bluewave

We started to see these ENSO disconnects when the 15-16 super El Niño California rainfall got delayed into the 16-17 La Niña. Then it was off to the races when the El Niño didn’t couple in 18-19. Last winter we saw weak El Niño conditions produce an 11-12 style La Niña pattern. So it’s no surprise we are getting an El Niño-like pattern with a moderate La Niña.

We have seen these strong trade wind surges and Niño 3.4 dipping temperatures back to the fall. All the competing influences have resulted in a more Niño-like pattern. Notice how the PNA is forecast to stay positive right into January. Beyond early January, it will be interesting to see which type ENSO pattern prevails.

Another El Niño-like feature that we are experiencing this month is the mild average temperature combined with the 10”+ snow in NYC . Since 1995 , all the 35.9° or warmer Decembers with 10”+ were warmer ENSO years. The snowy La Niña Decembers all had a monthly average around 32° degrees. This month is already 39.1° before the big coming solstice warm up. An interesting question to ask is how long can we keep this Niño-like pattern going? I looks like January will at least start out more Niño-like. So it will be interesting to see which ENSO type pattern shows up in February. Warmest Decembers highlighted...NYC snowfall....average temperature....Niño 3.4 temperature.... Time Series Summary for NY CITY CENTRAL PARK, NY - Month of Dec Click column heading to sort ascending, click again to sort descending. Rank Year Total Snowfall Temp/ Niño 3.4 1 2010 20.1 32.8/-1.6 2 2003 19.8 37.6/+0.4 3 2000 13.4 31.1/-0.7 4 2009 12.4 35.9/+1.6 5 1995 11.5 32.4/-1.0 6 2002 11.0 36.0/+1.1 7 2020 10.5 39.1/-1.2

NYC is on track for its first Christmas with a morning high of 55-60 and an evening low in the 20s. Our new era of big temperature swings continues. The previous coldest temperature on Christmas with such a warm high was 33° in 2008. Data for December 25 - NY CITY CENTRAL PARK, NY Click column heading to sort ascending, click again to sort descending. Date Max Temperature Min Temperature Precipitation Snowfall Snow Depth 2015-12-25 66 57 0.01 0.0 0 1982-12-25 64 41 0.02 0.0 0 1889-12-25 64 43 0.00 0.0 M 2014-12-25 62 44 0.09 0.0 0 1940-12-25 62 38 0.00 0.0 0 1979-12-25 61 50 0.87 0.0 0 1964-12-25 60 49 T 0.0 0 1994-12-25 59 41 T 0.0 0 1965-12-25 59 47 0.65 0.0 0 1932-12-25 59 47 0.13 0.0 0 1893-12-25 58 45 0.00 0.0 M 2008-12-25 57 33 0.13 0.0 0 1936-12-25 56 40 0.00 0.0 0 1933-12-25 55 34 0.00 0.0 0 1915-12-25 55 38 0.01 0.0 0

It’s been a big year for squall lines. This could be one of the most intense low topped squall lines that we have seen on Christmas. I wonder what the annual record number is for all types of squall lines in the region?

Yeah, this month looks like a continuation of the atmospheric coupling weirdness of recent years. The 500 mb pattern is much more El Niño-like. Especially the deep Aleutian Low and +PNA. I guess that this makes up for the last 2 winters which were technically El Niño with La Niña-like 500 mb patterns.

I am not sure why my original post was quoted so many times. All that I said is that the interior areas with the deepest snowpack would have to watch the shorter term forecasts for flooding potential. I didn’t go into any specifics since it was based on the longer range Euro.This is very similar to the BGM AFD. I also mentioned that the coastal sections would have time to gradually melt their smaller snowpack over the next several days before the storm arrives. 2 inch rainfall totals are not out of the question for those enhanced higher terrain locations. Combined rain and snowmelt could result in significant runoff and thus potential for flooding will need to be monitored closely. Finally, weight of old snow and new rain on rooftops where heaviest recent snow occurred, could lead to problems.

The Euro has one of the strongest south based blocks that we have seen this time of year. It has a whopping +6 SD 500 mb height anomaly just south of Greenland. A 584 dm ridge east of Newfoundland should be very close to the record in late December.

The timing of the Christmas cold front looks to be early Christmas morning. The EURO and new GFS V16 are pretty close. So the high for the day will be early with falling temperatures after. Another big temperature swing weather pattern.

