A special update this evening to cover tomorrow’s severe storm threat. A cold front will approach the region from the NW tomorrow driving cool Canadian air into a tropical airmass that will be reinforced tonight by increasing southerly flow. As a result, showers and storms will fire to our west tomorrow afternoon and move east into our area tomorrow afternoon into the evening hours. Heavy rain and damaging winds will be the primary threats along with frequent lightning though some small hail cannot be ruled out. Tornadoes appear not to be an issue.
There will be two surface features to trigger storms, a cold front, and a pre-frontal trough. Right now, guidance is indicating that a line of storms will form along the pre-frontal trough early in the afternoon just south of the mountains. This line then looks to move SE and strengthen as it encounters more instability. There looks to be very little in the way of morning convection but a warm front won’t be too far to our north so some clouds will likely be limiting instability in the morning in the far north. Farther south, this does not appear to be a concern. Winds will be out of the SW for the most part except perhaps for the midcoast where a southerly component will be present. The SW winds will act to keep the coast more or less in play for severe weather as there does not appear to be a strong sea breeze influence.
The pre-frontal trough will likely focus storms along the coast and south into SNE during the late afternoon/evening hours. Farther north, we will turn to the cold front for thunderstorms in the mountains. These storms will likely be more of a mid to late evening affair as it will take longer for that area to destabilize due to morning clouds. Storms along the cold front itself will likely be more hit or miss but could pack more of a punch as they won’t have tons of competition. These storms will fizzle as they head into the atmosphere cooled and stabilized by the pre-frontal trough convection.
There are three main ingredients needed for strong/severe thunderstorms: instability, an organizer, and a trigger. What will set the storms off, what will organize them into lines capable of damaging winds, and then what will ‘feed’ them? We already know a cold front/pre frontal trough will do that at the surface (see maps above). Now what will organize them? For organized lines of strong to severe storms to form, you need wind shear both in the low levels (0-1km) and in the upper levels (0-6km). The upper level shear will organize the storms and the low level shear will help mix down strong winds aloft so they can reach the ground.
Guidance is indicating a sufficient supply of both tomorrow as upper level energy (wind) moves in from the west associated with a short wave trough currently moving into the Great Lakes region. 25-30 kts of low level shear isn’t crazy but it will likely be enough to mix down some strong winds so be prepared for that threat. Now that we’ve established that there is sufficient low level shear, what about upper level shear?
That same short wave trough will bring some 0-6 km shear that is likely going to be enough to organize storms into lines capable of damaging winds. Also notice the winds heading away from each other (spreading out) as they approach the coast. This divergence aloft will aid in getting air rising tomorrow which will further assist in storm development. We now know that we’ll be able to trigger storms and get them organized but the next question is will there be any fuel for them to grow strong enough to introduce the threat for severe weather? For that we look to instability.
Instability has two components: heat and moisture. You need heat to provide enough energy for air to rise, and you need enough moisture to produce storm clouds when that air rises. Temps will climb into the 80’s tomorrow so that won’t be a problem but how about moisture? For that we turn to dew points which are forecast to be in the upper 60’s to low 70’s which is more than enough to trigger storms. Precipitable Water values (PWAT’s) will approach 2″ indicating a very moist atmosphere even well above the ground. Such high available moisture will support very heavy rain which could cause some minor flooding problems and will certainly be beneficial in easing our current drought.
The forecast sounding for PWM tomorrow afternoon brings it all together. Notice the red and green lines (temps, and dew points) remain close to each other through the first 35kft+ of the atmosphere (as a side note for the hardcore weather geeks out there, that temp trace looks mighty close to the moist adiabat). Notice how sharply the red line goes from right (hot) to left (cold) in the first 5kft. This indicates very steep lapse rates (of up to 11c/km!) which also supports strong storms. Strong winds throughout the atmosphere will help organize the storms into line segments capable of damaging winds. All in all, a pretty solid setup especially for New England.
To wrap up the instability part, we look at CAPE (Convective Available Potential Energy) which basically measures how much energy is available for thunderstorms to tap. CAPE values are forecast to exceed 1,000 j/kg which is more than supportive of strong storms. We have now established that we have the trigger (cold front/prefrontal trough), the organizer (strong 0-1km and 0-6km shear), and the fuel (70F+ dew points, 80F+ temps, 1,000j/kg+ CAPE) which means we have the green light for strong to severe storms tomorrow.
The SPC (Storm Prediction Center) agrees that the threat is there for strong to severe storms tomorrow and most of the area is under a slight risk for severe storms (a 2 on the 5 point scale). While that might not sound impressive, a slight risk is a pretty strong signal from the SPC for us. The exception is the immediate coast which is under a marginal threat (a 1 out of 5). Due to the wind setup tomorrow (mostly SW and not off the water), I wouldn’t be shocked to see that slight risk include the coast in the morning SPC update.
Is there anything that could interrupt the storms tomorrow? Yes there is bust potential. Should the pre-frontal trough move through a little early, it could trigger showers but not storms in the late morning which would limit instability leading to no fuel for any storms along the cold front. If the front is too close to the pre-frontal trough the storms along each would be prohibited from growing too big due to competition for instability. While this scenario is on the table, I would find it extremely surprising if no one in New England got a severe thunderstorm tomorrow. That doesn’t mean everyone will get one though.
I’m heading down to Mass tonight for a weeklong weather program at the Blue Hill Observatory and so won’t be quite as active this week. For more consistent updates tomorrow, be sure to check in frequently with the latest NWS forecasts as well as those from local media. Remember, when thunder roars, go indoors.