bluewave

The last time we had a December La Niña combined with a near +1 PNA and -4AO was 12-30-00. But notice how the usual 50/50 low east of New England has been replaced by a ridge. This used to be a decent KU signal for us this time of year. PNA 2000 12 26 0.921 2000 12 27 0.928 2000 12 28 0.963 2000 12 29 1.081 2000 12 30 1.075 2000 12 31 0.895 AO 2000 12 25 -4.343 2000 12 26 -4.208 2000 12 27 -4.239 2000 12 28 -4.635 2000 12 29 -4.688 2000 12 30 -4.115 2000 12 31 -3.606

The AO and the PNA improved from from forecasts 10 days ago but the big ridge east of New England also got stronger. So people aren’t going to want to hear about teleconnections anymore. This would have been more of a coastal storm with similar teleconnections in a earlier era. New Run Old Run

12-28-68 would probably be the closest -4 -AO match. But the PNA was negative with a deep Western Trough instead of ridge. So this could be the deepest cutter following a -4 AO and +1 PNA combo.

I believe the record was 8 days on the EPS with Hurricane Sandy. https://www.ecmwf.int/sites/default/files/elibrary/2013/10913-evaluation-forecasts-hurricane-sandy.pdf In this report, we have discussed the predictability for the landfall of hurricane Sandy, affecting the New York City area on October 30th 2012. The cyclone made an unusual turn toward the west before making landfall. The westward movement and rapid deepening from the 29th was most likely due to an interaction with a trough over the U.S. Therefore both the prediction of the tropical cyclone and the U.S trough were of importance. The results show that ECMWF operational forecasts 8 days before landfall gave a strong and accurate indication of what was to happen. From 7 days before the landfall the high-resolution forecasts were consistent in its prediction of the landfall. The results from the ensemble forecasts allowed a significant degree of confidence to be attached to these forecasts but also showed signs of a too slow movement of the cyclone, which led to a timing error of the landfall. The TIGGE archive has been used to compare predictions from different forecasting centres. The results shows that the ensembles from NCEP and UKMO started to pick up the risk for a deep cyclone making landfall 7 days (+168h) before the landfall and the ECMWF ensemble one day earlier (+8 days). Com- paring the performance between the centres, ECMWF had the highest probability for the cyclone landfall for most initial forecast times in the medium range. We also found that UKMO seemed to under-predict the depth of the cyclone compared ECMWF and NCEP and that the CMC ensemble had a large spread among the members. To evaluate whether the ensemble spread is reliable, the ensemble spread of trop- ical cyclone tracks needs to be evaluated over many cases. Comparisons between different forecasting centres for tropical cyclones can be found in e.g. Hamill et al. (2011); Yamaguchi et al. (2012).

So using that statistic, how many 970s mb or lower Great Lakes cutters have we had a few days after a -4 AO and +1 PNA in December?

We have had better luck within 5 days of St Patricks Day than Christmas recently. Time Series Summary for ISLIP-LI MACARTHUR AP, NY Click column heading to sort ascending, click again to sort descending. Ending Date Total Snowfall Mar 12 to Mar 22 Missing Count 2021-03-22 T 0 2020-03-22 0.0 0 2019-03-22 0.0 0 2018-03-22 25.0 0 2017-03-22 3.4 0 2016-03-22 2.3 0 2015-03-22 5.6 0 2014-03-22 T 0 2013-03-22 1.7 0 2012-03-22 0.0 0 2011-03-22 0.1 0 Time Series Summary for ISLIP-LI MACARTHUR AP, NY Click column heading to sort ascending, click again to sort descending. Ending Date Total Snowfall Dec 20 to Dec 30 Missing Count 2021-12-30 0.3 0 2020-12-30 0.1 0 2019-12-30 0.0 0 2018-12-30 T 0 2017-12-30 1.3 0 2016-12-30 0.0 0 2015-12-30 T 0 2014-12-30 0.0 0 2013-12-30 1.7 0 2012-12-30 0.6 0 2011-12-30 0.0 0

There are solid statistical relationships from the past when certain combinations of teleconnections were met and storm outcomes that accompanied them or lagged by weeks or even months. But the limit of these in the forecast range is about 8-10 days before we lose the useful skill. New features like the record warm pool to our east that didn’t exist when these relationships worked out have changed the equation. So marine heatwaves are altering ENSO, MJO, and teleconnection expectations in a warming climate. A -4 SD -AO and +1 PNA would produced a much colder and potentially snowy scenario rather than such a deep low tracking to our west. We get a different wavelength response now from the older days when the same numerical index values were met.

