***Significant Widespread Severe Weather Outbreak Likely Across Parts of the Northeast into Western Southern New England 7/26/12***

Several computer forecast models are coming into agreement that a fairly significant widespread severe weather event will unfold tomorrow across portions of the Northeast extending into western southern New England.  The potential for destructive winds, extremely large hail, and perhaps tornadoes are all possible from central PA to SE NY/N NJ and extreme western MA/CT.

An approaching mid/upper level trough which is not all that well defined will be sliding eastward.  Associated with this trough is a surface area of low pressure and a pretty strong piece of shortwave energy.  As the trough approaches weak height falls will begin to occur and the surface area of low pressure will initially be further deepening.  In response to the weak height falls and some continued deepening of the surface low early on, a warm front will be lifting northward and finally stalling out somewhere around the VT/NH/MA border.  South of the warm front a very warm and very moist airmass will work into place.  Dewpoints will be climbing through the mid to upper 60's...possibly even getting to 70F or a few ticks higher!  With 925mb temps climbing between +22C and +25C and 850mb temps climbing to around +20C full sunshine and full mixing would yield surface temperatures in the mid to upper 90's.  It appears likely there will be some cloud debris to begin the day and b/c there are uncertainties to how quickly we will get rid of these clouds we will likely not achieve maximum heating.  However, we should yield enough sunshine for temperatures to get into the mid to upper 80's...perhaps even some lower 90's across NE PA/SE NY/N NJ where cloud cover initially may be much less of an issue.

Some computer forecast models also are hinting at an elevated mixed-layer plume to advect into the region from the upper Mid-west and some forecast soundings to verify this.  It is very interesting to note, however, this elevated mixed-layer plume is actually located a bit more shallow in the atmosphere compared to where we normally see them located.  Normally with elevated mixed-layers you will find them from about 800mb up through 550mb or so.  This elevated mixed-layer plume looks to exist from about 850mb up to only 700mb or so.  Since elevated mixed-layers are associated with some pretty dry air it's preferred to have them located a bit higher aloft b/c this keeps the dry air toward the mid-levels of the atmosphere and keeps the low-levels of the atmosphere very moist.  Having the elevated mixed-layer plume this shallow could allow for drier air to end up mixing into the lower levels of the atmosphere drying things out a bit which could reduce low-level instability.  Given how the low-level flow is expected to be from the southwest and remain southwesterly this should keep a belt of strong moisture flowing into the lower levels preventing the drier air from working in.  The elevated mixed-layer plume being so shallow may also lead to some fairly poor lapse rates in the 700-500mb level.  Normally with elevated mixed-layers you have very steep >7-7.5 C/KM 700-500mb lapse rates as temperatures from 700mb to 500mb drop fairly significantly.  With the elevated mixed-layer plume so shallow the difference in 700mb and 500mb temps my not be as good as they normally would be, however, computer models continue to show that lapse rates here should still be on the order of 6.5-7 C/KM.

With the approaching trough/surface area of low pressure progged to be sub 1000mb and strong piece of shortwave energy wind fields aloft will also vastly increase and we will be dealing with dynamics we'd typically in the early spring or fall months.  Computer models are forecasting a low-level jet in the 925/850mb levels to be on average about 20-30 knots coming in from the southwest...helping to keep funneling in low-level moisture.  The mid-level jet at the 700/500mb levels is also expected to increase to as much as 35-40 knots at 700mb to 45-55 knots at the 500mb level with winds here turning to a more westerly direction.  The region will also be in the Right fornt quadrant of a 85-90 knots upper-level jet max with upper level winds over the region of about 50-60 knots coming from a more westerly/west-north-westerly direction.  500mb wind speed values of 45-55 knots should yield to about 35-45 knots of vertical shear.  With all this said there will be a great deal of both speed and directional shear present.  We will also have to watch surface winds across the region, especially the valley locations where surface winds may back to the south or even south-south-east thus further increasing helicity, especially in the 0-1km and 0-2km level.  The presence of speed/directional shear will also yield to large amount of helicity with 0-3m helicity forecasted to be in the 200-400 m2s2 range and 0-1km helicity in the 150-300 m2s2 range.  This is VERY favorable for supercell t'storms as well as tornadoes.

