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Steven Copertino

Severe Thunderstorms and Heavy Rain Inbound, Eclipse Details for Monday!

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Good Evening! 

As promised earlier in the week, today was looking like it was going to be the most active day in the forecast period, and it certainly did not disappoint! The large upper level trough and associated surface low pressure system located over the Great Lakes region pushed a warm front through most of the New York metro area and Northeastern states early this morning, which brought very heavy rainfall, lightning, and locally gusty winds. While the rain was quite heavy in some locations, widespread flooding was not an issue, and any flooding was confined to street and small stream activity. As this warm front moved through the region, it ushered in a very moist and unstable airmass characterized by 3000-4000j/kg^2 of convective available potential energy (CAPE) and very, very moist precipitable water values-above two inches in some locations. The main question was whether or not the low level jet associated with the sprawling low pressure system would be strong enough to wipe out some of the cloud cover of the region and allow for further destabilization and dynamics to take over, but as we saw early this afternoon, the sun did in fact make an appearence over eastern portions of the Northeast. As the sun heated the soaked grounds from this mornings storms, this created and even more unstable and moist airmass for any future storms to work with.

Later in the afternoon-around 4pm or so, discrete thunderstorms began to form over portions of Pennsylvania and the northern Mid Atlantic states in response to moderate to strong mid level forcing interacting with the very unstable environment present. Rapid growth into supercell structures occured in the strongest of storms, which went on to produce severe hail, damaging winds, and very heavy rains which caused flash flooding. Eventually the large scale forcing took over a good portion of the unstable warm sector and this led to the development of numerous showers and thunderstorms, which congealed into a more linear line of storms. As of 5pm, this line was located over portions of Pennsylvania and Maryland. While the line has lost some strength in the past hours, there are a few areas of stronger activity that are currently severe warned for damaging winds and potentially severe hail. With wind shear being in the moderate 25-45 knot range and cloud levels being quite low, these embedded stronger cells may pose a brief tornado threat as they head east into New Jersey within the next few hours.

This afternoons NAM model showing very high PWATS in addition to strong mid level winds, perfect for the development of heavy rain across the area this evening.

Otherwise, this line looks to propagate east at a gradual pace, with the main threats outside of the severe activity being torrential rainfall which can produce flash flooding within a matter of minutes. This threat has been exacerbated due to this mornings storms leaving behind very moist soil which may have trouble if rain rates do reach the 1-3″ range. As these storms head east this evening, they will begin to encounter a much more stable airmass from the Atlantic ocean, which should gradually shut down any strong to severe thunderstorm activity that may remain close to sunset. The heavy rain threat will still exist until these storms and the accompanying cold front push out past the region and into the open waters of the Atlantic.

For the severe risk this evening, the Storm Prediction Center in Norman, OK has issued a severe thunderstorm watch for the majority of the Northeast and Mid Atlantic States. The main threats include strong wind gusts up to 70 mph, locally severe hail up to ping-pong ball size, torrential rains, and frequent lightning.

In addition, the National Weather Service in co-ordinance with the Weather Predictions Center in Camp Springs Maryland has issued a Flash Flood Watch for rainfall rates of around 1-3″/hr possibly producing small stream and urban flooding. Rainfall totals may be in the 3-6″ range where the heaviest storms track over this evening.

This evenings latest high resolution Goes 16 visible Satellite imagery, regional radar mosaic, surface observations, and severe warning from the NWS (Courtesy of Simuawips.com)

This evenings latest high resolution Goes 16 visible Satellite imagery, regional radar mosaic, surface observations, and severe warning from the NWS (Courtesy of Simuawips.com)

Saturday and Sunday 

Conditions look to gradually improve beginning tomorrow morning as an are of high pressure begins to build off to our north and west. This will allow for decreasing cloud cover, warmer temperatures, and light and variable winds throughout much of the day. Expect highs to generally be in the mid to upper 80’s, with lows dropping down into the low to middle 60’s overnight. Conditions will likely be quite the same on Sunday as well, as the area of high pressure begins to move overtop the region. Conditions this weekend should be rather favorable for any outdoor activities, but we may have to keep an eye on the beaches as there may be at least a moderate risk for rip-currents and strong wave action along the coasts.

