Widespread t'storms expected Saturday, July 19th, 2013 with the threat for strong to severe t'storms

Saturday will be the final day of the high heat/humidity across New England as an amplifying trough moving through southern Canada and the Great Lakes will swing an area of surface low pressure through southeastern Canada with an attendant cold front towards and eventually through the region.  As the trough/cold front move closer to the region we will have to deal with the potential for t'storms and several of these are likely to become quite strong to severe.

Over the past few weeks, strong ridging has dominated much of the United States thanks to a very strong Bermuda High.  However, over the past several days, a trough has been developing and amplifying as it moves eastward and this is allowing for the ridge to slowly break down.  As this trough continues to amplify and the area of surface low pressure continues to deepen, the increased pressure gradient will allow for the winds aloft in the lower, mid, and upper levels of the atmosphere all to increase.  Throughout the day on Saturday we will see the low-level jet increase to 25-35 knots, with the mid-level jet increasing to 35-45 knots, even upwards of 50-60 knots across central/northern New England, and the region will be in the right entrance region of a 90-100 knot upper-level jet max which will really help to increase divergence and lift across much of New England.  With the increase in the mid-level wind fields, 0-6km shear values should increase to 30-40 knots, which is more than sufficient for organized t'storms.

While we may have more in the way of cloud cover on Saturday, temperatures are expected to reach anywhere from the mid-80's along the coast to as high as the low to mid 90's inland.  This coupled with dewpoints in the upper 60's to lower 70's will once again make for very uncomfortable conditions outside.  During the day on Friday steeper mid-level lapse rates (change of temperature with height) have advected into the region, and there is even a hint at a weak elevated mixed-layer (EML) across much of southern New England as mid-level lapse rates are approaching 7 C/KM and some forecast soundings do have an EML look.  As we move into Saturday, mid-level lapse rates are expected to remain quite favorable, in the 6.5-7 C/KM.  These stepper lapse rates coupled with surface temperatures ranging from the mid-80's to the lower to mid-90's and dewpoints in the upper 60's to lower 70's should yield to a moderate to extremely unstable atmosphere.  We could be looking at surfaced-based cape values ranging from 2500-4000 J/KG, mixed-layer cape values in the 1500-2500 J/KG range, MUcape values in the 2000-3000 J/KG range, and lifted index values as low as -6C to -9C...these numbers are indicative of an extremely unstable atmosphere and will be the fuel for the t'storms.

Out ahead of the cold front, a pre-frontal trough is expected to develop across the Hudson Valley and then slide east during/just past prime heating and this will be the focus for lift.  The combination of increasing lift along with adequate wind shear and a moderately to extremely unstable atmosphere will lead to the development of widespread showers and t'storms tomorrow afternoon and lasting through much of the evening and into the early overnight hours.  Winds aloft will be fairly unidirectional which suggests that the main storm mode tomorrow will be a linear squall line with bowing segments.  If we are to achieve the higher end of instability, then some supercells will be possible as well.  Forecast soundings also show an inverted-v signature.  Given all this, the main severe threat with thunderstorms tomorrow will be damaging winds in excess of 50-60 mph.  If we do see supercells, then the threat for large hail, possibly as large as golf balls would also be a possibility.  All storms will contain vivid cloud-to-ground lightning along with torrential downpours, perhaps leading to poor drainage flooding.

Below is a map indicting the risk area along with hazards:

***NOTE***  Not everyone in the shaded area may receive rain or even t'storms.  Not everyone will experience a strong to severe t'storm either.  The highlighted area outlines the area where severe weather may occur***

Strong to Severe t'storms possible on Saturday, July 20th, 2013

An amplifying trough will continue to strengthen as we move towards the weekend and slide a cold front east.  Once the cold front pushes east of the region we will usher in some "cooler" (in a sense of more seasonable temperatures) and drier air into the region as the pattern changes from ridge dominated to more trough dominated through much of next week.  Before this cooler and drier air works in though we will have to deal with the threat for t'storms and the potential exists for several t'storms to become quite strong to severe.

While isolated t'storms are possible both Thursday and Friday, Saturday is the day of greater concern for widespread t'storms along with an enhanced threat for some severe weather.  Saturday will not be a washout by any means, however, we may have some shower/t'storms activity around in the AM before we see a break.  This is when we see sun and lots of surface heating which will help to fuel the atmosphere for storms later on in the day.

A strong piece of energy moving through southern Canada along with the approaching trough/front will help to really tighten the pressure gradient over our region given the Bermuda High is centered to our south.  In response, the winds aloft will really begin to strengthen.  Forecast computer models show winds in the lower levels of the atmosphere to strengthen to 20-30 knots with winds in the mid levels of the atmosphere strengthening to 35-45 knots.  The region will also be in the right entrance region of a very strong upper-level jet which will really help to increase lift/forcing.  Given the strong mid-level winds, we are looking at vertical shear values to be in excess of 35-40 knots across the region.

At the surface, temperatures are expected to reach the mid 80's to around 90F with dewpoints right around 70F.  A piece of computer model guidance is showing mid-level lapse rates around 6.5 C/KM  on Saturday which is pretty decent.  The combination of high temperatures, high dewpoints, and steep mid-level lapse rates should lead to moderate amounts of instability region wide.  We are looking at the potential for SBcape values in excess of 2500-4000 J/KG, MLcape values 1500-2000 J/KG, and LI values in the -6C to -8C range.

As the cold front slides east on Saturday, there are some hints that a pre-frontal trough may develop out ahead of the cold front.  If this does come true, we could be looking at t'storms firing as early as 1-2 PM across western sections and then sliding east.  If a pre-frontal does not develop, storms may not fire until 2-3 or even 4 PM.  Typically, we almost always see pre-frontal troughs develop so I believe we will see action early on in the day along/ahead of the pre-frontal trough and then quite possibly again later on just ahead of the cold front.

Given the potential for moderate instability coupled with fairly strong winds aloft, and strong forcing from the front and/or pre-frontal trough and enhanced forcing from the right front quadrant of the upper-level jet, we are looking at numerous t'storms Saturday afternoon and quite possibly multiple lines of t'storms.  Any t'storms that develop will have the potential to become quite strong very rapidly and approach severe limits.  The main threat with any storms will be damaging winds in excess of 60 mph.  Small hail is also a possibility, however, the freezing levels appear to be rather high but if any supercells develop, which is a possibility if we can achieve 3000+ Capes, then there could be a secondary threat for large hail.  Torrential downpours are very likely with any storms so flash flooding will be a possibility.

This potential will continued to be closely monitored.