With surface temperatures expected to climb well into the 80's with dewpoints nearing 70F under the presence of an elevated mixed-layer plume with 700-500mb lapse rates on the verge of 6.5-7 C/KM the potential for moderate instability to develop is there.  In fact, several computer models are generating as much as 2500-4000 J/KG of SBcape, 1500-3000 J/KG of MLcape, 2000-3000 J/KG of MUcape, LI values of -6C to -9C and Ncape values as high as 0.25.  This is all indicative of a moderately unstable airmass.

During the morning hours as the warm front lifts northward showers/t'storms are expected to develop as isentropic lift increase as well as low-level warmth and moisture leading to an increase in elevated instability.  In fact, a mesoscale convective system is possible.  Some strong to localized severe storms are possible with this feature as it's moving through.  After this complex, if one develops moves through the question then will become how quickly can we get rid of the cloud debris and how quickly can we receive sufficient heating to rapidly destabilize the atmosphere to achieve those instability values just lifted above?  It does appear that we will indeed clear out the cloud debris and by early afternoon we see sufficient heating allowing for a rapid jump in temperatures/instability.  Once this occurs we should begin seeing the development of t'storms; first across PA/NY and moving towards southern New England with further development expected there.


Several rounds of severe weather are expected tomorrow and lasting well through the evening and overnight hours as the front slowly drags eastward.  Early on in the afternoon through the early evening hours numerous supercell t'storms will be possible with the threat for destructive winds, very large hail as well as tornadoes.  Once we near evening and near sunset we start losing those stronger helicity values  and the severe threat should shift from tornadic supercells to bowing squall lines capable of destructive winds and large hail...an isolated tornado with these can't be ruled out either.  Torrential rains will also occur with any storm as well as vivid cloud-to-ground lightning.  Flash flooding will also be possible.  


Everyone is strongly urged to pay extremely close attention to updates and latest forecasts throughout the day and take each and every warning extremely seriously.  This is no run of the mill typical Northeast severe weather event and we have a much higher than normal threat for severe weather with the potential for tornadoes.  


Below is a graphic displaying the areas with severe weather potential and areas of heightened potential:

Active weather week on tap (7/23-7/27)

If you love low humidity than this past weekend was just for you.  After an intense round of t'storms moved through southern New England on Wednesday a much direr airmass has settled into southern New England. This is all about to change.  A developing southwesterly flow is working to bring in a much warmer and more moist low-level airmass into the region.  In fact, dewpoints across the region are slowly beginning to work their way into the lower 60's and by tomorrow mid to even upper 60's dewpoints will have worked into much of the region and by Tuesday we could be looking at dewpoints approaching 70F!  With a warmer low-level airmass moving into the region as well temperatures both Monday and Tuesday should reach the mid to upper 80's and possibly 90F in the hottest locations.  With the increased humidity and moisture the threat for t'storms will also increase and both Monday and Tuesday feature interesting potential across southern New England.  We will take a closer and more detailed look at each individual day.

Overnight Sunday into early AM Monday:

As dewpoints continue to increase through the evening and overnight period and PWATS increase as well we will begin developing some elevated instability across the region.  In fact, a few computer models also develop some fairly weak surface instability overnight into early AM.  Later on this evening/overnight computer models are showing a weak piece of shortwave energy moving through aloft in the mid-level flow.  Given the presence of developing elevated instability and perhaps some weak surface instability some showers and perhaps a few t'storms are possible during the overnight and early AM period.  Winds aloft are on the weak side and mid-level lapse rates aren't particularly steep and with not much in the way of instability severe weather is not expected overnight.  Any storm though will of course carry a lightning risk along with torrential downpours given the increasing moisture.

