donsutherland1

The same physics is involved (conservation of angular momentum) in a rotating system be they celestial bodies or tropical cyclones.

Parts of the region again reached 80° today before clouds moved in. Central Park (80°), Islip, and Newark (81°) all reached at least 80°. A low pressure system will move along a nearly stationary front tomorrow. As a result, rain will develop tonight and continue into Friday. A general 0.50"-1.50" rainfall is likely during this period in New York City and nearby suburbs. Some locations could see higher amounts in excess of 2.00". The potential for an area of 1.00"-3.00" rainfall exists for parts of Connecticut and Massachusetts. Above normal temperatures will continue through at least the coming weekend. In addition, one or two tropical cyclones could develop in the western Atlantic in coming days. The extent of ridging to the north and interaction with a potential second system could ultimately determine whether one of these systems poses an eventual threat to a portion of the Southeast or stays out to sea. The potential Fujiwhara interaction would be unusual for the Atlantic basin. Typically, if the stronger system is to the right or east of the weaker one (possibly Invest. 94 as per most of the guidance at present), the stronger system will tend to deflect the weaker one northward and eastward sooner than might otherwise be the case. If both systems are of similar strength, they can rotate counterclockwise around a midpoint roughly between the centers of the two storms as they move along the steering flow. Then, there's low but not zero probability of a subsequent shift in the steering currents that could turn the storm back toward the coast as depicted on the 9/24 12z ECMWF. For now, the potential development and evolution of pair of tropical cyclones bears watching. The ENSO Region 1+2 anomaly was 0.0°C and the Region 3.4 anomaly was -0.4°C for the week centered around September 17. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.03°C and the ENSO Region 3.4 anomaly has averaged -0.38°C. La Niña conditions will likely develop during mid- or late-autumn. The SOI was +2.62 today. The preliminary Arctic Oscillation (AO) was +0.889 today. Based on sensitivity analysis applied to the latest guidance, there is an implied near 75% probability that New York City will have a warmer than normal September (1991-2020 normal). September will likely finish with a mean temperature near 69.8° (0.6° above normal). Supplemental Information: The projected mean would be 1.8° above the 1981-2010 normal monthly value.

Once one starts getting a large share of hours with above freezing temperatures during snowstorms, the amounts will be capped. So, at some point, even the blockbuster storms probably won't be as big as they have recently been or might be during the early to middle part of the transition. Snowfall extremes may have an asymmetric relationship to averages, at least initially, during the decline. In other words, the extremes e.g., top 1% of values, decline more slowly than the averages. One paper: https://pog.mit.edu/src/ogorman_snowfall_2014.pdf Prior to the decline, extreme values may increase relative to averages, even when averages start to decline. That might have been what happened from the mid-1990s through 2020 in New York City due to the benefit of warmer air holding more water vapor. Such developments are expected in the northern parts of the Northern Hemisphere. Figure 1: The paper from which the chart was taken explains: We find that, for the first half of the 21st century, the 99.9th percentile of daily snowfall, i.e., the largest daily snowfall in 1000 days, increases for large areas of the Northern Hemisphere by 10–20 percentage points until the decade of 2051–2060, while some areas, e.g. in Western Europe, show decreasing extreme percentiles (all percentages are relative to the historical baseline (1851–1920) values, Fig. 1, first column). Towards the end of the century (2091–2100), this trend diverges into sharply decreasing 99.9th percentiles for lower latitudes like Western Europe and parts of North America (up to points), and further increasing percentiles (more than points) in high latitudes. Substantiating these trends, the expected extreme magnitude, i.e., the average of daily snowfall events exceeding the historical 99.9th percentile (for details see the methods, Eq. (2)), increases by 5–10% points until the middle of the century (Fig. 1, second column). This indicates the strengthening of extreme snowfall events. The continued increase until the end of the century to 10–15% points of the baseline level shows that even with rarer extreme events as indicated by decreasing percentiles, the remaining extreme snowfall events are projected to intensify compared to the historical baseline... In contrast to this increase in extreme snowfall statistics, the mean daily snowfall diverges already in the near future. While snow-prone regions in high latitudes exhibit an increase of mean daily snowfall by 20% points until the middle of the century, we observe a sharp decrease for lower latitudes... These trends continue until the end of the century, yielding decreases of up to 80% points in large parts of Europe and parts of North America, while high-latitude regions like Siberia show a similar increase of around at least 50% points. Source: https://www.nature.com/articles/s41598-021-95979-4

