9:52 AM

June has seen some very impressive rainfall amounts with many locations well over an inch above normal.  Even some locations double the amount of normal rainfall.  While a continuous onslaught of showers and thunderstorms have impacted much of the Mid Atlantic, and specifically the New York City and Philadelphia metropolitan areas, other locations have been impacted with an above normal outbreaks of severe weather, especially tornado outbreaks from the southern Plains through the Mississippi Valley and Ohio Valley.  

Before going into the why, I want to show the rainfall anomalies throughout the region.

Connecticut:

Bridgeport, CT: +2.19″      Danbury, CT: +1.76″       Norwich, CT: +1.05″

New York:

Islip, NY:  +2.24″                New York City, NY:  +4.06″         JFK, NY:  +3.27″       LGA, NY:  +3.02″

White Plains, NY:  +1.05″

New Jersey:  

Atlantic City, NJ: +3.77″    Cape May, NJ: +1.90″          Freehold, NJ: +0.67″      Hightstown, NJ:  +1.31″

Millville, NJ: +1.74″      New Brunswick, NJ: +1.31″     Sussex, NJ:  +2.69″     Newark, NJ: +2.69″

Pennsylvania:

Allentown,PA: +2.23″      Philadelphia, PA: +1.28″    Lehighton, PA: +2.52″  

First, I want to address some of the differences seen in the magnitude of the above normal anomalies.  Some locations area barely a half an inch above normal while other locations are 3 to 4 inches above normal.  There is no trend in the precipitation anomalies that would support a synoptic or large scale pattern explanation, so I examined the radar trends over the past month and noted that the majority of the rainfall amounts were produced by convective precipitation events.  In other words, the extreme precipitation anomalies that did develop were supported by thunderstorms that happened to intensify over those locations.  The overall theme for the most part is that most locations ranged 1.5″ to 2.5″ above normal for June, and there is still 9 days left in the month with plenty of rainfall potential.  

So what is causing all of this rainfall?

To answer this question, I looked at two key factors that can drive a weather pattern, the stratospheric temperature anomalies and the Pacific sea surface temperature anomalies.  

El Nino: 

This summer, a weak El Nino has developed over the Pacific Ocean.  El Nino is when air pressure pattern in the South Pacific orient themselves where pressure is higher over Darwin, Australia is higher than at Tahiti which leads to a decrease in strength of the trade winds.  As a result, the water along South America is allowed to pile up, which inhibits up welling and allows the waters to warm.  The warming of the Pacific waters produces an atmospheric environment where the jet stream over North America is further south and more energized than normal.  The stronger the El Nino event, the further south and more energetic the jet stream.  

As we can see with the map to the left, El Nino is well on the way of developing and is certainly in a weak state of El Nino.  As a result, the jet stream over much of the United States is amplified and further south.  The position of the jet stream will certainly support the reason why the weather pattern has been so active.  A jet stream position further south would certainly support an environment where severe weather potential would be high.  Disturbances are able to have more interaction with the moist, warm Gulf of Mexico air mass, which can lead to wide spread severe weather out breaks.  However, the El Nino event alone would not explain why cold fronts have stalled over the forecast area, which has produced the heavy rainfall.  

Stratospheric Temperature Anomalies and the Negative NAO:

The key to understanding the cause of the wet pattern over the Mid Atlantic is to look at the interaction between the developing El Nino and the position of stratospheric temperature anomalies.  The stratospheric temperature anomalies have produced an environment the past twenty days that has produced a negative NAO and negative AO pattern.  The negative NAO pattern is key here as the higher pressures at 500 MB over northeast Canada and Greenland produces a blocking environment over North America.  As a result, instead of troughs simply lifting through the Northeast, the upper troughs and upper lows tend to stall over the Eastern Great Lakes.  As the upper lows and troughs begin to stall over the Great Lakes, the cold fronts drive towards the East coast, but lose support by the time they reach the forecast area.  

Meanwhile the combination of the strong jet stream over the Mississippi Valley and the stalled trough over the Great Lakes has enhanced a ridge over the southern Mississippi Valley and Gulf Coast.  This ridge over the Gulf of Mexico adds the ingredients of deep mid level moisture from the Gulf of Mexico and prevents cold front from moving much further south than the southern Mid Atlantic in most cases.  

