Today’s high temperatures will fall in the upper 70s to low 80s, under mainly cloudy skies and some showers moving in this morning. Winds will be SW 5-10 mph, except higher in thunderstorms. These showers are moving east into the region, accompanied by plentiful areas of steady rain and some areas of heavy rain.
In addition, an embedded thunderstorm or two is possible. When thunder roars, go indoors. But the main threat is some urban flooding, as the steadier to heavy rain is being sponsored by high PWATs streaming into the region along with high dewpoints. The high PWATs support some areas of very heavy rain, with values over 2″ flowing into the region:
Note – just because there is a 2″ or larger PWAT value doesn’t mean you’re going to see 2″ of rain at your house, but it does mean there is a strong potential for someone to see some heavy rain. Again, not necessarily 2″, but rather the potential itself for heavy rain is what needs to be the primary takeaway.
The above is the 11Z HRRR’s thoughts on how precipitation will proceed. It shows the batch of rain coming and going, then a few showers and storms forming behind the batch. The potential for formation of more showers/storms after this batch exists – there is a shortwave aloft, there is definitely enough moisture, and there will be increasing instability despite the presence of some clouds. Severe weather is not likely as a result of decreasing shear, but there is a possibility of an isolated stronger storm with gusty winds with 0-6km shear of 25-35 kt in spots.
The main threat in any cell that forms and isn’t overly weak is heavy rain, due to the aforementioned factors. As a result, urban flooding potential exists this afternoon as well in any cell with heavy rain.
A detracting factor to afternoon storms is weak mid-level lapse rates:
Generally, at least 6C lapse rates – preferably higher – are wanted to help with a storm threats and this is a swing and a miss in that department as much of the area hovers around 4.5 or 5 in this projection (with other high-res models spitting out similar output).
Any storm chance dwindles this evening as dry air aloft takes over, but there will be no relief in the low levels at the surface as dewpoints stay in the upper 60s to the low 70s.
High Heat Thursday-Saturday
With those dewpoints I just noted, combined with temperatures in the mid-90s tomorrow, there is a threat from this heat given heat indexes of 95-105. Similar conditions persist into Friday and likely Saturday. As such a heat advisory has been issued for Boston proper, plus areas to the north and south of Boston away from the immediate coast, between the NH border and areas just north of the South Coast, 12 to 7 PM tomorrow, with a strong possibility of expansion, as well as potentially future heat advisories for Friday and Saturday. I’ll have more on this later, but be in an air conditioned area if possible, and if an AC’d area is not available, or if you are working outside, drink plenty of water and take breaks. As for today, monitor the skies and head inside when threatening weather approaches. If driving and you encounter a flooded road, turn around, don’t drown.