Severe Weather Potential Friday, August 9th, 2013

Friday has the possibility to be a fairly interesting and perhaps active weather day across southern New England as the possibility will exist for showers and t'storms.  While the overall threat may be torrential downpours leading to pockets of flash flooding, the possibility of some strong to severe t'storms will also be possible, in which strong winds and even the possibility of a few isolated tornadoes will be possible.

During the overnight hours of Wednesday into early Thursday, a warm front lifted northward through southern New England and this allowed for dewpoints/humidity to rapidly increase across the region as dewpoints have risen into the upper 60's to around 70F.

On Friday, a mid-level trough sliding east passed the Great Lakes will allow for an area of surface low pressure to slide east through southern Canada along with a cold front towards southern New England.  Some computer forecast models are also showing a "secondary" warm front development just to the west of the region early tomorrow along with another weak wave of low pressure.  It is these two features which is opening eyes with regards to the possibility of severe weather tomorrow.

Some computer forecast models are developing a fairly potent low-level jet later tonight, lasting through much of tomorrow with winds in the 925-850mb layer exceeding 30-40 knots which is quite strong for this time of year.  Not only are these models developing strong winds here but these models also are indicting strong directional wind shear with winds at the surface coming from the south with winds turning towards the SW/WSW aloft with height.  Below are two hodographs taken from today's 18z GFS run for Windsor Locks, CT tomorrow afternoon.  I have circled four areas of interest.  The first two areas of interest are showing the hodograph, which measures wind speeds/direction with height.  The "longer" the hodographs, the stronger the winds aloft are, the more curved they are, the stronger the turning of winds with height are.  The second two areas of interest are the models suggesting the potential for several hundred to perhaps 1000 J/KG of Cape, which with combined with those hodographs insists the potential for severe weather/tornadoes would be a possibility tomorrow:

When it comes to the low-level jet, however, often times computer models can overdo the strength of them as computer models can sometimes suffer from what is called convective feedback.  This is one factor that makes this forecast/potential rather unclear at this time.

As far as instability goes, weak mid-level lapse rates and extensive cloud cover are expected to hold back instability values, however, with surface dewpoints near 70F and rich low-level moisture, surfaced-based instability values could exceed 1000-1500 J/KG along with mixed-layer cape values and lifted index values in the -3C to -5C range.

If this low-level jet verifies, or comes close to verifying, and we are able to manage even a few hundred J/KG of Cape, any t'storm that develops will have to be closely watched as atmospheric conditions will be quite favorable for strong winds and perhaps even some tornadoes, especially with extremely low lifted condensation levels, the level at which the cloud base is located (to put it simply).