Monday afternoon/evening:

Several computer models are developing a lee trough just to the west of southern New England during the late morning and early afternoon hours.  With surface temperatures into the mid to upper 80's, dewpoints into the mid to upper 60's, and mid-level lapse rates around 6.5 C/KM the atmosphere may become moderately unstable tomorrow afternoon.  While initially winds aloft are on the weak side and mid-level lapse rates won't be particularly steep both do increase as the afternoon goes on.  By the afternoon mid-level lapse rates should be between 6.5-7 C/KM with deep layer shear increasing to around 30-35 knots by late afternoon/early evening.  Some models also develop a LLJ across the region which helps to increase low-level shear and helicity.  At this point it's a bit unclear as to exactly how tomorrow will unfold, however, given some of the pieces in place some thunderstorms are possible tomorrow afternoon and evening.  Given some of the favorable parameters some strong to severe storms can't be ruled out.  Some simulated radar models actually develop a squall line by late afternoon and move it through western CT late tomorrow evening so this will have to be watched.  While the main threat should be torrential downpours/isolated flash flooding given the presence of high moisture damaging winds and large hail are also possible in some of the stronger storms.

Tuesday:

Tuesday appears a bit interesting although it appears we will dodge what could be a much more widespread and significant severe weather outbreak.  Over the past few days computer models continue to be faster with the frontal passage and move the cold front through much of the region by mid to late morning.  Due to this winds in the boundary layer shift from southwest to a more westerly/northwesterly direction and we start drying out the mid levels of the atmosphere and this dry air mixes down to the lower levels thus preventing stronger instability from developing.  Shear/lapse rates are expected to be fairly impressive on Tuesday as well.  While a major chunk of southern New England looks to miss out the potential for severe weather may still exist across eastern MA as the front may not move through here until mid-afternoon or so.  This would allow for this area to destabilize much greater and have a great combination of strong winds aloft/instability.  It's possible eastern MA could see quite a decent severe weather event.  We'll still have to closely pay attention to this though as if the front does slow down by several more hours this could increase the threat for severe weather for areas further west.

After Tuesday all eyes turn to Friday as some computer models have a fairly impressive setup for severe weather across New England.  However, given this is still several days out and alot can change we won't get into the details now.  We'll worry about tomorrow and Tuesday first but it does appear to be an active week coming up.

Threat for severe weather 7/18/12

Summer has once again returned to southern New England as a warm front lifted northward on Sunday bringing in much hotter and humid air to the region.  While the duration of the heat/humidity will be rather brief it will be fairly oppressive at times.  Temperatures on Monday reached 90F across much of the region and more of the same can be expected Tuesday and Wednesday.  In fact, Tuesday looks to feature many location reaching the mid to even upper 90's!  Wednesday will be the final day of 90's and heat/humidity, however, it will not go away quietly.

A fairly potent shortwave moving through eastern Canada will be swinging a cold front towards New England Tuesday and Wednesday.  On Tuesday, the cold front will be situated to where northern New England will see a threat for widespread t'storms, some of which will become quite strong to severe with damaging winds and large hail the main threat.  in fact, across extreme northern New England, especially near the US/Canadien border the possibility of an isolated tornado will exist as this area will be much closer to the strong piece of energy/associated surface low which will help to locally back the low-level winds and create some pockets of stronger helicity.

As Wednesday rolls around the front will continue sagging southeastward and this will place central/southern New England in the threat for storms/severe weather.  With a very warm low-level airmass in place and overnight lows on Tuesday expected to be very mild temperatures will shoot through the 70's/80's very early Wednesday and reach the lower 90's by early afternoon.  With a very moist low-level airmass in place dewpoints should be right around the 70F mark.  The combination of very hot temps and high dewpoints situated under a fairly steep mid-level lapse rate environment with 850-500mb lapse rates up around 6.5 C/KM will yield to a moderate to extremely unstable airmass across the region.  Several computer models are developing anywhere from 2500-3500 J/KG of SBcape along with 1500-2500 J/KG of MLcape, 2000-3000 J/KG of MUcape and LI values as low as -8C.

