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Steven Copertino

Bulk of Unusual July Storm Stays South, Locally Heavy Rain Still Possible

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Good Evening! 

Well the storm that we have been following for the past week or so is in its formative stages as of this afternoon/evening, with heavy rain beginning to break out over portions of the Mid Atlantic states. While the storm should continue to develop and mature through the evening, the overall evolution and impacts that will be accompanying this system have changed quiet a bit. As we outlined on Wednesday in the “What Could Go Wrong” section, we stated that the overall positive nature of the incoming trough was not conducive for phasing of the lead energy ahead of the system, thus making us question the overall likelihood of widespread heavy rainfall over the New York metro area. Since that time, the models have come into line with our previous thinking and have really shifted the upper level features around, which has in turn has had a rather dramatic effect on exactly where the heaviest rainfall will occur tonight and into Saturday morning.

This afternoons and evenings regional radar mosaic, high resolution visible satellite imagery, and surface observations, showing the development of widespread heavy rains over the Mid Atlantic region. This development will continue to ramp up through this evening (Courtesy of Simuawips.com)

This afternoons and evenings regional radar mosaic, high resolution visible satellite imagery, and surface observations, showing the development of widespread heavy rains over the Mid Atlantic region. This development will continue to ramp up through this evening (Courtesy of Simuawips.com)

So what has changed over the past few days? 

For starters, a positively tilted trough approaching the Northeast is never really a great start if you’re looking for a large, lumbering system with abundant precipitation to form off the coast. This is exactly what we have approaching the area this evening, as noted on the latest 500mb analysis as well as water vapor imagery from the non-operational GOES-16 satellite. The next factor preventing this region from seeing a long-duration system is the very fast flow out to the east of the main storm system. This is something that we usually have to deal with when forecasting winter storms, but it also applicable to summer storms as well. This speedy flow can be attributed to a large upper level low in the eastern regions of Canada. As energy in the base of the large upper level system over Canada begins to rotate and shift southeast, it begins to flatten heights to the east out, and this is the exact opposite of what we would need to slow this system down.

Ideally, we would want the Canadian system to be centered more to the west, mitigating the effects of lower heights ahead of the storm, which would allow the system to slow down a bit and have more northward progression. So putting these negative factors all together, we can expect the shortwave over the Ohio Valley to continue to move east quite slowly, and continue to cause the development heavy rainfall over portions of the Mid Atlantic. Instead of amplifying and strengthening to our south and east tomorrow, the mid level system will remain to our west and will likely wind up getting sheared out-which should cause the overall system to decay.

This afternoons NAM, showing the mid level evolution of the storm system over the next two days. Note the lack of phasing and eventual sheared-out look

This afternoons NAM, showing the mid level evolution of the storm system over the next two days. Note the lack of phasing and eventual sheared-out look!

So what are the impacts? 

Despite its obvious flaws, this is definitely an anomalous system for this time of year and it will bring some heavy impacts, but those do not look to be directly aimed at the NYC metro area. As precipitation continues to break out later this evening, they will be in an extremely moist environment characterized by PWATS over two inches in some spots. Additionally, soundings over the Mid-Atlantic region indicate that a deep “skinny” CAPE profile also exists with values around 1500-2300j/kg^2, which will add to the torrential downpour situation as the night goes on. The last major player that is on the table, is an extremely impressive upper level jet nosing in from the the Ohio Valley. As the wind maximum from this upper level jet streak begins to move further east, it will begin to greatly expand the amount of upper level divergence, which will be directly over the heaviest areas of convection (thunderstorms). This strong upper level divergence will support whats known as low level convergent boundaries-which could spark heavy storms with prolific and near-record rainfall rates over portions of the Mid Atlantic.

It is quite obvious that the numerous favorable factors are coming together in just the right manner so that flooding concerns will be extremely high for the MA region. In fact, the Weather Prediction Center has issued a rare High Risk for flooding over portions of the Mid Atlantic! In general, rainfall totals should be around the 2-5″ range, with some localized amounts of 7-10″ possible in the heaviest of rain bands. This will almost certainly create very serious flooding, so please stay tuned to your local NWS!

