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New England May Weather Outlook: Cool Start, Warm End

April showers may be lingering, but May flowers are coming, see the forecast below

May flowers spring up in a sign of warmer times coming.

We've officially made it to the "warm months", although it won't feel that way for a little while. The start of May is going to act like the start of April after much of April didn't act like April. With that in mind, here's how the month is looking to shake out.

The first week of the month is all but guaranteed to be a good 5-15 degrees below average with the exception of the very first day of the month. This cool start to the month could be a bit stubborn thanks to two major players: North Atlantic Oscillation and an Omega Block.

First off, North Atlantic Oscillation (NAO) has been in a negative phase since the end of our "summer preview" in the middle of April that sent temperatures shooting into the 80s and 90s. A negative phase of NAO tends to lead to cooler than average temperatures in the eastern United States along with more unsettled weather (this is especially true in winter). A positive phase tends to have the opposite effect. This can be seen by looking at the NAO index since the beginning of the year:

NAO Index for the year and forecast through May 15

When the index is above zero, it has been during generally mild times while times below zero correlates to generally cooler, stormier weather. Remember, not every single day will match exactly with this trend as there are many factors that determine what the weather will be from one day to the next. For example, our epic arctic blast in early February occurred during a positive phase (although the fact that this did occur in a positive phase is a reason that the arctic blast was so short). NAO gives a good idea of the overall trend.

With that said, you can see the NAO index forecast through May 15th on the chart above. The index dives even lower than it has been for the first week of the month. The index is expected to stay negative, but begin rebounding toward zero after week one. This could lead to a warmer week two, but certainly no summer like heat.

The other player in our stubbornly cool start is what's called an Omega Block. This block is so called because the jet stream often takes the shape of the "omega" symbol. These blocks lead to weather systems to get stuck in place, causing an area to see the same weather for days on end. More often than not, New England gets stuck on the cool and showery side of the block and this time is no exception.

Omega Block weather pattern that will hold over the United States

In the early part of May, this setup will block (hence the name) a low pressure system over the northeast. So, this blocking pattern combined with the negative NAO will lead to a rather miserable start to the month.

The good news for those looking for some more sunny, warm weather is that this pattern looks increasingly likely that it will begin to break down heading into the second week of the month. These patterns can be very stubborn and take a while to fully break down, so an expected warm up heading into the middle of the month will likely occur gradually, with average temperatures likely heading toward the middle of the month. This can be seen in the NOAA's 6-10 day temp outlook:

United States temperature outlook for week 2 of May

Despite forecast uncertainties, the jet stream is expected to "flip" heading deeper into the month with a ridge building in the east. This would support more above average temperatures as we head toward the meteorological start of summer. Chances increase for above average temps in the 8-14 day outlook.

United States temperature outlook for week 3 in May

One of the big forecast uncertainties is exactly when El Nino conditions will develop. El Nino has a more direct impact on weather in other parts of the world with the US seeing less noticeable effects, but this weather phenomenon can cause jet stream ridging in the western US, which would keep New England cooler. According to the NOAA, there is currently a 62% chance of El Nino conditions developing sometime from May through July.

Looking back at my April outlook, the general trend played out as expected. I wrote: "Overall, it looks like April will truly be a battle of warm and cold, with both sides winning at points this month."

This certainly ended up happening, with times of cool weather along with a taste of full summer heat. I did expect New England to see seasonable temps with "stubborn shots of cold air" in the middle of the month. Of course, mid-April is when things heated up big time.



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