donsutherland1

Colder weather will return tonight into tomorrow. Tomorrow will see increasing clouds. A cold rain will overspread the Philadelphia, Newark, and New York City. Some sleet or wet snow is possible at the onset. North and west of the cities, there could be some accumulations. Poughkeepsie and Hartford could see 2"-4" snow. Boston could pick up 1"-2". Afterward, a generally mild regime will likely prevail. The likely continuation of Arctic blocking through at least the first 10 days of January would typically provide a higher than climatological probability for above normal snowfall. Ideally, an expansive block would be anchored across the Baffin Bay or Greenland, not south of the Davis Strait. Blocks that are anchored near Newfoundland and Labrador can inhibit opportunities for snowfall and result in warmer to much warmer than normal conditions in the Middle Atlantic and southern New England regions. This is now the most likely scenario for much of the first half of January. Under this scenario, the first 10-15 days of January will likely be much warmer than normal across the northeastern United States, Quebec, and much of eastern Canada. After January 10, the EPO continues to show a possible transition that could lead to a trough's moving into the East. Arctic air will likely shift mainly to Eurasia. However, the evolving pattern could be sufficiently cold to afford opportunities for snowfall if the pattern evolves as currently shown on the EPS. The ENSO Region 1+2 anomaly was -1.1°C and the Region 3.4 anomaly was -0.9°C for the week centered around December 23. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.70°C and the ENSO Region 3.4 anomaly has averaged -1.15°C. La Niña conditions will likely prevail at least through the winter. The SOI was +18.05 today. Today, the preliminary Arctic Oscillation (AO) figure was -2.910. On January 1 the MJO was in Phase 3 at an amplitude of 0.640 (RMM). The December 31-adjusted amplitude was 0.568. A significant stratospheric warming event is now in its early stages. The highest levels of the stratosphere will continue to warm through the first half of the first week of January. The mean zonal winds will likely reverse at 1 mb, 10 mb, and 30 mb making this a major warming event. Toward the end of the first week in January, the top layers of the stratosphere will likely begin to cool. The significant December 16-17 snowstorm during what has been a blocky December suggests that seasonal snowfall prospects have increased especially from north of Philadelphia into southern New England. At New York City, there is a high probability based on historic cases that an additional 20" or more snow will accumulate after December. Were blocking to disappear, snowfall prospects would diminish. For now, blocking appears likely to continue through at least the first half of January.

I added the NAO, along with ISP.

HPN should get some accumulation. Probably not 4”, though. 80% of EPS members show 1” or more.

The AO/PNA data for the most recent storm (December 16-17) will be added once the final numbers are available in a few days.

Morning thoughts... Today will be partly sunny and mild. Temperatures will likely reach the lower 50s in much of the region. Likely high temperatures around the region include: New York City (Central Park): 53° Newark: 53° Philadelphia: 54° Clouds will increase tomorrow as another storm approaches the region. Some accumulating snow is a possibility to the north and west of New York City and Newark. From Philadelphia to New York City, mainly or all rain is likely. Boston could see some accumulation of snow.

@LibertyBell, I have created a quick table for the 6" or greater snowstorms for the NYC area locations. JFK had no snow data until 1959. The table can be found at: https://public.tableau.com/profile/don.sutherland#!/vizhome/NYCArea6Snowstorms/Dashboard1

