Severe Weather Possible Today 6/17

The radar is quiet this morning but by early evening it will light up across the state. We have a tremendous amount of wind shear in the atmosphere this morning from near the ground up to about 6km. While some of this is due to “turning” of the wind direction a lot of it is due to a very quick change in wind speed with height thanks to powerful winds aloft.

10 a.m. 0-6km Vertical Wind Shear is >40 knots. This is considered strong.

10 a.m. 0-6km Vertical Wind Shear is >40 knots. This is considered strong.

Vertical wind shear helps organize thunderstorms. It promotes storm longevity, organization, and in the right environment supercells. When vertical wind shear is too strong and instability is too meager the shear can actually disrupt updrafts rather than organize them. That’s today’s dilemma.

While today is starting out a bit on the muggy side our computer models are bringing down some drier air from aloft. This is what we call “mixing out” and relatively low moisture near the ground (known as the boundary layer) is not favorable for thunderstorms. Indeed, the presence of limited moisture in the boundary layer will result in only modest amounts of instability. Here’s the 12z NAM forecast for CAPE valid at 5 p.m. this afternoon.


5 p.m. 12z NAM mixed layer CAPE forecast. Generally 1000-1500 j/kg of MLCAPE across Connecticut.

CAPE forecasts for today are modest but given the strength of the vertical shear any storms that develop today are worth watching. The severe weather threat will be greater if better low level moisture is able to advect northeast from eastern Pennsylvania and New Jersey where dew points are still in the mid 60s.

I think storms may wind up being relatively widespread today with an upper level disturbance approaching around 5 p.m., some convergence near the surface which should help trigger storms, and widespread synoptic-scale ascent (QG forcing) in the left exit region of the jet streak moving across Pennsylvania.


12z NAM 5 p.m. 250 mb isotachs, wind speed (knots), and height. Note 100+ knot jet streak over central Pennsylvania and western New Jersey. The “left front quadrant” is an area where rising motion can be expected.

Here’s today’s severe weather threat…

  • Damaging Wind – Elevated
  • Large Hail – Elevated
  • Flash Flooding – Elevated
  • Tornadoes – Very Low

Note: Possible “threats” are very low, low, elevated, significant, and extreme. A higher threat level doesn’t necessarily imply widespread coverage. Coverage, likelihood, and especially significance (i.e. high end severe) are all factors in assigning threat categories.


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