bluewave

My guess is that there is an issue with the long range POU data since there may have been location moves over the years. BGM has also seen a similar temperature rise in December as White Plains and NYC. Other stations like ISP and New Brunswick have seen a steady warming also.

Yeah, the Pacific Jet backed off in January 2022 keeping the main ridge axis anchored near the West Coast for much of the month. So this allowed a coastal storm track to take over with cold air in place ahead of the storms. It was the last 30”+ snowfall month on Long Island.

The +PNA hasn’t been the issue this month. The much faster Northern Stream of Pacific Jet than forecast has been a very big player. So we keep getting storms racing across the country from west to east too far to the north. While it was nice getting those two light snowfall events over the past week, we missed out on the heavier 6”+ events which were common during past +PNA La Niña Decembers. Those years we didn’t have such a dominant Northern Stream which allowed for better coastal snowstorm tracks with enough cold air in place. The older runs for the first week of January underestimated the Pacific Jet influence and now they are correcting stronger. So you would want to see a weaker week two forecast actually make it to under 168 hrs for NYC to have a shot at a 4”+ event. New run stronger Pacific Jet into the West Coast than originally forecast for next week with lower 500 mb heights there Old run

Yeah, models continue to underestimate the strength of the Pacific Jet out beyond day 6-10. That’s why 500 mb heights next week along the West Coast will be lower than originally forecast. Same fast northern stream dominant flow from west to east across the country. It has been long range model bias this December and during recent years since 18-19.

Temperature pattern very similar to 2022. Warm up followed by a -20 departure close to Christmas. Then another warm up as we approach New Years. So the timing of the snow and cold worked out for the more wintry feel around Christmas. Since -20° departures are such a rarity these days, it will be interesting to see if this was the lowest for the winter like back in 22-23. The 2-4-23 departure came close at -19 but couldn’t beat the -25 on 12-24. I hope everyone has a great holiday period coming up. 2024-12-17 59 49 54.0 15.4 11 0 0.02 0.0 0 2024-12-18 53 42 47.5 9.1 17 0 0.32 0.0 0 2024-12-19 45 37 41.0 2.9 24 0 0.05 0.0 0 2024-12-20 37 33 35.0 -2.9 30 0 0.04 T 0 2024-12-21 33 19 26.0 -11.6 39 0 0.15 1.8 2 2024-12-22 21 13 17.0 -20.4 48 0 0.00 0.0 1 2024-12-23 31 13 22.0 -15.1 43 0 0.00 0.0 1 2024-12-24 39 28 33.5 -3.4 31 0 0.10 1.0 2022-12-23 58 8 33.0 -4.1 32 0 1.83 T 0 2022-12-24 15 7 11.0 -25.9 54 0 0.00 0.0 0 2022-12-25 28 14 21.0 -15.7 44 0 0.00 0.0 0 2022-12-26 29 18 23.5 -12.9 41 0 0.00 0.0 0 2022-12-27 35 29 32.0 -4.2 33 0 0.00 0.0 0 2022-12-28 47 33 40.0 4.0 25 0 0.00 0.0 0 2022-12-29 51 40 45.5 9.7 19 0 0.00 0.0 0 2022-12-30 62 46 54.0 18.4 11 0 0.00 0.0 0 2022-12-31 55 50 52.5 17.1 12 0 0.28 0.0 December thaw pattern to close out the year

The cooling was probably delayed due to the record background SST warmth. While global SSTs have cooled some in the last several months, they are still as warm as December 2015 which was a super El Niño. So this La Niña is definitely a late bloomer. EQSOI was on par with stronger La Ninas back in November. Plus CP OLR was the strongest on record for any La Niña during the month of November.

Only the 11th time that NYC had an 1” or more of snow on Christmas Eve. Data for December 24 - NY CITY CENTRAL PARK, NY Click column heading to sort ascending, click again to sort descending. 1912-12-24 32 24 1.10 11.4 12 1883-12-24 31 7 0.80 7.3 M 1966-12-24 26 22 0.74 6.7 T 1884-12-24 24 18 0.27 3.5 M 1961-12-24 33 25 0.35 3.4 3 1919-12-24 40 26 0.22 2.7 1 1998-12-24 32 23 0.11 2.0 M 1880-12-24 32 23 0.14 2.0 M 1930-12-24 35 29 0.07 1.9 3 1980-12-24 37 20 0.10 1.0 1 2024-12-24 39 28 0.10 1.0 1

There is still snow left in Central Park.