An area of sleet changing to rain moved into the big cities of the northern Middle Atlantic region this afternoon. Overnight, periods of rain are likely and the temperature will likely hold steady and then begin to rise. Tomorrow will become partly sunny and much milder. Readings will likely rise into the 50s in much of the region. The likely continuation of Arctic blocking through at least the first 10 days of January would typically provide a higher than climatological probability for above normal snowfall. Ideally, an expansive block would be anchored across the Baffin Bay or Greenland, not south of the Davis Strait. Blocks that are anchored near Newfoundland and Labrador can inhibit opportunities for snowfall and result in warmer to much warmer than normal conditions in the Middle Atlantic and southern New England regions. This is now the most likely scenario for much of the first half of January. Under this scenario, the first 10-15 days of January will likely be much warmer than normal across the northeastern United States, Quebec, and much of eastern Canada. After January 10, the EPO continues to show a possible transition that could lead to a trough's moving into the East. The ENSO Region 1+2 anomaly was -1.1°C and the Region 3.4 anomaly was -0.9°C for the week centered around December 23. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.70°C and the ENSO Region 3.4 anomaly has averaged -1.15°C. La Niña conditions will likely prevail at least through the winter. The SOI was +15.27 today. Today, the preliminary Arctic Oscillation (AO) figure was -2.950. On December 31 the MJO was in Phase 3 at an amplitude of 0.566 (RMM). The December 30-adjusted amplitude was 0.460. A significant stratospheric warming event is now in its early stages. The highest levels of the stratosphere will continue to warm through the first half of the first week of January. The mean zonal winds will likely reverse at 1 mb, 10 mb, and 30 mb making this a major warming event. Toward the end of the first week in January, the top layers of the stratosphere could begin to cool. The significant December 16-17 snowstorm during what has been a blocky December suggests that seasonal snowfall prospects have increased especially from north of Philadelphia into southern New England. At New York City, there is a high probability based on historic cases that an additional 20" or more snow will accumulate after December. Were blocking to disappear, snowfall prospects would diminish. For now, blocking appears likely to continue through at least the first half of January. In large account due to a very warm start to the month, January will likely feature above to much above normal temperatures across the northern Middle Atlantic and southern New England regions.

Sleet has commenced falling in Larchmont, NY.

I will have to work on that. It will require some time.

Morning thoughts... Clouds will increase this morning. The line of precipitation currently stretching from central Virginia northwestward into Indiana will reach the region this afternoon or evening. Some mixed or frozen precipitation is possible far north and west of New York City. Elsewhere, periods of rain are likely. Temperatures will likely reach the lower 40s across much of the region. Afterward, temperatures could hold steady or slowly rise overnight. Likely high temperatures include: New York City (Central Park): 43° Newark: 44° Philadelphia: 44° The first 10 days of January will likely feature much above normal temperatures. Afterward, a somewhat colder and potentially snowier pattern could evolve. 30-Day Verification: New York City (Central Park): Average daily forecast: 43.4° Average temperature: 43.8° Average error: 1.3° Newark: Average daily forecast: 44.6° Average temperature: 44.5° Average error: 1.2° Philadelphia: Average daily forecast: 45.1° Average temperature: 45.2° Average error: 1.3°

December is ending with a monthly mean temperature of 39.2° (1.7° above normal). The warmer than normal December was consistent with historic experience following an exceptionally warm November. Nevertheless, parts of the region are currently above normal in terms of seasonal snowfall. Seasonal snowfall amounts to date include: Albany: 27.4" (10.6" above normal) Baltimore: 1.6" (1.8" below normal) Binghamton: 51.6" (27.0" above normal) Boston: 17.3" (10.0" above normal) Bridgeport: 10.0" (4.2" above normal) Buffalo: 33.8" (2.4" below normal) Burlington: 10.9" (12.4" below normal) Caribou: 34.4" (0.7" below normal) Harrisburg: 11.7" (6.0" above normal) Islip: 7.5" (1.6" above normal) New York City: 10.5" (5.4" above normal) Newark: 11.9" (6.1" above normal) Philadelphia: 6.6" (2.9" above normal) Portland: 20.1" (5.0" above normal) Providence: 10.0" (0.2" below normal) Washington, DC: Trace (2.8" below normal) The preliminary AO average for December was -1.673. The AO was negative on 87% of days, at or below -1.000 on 77% of days, and at or below -2.000 on 35% of days. The likely continuation of Arctic blocking through at least the first 10 days of January would typically provide a higher than climatological probability for above normal snowfall. Ideally, an expansive block would be anchored across the Baffin Bay or Greenland, not south of the Davis Strait. Blocks that are anchored near Newfoundland and Labrador can inhibit opportunities for snowfall and result in warmer to much warmer than normal conditions in the Middle Atlantic and southern New England regions. This is now the most likely scenario for much of the first half of January. Under this scenario, the first 10-15 days of January will likely be much warmer than normal across the northeastern United States, Quebec, and much of eastern Canada. After January 10, there is some indication on the EPS that a transition that could lead to a trough's moving into the East could get underway. The ENSO Region 1+2 anomaly was -1.1°C and the Region 3.4 anomaly was -0.9°C for the week centered around December 23. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.70°C and the ENSO Region 3.4 anomaly has averaged -1.15°C. La Niña conditions will likely prevail at least through the winter. The SOI was +32.12 today. Today, the preliminary Arctic Oscillation (AO) figure was -2.984. On December 30 the MJO was in Phase 3 at an amplitude of 0.457 (RMM). The December 29-adjusted amplitude was 0.356. A significant stratospheric warming event is now in its early stages. The highest levels of the stratosphere will continue to warm through the first half of the first week of January. The mean zonal winds will likely reverse at 1 mb, 10 mb, and 30 mb making this a major warming event. Toward the end of the first week in January, the top layers of the stratosphere could begin to cool. The significant December 16-17 snowstorm during what has been a blocky December suggests that seasonal snowfall prospects have increased especially from north of Philadelphia into southern New England. At New York City, there is a high probability based on historic cases that an additional 20" or more snow will accumulate after December. Were blocking to disappear, snowfall prospects would diminish. For now, blocking appears likely to continue through at least the first half of January. In large account due to a very warm start to the month, January will likely feature above to much above normal temperatures across the northern Middle Atlantic and southern New England regions.