Overview:

The combination of the developing El Nino and the negative NAO patterns have lead to heavy rainfall that has been recorded over the forecast area.  The El Nino produced an energized jet stream that is much further south than normal, which leads to strong disturbances to move through the Mississippi Valley and then the Great Lakes.  The negative NAO pattern, which is supported by stratospheric temperature anomalies, has produced high latitude blocking.  The high latitude blocking has kept a trough over the Great Lakes in place rather than lift out, which means that disturbances around the trough continue to rotate through the Mid Atlantic and Northeast.  Meanwhile, the interaction between the El Nino enhanced jet stream and negative NAO pattern has produced a strong ridge along the Gulf Coast.  

The ridge along the Gulf Coast is forcing moisture through the Ohio Valley and into the Mid Atlantic.  The cold fronts, with weakening upper level support, stall up against the ridge.  The moisture, as a result, is lifted along the cold fronts and thus the constant threat for showers and thunderstorms for days and weeks on end.  

7:30 AM

The forecast for today through tonight is best described as significantly complicated and potentially very active.  The main theme is basically the progression of the warm front currently over southwesterly Pennsylvania through eastern Virginia.  I want to start with the fact that the surface low pressure system is further north and weaker than forecasted on any model guidance to this point.  The cold front is actually well to the west and will eventually become nothing more than a boundary between the very moist air that will move into the forecast area and the drier air that will eventually build in.  However, before I get to all of that, let’s discuss the events for this morning.

The majority of the showers and thunderstorms over western and central Pennsylvania have been weakening this morning due to the weakening of the low level jet stream at 850 MB.  The upper level winds this morning are also from the northwest throughout the Mid Atlantic, which is forcing much of the precipitation south and southeast thus slowing the progression of the heavy precipitation eastward this morning.  However, eventually the low pressure over the Great Lakes will move north and east into the eastern Great Lakes and St. Lawrence Valley this afternoon, which will drive the warm front through the forecast area this afternoon.  As the warm front moves north and east, showers and thunderstorms will redevelop throughout the region.  The showers and thunderstorms ahead of the warm front will be able to produce very heavy downpours. 

Note the surface map above.  Dry, cool air is in place over much of the New York City metro down through the northern suburbs of Philadelphia.  This dry, cool air will be slow to leave the forecast area, which will increase the development of isentropic lifting this afternoon.  In other words, the cool air over the forecast area ahead of the warm front will force thunderstorms to become elevated, which will lead to very heavy downpour over the forecast area this afternoon.  However, south of the warm front, a very different environment will develop. 

South of the warm front, a very warm and unstable air mass is in place.  The map to the left shows that atmosphere is very unstable south of the warm front and wind shear increases dramatically.  I don’t believe the warm front will clear all of the forecast area, in fact much of northern New Jersey and the New York City metro will remain north of this warm front.  However, all of central and southern New Jersey including all of southeastern Pennsylvania will be under the threat of strong to severe thunderstorms as the warm front moves north.  

So here is what to expect through today.  Clouds will continue to increase through this morning ahead of the warm front.  By around 11 AM, steady rain will enter the forecast area from eastern Pennsylvania and will push eastward through the afternoon hours.  The showers and thunderstorms will eventually have a significant impact on the Mets and Phillies baseball games this afternoon and evening.  As the warm front pushes north and east, a brief break in the rainfall can be expected.  Scattered showers and strong to severe thunderstorms will develop later this afternoon behind the warm front.  These thunderstorms will be capable of wind gusts exceeding 55 mph, large hail, very heavy downpours, and even an isolated tornado.  The combination of the heavy downpours over the forecast area that has already seen rainfall amounts two to three times what is normal will lead to the potential for flash flooding throughout the forecast area.  The low pressure system will move through New Jersey later this evening and slowly exit overnight.  As the low pressure system exits, lingering showers will remain over the forecast area especially along the coast.

By tomorrow, slow clearing can be expected with overcast conditions and scattered showers throughout the forecast area.  Clouds will break up by the mid to late afternoon hours with much drier conditions by the evening hours.