We are still getting the La Niña temperature progression which usually features below average temperatures in December. But the Pacific Jet was faster than other +PNA La Niña Decembers. So NYC and Newark weren’t able to get the 6-14” totals that we got during those Decembers. My guess is that the December +PNA will finish as the strongest of winter. You can see the PNA going more neutral in early January.

It really started in 2018-2019. Probably a combination of things. Forcing near the Maritime Continent, +EAMT events, and SST gradient over the Western to Central Pacific.

It has caught up in the last few days. Took longer than usual following past El Niños due to the record global SST warmth. So this one is a bit of a late bloomer.

In terms of temperatures we were colder than December 2023 and 2021 and not that much colder than 2022. Even though snowfall has been better than December 2021 to 2023, the snowfall has been well below what past La Niña +PNA Decembers had. This has been due to the faster Pacific Jet causing the +PNA ridge to drift further east at times. Our past +PNA La Niña Decembers were also mostly colder than this year with 7”-14” at Newark. The ridge also stayed anchored further west allowing better December very snowy benchmark storm tracks. So far from the very low bar of the last few Decembers were are doing better which is good. But very strong Northern Stream of the Pacific Jet didn’t relax enough for the colder and snowier outcomes in the other +PNA Decembers in 2020, 2017, 2005,2000, and 1995.

The 16-17 La Niña was one of the more unique that we have seen. It was our first La Niña winter around NYC averaging near 40° with many stations going over 30” on the season. We had a great benchmark storm track even when there was record warmth in the 60s within a day or two of some of the best snowstorms. It was also a snowy December for our area here in Southern CT. Many stations finished that December with 6” to 9”. Monthly Data for December 2016 for Fairfield County, CT Click column heading to sort ascending, click again to sort descending. DANBURY COOP 9.2 BROOKFIELD 3.3 SSE CoCoRaHS 7.8 BETHEL 3.5 NNE CoCoRaHS 7.8 NEWTOWN 4.3 E CoCoRaHS 7.8 BETHEL 0.5 E CoCoRaHS 7.4 SHELTON 1.3 W CoCoRaHS 6.7 IGOR I SIKORSKY MEMORIAL AIRPORT WBAN 6.0 TRUMBULL COOP 6.0 Monthly Data for December 2016 for New Haven County, CT Click column heading to sort ascending, click again to sort descending. PROSPECT 1.5 NW CoCoRaHS 9.9 CHESHIRE VILLAGE 2.2 SE CoCoRaHS 6.9 PROSPECT 0.5 SW CoCoRaHS 6.7 SOUTHBURY 2.0 NNE CoCoRaHS 6.2 MOUNT CARMEL COOP 6.0 HAMDEN 3.0 WSW CoCoRaHS 5.9 MILFORD 1.8 E CoCoRaHS 5.7

The actual cutoff value is closer to 4” like Newark since they measure later in NYC after the snow has had time to melt or settle since the airports measure more frequently. The official NYC values have been too low compared to the airports since the 1990s. Many times the measurement at the park occurs hours after the snow has stopped yielding a lower value than at the airports that measure when the snow stops. The December La Nina cutoff for above average seasonal snowfall at Newark is closer to 4”. It’s probably a little higher than the 3” at NYC since they measure more frequently than NYC before the snow has a chance to melt or settle. So the 3” measurements are actually closer to 4” in NYC if they measured at the end of the snow instead of waiting up to 6 hours. The only time that a lower December snowfall total worked out at Newark last 30 years was the 16-17 La Nina. The reason this works out for our area is due to La Ninas being frontloaded by nature. So the above average La Niña snowfall winters have a snowy start. Newark December La Nina snowfall and seasonal totals 2024….2.5”……? 2022….0.1”…..2.7” 2021…..0.1”….17.9” 2020…11.9”….45.7” 2017….7.7”…..39.4” 2016….3.4”….30.0” 2011…..0……..8.8” 2010….24.5”….68.2” 2008….8.3”…..27.1” 2007…..3.9”….14.6” 2005….11.0”….37.9” 2000….14.9”….39.3” 1999……T……..18.4” 1998……1.2”…..12.8” 1995……12.8”….78.4”

The timing worked out very well this year for the coldest and snowiest part of the month close to Christmas before we warm back to the 50s to close out the year.

This is why we should probably think about adjusting the snowfall measurements higher prior to 1990 since they measured less frequently. So snowfall during the older era was underestimated compared to today. While it doesn’t take away from the 2010s, the older era storms would be heavier if measured the same way. https://news.ucar.edu/14009/snowfall-measurement-flaky-history Many pre-1990 numbers would be higher using current methods 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.

