Concord, MA Tornado

Courtesy: NECN
Courtesy: NECN

Of all the tornado warnings that are issued in southern New England – it’s hard to think of one more perfectly executed than the one issued Monday morning on August 22nd for Middlesex County, Massachusetts by Hayden Frank at the National Weather Service in Taunton.

This was a classic low probability/high impact severe weather event that we deal with with some regularity over the summer. What the models did show was some minor instability and a moderately sheared environment in the lowest levels of the atmosphere. With very low cloud bases (low LCLs) and a moist flow off the ocean (where sea surface temperatures are at their climatological maximum) these are always worth watching but frequently don’t produce much of anything.

The meteorologists at Taunton mentioned this possibility in their 4 p.m. Sunday forecast discussion.

there is a very low
probability of a brief tornado or waterspout developing.  However,
this is a very low probability.

Hayden mentioned it again on NWS Chat to media and Emergency Managers later in the evening.

The 12z models on Sunday weren’t overly impressive – but they were enough to keep things interesting. Here’s the 12z 4km NAM which shows an impressively veered wind profile and a bit of surface-based instability (about 500 j/kg of CAPE).

bed1

The 00z 4km NAM had a similar story to tell with some surface-based CAPE and a similarly impressive veered wind profile in the lowest part of the atmosphere. What is most impressive here is that there’s over 200 m2/s2 of SRH in the 0-1km layer!

bedhodo

On radar you can see a hint of an MCV that forms just north of Worcester and tracks northeast toward the New Hampshire Seacoast. It’s certainly conceivable that low level shear was enhanced even further near this system. The storm began to look interesting around Marlborough.

KBOX 0654 BR/BV/CC/NROT
KBOX 0654 BR/BV/CC/NROT

A line of higher reflectivity echoes is moving northeast and there appears to be several embedded mesocyclones. None are particularly tight but the one on the Marlborough/Southborough line is concerning with ~59 knot of Low Level Delta V (LLDV) and nROT > 1.00. These values are higher than the median for recent northeast tornadoes – though the LLDV is not gate-to-gate and is relatively broad.

At 701 UTC some of these embedded mesocyclones persisted and the first Tornado Warning of the morning was issued. nROT continued around 1.00 as the line moved northeast with several mesocyclones appearing and disappearing until one really went to town over Concord. nROT peaked at 1.40 and LLDV peaked at 73 knots.

KBOX 0717 BR/BV/CC/NROT
KBOX 0717 BR/BV/CC/NROT
KBOX 0718 BR/BV/CC/NROT
KBOX 0718 BR/BV/CC/NROT
KBOX 0720 BR/BV/CC/NROT
KBOX 0720 BR/BV/CC/NROT

For a night where there was no tornado outlook from SPC (they did mention the threat for an isolated rotating storm, however) and an uncertain evolution this was a very impressive warning performance by the NWS. The warning was issued at 3:01 a.m. and the tornado hit Concord, MA at 3:20 a.m. That’s an impressive 19 minutes of lead time!

Kudos to the NWS to recognizing the threat and watching the radar so closely during the overnight shift. A job well done and a good reminder to watching things very, very closely this time of year when you have strong 0-1km shear, low LCLs, and even a small amount of instability.

The Almost Tornado Day

Yesterday was a busy day in the weather office! A cluster of storms straddling the Connecticut/Massachusetts border featured varying amounts of rotation and given the environment we were concerned about tornadoes.

bdlhodgraph

The good news is – no tornadoes developed! The environment appeared prime for tornadoes. Around 3 p.m. we had relatively strong low level shear with 0-3km helicity >200 m2/s2. The storm motion off this hodograph is just to the south of due east at about 12 m/s.

The storms that developed were moving to the northeast or east-northeast across Connecticut. My buddy Chris Legro at the National Weather Service in Gray, ME theorized that this could indicate that these storms may have not been rooted at the surface for at least some of their lifetime given their storm motion.

In Woodstock, storm chase Joseph Berger grabbed this shot of a wall cloud over the Woodstock Fairground. Video making the rounds on Facebook shows a rotating wall cloud with some scud hanging around underneath.  The video above from Joseph Berger shows a bit of rotation and a likely Woodstock nearby in Woodstock. This thing was close to dropping a tornado.

kbox_20160816_1954_BV_0.5

Here we have about 45 knots of low level delta V (LLDV) – though it is a bit broad. Keep in mind the average LLDV for northeast tornadoes in recent years of 50 knots LLDV. Had it been a bit tighter maybe we would have had a tornado?

Before the Woodstock wall cloud we had an even more dramatic jump in low level rotation – with 67 knots of LLDV that lasted for only one scan over Eastford and Pomfret.

kbox_20160816_1931_CC_0.5

The couplet appears legit – though those outbounds look a bit suspicious. Regardless, even tossing some of the higher values we’re close to 50 knots of LLDV which is concerning.

A second storm had a pretty interesting look over Simsbury as well. The radar from Boston was picking up ~45 knots of LLDV but the radar from Long Island was actually sampling the storm 2,000+ feet lower with only marginal rotation. A good reminder to use multiple radars!

kbox_20160816_1906_NROT_0.5 kokx_20160816_1905_BR_0.5

So what happened? I think these storms may have no been completely surface based. The storm in Woodstock/Pomfret/Eastford got awfully close to dropping a spinner but couldn’t quite get it done. While 0-3km shear was impressive winds were relatively weak in the lowest 1km of the atmosphere. That may have played a role. Several of the storms we saw had better mesocyclones 5-10kft up than they did at the lowest elevations.