A significant change in the pattern will finally unfold for next week.  The upper trough will reposition itself off the coast, which will produce a northwesterly to northerly flow over the forecast area.  The position of the trough will mean that most disturbances will remain east of the forecast area, which will mean much drier conditions for the region.  High pressure over the Ohio Valley will bring clear to partly cloudy skies and much warmer, more seasonable conditions for the forecast area.  In fact, locations over the Delaware River Valley may even reach 90 degrees for afternoon highs by mid week.  Locations along the immediate coast will have to contend with influence from the Atlantic, which may lead to overcast skies in the morning and cooler temperatures than surrounding areas inland.  A new disturbance will approach from the west on Friday as the trough retrogrades back towards the Great Lakes with scattered showers and thunderstorms.

So overall, a very active period of weather over the next 36 hours will be followed by more summer like conditions through much of next week.  Expect updates via twitter through the day!

The latest updates can be found at the bottom!

8:36:31 AM: The steady rain is close to coming to an end as the back end of the heavy rain moves through eastern Pennsylvania.

8:37:32 AM: The occluded front is still off to the west of the region and will continue to slowly progress eastward through the afternoon.

8:38:03 AM: Scattered showers are currently developing over much of central Pennsylvania and will continue to move east through the afternoon hours.

8:38:37 AM: I expect the steady rain to come to an end from west to east starting by 9:30 AM and exit the coast by around 2 PM east of Long Island.

8:39:07 AM: The occluded front and low pressure system should move east of the forecast area by this evening, which will put an end to the rain threat.

8:40:05 AM: The overcast conditions back through central Pennsylvania will prevent the atmosphere from destabilizing.
8:59:09 AM: Through this past hour, there is a wide variety of rainfall amounts from this system ranging from a little over a 0.10″ to as much as 2″!

8:59:37 AM: The heaviest rain has fallen over the central Hudson Valley towards Albany where the upper level lifting has been strongest.

9:00:26 AM: Meanwhile, portions of NE PA has received as much as a half an inch of rain, especially along the Delaware River.
9:23:18 AM: The steady rain is coming to an end over Wayne, Monroe, Carbon, Lehigh, Berks, Chester Counties of PA. Scattered showers remain a threat.

10:27:08 AM: Steady rain has ended in Philadelphia and much of eastern PA. Steady, heavy rain will continue for much of NJ and NYC metro through 12 PM.

10:29:37 AM: Much of Long Island will have heavy rainfall through at least 3 PM with rain tapering off by 5 PM over eastern Long Island and Connecticut.

10:36:47 AM: Doppler estimated rainfall rates with this finally heavy band of rain range from 0.25″ to 0.50″ per hour with isolated amounts to 1″/hr.

12:17:24 PM: Scattered showers and isolated thunderstorms are starting to develop over central and eastern PA.

12:18:51 PM: Most of the thunderstorms will be weak, however a few isolated thunderstorms could become strong just ahead of the low pressure system.

12:20:09 PM: The strongest thunderstorms will be capable of occasional lightning, small hail, very heavy downpours, and wind gusts up to 50 mph.

2:40:51 PM: The steady, heavy rain is just about over for Passaic, Bergen, Essex, and Hudson Counties for NJ and New York and Bronx Counties of NY.

2:41:47 PM: The rain will continue to progress move ENE through the afternoon with rain exiting Long Island and Connecticut by around 4 to 6 PM.

2:42:56 PM: Scattered showers and isolated thunderstorms are still moving through E PA and much of New Jersey.

2:43:29 PM: However, these showers and thunderstorms are weakening and will continue to do so as the afternoon progresses.

2:52:56 PM: An Urban Flood Advisory has been issued for Suffolk and Nassau Counties of NY due to rainfall amounts of half an inch per hour.

2:52:56 PM: An Urban Flood Advisory has been issued for Suffolk and Nassau Counties of NY due to rainfall amounts of half an inch per hour.

2:59:29 PM: A Coastal Flood Advisory is in effect through 7 PM to 12 AM for shore lines of the Delaware River around Philadelphia.

3:00:17 PM: The persistent southeast wind will continue to push water from the Delaware Bay into the Delaware River, which will cause minor flooding.

3:01:48 PM: The stable nature of the atmosphere is limiting the strength of thunderstorms moving east into the forecast area.

3:02:27 PM: While the southern Mid Atlantic will see severe weather this afternoon, I only expect scattered showers and an isolated strong thunderstorm.