Yeah, but it is was our coldest late December into early February in modern times. This week we make a run on 60° before Christmas and then drop to around 15° in NYC. So no subzero readings like January 1977.

Yeah, this will probably be one of the most underperforming extreme blocks for snow and cold in our in our area.

The 360 hr forecasts have been low skill. if the EPS finally gets one right, then it will be a mild start to January. But who really knows beyond 8-10 days. This is one of the few milder 360 hr forecasts since late November. Most of them have been too cold.

The SSTs are still almost 60° south of Nova Scotia where the ridge axis is located. https://www.ndbc.noaa.gov/station_page.php?station=44150 Water Temperature (WTMP): 59.5 °F

This is the warmest the NW Atlantic has been during mid-December.

I think it’s the record warm pool in the Western Atlantic causing more ridging there than we used to get. But notice how the day 8-10 forecast can’t detect it. We see it start to show up around day 5-7. So it makes looking at these ensembles beyond a week a low skill proposition. New run Old run

This is probably the first time in December that we could get a 974 mb Great Lakes cutter following a few days after a -4 AO and +PNA. So my guess is that the warming climate is causing wavelength changes that would have produced a KU event in the past. Just goes to show how past analogs from a colder era don’t work anymore.

Maybe if we are lucky a big cutter before Christmas can finally pull the trough into the East. That could leave the door open later in the month for a colder storm track. But I am hesitant to buy into a significant snow threat from more than 5 days out with how the models have been in full pushback mode since late November.

The models have changed quite a bit from what they were showing last weekend. So we could get out of December without much snow. But I still think the AO dipping under -4 this month will give us more blocking opportunities JFM.

The warm up around the solstice is an actual annual occurrence now. We have had 11 consecutive years going over 55° in NYC which has never happened before. Contrast that with the first 2 weeks of the month which hasn’t seen the big maximum temperature increase. But I couldn’t tell you why the timing within about 5 days of the solstice has been so reliable for these warm ups.

Yeah, it reversed right after the Boxing Day Blizzard. Just an amazing run of Decembers from 2000-2010. While we have an occasional snowy December like 2020, the snowfall is way down and the temperatures up since the 11-12 non winter. December snowfall has been only 30% of the previous decade or so. Monthly Total Snowfall for NY CITY CENTRAL PARK, NY Click column heading to sort ascending, click again to sort descending. Year Dec Season Mean 2.8 2.8 2022 T T 2021 0.2 0.2 2020 10.5 10.5 2019 2.5 2.5 2018 T T 2017 7.7 7.7 2016 3.2 3.2 2015 T T 2014 1.0 1.0 2013 8.6 8.6 2012 0.4 0.4 2011 0.0 0.0 Monthly Total Snowfall for NY CITY CENTRAL PARK, NY Click column heading to sort ascending, click again to sort descending. Year Dec Season Mean 8.9 8.9 2010 20.1 20.1 2009 12.4 12.4 2008 6.0 6.0 2007 2.9 2.9 2006 0.0 0.0 2005 9.7 9.7 2004 3.0 3.0 2003 19.8 19.8 2002 11.0 11.0 2001 T T 2000 13.4 13.4

Yeah, the Euro lost the southern stream low that was phasing in yesterday. It was that low that was keeping the baroclinic zone close to the coast. Now, it’s too northern stream dominant and the warm pool and ridge is really flexing east of New England. It could also be another reminder how we can’t seem to avoid the big warm up around the solstice.

If the storm wraps up too much and tracks over or west of NYC, then the coldest temperatures will miss to our SW.

We’ll see. The GFS has been struggling the most with the ridge. Remember last weekend when the storm today was forecast by the GFS to get suppressed to South Carolina? That’s why we rely so much on the Euro,EPS, GEM, and GEPS for longer range forecasts. GFS too suppressed with SE Ridge and storm from last weekend New runs closer to what the CMC and Euro were showing

The problem with the block is that it’s forecast to link up with the ridge to our east and become too south based. That’s why so many GEPS and EPS along with the 0z OPs track over or west of NYC. Seems like anytime the ridge can build over the record warm pool it does.

Yeah, the GEPS usually has a cold bias beyond a few days and the storm track is even further west than the EPS clusters and means.

The storm track coming right over or just west of NYC is a concern since that warm pool and ridge east of New England is continuing to be a factor. The PNA actually is pretty good with a nice ridge out West. So this looks more like and issue with the ridge to our east causing the low to come too far west for big snows near the coast.

Yeah, the weaker cluster is the one to the west with fewer lows. The most populated clusters track right over us or near the BM. Several lows in the 950s to 970s is unusual for this far out. So a high impact storm signal either way.