Before we get into the detailed severe weather business, highs today will range from the mid 80s to around 90 in northern MA and southern NH under partly to mostly sunny skies. It is oppressively humid at this moment with dewpoints ranging from 65-70, and most on the low end of that will climb to the high end of that scale. Winds will be primarily be SW to begin with, near 10 mph, but following the passage of a cold front these winds will turn NW this evening into the early overnight.
There will also be a chance of scattered severe thunderstorms this afternoon and evening. We have a good trigger in place – a cold front off to the west, to be exact – and as mentioned, there is a lot of moisture in the atmosphere. There is also strong wind shear overhead, which not only helps to provide strong winds but also could potentially lead to some organized storms. Aiding the wind threat, an area of mid-level dry air should assist in creating a damaging wind threat (and I don’t forsee it being too much mid-level dry air unlike a few weeks ago). Note the green line jogging to the left in this 6z Hi-res NAM sounding at Lawrence valid for 4 PM today – that green line is the dewpoint, and mid-level dry air supports damaging straight-line winds. There is still a lot of moisture no less, and that will likely support heavy rain in most cells, with very isolated urban flooding possible. PWAT’s (not shown) are also near 2″ which is an additional supporting factor for heavy rain. Anyway, here is that 6z Hi-res NAM sounding valid at 4 PM in Lawrence:
One question to resolve is that of instability. We have plenty of moisture to help in that department, but there is a line of showers extending from Worcester all the way through Northeast MA to areas offshore of the southwest Maine coast. This could pose an issue due to associated clouds stabilizing the atmosphere, and as such reducing the threat. If the clouds clear it’ll be game on for a *threat* of storms, but if not it will be quite difficult around here. These showers are moving roughly to the east-northeast, and so aren’t impacting Southeast MA/RI/CT as much with cloud debris, so the highest threat will be near and especially south of the Mass Pike (I-90). The radar also notes showers and storms already forming in northern New England and adjacent southern Quebec. Radar valid 11:15 AM:
The satellite data is on a ~30 minute delay, but this scan valid at 10:45 AM shows slightly fewer clouds than the one 30 minutes earlier not shown. So it is still very much a toss-up as for how much clearing occurs.
In short, the main threats today will be damaging winds, heavy rain and frequent lightning. An isolated hail report is possible as well. The best threat is south of the Mass Pike, where SPC has noted a slight risk of severe storms. SPC has also noted a slight risk in Downeast Maine and a marginal risk elsewhere in New England. I’ll be on Twitter @ncoram_wx discussing the situation, but please stay tuned to local media or the National Weather Service for warnings and statements. When thunder roars, go indoors as lightning can strike far away from a parent storm without rain; and lightning can be deadly even in a sub-severe storm. And in the event that you are driving and encounter a flooded roadway, turn around, don’t drown as you never know how deep the water can be, and less than one foot of water can harm your car. While urban street flooding is unlikely, it cannot be ruled out.