My thoughts on the next few months.

 

Summer is winding down. The Summer Outlook I tacked on to the end of my Hurricane Outlook, did very well in terms of the overall pattern.  Meteorological Fall starts September 1^st, with the autumnal equinox happening at 8:43 a.m. ET on Sept. 22. So, let’s take a quick look at what this Fall might have in store.

The La Nina is slowly developing, it isn’t impacting the pattern a lot…yet… This should  change as we move forward into Fall and Winter. Looking at the current Sea surface temperature (SST) anomalies we can see the cooling in the eastern equatorial Pacific. These SST have remained below normal in the eastern and east-central tropical Pacific, and remained above normal in the western tropical Pacific Ocean for the last several months. 

Image Credit: Climate Prediction Center / NCEP

Looking at the temperature charts we can see the SST are on the cool side of neutral. There is no doubt that the transition from these conditions to La Niña has been slower than originally anticipated. this has played a role in how the pattern has been behaving.

 The weather has been unusually warm and humid, but that has changed of late. Why is this the case? We can look at the two main culprits to see why.

The Madden-Julian Oscillation (MJO) is in phase 2, and will head into phases 3 and 4, this tends to place the trough right over the Northeast and Middle Atlantic.  This accounts for the cool down and diminished rainfall. It is also protecting us from Ernesto.

When we look at the MJO phase forecast, we see a lot of colorful squiggly lines. To the lay person this can be very confusing. So how do you read this diagram. The black line shows evolution and ensemble forecast of the MJO over the last 40 days. The yellow lines are the individual ensemble members, the red line shows the forecast for days 1-7, blue line shows the forecast for days 8-14, the purple line shows day 15 and beyond. 

We had a negative Pacific Decadal Oscillation (PDO) last winter into the spring. Here is a look at the global SST anomalies back in April. 

Since then, the SST have warmed in the Gulf of Alaska, we can see that in the current SST anomaly chart. But this warm blob is starting to cool back off. And we should see conditions in the Gulf of Alaska return to what we saw earlier this year. 

During the Summer, a negative PDO tends to produce a trough in the Gulf of Alaska and a trough in the Midwest into the Great Lakes.   This increases the odds for a western Atlantic ridge.  The late spring and first half of Summer was very hot and it was wet. Now the pattern has cooled off, and is starting to dry out.  The region has seen two heavy tropical rainfall episodes from Beryl and Debby. A lot of this had to do with the orientation of the PDO, where the ridge placement favored southern heat and moisture to invade our region.  

 Now these two teleconnections are signaling some changes in the long-term pattern.

The atmospheric wave lengths are changing as we move from Summer into the Fall. Generally, troughs are deeper and stronger during the fall, winter, into spring and tend to be Not as robust and weaker during the summer; due to the jet stream being farther northward on average. So, we’re entering the time when the strength of opposing air mass characteristics causes wavelengths to expand, allowing troughs and ridges to expand and lengthen in size. As most of us know, ridges are areas of higher pressure and typically mean hotter and drier conditions at the surface. Troughs are therefore the opposite as areas of lower pressure generally mean cooler and wetter conditions at the surface.

So, what we're going to be seeing moving forward, is  a blend of seasonal changes of the Jet, and the MJOs and PDO’s influence on the trough/ridge pattern. We're likely going to see more cold fronts moving through the Northeast, leading to variations in the temperature pattern, cooler outbreaks will become more common with episodes of heat become less of an issue.

There are signs in the Bay of Bengal and Arabian Sea, that will influence what goes on across northern Africa; this is showing the Atlantic is likely to become active again fairly soon. We’re also entering the heart of the Atlantic hurricane season. So once things start backup, the Atlantic is likely to become very active.  The overall problem going forward, is going to be ridging in the Atlantic, this could keep somewhat of a lid on the Atlantic during the height of the season, this could put my hurricane outlook in some jeopardy, time will tell.  

The trough setup is going to keep us coolish for the next two or three weeks. The trough should help keep many of the hurricanes away from the East Coast over the next month as well. This very well could be the general thyme for the first few weeks of the upcoming Fall season. With overall temperatures seasonal to slightly below average, with most of the hurricanes staying away from the East Coast.  This isn’t saying there won’t be warm temperatures with high humidity. But on average prolonged bouts of high heat and humidity are in the rearview mirror.    As La Nina takes hold, heat could become more of an issue toward the end of September into October as the Southeast ridge becomes more active.

In spite of La Nina taking its time developing. The idea I had in the Summer outlook, remains the same. This should be a weak La Nina.

Weak La Nina and negative PDO temperatures and rainfall patterns.

La Nina will likely make for October into at least the first half of November to be warmer than average. Areas closer to the Great Lakes and northern parts of the region could see overall well above temperatures. But remember above average temperatures in the Fall, are not as hot as they would be during July and August.  The pattern that looks to setup, will mean in spite of a coolish start, overall temperatures during Fall will be above average

Weak La Nina's tend to see below average precipitation across much of the Northeast.

So, how wet the rest of this year will be here in the Northeast and Middle Atlantic will depend on tropical activity. With SST in the Atlantic at near-record to record warmth.  The super warm SST in the Atlantic, mean the Atlantic Multi Decadal Oscillation is going to be staying warm for the Fall. Once La Nina finishes developing it will exert force on the pattern as we move toward winter. This will aid tropical storm development in the Atlantic Basin, increasing the odds for possible impacts on Middle Atlantic and New England from tropical activity. So, this Fall will likely see overall above average precipitation across the Middle Atlantic into New England. 

With the warm temperatures, outside of a slight chance for snow in the high elevations during October, The region's northern areas shouldn't see much in the way of snow until mid to end of November. The lack of a moderate or strong La Nina could have implications in the 2024-2025 Winter forecast. A weaker La Nina would bring a better chance for a winter around average.  But as has been the case for the last several winters, something could throw a monkey wrench into the works.

I hope you found this enjoyable to read.