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Stormy AM Commute - Severe Potential?

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It's not often that we deal with many thunderstorms at all in the month of October, let alone morning thunderstorms. But we have a bit of a unique setup for Wednesday morning. You felt the air change on Tuesday - more mild and humid. Combined with a very potent piece of energy swinging through that atmosphere - this could be enough to touch off some thunderstorms and perhaps even a spin-up tornado risk for early Wednesday.

The Setup:

There are some question marks still in place, even on the doorstep of any rain coming in. One thing we know we have plenty of is wind shear. Winds will be increasing with height and changing directions with height (veering) as well. One way of quantifying that shear is 'helicity.' - one variable we look at when judging a severe weather threat. Helicity values will be very high Wednesday morning, and *if* that can be tapped then you have the potential for rotating storms.

helicity

RAP Helicity values on Wednesday morning - some very high values for eastern New England. Source: WeatherBell

The one thing holding down that threat is instability - basically the inclination of air to rise. CAPE values aren't too impressive for Wednesday morning - generally at or below 1000 J/kg. Without colder air aloft and the timing of this being in the morning - there's not much room for those values to get any higher. So we may have an instance where there's tons of wind shear for storms to go severe, but not enough instability to generate storms (except maybe some low-top thunderstorms) in the first place.

Another piece of the puzzle is the energy itself that's coming along with this front - which we see as a vort-max in the atmosphere. The main push of this energy is to our north, across northern New England. So basically we have 3 factors that are all there, but not co-located. The highest CAPE will be across the South Coast/Cape/Islands. The highest helicity values will likely be more toward Metro West/southern NH. And the most vigorous upper-level energy will be across northern New England.

vort

Negatively tilted trough at 500mb swinging through, with much of the mid-level forcing heading across northern New England. Source: Weatherbell

The Timing:

It's only a small window of time that we're watching for downpours/storms in the area, and that window is approximately 4-9am. This system is racing out of the Ohio Valley, where it produced tornadoes on Tuesday evening. While the rain won't be around for too long, it's at a problematic time - rush hour. Most of these downpours/storms will be moving across the state while we send kids out to the bus stop or start the drive to work. The first to get in on the action will be WMass into Worcester County, then into the Boston Metro in the 6-8am time frame. By late-morning, the last of the rain should already be pushing off the coast. The rest of the day looks dry.

timing

Current computer model (RPM) forecast for Wednesday morning. If anything, I'd lean for a little faster solution bringing the rain in an hour or so earlier.

Of particular interest is that Wednesday is National Walk To School Day - and as you can imagine lightning/rain/wind is not ideal! I've heard of a few towns already postponing the walking commute until later in the week, which would be the best idea. And check back in with Danielle Niles during the morning for the latest on what this line of showers/storms is doing.

Main Impacts:

Gusty winds are probably the most set in stone concern for this event. The breeze will stay active overnight in the 10-25mph range, and will be picking up in the vicinity of the rainfall Wednesday morning. Gusts 15-35mph are likely out of the southwest during the morning...an ugly start to the day. If any thunderstorms can manage to develop, then winds up to 60mph could be dragged down to the surface. So some sct'd downed trees/limbs are possible - which lead to sct'd power outages. Even as the rain departs, winds will swing to the west and stay gusty all day in the 15-30mph range. A shame for our fall foliage! Probably not enough to knock everything down, but there will definitely be leaves to rake after this.

severerisk

While a tornado threat is rarely 'high' in New England, there are definitely enough warning signs to at least throw the potential out there for Wednesday morning. The variety we'd be talking about are brief 'spin-ups' that usually aren't on the ground for long. Again - the main idea is just to keep a close eye to changing weather conditions during the first half of the day so that you're not caught unprepared!

tornadoprob

As of this writing, the Storm Prediction Center had put much of Southern New England into their 'Slight Risk' area for Wednesday morning. This ALSO includes a 5% tornado probability for eastern CT, RI, and a sliver of Massachusetts. While 5% seems low, it's actually a pretty high figure for the SPC to put out for this part of the country. Point remains - we should stay vigilant during this short amount of time!

Locally heavy downpours are possible, but I'm not expecting anything more than briefly poor visibility/nuisance flooding as this moves through. Most rain totals should stay under an inch. Hail will likely be hard to generate too, since the instability is looking rather poor.

Rest Of The Week:

A gusty wind will bring in a drier air mass Wednesday afternoon, and after reaching highs near 70 it'll cool off in a hurry Wednesday night. Overnight lows should drop down into the upper 30s to mid 40s. Thursday looks dry and cool as most of the unsettled/showery weather stays off to our north. And most of Friday looks to be cool but in good shape as well. We'll just have to keep an eye on a warm front advancing from the south/west. Clouds should increase during the 2nd half of the day, and some showers could approach the South Coast/Islands by evening.

That same boundary is going to hang around all weekend - the question is where exactly will it rest? It's one of those classic '40 miles north and it's a rainout, 40 miles south and we're high and dry' situations. Currently thinking that our best shot for rain will be Friday night/Saturday morning - in particular areas south of the Mass Pike. Signs are that high pressure could push this boundary a little farther offshore for Sunday, giving us a cool, pleasant and bright day. Stay tuned for updates as we get closer to Columbus Day Weekend!

 

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