Periods of rain/storms overnight, more threat for storms on Friday 6/27/13

This entire week has been quite unsettled as we have had showers and t'storms across the region every single afternoon.  While not everyone has seen action, several places have been hit several times this week with showers and t'storms and as we move into Friday and into the weekend we will remain in a similar pattern, in fact, this pattern should remain in place through next week.  However, the point of this blog post is to discuss the weather for Friday.  You may hear forecasters every now and then (or even so often) say, "this is a highly complex weather forecast"...well this is yet another case of that and, well b/c it is.  As much as we know and understand the weather and the atmosphere and how they work, both are still a highly complex science and sometimes there is just so much chaos going on in the atmosphere that forecasting to certainties and exacts can be extremely difficult at times.  This is why I am always in favor of explaining the situation in as much detail as possible to hopefully help the reader have an understanding of the situation.

Anyways, this past week we have been dealing with several different meteorological phenomena's which have lead to some very difficult forecasts, some turning out good, and some getting completely wrong.  As we move through the remainder of tonight and through the day tomorrow, the forecast won't necessarily become much easier, however, confidence is increasing on several weather events we will see across the region.

During Wednesday, including the overnight period, a weak backdoor cold front (cold front which moves into the region from the north, northeast, or east) slid southwestward across the region and by Thursday morning, this weak boundary had washed out across central CT.  Typically when cold fronts move through, we usually see a refreshed airmass, it's cooler and less humid.  Well, that wasn't the case this time.  This front, as mentioned was very weak, and the cooler temps/dewpoints actually lagged well behind this front and you'd have to look all the way into northern New England to find this refreshing air.  During the day today, the washed up front left a boundary in place across the region and that was the focus for some afternoon showers/t'storms that popped up across the region.

Too our west, a deepening trough has been slowly sliding eastward dragging a cold front east as well.  As the trough/front slowly slide eastward, a surface area of low pressure has been developing and this will slide to our northwest during the day tomorrow.  As this occurs, the washed up backdoor cold front over the area will actually retreat north as a warm front, in fact this process is already occurring.  With the system strengthening to our west we have seen stronger winds aloft move into the region, especially in the lower levels of the atmosphere, these factors will lead to the development of showers and t'storms during the overnight hours.  While instability is very marginal, winds aloft are increasing so we can't completely rule out a strong storm overnight with some gusty winds.  The main threat though will be torrential downpours leading to pockets of flash flooding.

Now, as we move into Friday morning, there still should be numerous showers and perhaps some t'storms around.  After this activity moves out, this is where the forecast becomes much more complex and challenging.  Winds aloft will be quite strong tomorrow, also, with the possibility of the warm front being close-by, this will lead to some very high helicity values (change of wind direction with height), across the region.  There should be lots of clouds present, however, some computer model guidance is showing that there may be breaks of sun late tomorrow morning, especially across western CT/MA.  If we are able to get any sunlight, given how dewpoints will be into the upper 60's to lower 70's, and the potential for temperatures to get into the lower 80's, this will lead to a fairly unstable airmass tomorrow.  If we are able to achieve sufficient destabilization, the threat for more t'storms in the afternoon will vastly increase.  Given the presence of very strong wind shear and a very strong system to our west, the threat for strong to severe t'storms would exist.  Now, conditions right now don't appear to favor widespread severe weather, in fact, we are probably only looking at an isolated threat for a few of the storms to become strong to severe.  Any stronger storm will have to be closely watched though as it would have the potential to acquire rotation and the threat for an isolated tornado would be in the cards.  Again, the confidence in this right now is low b/c there are major questions with how unstable the airmass will become tomorrow.  Basically, the more sun we see tomorrow, the higher the threat for t'storms and the higher the threat for some of these storms to become strong to severe and be capable of damaging winds, large hail, and perhaps a tornado.  If we see little sun, the threat for t'storms will be much lower and the threat for severe storms will be extremely low, although still would have to closely monitor any storm that would develop.  Besides instability, the tornado potential would also depend on where the front front positions itself.  If the warm front is able to get well into northern New England, well then we would have less helicity values as the directional component to the wind would decrease and winds aloft would be blowing from the same general direction.