In coming days, a positive to strongly positive EPO will coincide with a negative to strongly negative AO. Since 1950, there were 13 cases where the EPO was +1.000 or above while the AO was -1.500 or below for at least three consecutive days during the January 1-20 period. In 3 cases, the Arctic block extended southward into the Middle Atlantic region (January 1-4, 1955; January 15-17, 1998; and, January 1-3, 2003). The composite pattern is one that favors above to much above normal temperatures and little or no snowfall through at least the first 10 days of January in much or all of the Middle Atlantic Region and southern New England. Central and Upstate New York, along with central and northern New England would have greater opportunities for snowfall. Based on the latest guidance, there is a growing possibility that New York City’s Central Park could have a mean temperature of 40° or above during the January 1-10 period. There have been 20 prior cases since record-keeping began in 1869. It still appears possible that the January 10-15 period could mark a transition to a potentially snowier pattern. The key will be the continuation of Arctic blocking. Following such warm starts to January, 60% of the above cases saw less than 2” snow during the second half of January. However, in the 30% of cases with 4” or more snow during the second half of January, the AO was predominantly negative. Half of those cases saw 10” or more snow during the second half of January (1907, 1966, and 2005). Although a continuation of Arctic blocking cannot ensure a snowy second half of January—1998 is one such exception, largely on account of the super El Niño—the general absence of such blocking has seen little or no snowfall during the January 16-31 period following such warm starts to January. Things to watch for going forward if a snowy second half of January is likely: Continuation or reinforcement of the persistent AO- regime Development of a trough underneath the block that extends into the Middle Atlantic region The coldest air in the hemisphere will likely be focused on Europe on account of what increasingly looks like a polar vortex-splitting major stratospheric warming event. Severe cold’s migration to Europe does not mean that snowfall will be lacking. In the meantime, inspired by the work of Hooralph, here’s a sortable table of New York City’s 10” or greater snowstorms (1950-present): https://public.tableau.com/profile/don.sutherland#!/vizhome/NYC10Snowstorms1950-Present/Dashboard1

The new normals are implemented simultaneously worldwide. The WMO is scheduled to meet on the new normals on May 25, 2021. The new normals should then be adopted and implemented, probably starting in June. January-May anomalies might then be retroactively adjusted to fit the 1991-2020 base period.

DCA _ NYC _ BOS __ ORD _ ATL _ IAH ___ DEN _ PHX _ SEA 4.1 4.4 4.6 5.0 2.5 1.8 1.8 0.7 3.5

Morning thoughts... Rain will end from west to east from late this morning to mid-afternoon. Temperatures will likely hold nearly steady in the middle 40s across most of the region. Likely high temperatures around the region include: New York City (Central Park): 45° Newark: 46° Philadelphia: 46° Clouds will increase tomorrow and rain could arrive.