Mondays Eclipse Weather 

With the models coming into their more-reliable range as of this afternoon, things still look quite favorable for the viewing of the partial eclipse on Monday afternoon. The area of high pressure that will be approaching the region this weekend looks to be just offshore of the NJ coast by mid-Monday afternoon, which should allow for onshore flow to overspread the area. This will create a rather stable airmass, with few prospects for cloud development.  To look at it another way if you do not trust a few models, the European ensembles, which consists of 50 different model runs and one control run, show less than ten-percent cloud cover for NYC during the event-which is near perfect! All in all, conditions look excellent on Monday for experiencing quite a rare event, but remember to do so safely with the special eclipse glasses that can be found online and at local stores.

ECMWF Ensembles showing a blend of 51 models with less than a 10% chance of cloud-cover for the eclipse on Monday!

ECMWF Ensembles showing a blend of 51 models with less than a 10% chance of cloud-cover for the eclipse on Monday!

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Have a great evening!

Steve Copertino

 

Steven Copertino

Heavy Rain Threat Friday, Monday’s Eclipse Conditions Coming Into View!

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Good Evening! 

Today was another warm and relatively clear day across the entire northeast, but we did see higher dew points begin to creep in which allowed for a “muggier” feel to the day during the peak of the afternoon. With a northwesterly flow in place in the mid levels of the atmosphere, more dry air from Canada allowed for mostly sunny conditions across the region, with come occasional cumulus clouds rising up and eventually collapsing due to any kind of support. As the day progressed, an area of  surface high pressure off to our north and west also helped to keep conditions relatively calm, with light northwesterly winds lasting the entirety of the afternoon. With light winds, dry mid levels, and predominately sunny skies, temperatures were able to gradually rise into the mid to upper 80’s across the area, with cooler temperatures located over portions of interior Connecticut and New York.

Conditions remain quite pleasant this evening as much of the metro area is sandwiched in between two stalled fronts, with the northern front holding back cooler temps and lower dew points, and the southern front holding back oppressive dew points and warmer temperatures. Regardless, as the sun begins to set, the area should be able to cool down quite a bit thanks to a hefty amount of radiational cooling likely to take place due to the aforementioned high pressure, light winds, dry mid levels, and clear skies. This should allow low temperatures to bottom out into the upper 50’s to lower 60’s-with some of the more interior locations possibly dropping into the lower 50’s by early tomorrow morning.

This evenings latest high resolution visible satellite imagery from the GOES 16 satellite, regional radar mosaic, surface observations, and frontal positions from the Weather Prediction Center (courtesy of Simuawips)

This evenings latest high resolution visible satellite imagery from the GOES 16 satellite, regional radar mosaic, surface observations, and frontal positions from the Weather Prediction Center (courtesy of Simuawips)

Thursday and Friday 

Tomorrow morning should start off much like how today did, with relatively clear skies and cool temperatures to begin with. The main difference will be that the area of high pressure will begin to retreat and move off the coast, which will lead to more winds coming from the Atlantic, which will lead to increasing dew points as temperatures begin to rise. While we will have prevalent onshore winds to deal with during the day, a mid level ridge axis will be moving over the area, which should allow temperatures to rise into the low to middle 80’s area-wide-with some cooler conditions possible along the immediate coasts. As the mid level ridge pushes up and over our area early tomorrow afternoon, a shortwave trough will be moving over the Great Lakes region, which will drape a warm front over portions of Pennsylvania and the Mid Atlantic states.