Yeah, having a much warmer than average NW Atlantic nearby makes a big difference. ISP has had 5 events of 17” or greater since the 09-10 winter. MSP recorded one 17” snowstorm over the same period. for ISLIP-LI MACARTHUR AP, NY 27.8 2013-02-09 24.9 2015-01-27 23.9 2009-12-20 23.7 2016-01-24 18.4 2018-03-22 for Minneapolis-St Paul Area, MN (ThreadEx) 17.3 2010-12-11

The Christmas storm wave break looks like it may be off the charts. The ridge just south of Greenland goes more than +5 SD. This could be one the strongest blocks in that region for this time of year. So we will be probably be looking at winter storm threats from the end of December right into January.

Walt, great thread as usual. As I posted in the other thread, the new parallel GFS v16 did much better than the current OP v15 with the major winter storm this week. It didn’t have the cold and suppressed bias that has been a problem in the current OP. So it’s not a surprise that the v16 is warmer and slower than the current OP for the major storm around Christmas. New parallel v16 GFS Current V15 OP

Moving into our annual solstice warm up this week. Followed by a return to cooler than normal after Christmas. The Southeast will turn out to be the coldest part of the CONUS relative to the means this month. Florida is on track for their first colder than average month since March 2018.

I guess that he only ran the model for that storm. But it’s also happening in places like Japan. https://www.washingtonpost.com/weather/2020/12/18/japan-snow-stranded-motorists/

The WAA aloft and dryslot did their thing.

I only got 4” here before the sleet arrived. So the bar is set pretty low. I will go out on a limb say that there will be a better storm than that at some point this winter.

The Christmas storm looks to really create a major wave breaking event. So I am taking a wait and see approach as to how long after that we can expect a colder and snowier storm track. I am hoping that we can finally get a cold enough storm for the South Shore to finally get into the jackpot zone like we were in 15-16, 16-17, and 17-18.

We can get rapid snowmelt under a variety ENSO conditions. The last one that made local headlines was in March 2015. But I am glad that areas near the coast will have a chance to gradually reduce the snowpack going into the late week storm.

It’s interesting how persistent this solstice warm up has been especially since 2011. It has occurred under a wide variety of weather patterns. Some years like 2013 had a snowstorm just before the warm up like the forecast for this year. Others like 2017 went into the deep freeze right after Christmas. 2015 featured the historic +13.3 departure and the warmest day of the month was on the 24th. Time Series Summary for NY CITY CENTRAL PARK, NY Click column heading to sort ascending, click again to sort descending. Ending Date Highest Max Temperature Dec 18 to Dec 25 Missing Count 2019-12-25 57 0 2018-12-25 61 0 2017-12-25 55 0 2016-12-25 58 0 2015-12-25 72 0 2014-12-25 62 0 2013-12-25 71 0 2012-12-25 56 0 2011-12-25 62 0

Areas to our north which were below 0° today may experience one of their biggest day 5-6 temperature rises for December. The records for December were set in 2013 and 2008.

Plenty of competing factors seem to be altering the typical La Niña forcing pattern for December. Notice how the VP anomalies are skewed further S and E than one would expect from a La Niña. The forcing near the DL is more of an El Niño look.

These are two great tweets on how the record warm SSTs to our east are providing extra moisture to snowstorms. But on the flip side, we also get bigger rain and wind storms when the intense lows track to our west. Especially with systems that can go negative tilt.

You can think of the warm storm track near Christmas as a sacrifice bunt. The ensembles are trying to use it as a means to shift the blocking from east of New England closer toward Greenland later in the month. We saw something like this with the warm storm track back around December 1st. So while the event around Christmas looks warm and wet, it could set the stage for a cold and snowy storm track as we approach and move into 2021.

There is plenty of missing climate data for some reason this month. But NYC is at +0.5 on the xmacis2 climate site. A very Niño-like temperature departure this month with warmth along the Northern Tier and cold South.

Thanks, Don. There is a long history of people making out of place responses to things that people didn’t actually say in their original post. BGM and ALY mention possible runoff issues in their AFD. People have to realize that we are having a long range model discussion. Obviously, the actual details that show up closer in will be the ones of most relevance. So to say that x model shows y solution a week out is not a 100% endorsement of what its showing. Just to begin to think about what types of impacts could arise if such solution was actually to verify. We have the existing significant snowpack to consider, and the implications for additional runoff. Milder temperatures and some rain will arrive for Thursday into early Friday. Deepending on how much rainfall and snowmelt occurs, there could be a response on area rivers and streams, although it is still too early to tell how much of an impact this will be.