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Health & Fitness

What Led to the Rainy August in Monmouth County?

An overview and discussion on the influences of last weekend's rainstorm on the current weather pattern.

On Monday I reported on the impressive rainfall totals that were recorded throughout Monmouth County ranging from 2.78 inches in Freehold to 5.10 inches in Millstone Township.  The following are rainfall totals taken throughout Monmouth County from the National Weather Service:

...MONMOUTH COUNTY...

MILLSTONE TWP 5.10 1217 PM 8/15

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ABERDEEN TWP 4.90 820 PM 8/14

SANDY HOOK 3.60 900 AM 8/14

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CREAM RIDGE 3.22 530 PM 8/14

FREEHOLD 2.78 800 AM 8/14

Since then another one to two inches of rain has fallen over Monmouth County, which is rapidly eroding the drought conditions that were in place over the county.  Before this new wet pattern became established, rainfall departures ranged from a foot below normal to four inches below normal or 50% to 25% below normal for this time of year.  However, since the start of this month, those deficiets are rapidly becoming neutralized as now departures are near to slightly above normal over western Monmouth County and only 25% to 10% below normal over southeastern Monmouth County.  This rapid change in conditions is even more impressive over northwestern New Jersey and the rest of the Delaware and Lehigh River Valleys where these locations have had the highest August rainfall amounts ever, and there is still another 12 days of the month left!

So first, what is causing all of this rainfall?  There are several factors that can lead to a new weather pattern, but one factor that stands out the most is the intensification of the negative state of the North Atlantic Oscillation or NAO.  A negative NAO is basically a pattern where a deep trough or upper level low develops over the Canadian Maritimes and an upper level ridge develops over Greenland and northern Canada.  This pattern acts like a stopper to the upper level pattern over North America, which slows down storms and forces storms to move over the same location again and again.  

In the case of this past major rain storm, three factors came into play.  The first was of course the negative NAO pattern discussed above.  The second was a strong disturbance that was forced to move further south than previous disturbances (because of the negative NAO pattern to the east), which produced a developing coastal low over the southern Mid Atlantic and New Jersey coastal waters.  This disturbance provided intense lifting over New Jersey.  Finally, the availablity of deep tropical moisture was the last ingredient that lead to these rainfall amounts.  Remember, there were several tropical disturbances and tropical storms moving over the western Atlantic durring that time period.  While these tropical systems did not directly interact with this storm, the upper level pattern in place that forced these tropical low pressure systems near Bermuda also was driving deep tropical moisture up the east coast of the United States, which interacted with this low pressure system.

So why did portions of southeastern Monmouth County miss out on the heaviest rainfall?  

Great question!  The orientation of the rainfall from this storm I believe is directly connected to the drought that was in place over much of coastal New Jersey leading up to this storm.  See, even though all of Monmouth County was running below normal in precipitation, there was still a gradient in the surface moisture from western Monmouth County to eastern Monmouth County.  This gradient, I propose, enhanced rainfall in the small scale or mesoscale.  While the drought conditions are being eroded away, this gradient will still exist as this is a natural configuration of our local atmosphere.  Let me explain.

Have you noticed that thunderstorms constantly weaken as the thundersotrms move from Mercer and Middlesex Counties to Monmouth County?  That's because the Atlantic Ocean is a stabilizing influence for the atmosphere along the coast.  As a result, more times than not, the thunderstorms weaken as they move from western Monmouth County to eastern Monmouth County.  This set up continues to happen through the warm summer months and typically lingers into the Fall and at times winter months.  

This past rainfall event though, has started to reduced the intensity of this gradient and has altered the mesoscale (small scale) dynamics in Monmouth County.  As a result, moving forward the difference in rainfall amounts in non-tropical storms between interior and coastal Monmouth County is not going to be as great.  This means that when thunderstorms develop, as they did yesterday, the thunderstorms are going to be able to sustain themselves as they reach the coast.  This new aspect of the weather pattern feeds back on itself as thunderstorms will now continue to reduce the drought conditions over southeastern Monmouth County as we have seen the past seven days.

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