Wednesday, July 1st, 2015 storm potential

A potent trough and attendant shortwave will slowly be pushing eastward during the day on Wednesday allowing for a cold front to also slowly push east.  Ahead of the trough the surface and low-level flow will be out of the southwest allowing for a warm and moist low-level airmass to set in place.

As the shortwave moves just to our north it will be associated with some pretty cold temperatures in the mid-levels of the atmosphere for this time of year.  The GFS computer forecast model suggests temps at 700mb (~10,000ft ASL) as cold as +4 to +6C with temps at 500mb (~18,000ft ASL) between -10C and -12C and perhaps as cold as -12C to -14C.  This should yield to some fairly steep mid-level lapse rates (change of temperature with height).

With surface temperatures projected to warm into the lower 80's and dewpoints expected to climb into the mid to perhaps upper 60's, the combination of warm temps, high dews, and presence of steep lapse rates should yield to a modestly unstable airmass to develop in the afternoon/evening hours.

There will also be some decent wind shear associated with the trough with winds a few thousand feet up off the ground as strong as 25-35 knots with winds around 10,000ft as strong as 30-40 knots and winds around 18,000ft as strong as 40-45 knots.

What all these ingredients point to is the potential for showers and thunderstorms to develop and perhaps become quite numerous as the afternoon progresses.  While typically we see the threat for showers and t'storms diminish as the sun begins to set, colder air working in the mid-levels of the atmosphere may keep activity going well through the evening hours.  Given the degree of winds aloft and such cold mid-level temperatures leading to low freezing levels, the strongest t'storms will be capable of producing hail and strong winds.  Even coastal areas will have the potential despite a sea-breeze.  Typically in these setups the sea-breeze is a non-factor and if anything can sometimes act as an enhancer.  All t'storms will also produce torrential downpours given degree of low-level moisture.

One thing to watch, however, is the potential for some rising heights across portions of southern New England later in the afternoon into the evening.  The GFS forecast model shows heights rising a bit later in the afternoon.  If this occurs this could diminish activity as the afternoon progresses or prevent further activity developing.

Noon severe weather update

Showers and some embedded thunderstorms pushed through portions of New England this morning, particularly along and north of Windsor Locks, CT.  While clouds are still in place, satellite trends have been breaking up the clouds west of southern New England and the sun has been in and out.  Given recent satellite trends, more and more breaks of sun should occur across much of MA and south into CT and RI.  

The combination of steep mid-level lapse rates, in excess of 6.5 C/KM, rich low-level moisture characterized by dewpoints climbing into the lower 70's, and surface temps rising into the 80's will all lead to quite an unstable airmass this afternoon.  It's still a bit unclear as to how unstable we become due to the presence of clouds but some areas could end up with over 1500-2500 J/KG of Cape...which would be quite significant given the degree of wind shear aloft.  

Some of the latest computer forecast models, more particularly the higher resolution models, develop what appears to be a broken line of supercells moving through MA and CT later this afternoon.  At this time the potential for severe weather is still certainly in place along with the possibility for some significant severe weather including damaging wind gusts in excess of 70 mph, hail as large as golf balls, and a few isolated tornadoes.  A strong tornado can't be ruled out either.  

The best timeframe appears to be between 2-6 PM (as late as 8 PM for RI and SE MA if activity sustains) for strong to severe t'storms capable of the hazards described above.  

The situation will continue being monitored with updates provided as needed.  

Below is the latest satellite image from noon showing breaks of clouds moving east:

Update on today's severe potential

Forecasting severe weather across New England can be quite challenging at times...in fact it can be challenging most of the time.  There are just so many mesoscale factors and features that need to come together perfectly and you really need to just watch things evolve pretty much as they are.  This situation is no different.

For the past few days computer forecast guidance has really been hammering the potential for a significant severe weather event to occur across portions of New England with the potential for widespread damaging winds, large hail, and even a few tornadoes all possible.  They key to realizing the highest potential is to get all the main ingredients to fall in place at the same time.

While all these hazards are still on the table, as the morning progresses confidence has decreased a bit and confidence is on the lower side as to exactly what will happen today and exactly how big the event will be.

Currently there is a large complex of t'storms just to the west of New England called a mesoscale convective complex (MCS) that is racing towards CT/MA/VT/NH.  This complex is associated with a great deal of cloud debris and if this complex does not fall apart quickly that could vastly reduce the degree of solar heating we achieve today which means less in the way of instability.  It could also mean for an earlier show of action which also means less instability and less potential for a major severe weather event.

While some early indications projected areas along and north of the MA Pike for the highest severe weather potential, this area may have shifted southward to the MA Pike including interior NE MA, and down into northern CT and northwestern RI.

Here are a few scenarios which are still on the table for today

1)  If the complex of t'storms to the west erodes soon and clouds begin to break up this would allow stronger surface heating to occur over New England.  Given the atmospheric environment that will be in place, this scenario would be the most likely to produce a major severe weather event with damaging winds which could be as strong as 70 mph in the strongest storms, large hail of golf ball or even bigger in the strongest storms, and perhaps even a few tornadoes.  In this scenario a strong tornado couldn't be ruled out somewhere.

