New England May Weather Outlook: Gradually Evolving

May is an interesting month weather-wise as summer weather tries to kick start amid typical spring conditions. This can often lead to an up and down month in the temperature department. Average high temperatures rise a good 8-12° from the start to the end of the month.

Summary: May looks to see a steady evolution in the pattern, going from cooler and unsettled to milder as the month goes on. This will likely come as persistent troughing gives way to ridging expanding eastward through the month, though a large uncertainty persists for the middle of the month. Read below for the in-depth discussion.

TEMPERATURES

The first week of May will be dominated by troughing, frontal boundaries and cutoff low pressure systems. This will keep plenty of clouds and showers around through next week. All of this will bring a rather persistent onshore flow as well, keeping temperatures on the lower end, especially for the coastal plain. This will generally be the case for much of this upcoming week.

The main driver behind the forecast for the first full week of May is an Omega Block that will be setting up across the United States. This will be a strong and classic Omega Block, which involves a large ridge flanked by two cutoff low pressure systems on either side of it. This can shape the jet stream to appear as the Greek letter Omega. Dry and mild weather is often observed under the ridge with unsettled and cooler weather in the troughs.

In this specific case, cutoff lows will be positioned over the southwest and eastern United States with a ridge over the northern plains. The cutoff low over the east won't be centered over New England, but the trough will influence the weather for the region. This cutoff will slowly drift east and likely make its closest pass to New England around midweek. This will keep the weather unsettled and on the cooler side to kick off the new month.

Below: 500mb flow pattern Monday afternoon, showing a pretty clear Omega Block situation:

Heading into late next week and into the second full week of the month, the Omega Block will steadily break down. This will likely be replaced with expansive ridging over the western and northern United States. A trough may undercut the ridge over east, which makes for a more uncertain forecast heading toward next weekend. Overall, it looks like a general warming trend will ensue moving through next week, but a trough undercutting the ridge will keep summer-like warmth at bay for New England.

Below: Height anomaly late next week showing the expansive ridging building over the west with a potential trough undercutting it across the south and northeast:

This can be seen on the Climate Prediction Center's 6-10 day outlook, which currently covers May 8-12. High confidence in above average temperatures is seen across much of the west and northern tier. This confidence drops quite a bit moving east and south due to the aforementioned potential troughing. All of this may lead to a rather seasonable middle of the month for New England. There could be up and down temperatures for New England during this time, which could lead to near average temperatures as a whole.

For the middle of the month, a general eastward progression of the overall pattern is the favored outcome. This pushes the troughing off the east coast and allows for more ridging to spread farther east across the northern tier. The troughing over the southern United States may reload and remain in place.

This is a large wild card for New England as this could continue to influence the northeast's weather depending on its exact location and setup (which is tough to say at this point, naturally). As of now, the ridging over the northern tier of the United States is expected to spread eastward as a trough enters the west. This puts New England to be slightly favored to see generally above average temperatures, but confidence is not high.

The end of May is pointing toward a milder finish for the month as signals are hinting at a trough-in-the-west-ridge-in-the-east setup as the steady eastward progression of the overall pattern continues. Remember this is the general large-scale pattern and does not take into account small-scale factors that will affect day to day weather and temperatures. Overall, we're predicting a steady warming trend through the month (relative to averages). The second half of the month is certainly favored to be warmer than the first half at this time.

PRECIPITATION

Generally unsettled weather likely to continue through the first week of May given the Omega Block and persistent troughing. This doesn't necessarily mean above average precipitation, however. While there have been unsettled times over the past month, New England remains in a rainfall deficit as these waves of activity haven't produced a whole lot of sustained, soaking rain.

This may end up being the case again for much of next week with a slowly meandering cutoff low and cold fronts. This will likely produce multiple rounds of showers with dry periods in between. With general ridging forecast for the second half of month, the first half may end up being more unsettled.

With that said, we're thinking the month will end up near average in the precipitation department. Should New England fail to see soaking rains over the next week or so, the month could very well end up below average despite the unsettled weather here at the start. Drought conditions have very slowly been improving, but with near to below average precipitation in April, moderate drought conditions persist for a portion of southern and central New England.

Below: Current US Drought Monitor status:

LOOKING BACK

April played out just about as expected. We predicted a generally cooler start to the month with milder conditions for the second half. This generally played with persistently cooler temperatures leading to generally below average temperatures from April 1-15. More frequent mild days in the second half of the month led to April finishing slightly warmer than average for most. This can be seen on the maps below.