Tomorrow will be mostly cloudy and milder. Some rain showers or a period of rain is likely, especially during the morning hours. Well north and west of Newark and New York City, there could be some mixed or frozen precipitation. The likely continuation of Arctic blocking through at least the first 10 days of January would typically provide a higher than climatological probability for above normal snowfall. Ideally, an expansive block would be anchored across the Baffin Bay or Greenland, not south of the Davis Strait. Blocks that are anchored near Newfoundland and Labrador can inhibit opportunities for snowfall and result in warmer to much warmer than normal conditions in the Middle Atlantic and southern New England regions. This is now the most likely scenario for much of the first half of January. Under this scenario, the first 10-15 days of January will likely be much warmer than normal across the northeastern United States, Quebec, and much of eastern Canada. After January 10, there is some indication on the EPS that a transition that could lead to a trough's moving into the East could get underway. The ENSO Region 1+2 anomaly was -1.1°C and the Region 3.4 anomaly was -0.9°C for the week centered around December 23. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.70°C and the ENSO Region 3.4 anomaly has averaged -1.15°C. La Niña conditions will likely prevail at least through the winter. The SOI was +25.01 today. Today, the preliminary Arctic Oscillation (AO) figure was -2.446. On December 29 the MJO was in Phase 4 at an amplitude of 0.355 (RMM). The December 28-adjusted amplitude was 0.163. A significant stratospheric warming event is now in its early stages. The highest levels of the stratosphere will continue to warm through the first half of the first week of January. The mean zonal winds will likely reverse at 1 mb, 10 mb, and 30 mb making this a major warming event. Toward the end of the first week in January, the top layers of the stratosphere could begin to cool. The significant December 16-17 snowstorm during what has been a blocky December suggests that seasonal snowfall prospects have increased especially from north of Philadelphia into southern New England. At New York City, there is a high probability based on historic cases that an additional 20" or more snow will accumulate after December. Were blocking to disappear, snowfall prospects would diminish. For now, blocking appears likely to continue through the first half of January. Based on sensitivity analysis applied to the latest guidance, there is an implied near 100% probability that New York City will have a warmer than normal December. December will likely finish with a mean temperature near 39.2°.

For the first time, the EPS is now forecasting the zonal mean wind to reverse at 1 mb, 10 mb, and 30 mb. Previously, the reversal was not forecast at 30 mb. If this forecast verifies, this will be a major stratospheric warming event. It will likely have an important impact on the second half of winter, but details as to whether that impact is focused on North America or Eurasia or both remain uncertain. More than likely, this warming event was the result of the strong blocking that had developed and persisted through much of December. It will likely reinforce the tendency for blocking as the polar vortex weakens dramatically. How the blocking evolves and where the block sets up will be crucial.

Morning thoughts... Today will be mostly cloudy and still cool. Temperatures will likely top out in the upper 30s and lower 40s across most of the region. Likely high temperatures around the region include: New York City (Central Park): 43° Newark: 43° Philadelphia: 43° Tomorrow will become milder with periods of rain.

Probably. We’ve seen that in parts of the eastern U.S. following La Niña winters.

Yes, that is correct.

After parts of the region experienced widely scattered snow showers this morning, today was fair and cold. Tomorrow will be mainly cloudy and perhaps a bit milder. The likely continuation of Arctic blocking through at least the first 10 days of January would typically provide a higher than climatological probability for above normal snowfall. Ideally, an expansive block would be anchored across the Baffin Bay or Greenland, not south of the Davis Strait. Blocks that are anchored near Newfoundland and Labrador can inhibit opportunities for snowfall and result in warmer to much warmer than normal conditions in the Middle Atlantic and southern New England regions. This is now the most likely scenario for much of the first half of January. Under this scenario, the first 10-15 days of January will likely be much warmer than normal across the northeastern United States, Quebec, and much of eastern Canada. After January 10, there is some indication on the EPS that a transition that could lead to a trough's moving into the East could get underway. The ENSO Region 1+2 anomaly was -1.1°C and the Region 3.4 anomaly was -0.9°C for the week centered around December 23. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.70°C and the ENSO Region 3.4 anomaly has averaged -1.15°C. La Niña conditions will likely prevail at least through the winter. The SOI was +20.86 today. Today, the preliminary Arctic Oscillation (AO) figure was -2.757. On December 28 the MJO was in Phase 4 at an amplitude of 0.160 (RMM). The December 27-adjusted amplitude was 0.061. Based on the latest guidance, a significant stratospheric warming event near or just after the start of January is likely. The highest levels of the stratosphere will warm during the closing days of December. The warming will begin to descend during the opening week of January. In addition, the mean zonal winds at 1 mb and 10 mb will likely reverse while the mean zonal wind at 30 mb will likely slacken considerably. Toward the end of the first week in January, the top layers of the stratosphere could begin to cool. The significant December 16-17 snowstorm during what has been a blocky December suggests that seasonal snowfall prospects have increased especially from north of Philadelphia into southern New England. At New York City, there is a high probability based on historic cases that an additional 20" or more snow will accumulate after December. Were blocking to disappear, snowfall prospects would diminish. For now, blocking appears likely to continue through the first half of January. Based on sensitivity analysis applied to the latest guidance, there is an implied near 100% probability that New York City will have a warmer than normal December. December will likely finish with a mean temperature near 39.1°.