5:19:10 PM: For the rest of the afternoon and evening, expect scattered showers to continue to move through the forecast area.

5:19:39 PM: The steady rain will continue over eastern Long Island and Connecticut through the late evening hours.

5:20:05 PM: However, the majority of the steady rainfall is over for the NYC and Philadelphia metros.

5:20:55 PM: Upper level energy rotating through the forecast area this evening will support showers with brief periods of moderate rainfall.

5:21:19 PM: Much drier conditions are expected tomorrow, especially in the afternoon.

Most recent updates on the bottom!

12:40:48 PM: The strongest thunderstorm of all the thunderstorms throughout the region is over western Burlington County.

12:42:03 PM: The thunderstorm over western Burlington County is producing very heavy rainfall and frequent lightning.

12:43:03 PM: These thunderstorms are below severe level, but can reduce visibility rapidly due to the heavy rainfall and strong wind gusts.

12:44:14 PM: A strong shower over Bergen and Essex County of New Jersey is tracking towards Bronx over the next few hours.

12:44:14 PM: A strong shower over Bergen and Essex County of New Jersey is tracking towards Bronx over the next few hours.

12:47:11 PM: Several lines of strong thunderstorms are developing over eastern Pennsylvania. The main threat from these thunderstorms is heavy rain.

12:54:24 PM: I’m becoming very concerned about flash flooding for western Burlington, Camden, and western Gloucester Counties of New Jersey.

12:58:42 PM: Doppler estimated rainfall amounts are approaching an inch and a half over western Burlington County, especially over the town of Crossroads

1:05:35 PM: A disturbance moving through NW Ohio and W PA will have an impact on the forecast area this evening with potentially strong thunderstorms.

1:05:58 PM: So the thunderstorms over the region now is due to the instability from the sun warming the lower levels.

1:06:26 PM: Meanwhile, more thunderstorms are possible due to the upper level disturbance approaching from the west this evening.

1:07:22 PM: Meanwhile, more thunderstorms are developing due to the outflow boundaries from the initial thunderstorms.

1:07:55 PM: An example of this can be seen over Bucks County as one thunderstorm is drifting to the northwest.

1:12:28 PM: So far the thunderstorms have stayed below severe levels. Given the lack of lightning so far, these storms are more like heavy showers.

1:28:57 PM: The Delaware River Valley is just being crushed by heavy rainfall right now.

1:29:25 PM: Meanwhile, the storms over the Bronx have weakened rapidly due to the marine layer in place.

1:33:06 PM: A flood warning for small streams and urban areas has been issued through 4:15 PM for Burlington County.

1:35:49 PM: The heavy rain is expanding towards Monmouth and Ocean Counties of New Jersey as the storm drift north.

1:33:06 PM: A flood warning for small streams and urban areas has been issued through 4:15 PM for Burlington County.

1:35:49 PM: The heavy rain is expanding towards Monmouth and Ocean Counties of New Jersey as the storm drift north.

1:36:30 PM: There is a weak sea breeze front over eastern Monmouth and Ocean Counties that will have to be watched.

1:36:58 PM: This boundary may produce very heavy rain for central and western Monmouth and Ocean Counties.

1:38:09 PM: Meanwhile, the thunderstorms over the southern Hudson Valley and Connecticut have basically fallen apart. This includes the Bronx.

1:40:33 PM: The nature of these thunderstorms is to move very slowly, which is why heavy rain is becoming an issue.

1:46:40 PM: A Flash Flood Warning has been issued for central Burlington County. Local law enforcement has reported roads being washed out.

1:47:20 PM: Doppler Radar estimates are now exceeding 2.5″ of rain so far from these thunderstorms.

1:51:36 PM: An impressive line of showers and thunderstorms has developed from SE to NW stretching from Ocean County, NJ to Luzerne County, PA

1:52:15 PM: These thunderstorms will continue to drift towards the northeast and east towards the coast and the New York City metro.

1:54:58 PM: Given the marine air mass along the coast, I think these storms will peak over central Sussex County down through central Monmouth County.

1:55:21 PM: Then the storms will weaken as they are elevated over the marine layer.

1:55:50 PM: Note that temperatures along and behind these storms are in the upper 70′s to lower 80′s with mid to upper 60′s for dew points.