Along with the potential for moderate to extreme instability winds aloft will also be fairly strong as well with a solid amount of speed shear (winds increasing with height).  While the strongest shear values will remain north of the front shear across central/southern New England is expected to be adequate enough.  Computer models have about 30-40 knots of vertical shear values across the region along with 850mb winds of around 20 knots, 700mb winds around 30 knots, and 500mb winds around 40 knots.  Winds throughout the atmosphere are expected to be unidirectional as there will be a westerly component to the wind at all levels.

While the combination of shear/instability looks rather impressive there are some factors that have to be considered.  Timing of the front may be an issue.  Over the past few days several computer models actually moved the front through the region too early which would certainly lessen the chance for severe weather across the region.  It does appear though that the front should not come through too early as models have slowed the front down.  In fact, some new data actually moves the front too slow which would actually mean it would be once again northern and central New England as the main threat area on Wednesday.
Another issue could be a wind shift occurring too early in the afternoon with surface winds shifting from southwest to a more westerly direction.  If this were to occur we could actually see some drier air aloft mix down to the surface resulting in dewpoints lowering a bit.  While the low-levels are rather moist dewpoints shouldn't drop more than 2-3F, however, that still is enough to really knock down instability numbers.
While not likely to be a major factor we do have to watch to see if there are any cloud debris from previous day convection.

Given the above the potential will exist on Wednesday for widespread t'storms across the region and the potential will certainly be there for severe weather.  Given how the winds aloft will be unidirectional with a good presence of speed shear we could see a pretty potent bowing squall line develop and the main threat for this would be damaging winds.  While freezing levels are a bit on the high side and helicity values expected to be on the lower side with the presence of the unidirectional flow large hail is not expected to be a major threat, however, it certainly can't be ruled out.  With high dewpoints and available moisture torrential rains will be likely with any storm and some isolated flash flooding can't be ruled out either, although this should be confined more to the normal flood prone/poor drainage areas.

Once the front clears the region we will return to more comfortable weather with much lower humidity.

Fire Works Mother Nature Style on July 4th?

With the July 4th holiday fast approaching and the weather in full summer mode the number of outdoor activities heavily increases as people have cookouts, go to the beaches, go golfing, yard work...the list can go on and on.  Not only are we entering the prime portion of the summer for outdoor activities but we are also entering the prime time of summer for thunderstorms and it looks like the 4th of July will provide us with the chance for widespread thunderstorms and some of them will have the potential to be very nasty.

Over the past 7-10 days we have seen an extremely active weather pattern with numerous severe weather events spanning from the upper mid-West into the Northeast and even down into the mid-Atlantic states where several days ago a derecho (an extremely damaging squall line of thunderstorms with an average forward motion of 58 mph or more for at least 250 miles) caused substantial damage as well as loss of life from just about IA/IL all the way to southern NJ down through northern VA.  The weather pattern over this time period has been a historically favorable setup for significant severe weather across the east coast of the United States and as we move through this next week this same weather pattern will be in place, however, there will be a slight shift in the pattern which will place the prime focus for severe weather and the possibility of significant severe weather across the Northeastern United States.  Over this next week there will be two chances for severe weather events, the 4th of July and Saturday, July 7th.  We will first deal with the potential the 4th of July holds and worry about Saturday once we get through Wednesday.

As we move through the next few days a massive ridge will continue building across pretty much the entire United States with the exception of the west coast and Northeast.  As this ridge continues to build a trough will be deepening and amplifying as it makes a move towards New England.  As this occurs the winds in the mid and upper levels of the atmosphere and going to begin coming more out of the west to northwest.  At the same time a strong s/w embedded within the trough along with an area of surface low pressure moving through Canada are going to begin swinging a cold front eastward.  Ahead of the cold front a warm front will be lifting northward through New England during the overnight hours of Tuesday into the morning hours of Wednesday.  With the approaching/passing warm front a very warm and moist warm level airmass will once again be moving into the region.  As the warm front nears we will also see a good burst of elevated instability along with increasing winds aloft.  This will yield to the development of a mesoscale convective system (MCS) either off to our west or northwest late on Tuesday.  Computer models are all pretty much in agreement of the development of an MCS, however, they begin to diverge a bit in the intensity of the MCS as it approaches our region as well as the timing of the MCS.  Historically, computer models do have trouble with these systems, especially with where they track.  With regards to the possible track of the MCS in the past MCS's tend to track along and ride the axis of strongest elevated instability (Most Unstable Capes) as well as following the ridge axis.  As of now computer models appear to have our region right in the bullseye for an MCS track.  The questions now are; 1) What time does the MCS work it's way through the region?  2) How strong will the MCS be?  3) What significance will the MCS have on the potential for storms later in the day?

