donsutherland1

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.

It’s possible that the climate data might be missing some of the events. That might be leading to the low figure for 1898-99.

Fantastic work, Hooralph. When you have time, you might want to check 1898-99. It seems to be off. https://www.weather.gov/media/okx/Climate/CentralPark/monthlyseasonalsnowfall.pdf Have a great New Year.

The EPO is positive and is forecast to soar to strongly positive levels early in the first week of January on the EPS.

I suspect that the frozen precipitation threat is well north of the big cities on January 3. Somewhere between Poughkeepsie and Albany might have a good chance of seeing ice and/or at least some snow. As for the transition, it’s still there on the overnight guidance. Sometimes, the guidance rushes things. At that time, the stratosphere will likely be cooling from what was a significant, but not major warming event. The zonal winds are forecast to reverse at 1 mb and 10 mb, but not 30 mb. The polar vortex will be weakening dramatically.

Morning thoughts... Today will be partly cloudy and mild. Temperatures will likely top out in the upper 40s and lower 50s across most of the region. Likely high temperatures around the region include: New York City (Central Park): 50° Newark: 50° Philadelphia: 50° Tomorrow will be fair and seasonably cold.

The extended range of the EPS currently shows an evolution that could lead to a trough in the East between January 10-15. So, unlike last winter when a stubborn EPO+/AO+ pattern locked in, things could be different this time around.

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). Below are the composite 500 mb height anomalies and temperature anomalies for that cluster: Mean temperatures were the following: Boston: 33.0° New York City: 37.7° Philadelphia: 40.5° Washington, DC: 43.9° Only Boston saw accumulating snow during the above cases. Its biggest snowfall was 3.5" during January 15-16, 1998. The dynamical guidance is even warmer than these composite anomalies. The closest match from the above cases is the January 1-4, 1955 period. In sum, the forecast pattern is one that will likely favor 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.

After a cold start, temperatures moderated this afternoon. Temperatures generally peaked in the middle and upper 30s across the region. Meanwhile, on account of the strong storm that impacted the region on December 24 into December 25, more record warmth occurred in parts of Canada. That storm also brought blizzard conditions to parts of Nunavut today while eastern portions of that Province experienced unseasonable warmth. Daily record high temperatures included: Badger, NL: 51° (old record: 42°, 2001) Churchill Falls, NL: 36° (old record: 22°, 2011) Corner Brook, NL: 52° (old record: 41°, 2010) Hopedale, NL: 42° (old record: 36°, 1976) Pangnirtung, NU (66.15°N, 65.71°W): 39° (old record: 26°, 2000) St. John's, NL: 50°/10.0°C (old record: 50°F/9.8°C, 1977) Tomorrow will be partly sunny and noticeably milder before cooler air returns to 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. There is growing ensemble support for the latter outcome. The probability that the block will be anchored near Newfoundland and Labrador in the extended range can inhibit opportunities for snowfall. This can especially be the case should the block attempt to link up to the western Atlantic ridge. In coming days, it will become clearer whether the recent and still ongoing shift in the ensemble guidance toward that outcome will become the most likely scenario for the first half of January. Under this scenario, the first 10 days of January could be much warmer than normal across the northeastern United States, Quebec, and much of eastern Canada. The ENSO Region 1+2 anomaly was -0.8°C and the Region 3.4 anomaly was -0.9°C for the week centered around December 16. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.57°C and the ENSO Region 3.4 anomaly has averaged -1.17°C. La Niña conditions will likely prevail at least through the winter. The SOI was +15.15 today. Today, the preliminary Arctic Oscillation (AO) figure was -1.954. On December 26 the MJO was in Phase 2 at an amplitude of 0.165 (RMM). The December 25-adjusted amplitude was 0.194. 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. There is the proverbial fly in the ointment. It appears that this warming event could produce a split in the polar vortex. Typically, when the polar vortex splits, Eurasia is favored for cold. Displacement of the polar vortex often favors North America. The recent 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 into the start of January. Based on sensitivity analysis applied to the latest guidance, there is an implied 99% probability that New York City will have a warmer than normal December. December will likely finish with a mean temperature near 39.0°.

Under bright sunshine, the temperature rose into the middle 30s by early afternoon. Some pictures from around Manursing Lake in Rye, NY:

It looks like a fairly strong event with the mean zonal winds reversing at 1 mb and 10 mb. It’s still uncertain about 30 mb. We’d have to see how the split occurs to have a better idea of its impact. Most, but not all, such cases favor Eurasia