1:56:28 PM: Ahead of these storms along the coast, temperatures are in the lower 70′s and dew points in the upper 50′s. This air is more stable.

2:17:08 PM: A perfect example of what to expect this afternoon can be found over Sussex, Morris, and Passaic Counties of New Jersey.

2:18:27 PM: As the thunderstorms moved east, they rapidly weaken to moderate showers and continue to weak as they move east. This may protect NYC.

2:33:55 PM: A severe thunderstorm has developed over New Castle, DE and will likely impact Salem County in New Jersey with nickel size hail.

2:36:23 PM: I am now convinced that the heaviest rain is going to remain from Eastern PA to west/central New Jersey, or the Delaware River Valley.

2:41:28 PM: Flash Flood Warnings are now issued for central and southern Bucks, northern Lehigh, Carbon, Northhampton, and Monroe Counties of PA.

2:55:42 PM: Very strong thunderstorm now moving through Hunterdon County of NJ right now. Still below severe level however.

2:57:08 PM: Note that as this line of storms moves NE that they continue to weaken and become elevated heavy showers.

3:08:57 PM: This evening is starting to look very interesting for E PA and W NJ as the lifting from the disturbance to the west approaches.

3:33:25 PM: A band of showers is currently moving over Yankee Stadium. These showers will likely linger through the game.

3:35:12 PM: The strong thunderstorms that were over W-C NJ have become an area of heavy rainfall slowly drifting towards New York City.

3:43:00 PM: There is a real potential for flash flooding over Middlesex, Union, Sommerset, and eastern Hunterdon Counties of New Jersey.

3:45:20 PM: Doppler estimated rainfall amounts of 1 to 3 inches are being produced by these thunderstorms, some isolated amounts to 5 inches.

3:48:53 PM: Locations from SE Montgomery through Union, NJ are receiving very heavy rainfall. A flood warning should be issued for these locations soon

3:49:43 PM: Another line of storms and strong showers is approaching from NE PA and will enhance the rain over N-C NJ this afternoon.
3:54:05 PM: As these two mesoscale boundaries interact, showers and thunderstorms will continue to develop and train over N-C New Jersey.
3:59:40 PM: For those at the Mets/Yankees game, get ready for some wet weather.

4:22:15 PM: Baseball update: Heavy rain is approaching from Essex County and should enter the Bronx in roughly 15 to 30 minutes.

4:22:15 PM: Baseball update: Heavy rain is approaching from Essex County and should enter the Bronx in roughly 15 to 30 minutes.

4:28:15 PM: Note that the DBZ over E-C New Jersey has collapse from 45 DBZ to 25 DBZ in roughly 30 minutes. This means the showers are weakening.

4:29:40 PM: Very interesting development. The boundary over Sussex, Warren, and Northhampton Counties is pushing the C-NJ boundary south.

4:30:13 PM: This MAY mean that the Bronx dodges much of the heavy rain, but I am not confident of that.

4:30:42 PM: Meanwhile, heavy rain has started to move into Monmouth County. Heavy rain is falling over Freehold right now.

4:32:32 PM: The sinking air from the northern boundary is helping to weaken the rainfall over Morris, Essex, Hudson, and Union Counties.

4:33:03 PM: The rainfall is MUCH heavier over Mercer County, NJ and Bucks County, PA.

4:51:03 PM: For those at the baseball game, showers are breaking up as the main focus of the line is pushed south towards central NJ.

4:52:00 PM: These mesoscale boundaries are acting a lot like Lake Effect snow bands in terms of movement. In a way, these seem like moisture plumes.

4:54:39 PM: The heavy rain is now focusing over Middlesex, Monmouth, and Mercer Counties of NJ and Bucks and Montgomery Counties of PA.

4:55:23 PM: Meanwhile, the disturbance well to the west is starting to produce thunderstorms over E-C PA, which are racing eastward.

4:56:03 PM: The interaction of the mesoscale boundaries and the upper level disturbance is going to complicate the forecast for this evening.

5:21:16 PM: A Flood Warning has been issued for Neshaminy Creek at Langhorne in Bucks County through 8AM Sunday.

5:45:04 PM: The upper disturbance is approaching fast with strong to severe thunderstorms developing over Wyoming County, PA and racing SE.

5:58:53 PM: Very strong thunderstorms are developing over Chester County and also starting to produce very heavy rainfall.