Going into further detail regarding the above mentioned MCS, the MCS may play a pivotal role with regards to the severe weather potential later on in the afternoon.  Associated with the MCS will be the presence of mid and high level clouds.  One of the keys here will be how much leftover cloud debris will be across our region and how quickly it would take to move these clouds out so we can generate strong solar heating to boost temperatures and instability.  Whether or not we can clear out and receive strong solar heating will ultimately depending on the timing of the MCS.  If the MCS moves through our region earlier in the morning and exits by about 9 AM...perhaps even 10 AM this will further increase the chances of clearing out any leftover cloud debris early enough to where we can achieve strong surface heating.  If the MCS moves through our region later in the morning and we're still dealing with cloud debris by noon-1 PM it would be extremely unlikely we will see enough heating to generate strong enough instability.

Another issue we may have to contend with regards to the MCS is if the MCS is quite large in aerial coverage and strong we may have to deal with subsidence on the wake side of the departing MCS.  While this would actually help us rid of any clouds faster it could actually hinder the development of more clouds/storms later on in the afternoon unless we are able to get some vigorous lift.  At this point in time this should not be an issue, however, this will have to be closely watched.

After the MCS...

As briefly mentioned above after the MCS departs the region we then turn our attention to how quickly we can get the cloud debris out of the region.  If we are able to see strong surface heating given how 925mb temps will be around +22C to +23C and 850mb temps around +18C temperatures should really jump across the region.  Temperatures would be in the 80's across the entire region with many inland locations possibly getting into the upper 80's.  Dewpoints are expected to be in the mid to upper 60's across the region as well making it feel quite uncomfortable outside.

The combination of high temperatures/high dewpoints would lead to an extremely unstable atmosphere.  In fact, some computer models which are showing less in the way of cloud debris are generating as much as 2500-3500 J/KG of SBcape, 1500-2500 J/KG of MLcape, 2500-3500 J/KG of MUcape, and LI values as low of -6C.  While the level of heating will strongly determine exactly how unstable the atmosphere will become, how steep the mid-level lapse rates are will play a major factor in how unstable the atmosphere can become and also play a role in how vigorous updrafts can become.  As of right now computer models are differing with regards to the steepness of the mid-level lapse rates.  Some computer models really weaken the mid-level lapse rates, most likely in response to the MCS moving through the region but do try to recover them very late in the afternoon.  Some other models really don't weaken them at all and keep mid-level lapse rates closer to 7 C/KM rather than a modest 6 C/KM.

While computer models are not really forecasting tremendous amounts of wind shear aloft there are some computer forecast models which have a good deal of directional shear leading to some fairly high helicity values.  Now it's possible this one model may be suffering from some feedback issues resulting in the model overdoing the speeds of the winds in the lower levels of the atmosphere.  On average computer models are generating around 30 knots of vertical shear, about 15-20 knots of wind in the lowest 7000ft of the atmosphere with 30-35 knots from about 7000ft up through 35,000ft.

All in all Wednesday has the potential to be a fairly active day across the region but at this time there are plenty of questions at hand which makes this potential very uncertain at this point.  If the extreme side of things work out then the potential will exist for a fairly significant severe weather episode across the region with the potential for large hail, damaging winds and perhaps even a tornado.  If we see less in the way of instability with weaker lapse rates and shear the severe weather threat would be greatly diminished and we'd only be looking at a few strong to severe storms.

With this Wednesday being a major holiday with lots of outdoor activities planed it is extremely important to pay close attention to all forecasts and listen for the latest updates.  Hopefully over the next 24 hours more of these issues discussed above can be resolved.