bluewave

The UHI in Manhattan has been in effect since the 1890s and Queens since the 1930s to 1950s. But it’s just more obvious when we have calm winds. Plenty of strong high pressure this month for radiational cooling.

A place like ISP made up enough rain to finish the year over +2.00” since they were so wet up until the end of August. But Philly is still down around -5.00 due to the drier year there before the fall and less rain since then . So some of the NJ reservoirs are still low. THE ISLIP NY CLIMATE SUMMARY FOR DECEMBER 25 2024... CLIMATE NORMAL PERIOD 1991 TO 2020 CLIMATE RECORD PERIOD 1963 TO 2024 WEATHER ITEM OBSERVED TIME RECORD YEAR NORMAL DEPARTURE LAST VALUE (LST) VALUE VALUE FROM YEAR NORMAL ................................................................... TEMPERATURE (F) YESTERDAY MAXIMUM 37 213 PM 65 2015 42 -5 53 MINIMUM 24 623 AM 3 1983 28 -4 37 AVERAGE 31 35 -4 45 PRECIPITATION (IN) YESTERDAY 0.00 1.29 2002 0.15 -0.15 0.00 MONTH TO DATE 4.62 3.87 0.75 5.47 SINCE DEC 1 4.62 3.87 0.75 5.47 SINCE JAN 1 47.19 45.15 2.04 47.10 PHILADELPHIA PA CLIMATE SUMMARY FOR DECEMBER 25 2024... CLIMATE NORMAL PERIOD: 1991 TO 2020 CLIMATE RECORD PERIOD: 1872 TO 2024 WEATHER ITEM OBSERVED TIME RECORD YEAR NORMAL DEPARTURE LAST VALUE (LST) VALUE VALUE FROM YEAR NORMAL ................................................................... TEMPERATURE (F) YESTERDAY MAXIMUM 40 3:16 PM 68 1964 44 -4 53 2015 MINIMUM 31 11:06 PM 1 1980 29 2 39 1983 AVERAGE 36 36 0 46 PRECIPITATION (IN) YESTERDAY 0.00 1.11 1945 0.12 -0.12 0.00 MONTH TO DATE 2.28 3.30 -1.02 6.23 SINCE DEC 1 2.28 3.30 -1.02 6.23 SINCE JAN 1 38.23 43.44 -5.21 40.50

Many spots will be approaching 10.00 or rain since November 20th which is a nice change from the drought last fall. Data for November 20, 2024 through December 26, 2024 Click column heading to sort ascending, click again to sort descending. CT PAWCATUCK 0.8 SE CoCoRaHS 9.39 CT COLCHESTER 0.6 ENE CoCoRaHS 9.16 NY MOUNT SINAI COOP 9.06 CT STONINGTON 1.4 NNW CoCoRaHS 9.02 NY NESCONSET 1.4 SSW CoCoRaHS 9.01 CT SALEM 3.6 SE CoCoRaHS 9.01 CT PAWCATUCK 1.8 SSE CoCoRaHS 8.94 CT OAKDALE 2.6 WNW CoCoRaHS 8.93 CT MYSTIC 1.6 W CoCoRaHS 8.76 NY ST. JAMES COOP 8.71 NY CENTER MORICHES 0.5 N CoCoRaHS 8.64 NY MANORVILLE 1.8 SSE CoCoRaHS 8.63 NY BAITING HOLLOW COOP 8.57 NY CENTERPORT 1.1 SE CoCoRaHS 8.54 CT EAST LYME 0.6 N CoCoRaHS 8.53 NY RIDGE 1.8 SE CoCoRaHS 8.51 CT NEW CANAAN 1.9 ENE CoCoRaHS 8.44 NY SELDEN 1.6 ESE CoCoRaHS 8.42 NY PORT JEFFERSON STATION 0.3 SSW CoCoRaHS 8.39 NY CENTERPORT COOP 8.37 CT CHESTER CENTER 2.7 WNW CoCoRaHS 8.37 CT NORWICH 5.4 SE CoCoRaHS 8.35 CT HIGGANUM 0.8 NE CoCoRaHS 8.31 CT HIGGANUM 0.7 N CoCoRaHS 8.29 CT STONINGTON 0.5 NNE CoCoRaHS 8.27 CT GUILFORD COOP 8.24 NY UPTON COOP - NWSFO NEW YORK COOP 8.23 NY FISHERS ISLAND 0.5 NE CoCoRaHS 8.23 NY CENTEREACH 1.3 NE CoCoRaHS 8.21 CT EAST LYME 0.5 SW CoCoRaHS 8.17 NY RIVERHEAD RESEARCH FARM COOP 8.13 NY ISLIP TERRACE 1.1 NNE CoCoRaHS 8.09 CT MOODUS 0.7 SSW CoCoRaHS 8.07 CT NORWICH 5.2 SE CoCoRaHS 8.03 CT NEW LONDON 1.0 NNW CoCoRaHS 8.01

The only thing that matters if you want to see a NYC 4”+ snow event is slowing the Pacific flow long enough for the ridge to stay anchored along the West Coast. That along with the increased AO blocking could allow for a coastal storm track with cold in place. But the period in question is still out closer to the 2nd week of January and requires it showing up in the shorter range in order to be believable.

Looks like the coming warm up will be a rainy one to close out the year.

We haven’t been getting the usual MJO composites this winter due to the competing influences like the +AAM. It’s possible the WPAC forcing has been driving the much faster Pacific Jet and the +AAM rise the +PNA. So we get the faster Pacific Jet pushing the +PNA ridge axis too far east. It’s not really a pattern combo we have seen before. Since other big +PNA Decembers featured a weakened Pacific Jet and ridge axis anchored along the West Coast allowing big 6”+ snowstorms near NYC. So it looks like more of the same into the first week of January. Beyond the first week we’ll have to monitor if the Jet can weaken enough to allow the ridge to hold near the West Coast. That would be the best chance to try and shift the storm track to more of a coastal track allowing 4” events in NYC. The question then becomes how long can we hold that pattern if we get it before the usual seasonal La Niña more -PNA pattern emerges during the mid to late winter. And if the recent +AAM can back against that La Niña seasonal tendency.

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.