Unfortunately, that's true. Humanity failed to make the efforts needed when the requirements were gradual. It is all but certain that the much steeper requirements needed to limit warming won't be pursued. The U.S. has left the playing field so to speak. Europe appears to be retreating.

New York City has had a string of low snowfall winters in the midst of ongoing warming. Arguably, New York City is in the early stages of a structural decline in winter snowfall. Washington, DC has already made a transition to lower snowfall. So, what might it look like if New York City has entered a transition to lower snowfall and how might the evolution unfold afterward? Early-Mid-Transition: Possibly 2020s to Mid-2030s: One can expect larger interannual swings (rising standard deviation in snowfall). Very lean winters will be mixed with some big winters, increasingly defined by fewer bigger events, as warmer air holds more water vapor. The 30-season moving average could increase, at least for a time, before turning downward. Mid-Late-Transition: Possibly Mid-2030s to Mid-2040s: As baseline winters warm further, more storms fall as rain, and snow becomes concentrated into fewer, well-timed cold shots. Interannual variability declines once precipitation falls increasingly as rain. Seasonal snowfall standard deviation flattens and then falls. New Low Snowfall Regime: Possibly Mid-2040s-Mid 2050s The mean is low and the standard deviation is lower than the volatile peak, because snow is infrequent. Big snowstorms don’t vanish outright but become rarer along the coast as rain wins more often. Regional studies for the Eastern U.S. find decreasing annual snowfall but continued potential for occasional high-impact even blockbuster events (fewer in number, increasingly conditional on strong cold air). Select Charts: 30-Season Moving Average Snowfall: The Great Rise during the first quarter of the 21st Century 30-Season Volatility: A Sharp Rise in Volatility: 30-Season Moving Average Snowfall for Biggest Daily Events: Bigger Events Grow Bigger: Percentage of Seasonal Snowfall from 10" or Above Days: Bigger Storms Contributed a Larger Share of Seasonal Snowfall (Mid-1990s-2020): 30-Season Moving Average of the Number of Measurable Snowfall Days: Fewer Days with Measurable Snowfall: In sum, some of the characteristics of an early transition toward lower structural snowfall seem to be present. More time will need to pass before one separates the long-term signal from the noise of internal variability.

Even for the NYC area, using a 0.0°-1.0° warm anomaly over the 1991-2020 baseline isn't a guarantee of a very bad (low) snow season. Looking at all December-March cases that fell within that range at Central Park: Mean seasonal snowfall was 16.9"; median seasonal snowfall was 13.4". In terms of seasons, 13.3% had less than 10" seasonal snowfall but 26.7% had 20" or more. Most common range: 10.0"-19.9" (60.0%). Highest: 38.6", 2020-2021; Lowest: 2.8", 1972-1973. And for 0.5°-1.5° above the 1991-2020 baseline: Mean snowfall: 19.0" Median snowfall: 17.9" Snowfall distribution: 36.4% had 20" or more 18.2% had 30" or more 45.5% had 10.0–19.9" 18.2% had less than 10" Extremes: Snowiest: Winter 2005-2006 with 40.0" Least snowy: Winter 1972-1973 with 2.8" If one were looking at a 40.0° winter, that would signal very bad prospects.

Yes, that's correct. I fixed it..

The preponderance of guidance had it either as a tropical depression or tropical storm. Intensity forecasts this far out aren't very accurate.

With respect to the EPO, the swing from the minimum value during the first half of September to the maximum value (so far) for the second half of September is the fourth largest on record. All of the prior 10 biggest such cases occurred during the evolution toward either La Niña or Neutral-Cool ENSO winters. The composite temperature anomalies for the Neutral-Cool ENSO Winters: The composite temperature anomalies for the La Niña Winters: However, warmth is not a slam dunk for the Northeast/Great Lakes. Winters 1964-65 and 1970-71 were cold in the Northeast, Great Lakes, and Northern Plains. Winters 2007-08 and 2020-2021 were cold in the Central Plains. Finally, the sample size is limited to 10 extreme EPO cases.

The early guidance on Invest 94 now shows a cluster of solutions indicating that the system could be approaching North Carolina in five days. Much can still change, as five day forecasts are subject to large errors. A near miss or out-to-sea solution remains viable.

No. The record is October 10, 2018.

2.0" fell on December 29, 1917.

I recall that event. There was sea smoke rising from the Long Island Sound even as the snow was falling. I doubled the size of an old photo from that storm and highlighted some of the sea smoke.

For Central Park: February 13, 1899: 16.0" snow (High: 11°; Low: 6°) For Newark: December 26, 1872: 16.0" snow (High: 11°; Low: 5°) For Central Park, 18.0" fell on December 26, 1872 with a high of 12° and low of 6°. Data for Measurable Snowfall: Coldest High for Measurable Snowfall: New York City: 8°, February 8, 1895 and December 29, 1917 Newark: 4°, December 29, 1917 Coldest Low for Measurable Snowfall: New York city: -6°, December 29, 1917 Newark: -6°, February 11, 1899 and January 14, 1912

Even as New York City sweltered in 97-degree heat on September 23, 1895, it was on the verge of a dramatic plunge in temperatures. The mercury still reached 90° on September 26, but a sharp cooldown followed. Just two days later, the high struggled to reach 68°, and by September 30, temperatures had dropped further, with a high of 60° and a low of 47°. The chill deepened as October began, with a low of 44° on the 1st, and the entire first week remained unseasonably cool. A reinforcing shot of cold air arrived on October 9, delivering a low of 38° and a daytime high of just 47°, followed by a low of 37° on October 10. The rest of the month continued a mainly cool pattern, with five more mornings dipping into the 30s, marking October 1895 as a notably chilly chapter in the city's weather history. With a monthly mean temperature of 52.4°, October 1895 was tied with October 1887 as the seventh coolest October on record. The last October that was at least as cool as October 1895 was October 1925.

It was. October 1895 was very cool.

Probably not. I am fairly sure that most or all of the NYC region have already seen their last 90s. In fact, there has been a big change in the ECMWF weeklies for the week of September 29-October 6 with the new run that just came out. 9/22 Run: 9/23 Run: Beyond that, all the weeks are warmer than the were on the preceding run.

Much of the region saw the mercury rise into the 80s today. The warmth will be trimmed in coming days. A weak cool front will trigger some scattered showers overnight. Afterward, the front will stall near the region tomorrow. Several waves of low pressure will move along the front bringing showers and rain Thursday from Saturday. A general 0.50"-1.50" rainfall is likely during this period. Some locations cuold see higher amounts in excess of 2.00". Above normal temperatures will continue through at least the coming weekend. The ENSO Region 1+2 anomaly was 0.0°C and the Region 3.4 anomaly was -0.4°C for the week centered around September 17. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.03°C and the ENSO Region 3.4 anomaly has averaged -0.38°C. La Niña conditions will likely develop during mid- or late-autumn. The SOI was +2.50 today. The preliminary Arctic Oscillation (AO) was +1.151 today. Based on sensitivity analysis applied to the latest guidance, there is an implied near 66% probability that New York City will have a warmer than normal September (1991-2020 normal). September will likely finish with a mean temperature near 69.6° (0.4° above normal). Supplemental Information: The projected mean would be 1.6° above the 1981-2010 normal monthly value.

Considering the magnitude of warmth that is likely for the second half of September, the odds are tilted toward a warmer than normal October. It has been a long time since a September 16-30 as warm as this year's will likely be, has been followed by a cool October, even against the 1991-2020 baseline. CFSv2 Forecast for October 2025: Current CPC Monthly Outlook: The most recent ECMWF weekly forecast showed the potential for a cool start to October but very warm conditions in central Canada. So, a warm outcome is not cast in stone, but the combination of the CPC outlook, CFSv2 forecast, and statistical odds suggest that a warmer outcome is more likely than a colder one. Nevertheless, the highest odds of a warmer than normal outcome are probably in the western half of the CONUS and Canada.

It will turn noticeably warmer tomorrow before a weak cool front crosses the region on Wednesday. Highs tomorrow will likely reach the lower 80s with some middle 80s in the warmer spots. The advancing front could trigger some showers or thundershowers. Additional rain is possible Thursday into Saturday as a series of low pressure systems move along the frontal boundary. The ENSO Region 1+2 anomaly was 0.0°C and the Region 3.4 anomaly was -0.4°C for the week centered around September 17. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.03°C and the ENSO Region 3.4 anomaly has averaged -0.38°C. La Niña conditions will likely develop during mid- or late-autumn. The SOI was -1.19 today. The preliminary Arctic Oscillation (AO) was +1.147 today. Based on sensitivity analysis applied to the latest guidance, there is an implied near 58% probability that New York City will have a warmer than normal September (1991-2020 normal). September will likely finish with a mean temperature near 69.4° (0.2° above normal). Supplemental Information: The projected mean would be 1.4° above the 1981-2010 normal monthly value.

For reference: Most Recent October Impacts:

Driest Year during the 1980s: National: 1988 Northeast: 1980 New York City: 1981 Driest September-August during the 1980s: National: 1987-1988 Northeast: 1984-1985 New York City: 1980-1981 Driest Summer during the 1980s: National: 1980 Northeast: 1983 New York City: 1981

No. The low at Mount Pocono was 48° as clouds moved in. Yes. For the Northeast it was the driest, but not for New York City. September 1, 1980-August 31, 1981 was the driest such period in New York City during the 1980s with 34.83" of rain.

With the rapid drop in temperatures last night, White Plains had its first mean temperature below 60° (59.5°) yesterday since June 2. The first such mean temperature last year occurred on September 24. The 1991-2020 normal first such mean temperature occurred on September 9. This morning, the temperature fell to 49° in White Plains. That was the second reading in the 40s this month. The last time there were at least two such days occurred in 2022 when there were four. The 1991-2020 average is 4.4 days.

Through the first three weeks of September, JFK Airport, LaGuardia Airport, and Central Park have all recorded a monthly high temperature of 86°. The last time all three sites had the same monthly high temperature was August 2024 when the peak temperature was 95°. The last time all three sites had the same highest temperature in September was 2011 when the highest temperature was also 86°. There is a single case where all three sites recorded identical highest and lowest monthly temperatures: November 1957 with a monthly high of 66° and a monthly low of 28°. Back to the present, temperatures will top out mainly in the lower and middle 70s tomorrow. It will then turn noticeably warmer on Tuesday before another cool front crosses the region on Wednesday. Highs on Tuesday will likely reach the lower 80s with some middle 80s in the warmer spots. The advancing front could trigger some showers or thundershowers. Additional rain is possible Thursday into Saturday as a series of low pressure systems move along the frontal boundary. The ENSO Region 1+2 anomaly was -0.2°C and the Region 3.4 anomaly was -0.5°C for the week centered around September 10. For the past six weeks, the ENSO Region 1+2 anomaly has averaged +0.10°C and the ENSO Region 3.4 anomaly has averaged -0.37°C. La Niña conditions will likely develop during mid- or late-autumn. The SOI was +0.12 today. The preliminary Arctic Oscillation (AO) was +0.667 today. Based on sensitivity analysis applied to the latest guidance, there is an implied near 51% probability that New York City will have a cooler than normal September (1991-2020 normal). September will likely finish with a mean temperature near 69.1° (0.1° below normal). Supplemental Information: The projected mean would be 1.1° above the 1981-2010 normal monthly value.