Wednesday, March 1st, 2017 low-topped thunderstorm potential

We have seen some very interesting weather over the course of the past month.  Near the beginning of the month we saw our biggest snowfall event of the winter, last week and the start of this weekend we saw record warmth with temperatures well into the 60's and even low 70's.  This past Saturday we had a low-topped line of thunderstorms approach ahead of a strong cold front which event prompted an EF-1 tornado in Conway, MA.  As we move through this week we are once again looking at the threat for thunderstorms on Wednesday and then back to winter on Friday as we may deal with snow squalls.  

The setup we are looking at for Wednesday is quite impressive for March 1st and one of the driving factors for this potential will be the advection of very steep mid-level lapse rates (strong decrease of temperature with height) associated with an elevated mixed-layer plume.  The question, however, is will the steepest mid-level lapse rates end up staying south of southern New England or will they advect in:

The importance of the steep mid-level lapse rates will be on the degree of instability which develops.  With a warm front expected to push at least into CT early Wednesday morning, south of this warm front temperatures should warm into the 60's with dewpoints into the mid 50's and possibly near 60F.  Much of this too will depend on the surface winds.  If surface winds remain more out of the southwest, surface temperatures are more likely to climb well into the 60's with dewpoints near 60F.  If winds remain more southerly or southeasterly, this will likely prevent temperatures and dewpoints from climbing that high.  With the possibility of temperatures into the 60's and dewpoints near 60F combined with the steep mid-level lapse rates, this could potentially yield several hundred J/KG of cape (measure of instability):

Normally these values aren't very impressive, however, we will have very strong lift and wind shear in place as well.  What cape does is measure an updrafts strength...so the more cape you have the stronger a storm's updraft can be if the cape is tapped into.  If you have strong wind shear and lift in place this can compensate for the lack of cape as strong lift and shear can also enhance a storm's updraft and this is the potential situation we would see more Wednesday.  

For the shear we are looking at a potentially 90+ knots mid-level jet at 500mb and 50-60+ knots at the 850mb level.  These values will help to enhance bulk shear and vertical shear values which will enhance updraft potential:

What can we expect?  These setups are always very, very tricky and getting the potential for any type of strong to severe thunderstorms in our area this time of year is nearly unheard of.  However, given the projected parameters and setup we will have to watch for a line of thunderstorms which will develop ahead of a cold front to our west.  If the above mentioned parameters pan out we will have to watch this line as it approaches southern New England.  Given degree of wind shear aloft strong to damaging wind gusts would be the main threat.  Also, given how winds do sort of turn with height we will have to watch for embedded areas of rotation as well.  

After this system passes we will begin a transition back towards winter and on Friday we have to watch for the possibility of heavy snow squalls with perhaps some thunder and lightning as well but we'll save this for a later post.