2)  If the complex of t'storms to the west does not weaken much and thicker cloud cover remains, the degree of surface heating will be much lessened across New England and the degree of instability will be much less.  This will mean a reduced threat for a major severe weather event and while some strong to severe t'storms will still be possible the threat would be much more isolated.  Some pockets of damaging winds would be possible, along with hail up to quarters, and still perhaps a tornado.

3) The MCS weakens as it moves into New England but comes through prior to peak heating.  In this scenario the severe threat is vastly reduce.  A few strong gusts perhaps knocking down weakened trees would be possible and small hail.  The threat for a tornado would be very small.

At this juncture the next few hours are very crucial and all we can do is watch current radar and satellite trends along with latest hourly computer data observations and go from there.  It's important to keep an eye and ear to the skies along with all the latest forecasts and updates.  

Strong to Severe T'storms Possible Tuesday 6/23/2015

Several pieces of computer forecast guidance are showing the potential for showers and t'storms across the region during the afternoon and evening hours on Tuesday.  Several atmospheric ingredients may move in place and move together to allow for the potential for some very potent thunderstorms posing a threat for strong to damaging winds, large hail, and perhaps even a few tornadoes.

Typically setups like this are on the rare side, however, what is opening some eyes with this setup is the potential advection of an elevated mixed-layer plume working in from out west.  For more on elevated mixed-layers, their origin, and importance in higher-end severe weather events please read this more detailed information provided here:

http://weatherwiz.blogspot.com/2015/06/what-is-elevated-mixed-layer.html

To explain EML's as simply as possible, they are capping inversions which prevent clouds, showers, and thunderstorms from developing too early and this allows for stronger heating and can lead to extreme amounts of instability developing below the capping inversion depending on how high the temperatures are and how high the dewpoints are.

Computer forecast models are showing a very strong piece of shortwave energy passing just to the north of the US/Canadien border.  This track may favor the greatest risk of severe storms being further north across New England, (central/northern MA into portions of VT/NH and into ME so the track of the s/w will have to be watched closely:

Computer forecast guidance also develops some extreme instability across the region with Cape perhaps as high as 2500-3000 J/KG and very strong wind shear, both speed shear (increase of winds with height) and directional shear (change of wind direction with height).  This combination could lead to the potential for any thunderstorms that develop to become quite severe and have the potential to produce damaging winds, large hail, and perhaps even a tornado.

Below is the 12z NAM bufkit for Windsor Locks, CT late Tuesday afternoon.  This is a pretty alarming sounding with extreme instability, high shear, and very high helicity being forecasted.  This is something seen more typically in the plains than here in CT.

This definitely needs to be watched!

What is an Elevated Mixed-Layer?

The origin of the EML

An elevated mixed-layer (EML) is a region of extremely warm and dry air typically located between about 800mb and 600mb (~6,500ft to ~16,000ft ASL) in the atmosphere.  In the United States, EML's originate and develop across the Rocky Mountains region near the desert southwest and across the high plains.  Air moving up and over the Rocky mountains warms and dries significantly.  This allows for very steep lapse rates (change of temperature with height) to exist between the surface and 600mb in the atmosphere.  

Importance of the EML and severe thunderstorms 

The EML is also known as a capping inversion because of the warming and drying of the atmosphere where the EML begins.  Remember, air will continue to rise as long as the surrounding air is cooler than the temperature of the parcel.  Once the air begins to warm, this process is no longer occurring, thus the air parcel no longer rises.  This process inhibits clouds, showers, and thunderstorms from developing which allows for the sun to continue shining strong.  If the low level airmass is warm to hot and there is a rich presence of moisture (high dewpoints) the combination of the very steep lapse rates, high surface temps, and high dews leads to the development of extreme instability, a key ingredient in the development and strength of thunderstorms.  

If some sort of lifting mechanism, such as a cold front, potent shortwave, etc, pushes in and provides enough lift to break through the cap, thunderstorms would begin to initiate and become very strong very quickly.  If the capping inversion is too strong then one of two things can happen; 1) No activity develops and all the instability goes to waste or 2) Only a few storms develop.  This can actually enhance the severity of the storms that develop because with only a few storms around the updrafts are not fighting each other.  

How can EML's be observed?  

An EML is easily observed on a skew-t forecast sounding which gives you a look at the entire atmospheric profile.  Below is an example of what a true EML looks like on a forecast sounding:

In the above sounding you can see the region of extremely warm (where the black temperature line jets off to the right) and extremely dry (where the green dewpoint line shoots off to the left) air.  

EML advection into the Northeast

While EML's don't often survive the journey into the Northeast, every now and then a plume of EML air can break off from the source region and ridge up over ridging at 700mb and remain in tack all the way into the Northeastern United States.  However, ridging at 700mb does not always guarantee EML advection into the Northeast.  In a research paper published by meteorologists Michael Ekster and Peter Bannacos, both identified 36 EML events and the corresponding 700mb pattern in place 36 hours prior to the event, 24 hours prior to the event and the observed day of the event.  As you can see ridging at 700mb appears dominant across the southeastern United States extending northward into the mid-At;antic and into the Northeast: