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Steven Copertino

PM Rain Exits, Storms and Cooler Weather Possible This Weekend!

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Good Evening!

After a relatively hot and humid day across the entire Northeast and Mid Atlantic regions, we have seen large-scale thunderstorm development courtesy of the mid level shortwave that we discussed back on Monday. This shortwave was located to the west of the area, and with CAPE values of around 2000-3000 j/kg^2, PWATS near 1.5″, and strong low level lapse rates, the weak amount of energy associated with the disturbance was more than enough to set off numerous showers and thunderstorms around 2pm. Due to a significant lack of vertical wind shear, these storms were not able to move all that much from where the initial updrafts formed, and thus most cells rained themselves out over time. However, when these updrafts collapsed and the rain cooled air rapidly sunk to the surface, it created new boundaries which served to provide breeding grounds for renewed storm development. In some locations we saw this happen three to even four times over, which caused serious street flooding.

In addition to the street flooding, cool/dry air aloft allowed for some of the more robust thunderstorms to develop marginal to severe hail stones, with some reports of stones reaching the 1.75″ mark! Some folks may have noticed that these storms were also producing a cooler rain than one would expect during August, and this also can be attributed to the cooler air aloft. These drops were whats know as “Big Drops” (I know, very creative) and form when an updraft just isn’t able to keep ice suspended long enough to freeze sufficiently to reach the ground as hail. However, a few thousand feet up in the atmosphere, these drops were frozen, and when they crashed into one another, they create a negatively charged particle in the atmosphere, called an electron. When enough of these electrons build up in an updraft, they connect with positively charged particles (protons) at the surface, the path they take to get to one another creates lightning! With a ton of ice in the atmosphere from numerous updrafts this afternoon, there was a very impressive amount of lightning that went up across the area. Some of this lightning even caused damage to houses and property, with some areas losing power. Otherwise, today’s storms mainly posed a very heavy rain, lightning, and gusty wind threat.

As of six o’clock this evening, most of the showers and thunderstorms that formed early this afternoon were finally beginning to wind down. Lightning activity over the area has greatly weakened over the past hour or so, and this also points towards a gradual dissipation of the rest of the thunderstorm activity. As this time, we think that the heaviest showers and thunderstorms should dissipate by sundown, with only a few lonely cells remaining thereafter. Some residual cloud cover is likely as we head into the overnight hours, and this will help to keep overnight lows rather mild, with temperatures dropping to around the low to middle 60’s.

This evenings latest regional radar. high resolution visible satellite, surface observations. and severe warnings, showing the bulk of the heavy rain dying off. This trend should continue as we head deeper into the evening (Courtesy of Simuawips)

This evenings latest regional radar. high resolution visible satellite, surface observations. and severe warnings, showing the bulk of the heavy rain dying off. This trend should continue as we head deeper into the evening (Courtesy of Simuawips)

Thursday and Beyond

Thursday should start off just as the past few days have, with any low clouds quickly burning off by the mid to late afternoon hours. This will allow afternoon temperatures to climb significantly during the afternoon hours, with highs likely reaching the middle 80’s to lower 90’s across the area. With a moist airmass in place tomorrow, we should see the region destabilize once again by the mid to late afternoon hours, similar to as we saw today. The main difference will be that tomorrow does not look to have a coherent area of forcing to initiate thunderstorm development, so any thunderstorms tomorrow afternoon will likely have to rely on convergent boundaries from local sea breezes. This activity will likely be limited to a heavy rain and lightning threat given the lack of favorable parameters for anything outside of heavy thunderstorms. Storms should gradually fade tomorrow evening, leaving generally mild and humid conditions in their wake.

As we head into the first half of the weekend, a large shortwave trough diving out of Canada will provide a threat of heavy rain and possibly some thunderstorms over the area as moisture begins to surge north from the Gulf States. This system will be watched closely, as it does have a very favorable upper level jet streak which could potentially enhance a heavy rainfall threat over our area. A strong cold front looks to push through late Saturday and into Sunday, which should bring not only cooler temperatures to our area, but also much lower humidity compared to what we’ve been seeing-which should make for an enjoyable end to the weekend!

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Steve Copertino & Doug Simonian 

 

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

Rare July Nor’Easter Possible This Weekend With Heavy Rains and Wind

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Good Evening! 

Today was yet another in a series of days with below average high temperatures and low humidity. The difference with today, was the fact that we were able to mix out some early morning clouds and have a mostly sunny day across the vast majority of the area, as well as the Northeast. The area of high pressure that contributed to some pleasant and cooler weather yesterday moved directly over the NYC metro area this afternoon, and this further helped to conditions quite pleasant, with light winds generally from the south. With low humidity, clear skies, and a fresh high pressure system over the area, we saw highs limited to the mid to upper 70’s, with a few locations off to the west and north over Pennsylvania and northern New England reaching into the lower 80’s this afternoon. Overall these temperatures were around 5-10 degree below the norm for this time of year, and quite the contrast to the miserable heat and humidity we had over the entire Northeast just a week ago.

As the area of high pressure that was in control for most of the day continues to head eastward, it will begin to kick up more of a southerly flow in its wake later this evening. As it does so, this will lead to increased low level moisture building over the region as well as mid level temperatures increasing ever so slightly. The gradual increase in low level moisture should allow for the development of cloudier conditions for the rest of the evening, which should allow for overnight lows to not be quite as low as they were the past few nights, with temperatures getting down into the lower to middle 60’s across much of the area.

This afternoon/evenings latest high resolution visible satellite imagery, regional radar mosaic, and surface observations, showing relatively pleasant conditions with some high clouds beginning to move in as the high pressure to our east gradually fades (Courtesy of Simuawips.com)

(Click To Animate) This afternoon/evenings latest high resolution visible satellite imagery, regional radar mosaic, and surface observations, showing relatively pleasant conditions with some high clouds beginning to move in as the high pressure to our east gradually fades (Courtesy of Simuawips.com)

Thursday into Thursday Evening

Thursday should start off with mostly broken clouds, with some overcast further inland as the area of high pressure really begins to back off to the east and begins to introduce a brand new moist airmass into the Mid Atlantic and Northeast regions during the late morning and afternoon hours. Additionally, south-southwest flow will also begin to increase markedly ahead of a mid level shortwave that will be located over the Great Lakes region as well as the Ohio Valley. As this south-southwest flow begins to increase, mid level temperatures will begin to increase once again, which should support a round of warmer temperatures by the mid afternoon hours. Despite the possibility of some cloud cover, highs should be able to get into the low to middle 80’s-which will be right around normal for the area during late July. It should be noted that some influence from afternoon sea breeze activity could work to cut highs along coastal sections tomorrow by five or so degrees. In addition to the increasing temperatures, the dreaded dewpoints will once again be on the rise tomorrow afternoon-with values ranging in the mid 60 to lower 70 range, making for a muggy afternoon tomorrow, reminiscent of the days last week.

With all of this increasing moisture in the low levels of the atmosphere, we will likely also see instability begin to increase tomorrow afternoon and into the evening hours. Though not as impressive as it looked a few days ago, tomorrows setup should still yield a marginal risk for severe thunderstorms across the Northeastern states as a weak mid level system will gradually work its way east and potentially spark some showers and thunderstorms during the late afternoon and evening hours. The main limiting factors for any widespread/impressive thunderstorm development will be relatively meager mid level lapse rates, as well as any impressive shear to help keep potential updrafts organized and coherent. However, more favorable parameters may come together over portions of Pennsylvania and New York, so those with interests in those locations may have a better shot at seeing strong to severe thunderstorms capable of damaging winds and possibly a brief tornado. Otherwise, any storms that form near the NYC metro area will likely be capable of producing gusty winds, heavy rainfall, and frequent lightning before they gradually fade to the east by sunset. Expect another humid evening with lows ranging in the middle to upper 60’s once again.

