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.
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