Once again, a severe weather outbreak is not expected at this time, however, the threat for t'storms will exist and if we see enough sunshine to really destabilize the atmosphere, the threat for t'storms and threat for some of these to become strong to severe increase.  Less or no sun, low threat for storms and even lower threat for any of these to become strong to severe.

More t'storms in the forecast for Wednesday, JUne 25th, 2013 with increased potential for strong to severe t'storms

We are fully engaged in a summer-like pattern as temperatures the past few days have been in the upper 80's to lower 90's across the region and with dewpoints into the upper 60's to lower 70's it has felt very uncomfortable outside and of course, we've had our daily t'storms, although not everyone has seen rain or storms.  This pattern of heat/humidity and storms will continue through the week and into the first part of the weekend and each day there will be a threat for strong to severe storms.  However, on this post we will just focus on Wednesday and what tomorrow may bring.

We are looking at yet another hot and humid day across the region tomorrow as temperatures away from the immediate coastline will once again get well into the mid to upper 80's to perhaps even 90F, in fact, places like Windsor Locks and Hartford could be looking at a 4th consecutive of 90F+ temperatures!  Dewpoints will once again remain in the mid to upper 60's so it will feel very humid outside.  

The big difference between tomorrow and the past two days is we are looking at more of a widespread threat for showers/t'storms, along with an elevated risk for stronger to severe t'storms.  Remember, when talking about strong t'storms, we are referring to t'storms which are capable of producing winds 40 mph or greater and/or small hail and when talking about severe t'storms, we are referring to t'storms capable of producing winds of 58 mph or greater, and/or hail 1'' in diameter or greater.  Also, not every thunderstorm becomes severe and the majority of people don't usually experience "severe" t'storms.

With that said, tomorrow, computer forecast models are showing a fairly strong piece of shortwave energy, embedded in the mid-level wind flow moving east-northeast from PA to NY right through southern New England during the afternoon hours.  As this shortwave energy approaches the region, winds well up in the atmosphere will begin to increase and provide us with sufficient wind shear, something we have lacked the past few days.  This wind shear will really help for storm to become better organized and sustain themselves much longer.  The shortwave energy will provide a focus for strong lift, something we've also lacked and that will allow for t'storms to be more widespread than they have been the past few days.  

With very little in the way of a cap tomorrow we should begin to see cumulus clouds beginning during the mid to late morning hours and this will really prevent temperatures from reaching into the lower 90's and may prevent anyone from seeing 90F tomorrow.  The combination of heat/humidity will once again provide us with quite an unstable airmass tomorrow, although probably not as unstable as the past few days b/c we will be looking at weaker mid-level lapse rates tomorrow.  This instability will provide the fuel storms will need to develop and strengthen.  

The combination of stronger wind shear and an unstable airmass will lead to the development of scattered t'storms during the early afternoon hours and we should see storms last well into the evening hours as the energy will be slow to move through.  The increased wind shear, along with instability will also increase the threat for stronger to severe t'storms tomorrow across the state.  Some of the storms that develop tomorrow will have the potential to produce winds of 40-50 mph and possibly as high as 60 mph.  While hail may not be a huge threat due to weaker lapse rates, the strong wind shear, along with some directional wind shear (change of wind direction with height), we could see some isolated supercell t'storms tomorrow, storms which have stronger rotation.  If any supercells do form, the threat for hail would locally increase and we could be looking at the potential for hail up to 1.5'' in diameter.

While confidence in the threat for t'storms is very high, confidence in how strong/severe the storms become or how many of these storms become strong to severe is lower.  This is due to the fact that the lapse rates will be so weak.  When mid-level lapse rates are weak, below 6 C/KM, (this means that for every km you rise in the atmosphere, the temperature on average decreases by 6C), air parcels aren't really rising all that rapidly.  Steeper mid-level lapse rates (higher rate of temperature change) allow for much more vigorous upward parcel acceleration and this translates to a much greater severe weather risk and t'storms have the ability to grow very tall, tapping into the much colder air in the mid/upper troposphere and stronger winds.  

