A Scorching Summer Gives Way To A Warm September Outlook
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You sweatin' enough yet? It was a superlative summer of heat in New England, and we'll continue to ride the trend and rock the flips flops well into autumn. Several records have already been set and more are in sight as September begins. Let's dig into how unusually hot the past few months were before looking ahead to the new season.
Graphic: WBZ-TV
*Data from the final day still to be updated, but standing will remain the same
August was the showcase month of our hot summer. It was handily the hottest on record in Boston, dating back to 1872. Not only was it the hottest August by a mile, it was also the 2nd hottest month all-time in the city. Only July of 1983 was hotter, and you'll note that it's unusual to have August contend for a hottest overall month record. Our peak heat is typically July as we lose significant daylight in August and the averages are trending downward.
It was the 2nd hottest August at Blue Hill Observatory in Milton, just barely trailing behind 2016. To update the running tally this decade, we are up to 35 months achieving 'Top 10 warmest' status compared to just 1 'Top 10 coldest' month, which was February of 2015. In other words, we can still get cold months, but odds heavily favor hotter than average ones as the climate continues to warm.
Graphic: WBZ-TV
A major part of what made this a hot summer was the humidity. This has been the number one question we've been getting in the weather office recently. "Is it just me or has this been ridiculously humid?" It's not just you...it truly has been! Dewpoint records are a little tough to come by, but the data we have shows this year is blowing away all previous ones when it comes to the truly oppressive stuff. We've spent ~500 hours with dewpoints of 70F or higher so far this year. These fantastic charts from Iowa State University really help illustrate what we've gone through.
Source: Iowa State University Mesonet
Considering there are still many days with potential dewpoints >70F in the forecast, we'll continue to build on this record. What really sticks out are the 'creme de la creme' high dewpoints of 72F or greater. The histogram shows that an average year in Boston has about 71 hours per year with dewpoints that high. This year has already featured a whopping 220 hours! We'll see what the final numbers are but this has undoubtedly been what's made for an uncomfortable summer season around here.
Graphic: WBZ-TV
The high humidity has played a role in our overnight lows, of which we are also setting new records. There have been 30 days so far this year which have failed to drop below 70F in Boston. The high humidity blocks us from cooling off effectively. At the moment we're time for the all-time record but we should easily break the tie next week.
The warm lows have played a big role in the overall summer averages. This was a Top 5 hottest summer in Boston, and the lows carried more weight than the highs. We broke the record for the hottest average minimum temps by a wide margin during climatological summer. The highs were notable for sure, but come in at 9th hottest for the season.
In general we see a larger warming trend with overnight lows versus daytime highs. A big part of this is likely due to the fact that our highs are largely dependent on incoming shortwave radiation from the sun, while our lows are controlled more by trapped longwave radiation. The more heat trapping gasses there are in our atmosphere, the more we are able to hold on to heat at night. This isn't just a city trend...you'll find the same if you look at the top of Mt. Washington which also just had a Top 5 hottest average minimum temp for summer and is trending up over the decades.
Source: xmACIS
So what comes next? Well there are some more notables and streaks to watch as we head into September. For starters, Boston hasn't gone below 60 degrees since June. We're already at the 4th longest streak on record and it would be a huge surprise if we had one through next weekend. So basically, you can lock in a 2nd all-time or 1st all-time finish. I'd say the top spot is very much in jeopardy given the outlook. You'll also notice this list is dominated by recent years.
The opening week of September is going to sizzle as another large ridge builds in across the eastern U.S. The only real question mark for us is Wednesday, which may be able to push down a backdoor cool front and allow for a day of 70s and 80s to break up an official heat wave. Otherwise, we're talking 90s on Monday, Tuesday, and Thursday at the very least and the potential for a couple record highs during that stretch.
In case you're wondering, the record number of 90 degree days for September in Boston is 6 of them set in 1983. And for a year, 1983 is also the record holder with 30 (we're sitting at 20 now, and unless September is absolutely ridiculous we should fall short of that all-time mark). There have only been a handful of heat waves in Boston during September but we may have a shot at one.
Another large ridge develops next week. Source: Weathermodels.com
After that, there is a strong model consensus of more heat building back during the middle of the month after a brief dip next weekend. In fact, most of the country could see very warm conditions develop. Keep in mind that the averages continue to drop in September, so it's not necessarily scorching summer heat. By mid-September an average high in Boston is just 72F! But we should keep a general summer feel around for a large portion of the month, and perhaps challenge another top 10 warmest month overall.
The Climate Prediction Center has certainly noticed and updated accordingly. Their monthly outlook, released Friday, shows New England carrying the highest odds of warmer than average temperatures in the U.S. So the bottom line is that you should probably keep the pool open a little longer, keep the AC units in the windows, and prepare for another warm start to fall. A late foliage season can likely be expected again this year as well.
September outlook made by the Climate Prediction Center