Probably a quick coating to 1” where the best snow bursts occur.

The December La Nina cutoff for above average seasonal snowfall at Newark is closer to 4”. It’s probably a little higher than the 3” at NYC since they measure more frequently than NYC before the snow has a chance to melt or settle. So the 3” measurements are actually closer to 4” in NYC if they measured at the end of the snow instead of waiting up to 6 hours. The only time that a lower December snowfall total worked out at Newark last 30 years was the 16-17 La Nina. The reason this works out for our area is due to La Ninas being frontloaded by nature. So the above average La Niña snowfall winters have a snowy start. Newark December La Nina snowfall and seasonal totals 2024….2.5”……? 2022….0.1”…..2.7” 2021…..0.1”….17.9” 2020…11.9”….45.7” 2017….7.7”…..39.4” 2016….3.4”….30.0” 2011…..0……..8.8” 2010….24.5”….68.2” 2008….8.3”…..27.1” 2007…..3.9”….14.6” 2005….11.0”….37.9” 2000….14.9”….39.3” 1999……T……..18.4” 1998……1.2”…..12.8” 1995……12.8”….78.4”

With the consistent undermeasurement of snowfall there since the NWS left in the early 90s, maybe the actual December cutoff for above or below seasonal snow during a La Niña is somewhere between the 3”-4” range. I have been using 3” since that is how the measurements have gone in the official records. So the actual level if there was correct measurement would probably be somewhere in the 3.0” to 4.0” range.

While the long range ensemble forecasts beyond a week can vary quite a bit, the model error has been very consistent. The December long range forecasts issued back on December 1st underestimated the strength of the Pacific Jet. This lead to the 500 mb heights verifying lower than forecast over the EPAC south of Alaska. This allowed the ridge to drift east at times instead of staying anchored out West. We have seen some version of this model error every winter in the Pacific since 15-16. So now if the models are showing lower heights over Western NA than there were from earlier runs, it’s possible we see some version of this pattern repeat heading into January. The other issue has been the tendency for long range -AO forecasts to link up with the Southeast Ridge at least transiently when lows are ejecting from the trough which has been so frequent in the West. Just hoping we can get enough overlap between the fading +PNA and the emerging -AO to at least get a shot at a 4” snow event for NYC. But there are no guarantees with the way the Pacific has been acting as the spoiler since 18-19.

The closest wall to wall winter for snow and cold in the East at under at under 1000 feet elevation would probably be Caribou, Maine in 76-77. Every day from 12-1-76 to 2-28-77 had at least 1” of snow on the ground. The highest temperature over that stretch was only 37°. With a total of just 6 days going above 32° for a high.

It would be nice to see snow on a SW flow which we haven’t had in a while.

Hopefully, NYC can get it done this winter so they don’t challenge 1932. Number of Consecutive Days Snowfall < 4 for NY CITY CENTRAL PARK, NY Click column heading to sort ascending, click again to sort descending. 1 1394 1932-12-16 2 1063 1952-01-27 3 1058 2024-12-22 4 1051 1963-12-22 5 794 1956-03-15

The trough showing up at the end of the runs is the same long range Pacific bias we have seen all winter. Heights over the EPAC and WNA have been verifying lower than forecast beyond 10 days. So this is nothing new since it’s the seasonal pattern so far. The one change we have seen in the models due the Hudson Bay warming event has been more blocking east of Hudson Bay. So at least we will get some blocking help from the Atlantic side in early January. The big question is how long does the early January window last? So far this winter is staring out on the La Niña temperature path of colder December 1 to estimated January 15th. Most of the time in this progression we see a warmer back half of winter on the La Niña progression path. Its also typical for mismatch December La Niñas with a +PNA to become more -PNA over time. My hope is that the Pacific can back off enough with some help from the Atlantic blocking to allow NYC to break the 4” snow losing streak in early to mid-January. I wouldn’t put it past the Pacific to spoil things again so there are no guarantees.

The global average temperatures are based on the older averages. This makes more sense for keeping track of long term trends. 30 year climate normals are great in a stable climate. But they mask the warming when they update every 30 years in such a rapidly warming climate. Now that the climate normals are so much warmer, the bar is lower to get a colder than average winter. So it’s possible we could eventually get a colder than average winter in the 91-20 normals, but it could still have been a warmer winter using 81-10 normals.