NY NJ PA Weather Forecasts

What has caused the heavy rainfall this month?

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.  

 

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