5:59:37 PM: I think the upper disturbance approaching from the west will enhance this heavy rainfall. I’m watching to see if the rain band moves north.

6:15:31 PM: Strong thunderstorms approaching Lackawanna and N Luzerne Counties of PA.
6:25:52 PM: A severe thunderstorm warning has been issued for S Lackawana and NE Luzerne Counties of PA for nickel size hail.
6:32:30 PM: As the sun sets, the mesoscale boundaries that have produced the heavy rain will weaken.

6:33:30 PM: However, showers and strong to severe thunderstorms will remain a threat as the upper disturbance approaches from the west.

6:44:56 PM: The severe thunderstorm warning has been extended to Monroe County of PA. This storm will also impact Warren and Sussex County.

7:02:49 PM: Scattered thunderstorms are redeveloping throughout eastern PA, however as the sun sets these storms will remain below severe levels.

7:03:13 PM: The thunderstorms however will impact the Phillies/Boston game this evening and may even cause a postponement.

8:05:07 PM: As the upper disturbance approaches, a line of strong to severe thunderstorms are driving south and east towards the forecast area.

8:06:23 PM: These thunderstorms will continue to dive south through NE PA and through much of NJ, SE PA, and much of the NYC metro.

8:07:49 PM: These thunderstorms are capable of frequent lightning, nickel to quarter size hail, and very heavy downpours.

8:08:33 PM: The heavy rain over central NJ and the Philadelphia metro has weakened considerably as the sun sets and the mesoscale boundary weakens.

8:15:00 PM: These thunderstorms will only intensify the flooding conditions that have developed throughout E PA and much of NJ.

8:15:30 PM: The good news though is that the storms are moving relatively fast and will be weakening as the evening progresses.

8:20:34 PM: The lower end of this line of thunderstorms over Schuylkill County, PA is becoming very impressive.

8:21:06 PM: This section of the line of thunderstorms should impact SE PA including Philadelphia and much of central/southern New Jersey through tonight

8:21:40 PM: The line of thunderstorms will be capable of frequent lightning, small hail, wind gusts over 50 mph, and very heavy downpours.

8:42:42 PM: As this line of thunderstorms approach, they will be lifted by the marine air mass.

8:44:04 PM: As a result very heavy rain and strong wind gusts can be expected overnight as the thunderstorms drop south and east.

8:52 AM

This summer is not shaping up to be one that works out well for beach lovers.  This morning, yet another cold front is slowly losing steam over the forecast area as scattered showers continue to move through the region.  

The majority of the rainfall is now off the coast as the best dynamics race towards the Canadian Maritimes this morning.  This means that once again, the cold front that had so much life over the Mississippi Valley and Ohio Valley is now dying over the Mid Atlantic.  A weakening area of low pressure is moving towards the Albany District and will push into New England this afternoon, as a result the cold front will slowly drag through the forecast area through the day.  However, already the cold front has stalled over portions of West Virginia and Virginia with a weak wave of low pressure developing along the boundary, basically of herald of things to come for the forecast area.  

Through this weekend, the cold front will stall right over the forecast area, which will mean that moisture will converge and focus over the region, guarenteeing a good amount of cloudy conditions over the forecast area.  As weak waves of low pressure, like the one over the Hudson Valley and West Virginia this morning move along the stationary front, scattered showers will develop and move through the region.  Which will produce another hit or miss type of weekend for many.  In short, there is a chance that your location will remain dry through the entire weekend, you might also want to consider buying a lottery ticket if you are that lucky.  However, most locations will be impacted by scattered showers at times through the weekend.  This is not a case where wide spread rain develops, but just a dreary; foggy; cool; almost fall-like weekend can be expected.  Personally, this type of weather makes me feel like playing football rather than baseball.  

So is this pattern going to break?  Well, looking at the water vapor this morning back through the Pacific Ocean, I see no mechanism to change this pattern.  In fact, with the growing strength of El Nino, this type of pattern will only be reinforced through the next several weeks.  

Going forward for next week, the stationary boundary will remain over the Mid Atlantic, wavering north and south depending on individual low pressure tracks and strengths.  However, the basic overall theme that we’ve seen the past several weeks will continue into next week.  A threat of showers and thunderstorms can be expected each day with temperatures averaging near to below normal.