Further north for the NYC metro area, things will be a bit more tame as we appear to be on the northern fringes of this system. This does not mean that we are exempt from seeing any heavy rains at all, as we will still have a very moist air mass to work with, and the same excellent upper level divergence to promote some convergent boundaries-which may spark some showers and storms. However, we will have to deal with some sinking air over the NYC area, which should prevent any widespread heavy rain from developing. Some isolated pockets of 1-2″ may be possible with the heaviest showers here, but general totals should be around an inch or less.

By tomorrow afternoon, the disturbance should begin to decay and get sheared out, which should gradually put an end to the heavy precipitation across the entire region.

(Again, if you are located in an area prone to flash flooding, please be aware of any warnings issued-ESPECIALLY during the overnight hours) 

Loop of the development and progression of the heavy rain over the Mid Atlantic over the next day and a half

Loop of the development and progression of the heavy rain over the Mid Atlantic over the next day and a half

We will be back Monday with a look at next week and beyond!

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Have a great weekend!

Steve Copertino

Steven Copertino

Public Analysis: Improving Conditions This Evening, Excellent Weekend Ahead!

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Good Evening! 

Today we saw the development of numerous showers and thunderstorms across the southern half of the metro area, with some of the storms being on the strong to severe side. These storms were fueled by the remnant frontal boundary that sank southward to portions of southern NJ earlier this morning. Deep tropical moisture was able to remain anchored within this boundary, and as daytime heating took over, instability was able to increase quite markedly over portions of southern Pennsylvania and New Jersey. As the last in the long series of mid level disturbances embedded in the west to east flow approached this very moist and unstable airmass, thunderstorms rapidly developed-bringing large hail, damaging winds, and very heavy rainfall. Some localized reports of wind damage were reported in Pennsylvania, Delaware, and portions of New Jersey, but overall it wasn’t a huge day for severe weather.

The showers and thunderstorms gradually moved into a more stable marine airmass near the coast, which caused them to weaken below severe thresholds and limited impacts to mainly heavy rain and frequent lightning. These showers will continue to head off the coast later this evening, giving way to more mild and cloudy conditions. As we head deeper into the evening, a shortwave trough and the accompanying energy will begin to push into the region, and due to the fact that there is still some residual moisture/instability, we cannot rule out a couple showers and even a late thunderstorm developing. Any storms that form would almost certainly remain below severe limits, with the main threats just being heavy rain and gusty winds.

Lows this evening will remain right around what the current temperatures are due to thick cloud cover and residual low level moisture trapped due to a temperature inversion a few thousand feet up in the atmosphere. Some patchy fog may try to develop this evening for some locations, but overall development should not be all that widespread.

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This evenings latest regional radar mosaic, surface observations, and high resolution visible satellite imagery, showing the bulk of the showers and thunderstorms moving off the Mid Atlantic coast (Courtesy of Simuawips)

This Weekend

Improving conditions are expected starting on Saturday as the upper level trough that has been providing the area with numerous rain chances finally swings through early in the morning hours. This will cause any residual fog to quickly dissipate as the low level moisture is quickly swept out of the region. Clouds should also begin to decrease in earnest as an area of high pressure begins to gradually build in from the west, centered over the Ohio Valley. Conditions tomorrow should feel much improved from the past few days, with dewpoints dropping quite considerably, giving way to a much more comfortable airmass. With increasing sunshine, a warm mid level airmass, and low moisture content in the atmosphere, temperatures should be able to rise to around seasonable levels-with lower to middle 80’s likely areawide.

High pressure will continue to build over the area as we head into the evening hours, allowing for even clearer conditions to develop just in time for sunset. The high pressure will also allow winds to become quite calm by the early evening hours, and with calm winds, low dewpoints, and clear skies, its looking quite likely that some radiational cooling will take place tomorrow evening, which should allow temperatures to drop into the middle 60’s across the entire area, which is just around normal for this time of year.

Sunday will likely be repeat of Saturday, with clear skies, low dewpoints, and light winds being the theme once again. The mid level airmass will be a little warmer on Sunday due to winds shifting to the west/southwest, so we expect highs on Sunday to be a little warmer, with temps reaching into the upper 80’s and lower 90’s across the region. Sunday evening will also be near-perfect, with clear skies and light winds, which will allow for cooler temperatures to return. Overall, it’s looking likely that this weekend will be quite beautiful and should be great for any outdoor activities as we undergo a temporary airmass change!

This afternoons high resolution North American Model, showing light northwesterly winds, warm temperatures, and an overall pleasant airmass over the area, which should make for a beautiful day!

This afternoons high resolution North American Model, showing light northwesterly winds, warm temperatures, and an overall pleasant airmass over the area, which should make for a beautiful day!

Next Week

The next threat of rain looks to come early in the week next week, as an upper level trough approaches from the west and drags another humid airmass from the south. This will greatly increase instability over our area, which will set the stage for afternoon thunderstorms, capable of heavy rain and gusty winds. This afternoons model guidance disagrees with regards to the handling of this piece of energy as it heads towards our area, but it seems possible that the threat of showers and thunderstorms stays around until at least Tuesday. Temperatures for the beginning half of the week should be around-normal, but this will be highly dependent on the amount of cloud cover and rainfall that we may receive.

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Have a great evening!

Steve Copertino

Steven Copertino

Public Analysis: Heavy Rain Exits, Calm Weather Sets In

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Good Evening! 

After a relatively calm few days, the area of low pressure that developed along the leftover stalled frontal boundary this morning sparked numerous showers and isolated thunderstorms that tracked over eastern Pennsylvania, New Jersey, New York, and portions of Connecticut. Deep tropical moisture ahead on the order of about 2.3″ of PWATs allowed for the overall complex to contain very intense rain rates, upwards of two inches per hours in some locations! To make matters worse, these storms were chugging along with a weak low level flow, so areas of heavy rain were rather slow and this allowed for some spots to experience flash flooding in areas of poor drainage. As the dynamics began to really ramp up as the low pressure intensified, some gusty winds began to be reported with a few embedded storms that tracked through northern New Jersey and into Long Island. There were some reports of small tree limbs being taken down with localized power outages, but due to the time of day, lack of greater instability, and weak mid level winds, these gusts luckily weren’t anything too serious.

As the low moved off the coast and began to intensify some more, the area of rain associated with the low began to accelerate, and this is what likely saved the area from seeing much higher rainfall totals that would have caused more widespread flooding. The rain gradually tapered off as the low pulled away and some more dry low level air worked its way in behind the system, which finally allowed for conditions to calm down. The cloudy, tropical air-mass gave way to mostly sunny skies, which allowed temperatures to rise into the upper 70’s and lower 80’s across the entire area, with light westerly winds. With daytime heating taking place, instability was able to build back to limited values, which promoted the development of some more showers and thunderstorms to our North and West this evening. These showers will gradually slide east/east southeast and likely diminish in intensity as the sun fades, and with it-the fuel they need to survive. The main threat with these storms will be some additional, brief heavy rain, but these storms will be moving at a decent pace, so the overall flood threat is low. Some patchy fog should develop this evening and into tomorrow morning as an inversion in the atmosphere traps the leftover humid airmass. With the inversion in place, lows should be quite mild with most of the area seeing temperatures in the mid 60’s to lower 70’s.

This afternoon/evenings latest regional radar mosaic, surface weather plots, and high resolution visible satellite imagery of the Northeast. showing the remnants of this mornings system exiting to the east.