All in all, we will once again see t'storms tomorrow afternoon/evening with activity being more widespread than the past two days.  The threat for stronger/severe storms will also be a bit more elevated than the past few days.  If outdoors, keep a close eye to the sky and up to the minute weather forecasts and prepare to seek shelter inside to avoid lightning.  

Threat for t'storms later today; some potentially becoming strong to severe

As highlighted yesterday, the potential exists later on for showers and t'storms across the region as a weak secondary cold front slowly shifts eastward.  For the past few days, computer model guidance has suggested that much of the morning hours would be filled with clouds, and this would be one factor limiting how unstable the atmosphere would become.  However, late last evening, cloud cover which blanked the area for much of Sunday drifted off to the east/northeast leaving us with clear skies overnight as well as now.  With much more sunshine than expected, this will allow the atmosphere to become a bit more unstable than previously thought, thus increasing the threat for storms and stronger storms later on.  It is unclear though as to how widespread t'storm activity will be along with how strong or severe the storms could become as there are some question marks which we'll highlight below:

1) Currently dewponts are into the lower 60's south of the MA Pike, lower north of the Pike.  Computer model guidance the past few days have indicated that these dewpoints will mix out...or in other words, decrease, which is beginning to occur now.  This is occurring due to the fact that above the surface, just a few thousand feet ASL, there is some drier air in place, dewpoint temperatures at 925mb (~2500') and 850mb (~5000') are only in the mid to upper 40's.  Typically you'd like to see them into the lower to mid 50's.  As we're moving through the morning hours, and the sun really heats up the atmosphere, allowing for stronger atmospheric mixing, that drier air will begin to mix down to the surface, thus lowering the dewpoints some.  The question is, how much will the dewpoints lower?  This will have major implications as well with regards to how unstable the atmosphere will become.  
Even if dewpoints do lower, there is potential for them to actually increase later on this afternoon.  We are noting some higher theta-e air working into the state, which would allow for dewpoints to increase.  A sea-breeze developing later on may help to increase them as well along with surface winds perhaps backing more to the south or southeast.  
2) As mentioned yesterday, the cold front is not particularly strong, so there isn't a great deal of forcing along it.  What this means is, that could limit how widespread the activity becomes and how much action we see develop.  On the other hand, since this would mean storms are more isolated in nature, this could allow them to become even stronger/more severe given they will have much more energy to tap into.
3) Forecast soundings show some funky warm spots in the mid-levels of the atmosphere. While some colder air will work in aloft, that doesn't necessarily mean these warm layers will dissipate.  These warm layers could affect storm updrafts in that it will be a bit more difficult for storms to grow very large, disallowing them to really tap into the wind energy and much colder air aloft, decreasing the threat for stronger winds and larger hail. 
With those factors mentioned, today, we are looking at temperatures reaching well into the mid 80's away from the immediate coast with dewpoints hovering around the 60F mark.  The combination of temps/dews, along with colder air working into the mid-levels of the atmosphere, will allow for a fairly unstable airmass with Cape values perhaps reaching 1500-2000 J/KG, especially if dewpoints increase later on and lifted index values as low as -3C to -4C.  Winds aloft are also quite strong, evident by vertical shear values in excess of 35-45 knots.  The combination of instability/shear will lead to the development of showers/t'storms later on and given the mentioned parameters, some of these storms will become quite strong to severe with the potential to produce damaging winds and large hail.  We will also have to keep an eye on for any rotating t'storms, increasing the hail threat.  Flash flooding will be a concern too, especially in low lying areas given amount of rain we have had.  The timeframe for t'storms will be from about 3-10 PM tonight.  Originally, it was thought storms would die as the sun would begin to set, however, with colder air working in aloft through the evening, this will help to keep instability around a bit longer.  
The front is expected to stall either over the state or just west and this will once again allow the potential for t'storms tomorrow with a renewed threat for strong/severe storms.  

Threat for t'storms; Monday, June 17th, 2013

A weakening storm system moving west to east across Quebec Provence in southeastern Canada will push a series of two cold fronts through southern England over the next two days.  The first cold front, is weakening as it approaches southern New England and as the front nears CT during the overnight hours this front is expected to wash out.  With the front approaching, we can expect some isolated to scattered showers through the overnight hours along with extensive cloud cover.  The combination of the front washing out along with extensive cloud cover will allow for overnight temperatures to remain around 60F with dewpoints hovering the 60F mark as well. 