There are some fluctuations, but the long-term trend is warmer on account of the rising greenhouse gas forcing. 2021 might be somewhat cooler globally than 2020 on account of the La Niña, but it will very likely still rank among the warmest years on record.

Morning thoughts... At 7:30 am, a narrow band of snow extended from Lake Ontario east-southeastward just past the Massachusetts border to North Egremont. A few snow showers were also moving southeastward across parts of central New Jersey. Most of the region will not be impacted by the snowfall. Today will be partly cloudy and cool. Temperatures will likely top out in the upper 30s and lower 40s across most of the region. Likely high temperatures around the region include: New York City (Central Park): 39° Newark: 40° Philadelphia: 39° Tomorrow will become mainly cloudy with similar readings.

Following a frontal passage, the sun returned and readings rose into the upper 40s and lower 50s. Tomorrow will be fair but colder with highs mainly in the upper 30s and lower 40s. As of December 28, the preliminary monthly Arctic Oscillation (AO) average has been -1.560. 75% of days have seen the AO at or below -1.000. This is a dramatic change from Winter 2019-20 when Arctic blocking was virtually non-existent. The last comparable December was December 2012 when the December 1-28 average AO was -1.786 and 79% of days saw the AO at or below -1.000. However, that December was even warmer, on account of a mainly positive EPO and negative PNA. The likely continuation of Arctic blocking through at least the first 10 days of January would typically provide a higher than climatological probability for above normal snowfall. Ideally, an expansive block would be anchored across the Baffin Bay or Greenland, not south of the Davis Strait. Blocks that are anchored near Newfoundland and Labrador can inhibit opportunities for snowfall and result in warmer to much warmer than normal conditions in the Middle Atlantic and southern New England regions. This is now the most likely scenario for much of the first half of January. Under this scenario, the first 10-15 days of January will likely be much warmer than normal across the northeastern United States, Quebec, and much of eastern Canada. After January 10, there is some indication on the EPS that a transition that could lead to a trough's moving into the East could get underway. The ENSO Region 1+2 anomaly was -1.1°C and the Region 3.4 anomaly was -0.9°C for the week centered around December 23. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.70°C and the ENSO Region 3.4 anomaly has averaged -1.15°C. La Niña conditions will likely prevail at least through the winter. The SOI was +19.15 today. Today, the preliminary Arctic Oscillation (AO) figure was -3.452. That is the lowest figure since November 26, 2018 when the AO was -3.629. On December 27 the MJO was in Phase 6 at an amplitude of 0.061 (RMM). The December 26-adjusted amplitude was 0.165. Based on the latest guidance, a significant stratospheric warming event near or just after the start of January is likely. The highest levels of the stratosphere will warm during the closing days of December. The warming will begin to descend during the opening week of January. In addition, the mean zonal winds at 1 mb and 10 mb will likely reverse while the mean zonal wind at 30 mb will likely slacken considerably. Toward the end of the first week in January, the top layers of the stratosphere could begin to cool. The significant December 16-17 snowstorm during what has been a blocky December suggests that seasonal snowfall prospects have increased especially from north of Philadelphia into southern New England. At New York City, there is a high probability based on historic cases that an additional 20" or more snow will accumulate after December. Were blocking to disappear, snowfall prospects would diminish. For now, blocking appears likely to continue through the first half of January. Based on sensitivity analysis applied to the latest guidance, there is an implied near 100% probability that New York City will have a warmer than normal December. December will likely finish with a mean temperature near 39.0°.

If it weakens or is displaced, that would be better for us.