As this front continues to advance east, it will bring an increasing chance of showers and possibly a few thunderstorms over the Northeast. Conditions will likely stay more tame back towards the metro area as the onshore flow keeps the atmosphere relatively stable, so only a gradual increase in mid to high level clouds is expected through the mid to late afternoon hours. Later in the evening, the threat of showers and an isolated thunderstorm will be increasing as the front draws closer and moisture convergence gradually increases. Expect lows to be much more mild Thursday night due to increased cloud cover and the threat for showers-with temps likely staying in the upper 60’s to lower 70’s.

Friday looks to be the most unsettled day in the forecast period as the area of low pressure associated with the large trough over the Great Lakes continues to move to the north and east. This will allow for the warm front to completely overspread the region by early Friday morning, likely bringing a period of locally heavy rainfall with it as it marches north and east. At this time it appears that the heaviest rain may fall over portions of New Jersey, Pennsylvania, C/S New York, and Connecticut, but this could easily change if thunderstorm development becomes more prominent during the morning hours further to the south of the NYC area. Otherwise, Friday will likely feature at least a moderate chance of showers and an isolated thunderstorm up until the later portion of the afternoon, when a renewed threat for heavy showers and thunderstorms may redevelop over Pennsylvania and warm air advection surges north over the Northeast. It seems that the overall severe threat on Friday will be quite low due to the increased cloud cover, but if some patches of sun do develop, then stronger destabilization of the atmosphere may take place, which could lead to more widespread/strong storm development. The primary threat will be heavy rains on Friday, which should last into the evenings hours and possibly even into the early morning on Saturday. Overall, confidence in flash flooding is low at this time, but with deep tropical moisture nearby and forcing from the warm front present, it would not take that much to chance for heavy showers and storms to develop and drop torrential rainfall within a short period of time. Details will likely need to be fine-tuned over the next 48 hours, so be sure to check back for the latest.

This afternoons European model, showing very high precipitable water values surging in behind a warm front progged to move through the area Friday AM with a chance at some locally torrential rainfall.

This afternoons European model, showing very high precipitable water values surging in behind a warm front progged to move through the area Friday AM with a chance at some locally torrential rainfall.

Saturday and Beyond 

Most of the area should still be within the residual warm sector on Saturday morning, but with the best jet dynamics and lift displaced off to the west, the threat for heavy rain will begin to gradually fall. The shortwave trough should continue to move east during the day, with conditions likely improving as the day goes on. Highs will likely be able to reach into the low to middle 80’s ahead of a cold front tracking east from the Ohio Valley. This front should usher in much drier air and bring in some cooler temperatures as well, as it mixes out the atmosphere.

Sunday will likely be much-improved from the previous two days as high pressure begins to build over the area. This should allow for clear skies, light winds, and generally pleasant conditions across the region, with highs likely reaching into the low to middle 80’s across the area-which is right around normal for this time of year. Conditions will be ripe for radiational cooling on Sunday evening, so expect low temperatures to drop down into the lower 60’s with relatively low dewpoints.

Monday will be the day that will have the most focus on the weather, not for any severe weather or tropical cyclones, but for the partial solar eclipse that will take place during the afternoon hours. While we are not in the path of totality, most of the area will still be able to experience a rather cool and rare occurrence. However, the big question right now looks to be if there will be any cloud cover. As of right now, the conditions look to be quite favorable for safely viewing the partial eclipse, with mid level ridging overhead and surface high pressure just offshore-providing light onshore winds over much of the metro. This is however five days away, and predicting cloud cover for a three-hour window is rather difficult, but we can say that signs are encouraging at this time for a pleasant viewing experience! Be sure to check back over the next few days for updates!

The European 51 member ensembles showing cloud cover percentage. Monday the 21st is highlighted, with little cloud cover expected from a vast majority of the ensembles! (Courtesy of WxBell)

The European 51 member ensembles showing cloud cover percentage. Monday the 21st is highlighted, with little cloud cover expected from a vast majority of the ensembles! (Courtesy of WxBell)

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