Severe Weather Likely Today



Storms today will mean business! We’re expecting a fairly sizable severe weather event in the northeast this afternoon and while we can never say with certainty that the severe will strike our backyard – odds are pretty good we’ll get some nasty weather in Connecticut. Any storm that develops today has the potential to produce winds strong enough to take down trees and power lines and could produce a tornado or two. We’re expecting a “high impact” day of weather across the state.

500_140715_12A deep and anomalous closed low over the Great Lakes has allowed copious amounts of moisture to come north under a strengthening mid level flow on the eastern seaboard. Dew points in the low to mid 70s are common across the region. This is a climatologically favored pattern for severe weather in Connecticut – even down to the beaches.



The morning weather balloon launch on Long Island shows a fairly unstable environment that’s loaded with moisture. There is a bit of a cap this morning for surface based convection that models show eroding by early afternoon. High resolution models develop some nasty storms in western Connecticut that move in later today. In addition, models show a spike in storm relative helicity (a way to measure the shear or turning of winds in the lower atmosphere) as the storms approach.

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With strengthening wind fields just above the surface any storms will have to be watched closely today. Below is the 12 hour NAM forecast valid at 18z today. Note the >30 knot low level jet at 850mb that has developed from NYC to central Massachusetts. Any time the 850mb flow increases above 30 knots in a setup like this it’s worth watching closely – especially on a day like today with lifted condensation levels as low at 500 meters.



Damaging winds at the ground, lots of lightning, flooding rains, and tornadoes are all possible today. Keep an eye to the sky – or to Twitter!

Severe Weather Likely Monday & Tuesday

There are a few kinds of severe weather setups here in Connecticut. There’s the classic high-end severe weather outbreak that is generally characterized by northwesterly flow aloft, high/extreme instability, and strong wind shear (e.g. July 1989, May 1995, May 2010). There’s the more common borderline instability/shear setup that can favor borderline “pulse” storms or weakly organized line segments. And then there’s the Monday/Tuesday setup.


The warm season cut-off low can be a prolific severe weather and flooding maker. While high-end or significant severe is unlikely it’s not out of the question. I blogged a bit about this kind of setup last June and a good example of a high-end severe weather day from an anomalous cut-off in July is the 2008 New Hampshire F2.

So what’s the deal with Monday and Tuesday’s severe weather threat? The NAM and GFS both show a strengthening low level jet on Monday and several shortwaves embedded in the southwesterly flow aloft moving through. The 9z SREF shows MLCAPE around 1000 j/kg Monday afternoon.

9z SREF / Courtesy: SPC

9z SREF / Courtesy: SPC

With modest instability and strengthening low level shear low topped supercells are quite possible tomorrow. Damaging winds and tornadoes will be possible. Lifted condensation levels will be quite low as 0-1km shear ramps up – that means we’ll have to watch the radar closely!

If that weren’t enough it looks like we could do it all over again on Tuesday with another low CAPE/high shear environment. The details will come down to mesoscale details that are tough to figure out this far in advance. In addition, flash flooding is a possibility with excessive rain possible. Here’s the WPC/NCEP precipitation forecast for the next 72 hours.


As I like to say – beware the Upper Level Low. These cut-offs can produce pretty nasty weather in the warm season, particularly in July and August when water temperatures in the Sound and south of Long Island are near their climatological peak.

Here’s something interesting based on this morning’s NAM solution for Monday. The CIPS analog guidance looks for other days with similar synoptic setups and looks at what kind of weather was reported that day (damaging winds, tornadoes, or even snow amounts in winter). Below I’ve posted the probability from CIPS of probability >5 severe reports and also >1 tornado.



25 Years Ago – The 1989 Tornado Outbreak

There are only a few “classic” northeastern U.S. tornado outbreaks that jump out in your mind. The 1985 Pennsylvania outbreak is one, the 1998 Pennsylvania/New York outbreak is another, and so is the 1989 northeast outbreak. The epicenter of that outbreak was right here in Connecticut with an F4 tornado touchdown in Hamden and New Haven.

The atmosphere couldn’t have been more primed for a big tornado event. Here’s the morning weather balloon launch and sounding from Albany.


A classic elevated mixed layer with a dry adiabatic layer from 625mb through 750mb is present. What is most striking, however, is the exceptional wind shear in the atmosphere. Winds at  500mb are out of the northwest at 80 knots while in the boundary layer winds are out of the south-southeast at 10 knots! That’s about as strong as it gets.

The presence of the “EML” allowed for significant instability to develop during the heating of the day. Prior to the tornado in Connecticut temperatures reached the low and middle 80s with dew points in excess of 70F. A quick and dirty modification of that sounding for 30/22 shows just how explosively unstable the atmosphere was.