As we move into and through the first half of Monday, one of the biggest questions is how much clearing and how much sunshine will we see?  There is decent agreement within the computer models, that there will be breaks of sun, along with clearing, moving from west to east during the late morning and early afternoon hours.  The greatest likelihood of seeing breaks of sun and sunshine will be along and just west of the CT River Valley.  In fact, we may see enough sunshine to where temperatures could reach the mid-80's.  

With temperatures, potentially reaching the mid-80's, dewpoints around 60F, and mid-level lapse rates between 6-6.5 C/KM, computer models are indicating we could see Cape values reach anywhere from 500 J/KG to as much as 1500 J/KG along with lifted index values getting to around -2C to even -3C.  This is all indicative of an atmosphere that will be marginally unstable tomorrow afternoon.  We will have to watch trends tomorrow with sun as any more sun than forecasted could lead to higher values.

A secondary cold front will slide towards CT tomorrow and with the potential for a marginally unstable airmass we will be looking at the threat for showers and t'storms.  The lift associated with the cold front is not very strong and the system moving through southern Canada will be weakening so we will not be looking at widespread showers/t'storms but more of the scattered nature.  

There will be some pretty impressive wind shear aloft as well with forecast models indicating 0-6km shear values as high as 35-45 knots.  These values are supportive for updrafts which can organize and also supportive of updrafts which could slightly rotate.  

Given the potential combination of instability/strong wind shear aloft, and a cold front sliding east, we should see some showers/t'storms develop during the early to mid afternoon hours.  While activity may not be all widespread, the presence of 35-45 knots of 0-6km shear will allow any activity to potentially organize into a squall line.  We will also have to watch out for a few strong to potentially severe t'storms with a threat for strong gusty winds and perhaps even some hail, especially if stronger cores can develop and acquire some weak rotation.  While not a major threat, given how much rainfall has occurred the past few weeks, there will be a flash flooding risk with any storms given there will likely be torrential downpours.  

The best timeframe for storms should be from 2-8 PM, although activity could persist until past 9 PM or so, however, as the sun goes down, the atmosphere will begin to stabilize. 

60th anniversary of the Worcester, MA tornado: The "Granddaddy" of Southern New England tornadoes.

When one thinks of southern New England one does not think of strong/violent or killer tornadoes here, and for great reason, as they are not very common, though not unheard of.  In fact, just recently, on June 1st, 2011 we had a strong/killer tornado rip through New England.  This tornado was on the ground for 39 miles, beginning in a section of Westfield, MA and finally lifting back up in a section of Charlton.  During it's 39 miles, 70 minute journey, at the peak the tornado was rated as a high end EF3 tornado with maximum winds of 160 mph, just shy of EF4 status and killed 3 people and injuring 72.  Prior to this 2011 tornado you have to go back to May 29th, 1995 when an F4 tornado struck the town of Great Barrington, MA.  In the 10 minutes this tornado was on the ground, 3 people were killed.  Prior to 1995, there was another F4 tornado (although there are major uncertainties regarding whether or not this should have been rated so high) which struck Hamden, CT on July 10th, 1989.  While only on the ground for several minutes, it produced major damage and injured over 40 people.  Then we'll go back to 1979, on October 3rd an extremely rare October tornado struck Bradley Airfield in Windsor Locks, CT.  While this was yet another short-lived tornado, it produced extreme damage along with killing 3 and injuring over 500 people.  This was yet another F4...even borderline F5 tornado.  While there were a few other tornadoes with a rating of F3 or higher we'll get right to be the big one, the Worcester, MA tornado of June 9th, 1953.

Below is a projected surface map on the day of June 9th, 1953.  An area of surface low pressure (circled in black) moving through Quebec Provence in southeastern Canada is pushing a cold front (highlighted by the blue line) to slide eastward towards southern New England and allowing for a warm front (highlighted by the red line) to lift northward through southern New England.  As the warm front would lift through CT and MA into northern New England, this allowed much warmer and humid air to work into the region, becoming two contributors on what was going to make for quite the unstable airmass later that afternoon.