Saturday looks to start off rather clear despite some areas of patchy fog, which should quickly burn off as the morning progresses. This should allow skies to become mostly sunny, with some spotty clouds as we head into late afternoon. To our north and west, a large mid level trough will be progressing east with a disorganized piece of energy. As the energy from this trough nears the area by the afternoon hours, the leftover tropical airmass and sunshine will allow instability to build to modest levels, with some decent wind shear allowing for the potential of muti-cell thunderstorms to develop over the area. With the upper level jet streak and greatest energy for storm development located to our north over New England, the main focus for storms looks to be over that region, but we expect scattered development at the most, with the main threat being gusty winds, small hail, and heavy rains.

Otherwise, the afternoon will likely be a hot and humid one as temperatures likely reach into the upper 80’s and even to lower 90’s across the region. The area should gradually dry out during the late afternoon hours and early evening hours as the cold front to our west begins to push through. As the front moves through tomorrow evening, the humidity should drop a bit as more stout westerly winds develop, so it should be quite pleasant with lows dropping down into the upper 50’s and lower 60’s.

Sunday will likely be a good deal quieter than the previous day as the cold front moves well to our east and the dry air to the west really begins to work its way into the area. A surface high to the south will begin to build, which will likely allow for mostly sunny skies and low dew points. The overall drier atmosphere will work to negate any threat for afternoon thunderstorms over the entire area. High temperature s on Sunday should be near-normal, with highs likely reaching into the lower 80’s across the entire area. Sunday evening should also continue the theme of being rather calm, with clear conditions and light winds expected, lows should be able to drop into the upper 50’s area-wide.

This evenings Rapid Precision Model showing the development of some afternoon thunderstorms tomorrow, followed by clearing conditions in time for the evening hours (Courtesy of WSI)

Extended Range

A strong heat ridge is expected over Western/Central parts of United States this weekend and into early next week. This will support a more longwave trough over much of the Great Lakes and Northeast with temperatures closer to normal overall. There are some timing differences on model guidance currently with individual disturbances embedded within the longwave trough could still produce some isolated-scattered convection over the region. But much of the time, just warm and dry  with some sunshine each day, as high pressure gradually builds in from the Ohio Valley.

Later in the week, we may have another infusion of deep tropical moisture into the area, with increasing temperatures. Whether or not we see another period of heavy rain will be highly dependent on if we have a trigger mechanism to set off any showers and thunderstorms. At this time, the chance for anything outside of afternoon thunderstorms seems low, and a general increase in both temperatures and humidity seems likely at the very least. We will continue to monitor this period in case anything does try to take shape.

This afternoons ECMWF model showing near-zonal flow across the country, with the main weather features located in the central regions of Canada. (Courtesy of Accuweather Pro)

This afternoons ECMWF model showing near-zonal flow across the country, with the main weather features located in the central regions of Canada. (Courtesy of Accuweather Pro)

For more information and posts like this one, make sure you sign up for Premium Forecasts — where multiple detailed articles, videos, and interactives are posted each day. Also, come interact with our staff and many other weather enthusiasts at 33andrain.com!

Have a great evening!

Steve Copertino

Steven Copertino

Public Analysis: Increasing Heavy Rain Threat Thursday, Seasonable Conditions Likely This Weekend

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Good Evening! 

We hope you had an excellent holiday weekend over the past few days and managed to dodge any showers and thunderstorms that popped up during Monday and Tuesday!

After some relatively hot and humid days on Monday and Tuesday, today was quite the improvement as a weak area of high pressure off of the Northeast coast moved offshore this morning and ushered in winds from the ocean, which were able to drop dewpoints by quite a bit across the area. A frontal boundary rode northward through the area earlier this morning and into the afternoon hours, which increased clouds briefly, but with the easterly winds coming off the Atlantic, the atmosphere was able to stay relatively stable and prevented the development of showers. The cooler marine airmass and relatively sunny skies allowed temperatures to climb into the upper 70’s and middle 80’s across the region this afternoon, which is just slightly below normal for this time of year.

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