Later in the afternoon and evening the setup looked awfully juiced for severe storms. Long, curved hodographs and plenty of instability and it went to waste! This HRRR forecast sounding for Hartford valid yesterday evening shows fairly substantial convective inhibition or CIN. In the absence of strong forcing we were unable to eliminate this cap for surface-based storms.

hfdcin

These events are always fascinating and always challenging to forecast. In retrospect there’s probably not a lot we could have done differently – we were pretty lucky we didn’t have 1 or 2 tornadoes in the state yesterday afternoon as we were really close!

North Haven Tornado

It would be fair to say I wasn’t expecting severe weather today but I also wasn’t surprised to see a tornado hit North Haven. We had a somewhat unstable environment and a bit of low level wind shear. We get these setups with a fair amount of regularity in the summer and once in a great while they’re able to produce a tornado. Today was one of those days.

On Monday I sent this Tweet out about the conditional severe weather threat.

What transpired was this. Shortly after 1 p.m. we had two mini-supercells develop – one over New Haven and another over Long Island Sound. The storm that moved toward North Haven was rotating – and right around the threshold where you would have to be concerned about a developing tornado.

I sent out this Tweet at 1:31 p.m. as the storm moved through Hamden. The National Weather Service issued a Severe Thunderstorm Warning that included a “tornado possible” tag but the rotation was not quite strong enough to issue a tornado warning but boy was it close!

The rotation shown by this storm (approximately 40-50 knots of delta-V) is quite close to the median value for all northeast tornadoes in recent years. Here’s a look at the radar data from the storm that really developed rotation over the Westville section of New Haven and produced the tornado over the Quinnipiac River on the Hamden/North Haven line.

okx1
OKX Radar 1705 UTC
okx0
OKX Radar 1716 UTC
okx3
OKX Radar 1725 UTC
okx4
OKX Radar 1727 UTC
okx5
OKX Radar 1735 UTC

It does appear that the lofted debris from this tornado produced a subtle tornado debris signature. This means that our radar was able to detect tornado debris more than 2,000 feet in the air tumbling through the cloud. Incredible!

The 16z HRRR sounding for New Haven does show an environment that was favorable for tornadoes.

bufkit

Here you can see about 200 m2/s2 storm relative helicity in the lowest 3km of the atmosphere along with sufficient surface-based instability. What is important to note here as well is the very low lifted condensation level (LCL) at about 900 feet above the ground! This is a somewhat classic setup for weak/spin-up tornadoes in southern New England with a super humid and somewhat sheared atmosphere.

The SPC mesoanalysis shows this as well with just shy of 200 m2/s2 effective storm relative helicity on the Connecticut shoreline.

This tornado is a great example of how important it is to be aware when these low probability/high impract severe weather events can occur. There was no tornado watch in effect – in fact there was no part of Connecticut even mentioned by the Storm Prediction Center as an area where severe storms were expected (not even a marginal risk). Occasionally, a small area of sufficient shear and instability can overlap on an otherwise unimpressive severe weather day and result in a spinner. Thankfully, no one was hurt.

 

Uncertain Severe Weather Threat Today

I’m not overly enthused about the severe weather threat today. That said, there are several mixed signals. The key has been all along how much low level moisture we’ll see over Connecticut. Rich low level moisture means big storms while less moisture should effectively eliminate the threat for strong thunderstorms. Take this morning’s HRRR for example.

hrrr_dew2m_neng_9

That’s some impressive drying!

Screen Shot 2016-06-07 at 6.13.25 AMThe SREFs on the other hand show much higher levels of low level moisture with dew points >60F and CAPE around and in excess of 1000 j/kg.

You can see here there’s decent clustering of the ensemble members with dew points in the 60s for BDL. No member is anywhere near the HRRR with dew points dropping into the 50s and 40s by afternoon.

The 6z NAM has also trended a bit drier and a bit less impressive.

So today will be all about watching how quickly the low level moisture moves out of the state. With strong wind shear any storms that develop could become severe with adequate CAPE. The best chance for severe weather today will be in eastern Connecticut, Rhode Island, eastern Massachusetts up through portions of New Hampshire and Maine.

 

 

Memorial Day Forecast

This weekend forecast has been a struggle! For the last few days we’ve been trying to pin down the location and strength of a backdoor cold front which is nearly impossible to do. This morning when I first checked the models I knew our forecast probably was going to have to change again. The reason? A tropical wave off the Bahamas moving toward the Carolinas.

two_atl_5d0

Whether or not this tropical wave becomes Tropical Storm Bonnie it doesn’t really matter. The low is going to open a conduit for a plume of tropical moisture to stream north into southern New England on Monday. You can see that here looking at precipitable water or PWAT forecast for Monday morning on the GFS (the Euro is actually even more impressive).

GFSUS_prec_pwat_090

So what happens with that plume of moisture? Well, as the tropical juice streams north there’s also a jet streak in southern Quebec as a shortwave trough approaches from the lift. In the right entrance region of that jet streak air tends to rise and that will be right over southern New England.

GFSUS_250_spd_096

The European model shows over 1″ of rain on Memorial Day here in Connecticut. A washout. The GFS shows rain but not to that extent. The European ensembles have an approximately 50% chance of >0.25″ of rain on Monday. Decent odds.

eps_qpf24_0.25_ma_20

The actual “numbers” being printed out by the models aren’t particularly important at this point. The setup being forecast is one that looks to me like a wet one. Saturday and Sunday should be much better days. 2 out of 3 for a holiday weekend ain’t bad for the time of year!