9:04 AM

Premium Content

There can be no doubt that change is in the air as the Arctic Oscillation relaxes and the Sub Tropical jet stream gets some breathing room to flex its muscles now.  The result of which is a pattern that by next week leads to a ridge moving through the East and a deep trough extending from the Gulf of Alaska to the West coast.  This pattern will bring a significant amount of rain into California and a threat of severe weather for the Plains, but is this the end of winter?  Is it time for those in the northern Mid Atlantic to throw their hands up in despair as another winter exits without everyone getting into the “party” of snow and cold?

Well, while it is true that the pattern is trending towards a return to warmer temperatures for the Philadelphia and New York City metropolitan area, I don’t think this pattern has much legs beyond a few days and I’m going to explain why.

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!

7:15 AM

As low pressure exits the forecast area this morning, a brief break in the clouds and rainfall can be expected through the day.  In case anyone forgot, that big bright ball in the sky is called the sun, and no we are not in Seatlle.  Weak high pressure over the eastern Great Lakes will continue to move north of the forecast area with a weak northwesterly to westerly wind along with clear to partly cloudy skies.  The clear skies will also support significantly warmer temperatures as highs return to near normal levels once again, which is the mid 70′s to lower 80′s.  

However, another low pressure system is developing over the Great Lakes and western Ohio Valley this morning.  This low pressure system has the potential to produce a very rainy day for 

Saturday, however I have some doubts on the idea of Saturday being a complete wash out.  There is a large spread in the model guidance for Saturday.  When looking at the water vapor this morning, notice the large complex of storms seen in blues, oranges, and reds.  This complex of storms is called a MCS or a mesoscale convective system.  Why is this important?  Well, a MCS can tend to be unpredictable in upper level patterns that are in a state of change.  MCS storms move in the direction of the upper level winds and in this case winds will be backing from the west to northwest.  This is an important key.  The idea on some model guidance is that the MCS will move towards the forecast area with widespread showers and thunderstorms that will produce heavy rain throughout the forecast area on Saturday morning through early Saturday evening.  However, if the MCS slides just slightly further south, then the majority of the rainfall will miss much of the forecast area.  So the key for the forecast tomorrow morning will be the position and track of the MCS tonight.  I’m still expected scattered showers and thunderstorms through much of Saturday.

On Sunday, the strong low pressure system responsible for the showers and thunderstorms will exit the forecast area with a few lingering showers and thunderstorms on Sunday morning.  The upper level trough associated with this surface low pressure system will move off the coast while a ridge builds over the Mississippi Valley and Gulf Coast.  This transition in the pattern will finally bring more summer like conditions to the forecast area.  The combination of the ridge to the west and the upper low/trough to the east will produce a north to northwesterly wind from the surface to 500 MB, which will bring dry, cool air into the forecast area.  The position of the winds throughout the atmosphere will downslope off Appellation Mountains, which will produce much warmer conditions over the forecast area through next week.  In fact, there is some potential for temperatures to hit 90 degrees over the Delaware River Valley by the end of the week for the first time in June.  Through this period there will also be a threat for a sea breeze along the immediate coast, which would produce significantly cooler conditions along the immediate coast.  There is also a potential for low clouds and fog developing for each morning along the immediate coast as well.

8:15 AM

The one good aspect of El Nino this summer is that development of tropical systems continues to be inhibited by strong westerly shear and a lack of any organized disturbances.  Strong westerly and southwesterly shear dominates the central and wester Caribbean and over the western Tropical Atlantic.  The combination of a trough over northern Caribbean and off the eastern United States coast and a ridge over the ITZC will continue to support strong shear over the next 48 hours.  Meanwhile, dry air dominates much of the Gulf of Mexico with little sign of any possible development for at least the next 48 hours.