows the jet stream early Tuesday. The winds at 30,000 are blowing over 100 knots. This is more typical for any other season, but not summer. These strong winds will help bring lots of moisture north.

jet stream tuesday.png

There are flood watches up for much of New England overnight and these are expected to continue into Tuesday and Wednesday in some cases. Flooding comes in all different forms and this watch is mostly for urban street flooding and also some smaller streams.

How much rain are we expecting? As I write this some areas have already seen over an inch of rain since 4PM this afternoon. Clearly, these areas are going to see well over an inch of rain between now and Wednesday afternoon.

I'll update the location of the heavier downpours during the day tomorrow on Twitter @growingwisdom Feel free to chat with me there.

The map below is one models prediction for rainfall into Wednesday evening. A few things jump out at me. First, the model is already incorrect about the amount of rain in parts of the area. For example, I have already had more rain at my house than the model is predicting the entire time between now and Wednesday.

rainfall through Wed evening.png

The model shows pockets of heavy rain, up to 6 inches in some places and this means that the potential for some extreme amounts of rain is possible. This is why the flood watches are posted as well.

flood watch.png

Models in these situations can predict heavy amounts of rain pretty well, but do a poor job with regard to the exact location of the heavy rainfall.
Itís not going to rain the entire time between now and Wednesday evening. Our best chance for showers and storms is overnight tonight, tomorrow afternoon and again during the first half of Wednesday.

At times itís going to rain so hard, if you are driving you may have to pull off to the side of the road. Then there will be several hours of dry weather between the rain. The one constant the next two days will be the humidity which is going to be oppressive.

Temperatures wonít be overly hot due to the lack of sunshine, but all that moisture in the air is going to make it quite difficult to sleep at night without air conditioning.

I expect a drying trend Wednesday evening into Thursday when the weather turns much better, drier and more typical of July. The weekend looks good, with a chance Sunday is not as nice as Saturday, but we have plenty of time to refine that part of the forecast.

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