With summer nearly in the record books, the first month of meteorological autumn is just ahead. September promises to bring shorter days, longer, cooler nights, and bit less humidity – that we can be certain of! However, will the month average out warmer than normal or cooler than normal? Pattern persistence argues for a continuation of the mild regime across much of the United States, as we’ve seen higher than avg temp departures for most of the past 12-16 months. Over the last 1-2 weeks, there has been noticable changes in the pattern, with significant cooling into the Mid-west and Plains states. That coolness has attempted to propagate east, and has succeeded to bring temperatures down to near/slightly below normal levels in the Northeastern states. With that being said, the remainder of this week into the weekend will feature gradually climbing temps, but still within the normal range (about 83-88F). The West Atlantic ridge has been strong and very resistant to cold fronts on its western periphery.

The NAO/AO have been negative all summer — plentiful blocking in the northern hemisphere. However, due to the enormous amount of warmth across North America, the high latitude blocking has only managed to create pockets of negative temp anomalies. Additionally, blocking in summer-September can yield a much different pattern than during the cold season. For example, over the coming weeks, the NAO is likely to stay neutral to negative, but it’s probable that it will connect w/ the West Atlantic ridge, keeping the East Coast on the mild side. The AO looks to neutralize in the medium range, possibly jumping positive into September, while PNA values should be positive as we begin September.

What does all this mean? I think it’s important to examine similar years in the past to form an opinion about what may happen in our future. The 1950s parallel the current year quite well, due in part to the warm Atlantic and cool Pacific ocean phase we’re currently experiencing. When we throw in the variables of ENSO state and other teleconnection modality, we can form a more accurate picture of the potential weather pattern.

Given the oncoming El Nino (readings are now +0.6c in region 3.4 indicative of weak Nino criteria), the years 1951, 1957, and 2002 are of particular interest. 1951 was an analog that I utilized for this summer, and it seemed to work well in predicting the overall warmth across America. 1958 and 1953 are two other analogs which fit the current and forecast regime into mid September.

The analog package paints the following picture for temperature anomalies in North America:

A closer look at the United States temp departure forecast vas per the analogs:

Note that the mean trough is probable to position itself in the Upper Mid-west/Great lakes, with positive heights in the West and East. As an aside, this pattern is one that may be conducive for tropical cyclone activity (direct or indirect influence) in the Southeastern US, and up the East Coast. Forecasting a cyclone in the Northeast is like throwing a dart blindfolded, but one can say that the pattern is more favorable than normal for direct or indirect effects — maybe remnants.

If we take a look at the analog precip forecast, it suggests a wet SE US, but not so much in the Northeast. Maybe indicative of recurving cyclones.

The NAEFS suggests a warm East coast pattern will ensue in the medium range:

All in all, it appears likely that the warmer than average pattern will continue for another month in the Northeast. My guess would be +1 to +2 in NYC for temperatures (slightly warmer than what August will finish at). The Mid-west is the one region of the country that may experience a cooler than normal September. Precipitation should be above average in the Southeast (tropical activity), with slightly below avg precip in the NYC-NE corridor. It’s too early to say whether Isaac will have any impact up here; however, the pattern is more conducive than usual for an East Coast hit. More likely, we’ll see remnants from Isaac, after it hits the Eastern Gulf or Southeast States.