Below we will take a look into the 700mb and 500mb levels of the atmosphere, which is roughly anywhere from around 10,000ft above sea-level to around 18,000ft above sea-level.  First off, is an image of the 700mb pattern over the Continental United States and much of Canada.  The red arrows indicate where the weather pattern is deriving from.  As you can see by the arrows, our airmass at 700mb originated from the desert Southwest.  This is actually pretty significant as research done over he past decade has shown that an atmospheric setup like this is conducive into allowing an elevated mixed-layer to sustain itself from the southwestern United States all the way to southern New England.  We could go into lengthy discussion into what an elevated mixed-layer is, but too make matters simple, elevated mixed-layers just lead to extremely unstable atmospheric conditions which is a major ingredient towards severe thunderstorms.  We also take note of the wind direction/strength depicted by the wind barbs.  We notice that the winds at this level are coming more from the northwest 25-35 knots, with even 40 knots over Ohio.  As the area of low pressure continued moving northeastward and then eastward into and through Canada, the system continued to actually strengthen and this allowed the wind fields to increase at this level on the day of the event.  

  

Below is a reanalysis map of the 500mb pattern along with wind strength/direction for roughly 8:00 AM eastern time on the morning of the 9th.  Just like at 700mb, we notice a trajectory of the pattern with the airmass originating from the southwestern United States and we also note a westerly flow with 40-45 knows over southern New England, however, just further off to the west, winds increase to 50-60 knots, and this would arrive over southern New England later that afternoon.

With the surface pattern, 700mb pattern, and 500mb patterns looked into we can now put all of this together and explain why such a violent tornado occurred in a region where they are oh so rare.  

The 500/700mb pattern which had been in place across the region for several days allowed an elevated mixed-layer plume to eject from the southwestern United States and survive it's journey to southern New England.  When the approaching storm system from the west allowed for a warm front to push northward through southern New England.  When the warm front pushed through this allowed dewpoints to soar into the 80's across spots and dewpoints to climb into the 60's.  The combination of temps into the 80's, dewpoints and into the 60's along with the elevated mixed-layer lead to extreme instability across the region. The warmfront, also in close proximity to the region, also allowed for winds at the surface to likely be from a more southerly/southeasterly direction.  With winds at the surface from the south/southeast, turning to the west/northwest aloft in the mid/upper levels of the atmosphere this created large amounts of helicity, or directional wind shear across the region...an extremely important ingredient for supercells and tornadoes.  

As the afternoon went on and the cold front and energy aloft associated with the system progressed eastward, thunderstorms began to develop.  One thunderstorm in particular became extremely severe and reaching supercell status, a supercell is a thunderstorm with very strong rotation.  At the height of the storm, the supercell thunderstorm produced a tornado which too this day is known as the granddaddy of southern New England tornadoes.  Along it's path, the tornado was on the ground for 90 minutes, traveling 48 miles tracking through eight different towns in central MA.  The tornado produced F4 damage across six of the eight towns affected, and even producing borderline F5 damage in spots.  When all said and done, 94 people were killed and over 1,000 people were injured.  Thousands of buildings/structures were heavily damaged with many completely destroyed.  

This was not the only killer tornado this system would produce, however.  Just the day before, the same system produced an F5 tornado in the town of Flint, MI.  That tornado killed 116 people and injured over 800 people.  This storm system, which began producing severe weather across NE/IA/WI would end up producing a total of 46 tornadoes, with 13 of them F3 or higher.  To this day, this series of outbreaks remains one of the most devastating and deadly in recorded history.  

Below is a slideshow, created by the National Weather Service in Taunton, MA, which also explains more regarding the setup, as well as showing numerous photos of the damage produced by the tornado, and even some images of the tornado itself.

http://www.erh.noaa.gov/box/papers/WorcesterTornado53_files/WorcesterTornado53.html

*Data was obtained through various National Weather Service Products and the images were obtained from the reanalysis site from Plymouth State and the Daily Weather Map Archive's ran by NOAA.*