Potential for strong to severe t'storms Wednesday, May 29th, 2013

Memorial Day is usually considered the "unofficial" beginning of summer, however, this past Memorial Day felt far from summer as temperatures hung into the 40's and 50's for just about everyone across southern New England with cool and damp conditions.  However, as we move through the next 12-18 hours we will begin seeing major changes in the overall weather pattern, giving many a taste of summer.  

While it is rather cool and cloudy outside with periods of showers, all of this is in advance of a very strong warm front which is slowly approaching southern New England.  As we move through the overnight hours and morning hours tomorrow, the warm front is expected to blow through CT and  into central/northern New England where it will become hung up.  South of the warm front, hot and humid air will begin working into the region.  

As of south it appears that all of CT, and much of western/central MA, including extreme southern VT and southwestern NH will end up in the warm sector with the strongest warmth tomorrow occurring down across southwestern CT, northward into western MA...perhaps even into central CT.  Across these areas temperatures are expected to surge well into the upper 70's...perhaps even into the lower 80's if there is enough sunshine and dewpoints should climb into the mid 60's.  

As the warm front lifts northward there will be showers and perhaps some embedded t'storms along the warm front.  As the morning goes on, this activity is expected to clear through CT/western MA/southern VT and southwestern NH.  Once this morning activity clears the areas, the million dollar question becomes, how much clearing do we see?  The amount of clearing we see will be extremely critical to t'storm development/strength later on in the afternoon.  

Computer model guidance is in agreement that during the mid to late afternoon, a s/w trough will slide southwest from Canada, in response we will see some weak height falls from 700-500mb along with some slight cooling at the 500mb level.  We will also see an increase in the wind fields aloft with bulk shear in the 0-6km level nearing 35-40 knots which is sufficient for organized storms along with the possibility of rotating updrafts.  With the warm front also just to the north, there will be fairly high amounts of 0-1km and 0-3km helicity values, especially in the vicinity of the warm front.  Currently, computer models have around 150 m2s2 of 0-1 km and 0-3km helicity which is noteworthy.  We will also have to watch surface winds throughout the day as if surface winds end up backing more to the south or southeast, this could vastly increase these helicity numbers, especially locally.  

As far as instability is concerned, there is some differing within the models as to how unstable the atmosphere will become.  The latest run of the GFS model is very robust, developing as much as 2000-300 J/KG of SBcape across western CT/western MA and portions of eastern NY.  How unstable the atmosphere will become all depends on two factors; 1) Amount of solar heating we receive and 2) How steep the mid-level lapse rates will be.  

As for surface heating, it's seems quite likely, there will be at least breaks in the cloud cover, especially across western sections.  Given the warmth of the low-level airmas advecting into the region, any breaks of sun would really make the temperatures shoot up.  As for lapse rates, the GFS forecast model is also fairly robust with mid-level lapse rates as 750-500mb lapse rates are just shy of 7 C/KM!  

Given all the above, scattered t'storms are expected to develop across NY/PA and then slide eastward into southern VT/western and central MA/southern NH and into western CT later in the afternoon.  Given the potential for at least weak surface-based instability, the potential for a few of these storms to become strong to severe is there.  If we are able to achieve moderate instability, we could be looking at a bit more of a stronger severe weather threat, with a higher chance for several storms to become severe.  The intensity of the storms will again depend on how much instability we see.  While the storm mode should be mainly linear, some supercells are also possible given the highly sheared environment.    

Besides the potential for torrential downpours and poor drainage flooding, the strongest storms will be capable of damaging winds in excess of 60 mph, large hail of up to 1'' in diameter (perhaps even up to 1.5'' if we see supercells) and a brief spin-up can't be ruled out either, especially if we maximize instability.  

Below is map of where the risk areas are located: