bluewave

The NYC UHI intensity hasn’t changed much since the 1980s on clear and calm nights when we get radiational cooling in the surrounding areas.

https://news.ucar.edu/14009/snowfall-measurement-flaky-history But when we turn to snowstorms in the Northeast, or elsewhere in the U.S., there is an additional factor at work when comparing modern numbers with historical ones. Quite simply, our measuring techniques have changed, and we are not necessarily comparing apples to apples. In fact, the apparent trend toward bigger snowfalls is at least partially the result of new—and more accurate—ways of measuring snowfall totals. Climate studies carefully select a subset of stations with consistent snow records, or avoid the snowfall variable altogether. Earlier in our weather history, the standard practice was to record snowfall amounts less frequently, such as every 12 or 24 hours, or even to take just one measurement of depth on the ground at the end of the storm. You might think that one or two measurements per day should add up to pretty much the same as measurements taken every 6 hours during the storm. It’s a logical assumption, but you would be mistaken. Snow on the ground gets compacted as additional snow falls. Therefore, multiple measurements during a storm typically result in a higher total than if snowfall is derived from just one or two measurements per day. That can make quite a significant difference. It turns out that it’s not uncommon for the snow on the ground at the end of a storm to be 15 to 20 percent less than the total that would be derived from multiple snowboard measurements. As the cooperative climate observer for Boulder, Colorado, I examined the 15 biggest snowfalls of the last two decades, all measured at the NOAA campus in Boulder. The sum of the snowboard measurements averaged 17 percent greater than the maximum depth on the ground at the end of the storm. For a 20-inch snowfall, that would be a boost of 3.4 inches—enough to dethrone many close rivals on the top-10 snowstorm list that were not necessarily lesser storms! Another common practice at the cooperative observing stations prior to 1950 did not involve measuring snow at all, but instead took the liquid derived from the snow and applied a 10:1 ratio (every inch of liquid equals ten inches of snow). This is no longer the official practice and has become increasingly less common since 1950. But it too introduces a potential low bias in historic snowfalls because in most parts of the country (and in the recent blizzard in the Northeast) one inch of liquid produces more than 10 inches of snow. This means that many of the storms from the 1980s or earlier would probably appear in the record as bigger storms if the observers had used the currently accepted methodology. Now, for those of you northeasterners with aching backs from shoveling, I am not saying that your recent storm wasn’t big in places like Boston, Portland, or Long Island. But I am saying that some of the past greats—the February Blizzard of 1978, the Knickerbocker storm of January 1922, and the great Blizzard of March 1888—are probably underestimated. So keep in mind when viewing those lists of snowy greats: the older ones are not directly comparable with those in recent decades. It’s not as bad as comparing apples to oranges, but it may be like comparing apples to crabapples.

The greater issue is the rapid subsurface warming across the Western Pacific mid-latitudes. This resembles more of a shift rather than a function of the old PDO cycles that we have become familiar with in the past. The new subsurface and H300 PDO index does a better job reflecting the magnitude of the shift. https://www.cpc.ncep.noaa.gov/products/GODAS/ocean_briefing_gif/global_ocean_monitoring_current.pdf

This has nothing to do with the SAI. The most recent 30 year data reflecting the warmer climate leaves no doubt as to the relationship. Back in the colder climate era the relationship was much weaker than it has become. Part of this is due to weather patterns becoming more repetitive in a warmer climate probably owing to local tropical SSTs resulting in non-linear convective temperature forcing thresholds being crossed. I understand that the most extreme warming has only occurred over the last 15 to 30 years. So we have a new emergent climate state that is different from the previous colder era. We don’t have the luxury anymore of a relatively stable global temperature regime as was the case from 1880 through 1982. So you had a much longer period where there was only a small increase in temperatures. It gave us a 100 years of correlations to work out and use. I understand that you may have some hesitancy in using the newer correlations derived over the shorter period. But I have been using numerous relationships from this new and warmer period that have been serving me well. But it’s not a 100 year data set to draw from like we had back in the older and colder climate era.

What you call an agenda is simply the recognition of how much warmer our climate has become. We have had over 50 top 10 warmest months since 2010 with only 1 top 10 coldest. So our colder periods like December 1 through December 16th this month have been few and far between. The link below shows just how much of a novelty this cold period has been compared to all the warm months in the 2020s so far.

Since 1995-1996 we have had 15 La Niña winters as defined by the RONI index. 14 out of 15 of those winters followed a repeating pattern which has been common the last 30 years. EWR, NYC, and LGA December snowfall pattern repeated throughout the entire winter. The Decembers with under 4” of snowfall at those stations went on to below average seasonal snowfall. With the Decembers over 4” or snow featuring average to naive average snowfall. You might ask how can this work out over 90% of the time? My guess is that La Ninas tend to show what they are capable of early on in the season. Plus as our climate has warmed it has lead to more repeating and persistent patterns. So I view this December to winter snowfall relationship more as a marker of a deeper underlying shared pattern rather than something that is directly causing the outcome. What is misleading about showing the long term snowfall trend in NYC since the regular observations began in the late 1800s? As the long term climate has warmed, the snowfall has gone down. There have been shorter term up trends like from the 80s to 10s against the long term decline. Plus snowfall measurements prior to the 1980s would be higher if measurements were taken as frequently as we do today.They also substituted melted down snow gauge measurements at times like in 1888 blizzard which undercounted the higher ratios back in the much colder era. So the actual downward trend line is steeper if we correct for the different way we measure snow now.

Exactly which part of the historic snowfall record do you disagree with?

In a warming climate we can get larger snowstorms which is what happened during 2010 to 2018. The main caveat was that we needed the dominant storm track to be colder to our southeast in order to realize the higher snowfall potential. With the northwest and warmer shift to the storm tracks since 2018-2019, we have resumed the long term downward trend in the snowfall setting all-time 7 year record lows for snowfall.

Over 90% success rate for 14 out of the last 15 La Ninas spanning 30 years.

That standing wave is why I have been pointing out the warmer risks to the EPS extended forecasts after December 16th. Notice how the long range EPS missed the forcing there back in November. This is has been a regular EPS forecast error beyond 10 days in recent years due to the record marine heatwaves near the Maritime Continent. New run shows current MJO 4-6 standing wave Old run missed the current standing wave

This is similar to what happened last year at this time with the models. Last December started with the extended guidance locking in the -EPO for the whole month. But as we approached the December 10th, the models caught onto how much stronger the Pacific Jet would be and the warmer +EPO pattern. Now the Pacific Jet is forecast to be much stronger mid-December than was indicated last week. Notice how much deeper the +EPO vortex is forecast to be now even with a near record -WPO 500 mb block. In the old days we would usually have the Pacific Jet weaken and we get more of a -WPO -EPO block. So we are on track for our annual warm up from December 17th to 25th. The +EPO is now forecast to becomes more dominant for our sensible weather than the -WPO as the Pacific Jet overpowers the pattern again. New run Old run

The most measurable snowfall in NYC in the post Boxing Day Blizzard era since 2011 has been between the December 11-20th. Time Series Summary for NY CITY CENTRAL PARK, NY 12-01 to 12-10 snowfall Click column heading to sort ascending, click again to sort descending. 2024-12-10 T 0 2023-12-10 T 0 2022-12-10 0.0 0 2021-12-10 T 0 2020-12-10 T 0 2019-12-10 1.6 0 2018-12-10 0.0 0 2017-12-10 4.6 0 2016-12-10 0.0 0 2015-12-10 0.0 0 2014-12-10 1.0 0 2013-12-10 2.1 0 2012-12-10 0.0 0 2011-12-10 0.0 0 Time Series Summary for NY CITY CENTRAL PARK, NY 12-11 to 12-20 snowfall Click column heading to sort ascending, click again to sort descending. 2024-12-20 T 0 2023-12-20 0.0 0 2022-12-20 T 0 2021-12-20 0.0 0 2020-12-20 10.5 0 2019-12-20 0.9 0 2018-12-20 T 0 2017-12-20 2.4 0 2016-12-20 3.2 0 2015-12-20 0.0 0 2014-12-20 T 0 2013-12-20 6.5 0 2012-12-20 0.0 0 2011-12-20 0.0 0 Time Series Summary for NY CITY CENTRAL PARK, NY 12-21 to 12-31 snowfall Click column heading to sort ascending, click again to sort descending. 2024-12-31 2.8 0 2023-12-31 0.0 0 2022-12-31 T 0 2021-12-31 0.2 0 2020-12-31 0.0 0 2019-12-31 T 0 2018-12-31 T 0 2017-12-31 0.7 0 2016-12-31 T 0 2015-12-31 T 0 2014-12-31 T 0 2013-12-31 T 0 2012-12-31 0.4 0 2011-12-31 0.0 0

Hopefully, we can at least put a few inches on the ground while we still have the cold this weekend.

Models have a situational bias based on the specific weather regime. Long range models tend to underestimate the warmth in warmer patterns. None of the long range models saw the warmth in December 2015 from November forecasts. Same for the too numerous to count warmer periods since then. They also underestimate the cold from long range forecasts at times in the fewer colder patterns we get. As the recent November forecasts missed the first half of December cold. This was also the case before the transition to colder in 2014-2015. It’s just that the climate has become so skewed toward warmth, as Guy shows below, that there is much more opportunity to underestimate warmth than cold since it occurs so much more frequently.

They also got down to -22° on the 5th. It was there 2nd earliest behind 2019 for going under -20°. That station was only there since 1998. First <-20° Summary for SARANAC LAKE ADIRONDACK REGIONAL AP, NY Each section contains date and year of occurrence, value on that date. Click column heading to sort ascending, click again to sort descending. 2018 03-18 (2018) -22 11-23 (2018) -21 249 2025 03-03 (2025) -24 12-05 (2025) -22 276

First time since 2006 and 2008 that NYC dropped below 20° this early in the season. Central Park CLEAR 19 -1 41 CALM First low < 20° Summary for NY CITY CENTRAL PARK, NY Each section contains date and year of occurrence, value on that date. Click column heading to sort ascending, click again to sort descending. 2025 02-20 (2025) 18 12-09(2025) 19 - 2024 01-20 (2024) 18 12-21 (2024) 19 335 2023 02-04 (2023) 3 01-17 (2024) 17 346 2022 02-15 (2022) 16 12-23 (2022) 8 310 2021 02-08 (2021) 17 01-04 (2022) 19 329 2020 02-15 (2020) 14 01-29 (2021) 14 348 2019 03-07 (2019) 18 12-19 (2019) 16 286 2018 02-03 (2018) 16 11-22 (2018) 17 291 2017 03-12 (2017) 19 12-27 (2017) 17 289 2016 02-15 (2016) 13 12-15 (2016) 19 303 2015 03-07 (2015) 18 01-04 (2016) 14 302 2014 03-13 (2014) 18 01-06 (2015) 19 298 2013 02-18 (2013) 17 12-25 (2013) 19 309 2012 01-22 (2012) 18 01-22 (2013) 13 365 2011 02-22 (2011) 16 01-03 (2012) 15 314 2010 02-07 (2010) 17 12-14 (2010) 19 309 2009 03-04 (2009) 18 12-18 (2009) 16 288 2008 02-29 (2008) 17 12-08 (2008) 19 282 2007 03-09 (2007) 13 01-02 (2008) 17 298 2006 02-27 (2006) 18 12-08 (2006) 18 283

In great winter storm patterns over the years for our area, it wasn’t unusual for at least one of the long range model runs to correctly see the storm potential. During late January and early February 2021, it was an OP GEM run that had a correct KU forecast from a week out. With the poor storm track patterns for big snows since February 2022, none of those OP runs showing numerous KUs long range have verified.

I suspect that the issues the models have been having long range are related to the much faster Northern Stream of the Pacifc Jet which continues. It’s not typical to get a strong -WPO +EPO couplet at the the same time in December. So the shortwaves in the fast Pacific flow are lowering heights over Eastern Alaska while the -WPO ridge stands firm. At least into mid December the +EPO and stronger Pacific are trying to have the ridge roll out into the Plains while the -WPO would have the ridge axis more in the West. So a tug of war between two competing influences that we normally don’t see at the same time. My guess is that the ridge eventually comes east at some point during the 2nd half of December as the Pacific Jet eventually wins out in these situations. Notice how the 10 year December strong -WPO composite features a more -EPO and a relaxed Pacific Jet. Much stronger Pacific Jet and unusual +EPO in mid-December for a very strong -WPO block Past 10 strongest December 500 mb -WPO blocks didn’t have to compete with such a strong Pacific Jet and +EPO

Models usually don’t get exact timing for the relaxation of a pattern which has persisted for several weeks. This is why I have been using the general recent climatology in December. Every year since 2011 from Philly to NYC there was at least 1 day reaching 55 or greater from the 17th to the 25th. That period is still 9-17 days out which is beyond the reliable model skill. It doesn’t mean every day or even most days were this warm since 2011. Some years it was only a 1 day warm up with other years more than a day reaching these temperatures. But the general December patterns over the years which have had a cold December 1-16th had moderated in the departures from the 17th through the 31st. The degree of this moderation has also been variable.

Miss the 2010-2018 KU benchmark era when we could lock in a 168 hr euro control run showing a near 950 mb low close to the benchmark and have it be correct. https://www.weather.gov/okx/Blizzard_Jan42018

Occasionally, we do get a colder month across the CONUS. This is the first several weeks of colder conditions since January. But these colder periods have been few and far between. So they tend to stand out when they occur and get more attention than the persistent warmth which has become the new normal. I will update the chart below when December data gets posted.

30 years since the mid 90s and over 100 years prior to that period isn’t a small sample size. What you describe as rigid is actually a very comprehensive approach which I have used with great success for long range outlooks. Since my approach is data driven, I haven’t come to any firm ideas yet on whether we can eventually shift back to at least some occasional benchmark tracks in the future. But the long term decline in snowfall has been unmistakable. So I leave open the idea of a bounce off these lows in coming years if we can weaken the Pacific Jet a bit. But this is very uncertain since the recent shifts have been so extreme.

We haven’t had a single average or above average snowfall season since the mid 90s without having at least one NESIS 1 or higher BM KU snowstorm. So KU benchmark storms have been mandatory for reaching average to above average snow since 93-94. Prior to the mid 90s this wasn’t the case in the much colder climate. We could string together a series of smaller snow events and reach into the 20s for snowfall.

Fits the Great Lakes cutter, I-78 to I-84 hugger and Rain-snow gradient, and suppressed Southern stream which has dominated since 2018-2019. Very difficult to achieve a true benchmark KU track with such a fast Pacific Jet. Way too many issues with kicker lows, amplified ridges pushing the gradient just north of NYC, and generally very poor wavelength spacing with so many shortwaves racing through the fast flow.

Ski resorts continue to be the big winner the overpowering Northern Stream of the Pacific Jet.