Courtesy: Schoharie County Emergency Management

Courtesy: Schoharie County Emergency Management

The first tornado touched down in upstate New York west of Albany and was on the ground for an incredible 42 miles. That same supercell went on to produce a series of tornadoes in Connecticut. The first tornado touched down near Route 4 in Cornwall and continued south into Bantam. The second tornado touched down in Watertown and Waterbury. The most violent of the tornadoes touched down in Hamden and continued south into New Haven.

The weather charts during the event were just incredible for a northeastern U.S. tornado outbreak with a strong disturbance moving out of southern Quebec into northern New England.



Through the day 500mb heights actually rise over southern New England with the best QG forcing displaced far to the north. Still, after the initial convective initiation in the morning those storms were able to propagate southeast into our area. The elevated mixed layer not only allowed strong instability to develop – it likely also helped keep convection relatively discrete. The atmosphere in many locations was “capped” – just enough CIN to prevent widespread convective initiation – but not capped enough to prevent all convection. The best QG forcing to the north also helped convection remain relatively scattered.

The damage in Connecticut was substantial with hundreds and hundreds of homes and businesses destroyed. Many people in the New Haven suburbs – including North Haven and North Branford were caught in the hail core of the storm with golf ball size hail or larger. Southeast of where the tornado lifted in Newhallville substantial wind damage occurred with many of the pine trees near Lake Saltonstall on the Branford/East Haven line snapped in half.

Here is some of our coverage from the 11 p.m. news on July 10, 1989 and the 6 p.m. news on July 11, 1989.

There were other tornadoes that day – some in northern Massachuetts, others just west of Danbury in Putnam County, and another swarm in northern New Jersey. If we were able to look at radar data (which sadly, we cannot) we’d probably see a line of supercells across the region.

On a personal note, the 1989 tornado event is my first weather memory as a kid. At the time I was living in Branford but on vacation with my family on Cape Cod. When I heard about the tornado back home I was devastated! I couldn’t believe that I missed “the big one” back home. I guess I’ve been a weather weenie for 25 full years now!

Arthur Strengthens, Will Miss Connecticut


The 11 a.m. National Hurricane Center advisory brings Hurricane Arthur fairly close to the 40N/70W benchmark but continues to show the storm passing well south of Connecticut.

We continue to expect no direct impact from Arthur. Even indirect impacts may be tough to come by. The Predecessor Rain Event or “PRE” as I discussed earlier doesn’t look like a major weather-maker here but I can’t say that yet for sure. We’ll have to watch how storms behave later tonight – can’t rule out a flood threat tonight/this evening.

As for thunderstorms, a combination of moderate instability and modest/moderate wind shear today will be enough for scattered severe weather today – particularly inland. The best forcing will be across northern areas up through western Massachusetts and there will be a bit of convective inhibition (or CIN) to overcome along the shoreline.

On Friday, as Arthur makes its closest pass a period of heavy rain is possible as some moisture gets involved with a slow moving cold front but our computer models are really struggling with this potential. Stay tuned for more on Friday!

Arthur to Miss Connecticut


As expected, Tropical Storm Arthur will pass well south of New England early Saturday. Virtually no direct impacts from Arthur are expected here. The storm may even pass east of Cape Hatteras as it brushes by North Carolina later this week.

In terms of what we can expect here the much bigger story will be the possibility of strong thunderstorms on Thursday and possibly a period of flooding.

A combination of instability and increasing shear will promote vigorous thunderstorm development during the day Thursday. The strongest storms will be capable of producing strong winds, large hail, and a tornado, while quite unlikely, isn’t out of the realm of possibility.

When tropical storms are lurking to the south frequently a “Predecessor Rain Event” develops well to the north as some of the moisture interacts with the jet stream and a surface cold front. The thunderstorms on Thursday and Thursday night may congeal into a “PRE”. Because of that, there is a threat for flooding but confidence in timing and location is not high. Additionally, it has been quite dry lately and rivers are running low. This will mitigate the flood threat somewhat.


As Arthur pulls away rain will remain possible during the day on Friday not from the tropical storm but from the cold front responsible for Thursday’s storms. A sharpening jet streak (an area of strong winds way up in the jet stream) will put us in a spot that is favorable for lift. Upward vertical motion can be expected in the right entrance region of a jet streak and that will be over New England as Arthur passes to the east.

The good news is the weather looks fantastic for Saturday and Sunday as Arthur pulls away. The sinking air (subsidence) behind tropical systems can provide brilliant blue skies and pleasant dry air for the time of year. This appears to be the case for the weekend.

Bottom line – the weather is going to get pretty crummy here Thursday and Friday. Don’t blame Arthur though – he’s not at fault this time.