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Under partly sunny skies, temperatures rose into the 80s across much of the region today. Through midweek, temperatures will average somewhat above normal. Afterward, the coolest air mass so far this season will likely overspread the region. Noticeably warmer air could again return early next week.

In the 6 past cases when the June AO averaged +0.750 or above (1950-2021), 67% of the following August and September cases featured above normal temperatures. The August ECMWF forecast shows a warmer than normal September in the Northeast. This warmth would be consistent with the ongoing warming that has been occurring in September.

On August 18, the SOI fell to -32.90. Since 1991, there were 8 cases when the SOI fell to -30 or below during the August 10-25 period. That outcome has often preceded a wetter than normal September in parts of the Northeast. Mean September rainfall figures for those 8 cases: Boston: 4.38" (normal: 3.55"); New York City: 5.08" (normal: 4.31"); and, Philadelphia: 5.12" (normal: 4.40"). Very wet years outnumbered very dry ones by a 2:1 ratio in Boston and 3:1 ratio in both New York City and Philadelphia. 63% of cases saw at least one day with 1" or more rainfall in Boston. 88% saw at least one day with 1" or more in New York City and Philadelphia. 50% of those cases saw at least one day with 2" or more daily rainfall in Philadelphia. In sum, the SOI may be offering a signal that there will be some drought relief for the northern Mid-Atlantic and southern New England regions in September. On September 7, Philadelphia picked up 1.22" of rain.

The ENSO Region 1+2 anomaly was -0.9°C and the Region 3.4 anomaly was -1.0°C for the week centered around September 6. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.62°C and the ENSO Region 3.4 anomaly has averaged -1.00°C. La Niña conditions will likely persist through the fall.

The SOI was +19.67 today.

The preliminary Arctic Oscillation (AO) was -0.510 today.

On September 10 the MJO was in Phase 5 at an amplitude of 0.244 (RMM). The September 9-adjusted amplitude was 0.446 (RMM).

Based on sensitivity analysis applied to the latest guidance, there is an implied 68% probability that New York City will have a warmer than normal September (1991-2020 normal). September will likely finish with a mean temperature near 70.8° (1.6° above normal).


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