This evenings latest RPM model showing the general evolution of Thursday weather, with some showers and thunderstorms possible. Widespread severe thunderstorm development is not anticipated (courtesy of WSI)

This evenings latest RPM model showing the general evolution of Thursday weather, with some showers and thunderstorms possible. Widespread severe thunderstorm development is not anticipated (courtesy of WSI)

This Weekends Potentially Impactful Nor’Easter 

Back on Monday we highlighted the possibility that the Northeast would have to deal with an area of heavy rain moving in from the west during the evening on Friday and into the early morning hours of Saturday, however since that time there have been some marked changes with regards to the duration, timing, and overall severity of this potential system. While the overall main players remain the same, with the initial monsoon energy moving up and around the periphery of the large heat ridge located in the Plains, as well as a small, but potent shortwave diving out of Canada-how these two interact remains to be far from settled at this time.

This afternoons model guidance suggests that as the initial monsoon energy heads into the Ohio valley, the potent Canadian shortwave trough will begin to interact with this energy and begin to phase together to some degree. This setup is something we’d typically be talking about in the Winter months, but the overall outcome may wind up being quite similar-just with warmer temperatures this time around. As these mid level systems interact and potentially phase, this will cause a surface low to develop over the Mid Atlantic states during the afternoon hours on Friday. As this surface low begins to become more established, it will have a very warm and moist feed of tropical air pumping into it from the south. Additionally, it looks quite likely that there will be a favorable upper level jet configuration overhead of this system, which should promote a healthy amount of upper level divergence and convergence at the surface levels. In short, when all of these factors come together in just the right manner, its a rather strong signal that very heavy rain will be likely for portions of the Mid-Atlantic.

So what could go wrong? Though this system will have some potentially favorable synoptic support, there are some serious questions that still remain with this system that make this a very difficult forecast. First off, the trough diving down from Canada is whats known as “positively tilted” in nature, meaning that the majority of its energy is located in the bottom left quadrant of the trough, which often allows the system to be progressive in nature. It also does not help that we have been stuck in a rather progressive west to east pattern for the past week or so, with no upstream blocking to help slow things down. This pretty much eliminates the chances of a large, wrapped up system along the coast unless something drastically changes. Next off, the initial monsoon energy out ahead of the Canadian system will be moving quite quickly to the east, and as the Canadian system drops south, it may wind up shearing out the energy ahead. This would create a much weaker and disjointed system that would likely have the heaviest rain offshore.

Taking all of this into account and carefully looking at this afternoons model guidance, I have decided to go with a blend of the ECMWF/NAM/ECMWF-ENS/UKMET at this time. This would lean towards a more progressive solution for this system, bringing the surface low from the Mid Atlantic coast, to well southeast of New England. This track would likely still bring moderate to heavy rain to the NYC area, but areas south of NYC would likely feel the brunt of this system (PA/MD/SNJ). At this time, it seems likely that the heaviest batch of rainfall should be able to drop 1-2″, with some higher amounts possible along the coasts of NJ and LI. This system is forecast to exit the region by Saturday afternoon, with some showers and gusty winds lingering around thereafter.

We will be carefully monitoring this system over the next few days and providing updates as needed!

This afternoons ECMWF model showing a rather impressive Nor'Easter for this time of year affecting the Mid Atlantic and portions of the Northeast with heavy rain and gusty winds (Courtesy of Accuweather Pro)

This afternoons ECMWF model showing a rather impressive Nor’Easter for this time of year affecting the Mid Atlantic and portions of the Northeast with heavy rain and gusty winds (Courtesy of Accuweather Pro)

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Have a great day!

Steve Copertino

Miguel Pierre

Premium: Coastal Storm with Heavy Rainfall Possible Late Week

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After the heat wave last week, more cooler, unsettled weather has returned to the region, as a trough moves through the Northeast.  More clouds and onshore flow this week have kept high temperatures well below average over the region — it certainly doesn’t feel like the middle of summer. High pressure will support more pleasant, warmer weather on Wednesday. Then it looks like something we often see during the fall and winter months: a coastal storm with the potential for heavy rainfall by Thursday night and Friday and perhaps into this weekend as well. But uncertainty is still quite high with exactly how this storm evolve, its timing, and how much impact it will have on the region.

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