bluewave

Yeah, we would want to see runs cutting the low of to our SW like the 12z CMC and 0z Euro disappear over the next several days to have a shot a significant snow at the coast from this one. But it all depends on how much the cutoff digs before turning the corner. While the cluster scenario can be more helpful, the means need to start backing off a bit so we have some room for them to come NW under 120 hrs and not be too warm.

We would need a lost and found scenario where the next several runs start backing off the amplitude only to come back in later runs.

The ECMWF has a product called cluster scenario. This breaks down the EPS members into clusters. So it gives more details than just a mean that combines everything. I believe some of the mets in the New England forum have been posting these. Unfortunately, the free site doesn’t have the zoomed in NA chart but the whole NH. It’s more helpful than an ensemble mean. https://charts.ecmwf.int/products/cluster_plot_legA?base_time=202212150000&cluster=192_240&parameter=500

I am not sure if the EPS is further east due to the smoothing out of the various TPV locations. The members like the OP that dig more to the SW over the Tennessee Valley are warmer with less snow. The colder members that plow more across the Ohio Valley have a BM Miller B with more snow. Notice the ridge poking into New England from the east on the OP.

It’s pretty variable on Long Island for which combination or sequence ENSO states produces the most snow. Time Series Summary for ISLIP-LI MACARTHUR AP, NY - Oct through Sep Click column heading to sort ascending, click again to sort descending. Rank Season Total Snowfall Missing Count 1 1995-1996 77.1 La Niña after El Niño 2 1977-1978 68.0 El Niño after El Niño 3 2017-2018 65.9 La Niña after La Niña 4 2014-2015 63.7 El Niño after neutral - 2013-2014 63.7 neutral after neutral 5 2004-2005 58.8 El Niño after neutral 6 2010-2011 55.3 La Niña after El Nino 7 2002-2003 54.6 El Nino after neutral 8 2009-2010 53.8 El Nino after La Niña 9 1966-1967 50.8 La Niña after El Niño 10 2012-2013 46.9 neutral after La Nina

https://xmacis.rcc-acis.org

Yeah, the rising PNA and record block over the pole is the storm signal. But the snowfall forecast for the coast will come down to the exact TPV track. If the TPV cuts off to our SW like the Euro, we will have less snow. But the CMC track across the BM would be snowier.

This will be one event when it’s tough to use ensemble means since there are several areas of low pressure. The Euro has a Miller B and an attempted phase of a southern stream low. So the mean is trying to smooth out too many different low positions. The one thing we know is that the 22-24th will have the highest tides of the month with the new moon. So coastal flooding could be an issue if the low cuts off to our SW. The only thing that I can say about the EPS mean is that an unusually high number of members have lows under 980 mb from 8-10 days out. This would mean that this could be a very intense storm with high winds and heavy precipitation. We almost never see so many deep lows on an ensemble mean this far out.

We would need for the diving PV to get out near the benchmark like in February 1978 to make people happy. The alternative is it gets stuck over land like January 1978 and we are on the warm side for much of the precipitation. Either way, it should get really cold behind the storm. http://greatlakes.salsite.com/maps/jan2427_1978_500_loop.gif http://greatlakes.salsite.com/maps/feb58_1978_500_loop.gif

This would be a new 500 mb amplitude record if it wound up verifying. The OP Euro has a near record PV just to our SW at -5.8 SD. The block at the pole is also in near record territory at +6.5 SD.

NYC hasn’t had 1”+ of snow in the week leading up to Christmas since 2008 and 2009. Time Series Summary for NY CITY CENTRAL PARK, NY Click column heading to sort ascending, click again to sort descending. Ending Date Total Snowfall Dec 19 to Dec 25 Missing Count 2021-12-25 0.2 0 2020-12-25 T 0 2019-12-25 0.0 0 2018-12-25 T 0 2017-12-25 T 0 2016-12-25 0.0 0 2015-12-25 0.0 0 2014-12-25 T 0 2013-12-25 T 0 2012-12-25 T 0 2011-12-25 0.0 0 2010-12-25 0.0 0 2009-12-25 10.9 0 2008-12-25 4.7 0

If suppression does turn out to be an issue after Christmas, then this December would be one of the low NYC low snowfall outliers for the AO dipping below -3 since 2000. December -3 or lower daily values since 2000 and NYC snowfall 12-11-22….-4.2378……? 12-28-20….-3.197……..10.5” 12-8-12…….-3.902……0.4” 12-18-10……-5.265…..20.1” 12-21-09……-5.2821….12.4” 12-05-05……-3.569….9.7” 12-28-01…….-3.293….T 12-29-00……-4.688….13.4”

The big block pulling back to Siberia could allow more of a Western Trough like the OP or one in the Plains like the EPS. While we would avoid the big OP warm up before Christmas on the EPS, still have to deal with that Great Lakes primary low. Any secondary development near the coast would probably favor the interior like we are seeing for Friday. Need a slower retrogression of the block back into the WPO domain to keep the heights up along the West Coast. Too fast an EPO rise could allow the PV to dig more out West.

The record SSTs in December 2020 along with the 500 mb pattern probably caused us to miss the 40” jackpot which ended up in Binghamton. The low was too tucked in for those amounts near the coast. But the historic January 2016 blizzard was able to feed off those same record SSTs. So the colder readings a month later plus the flatter SE Ridge and BM track worked to our advantage.

It’s pretty much the same thing every December. Late November models start showing a great pattern in early December. Then the first NYC 4-6”+ snowstorm gets pushed back into mid-December. Other years like this one gets pushed back to late December. In reality, NYC has only had a few 4”+ snows between December 1-10. Some years like last, the late December snow got pushed back to early January. But at least it eventually arrived. NYC Dec 1-10 snows over 4” since 2000 Maximum 2-Day Total Snowfall for NY CITY CENTRAL PARK, NY Click column heading to sort ascending, click again to sort descending. Rank Value Ending Date Missing Days 1 14.0 2003-12-06 0 0 0 2 6.0 2002-12-06 0 - 0 3 5.8 2005-12-10 0 0 4 4.6 2017-12-10 0 0

Anytime we get a Rockies Ridge following a strong -AO drop we can get something 12”+. We didn’t even need the -AO for Islip to have the record snows last January. All it took was a solid +PNA pattern. But historic events like January 2016 require both a favorable Atlantic and Pacific. Long Island did historic in March 2018 with the March wavelengths. Getting the trough out of the West is more important in December than JFM. While this current pattern is technically becoming more +PNA, there is still a lingering trough in the Western and Central US kicking up the SE Ridge. So sometimes looking at the raw indices doesn’t tell you everything we need to know about the pattern. Primary’s tracking to the Great Lakes and secondary’s hugging too close have been the preferred storm track in this regime. A pattern change will mean height rises in the Rockies and more of a benchmark track.

The primary low is a Great Lakes cutter so the coast will be too warm. If the primary got squashed to Pittsburgh, then the secondary would take a colder track SE of LI with good coastal snows. The secondary tracking right over NYC is only great for the interior crew.

Remember, getting pushed back doesn’t mean we won’t eventually get a 4-6” snow in NYC. While we don’t have the 2010 record blocking, models were showing snowstorms long range for all of December 2010. So the snowy pattern got delayed by 3 weeks or more depending on the model. But sometimes pushback patterns ultimately disappoint. So we would be happy with something in between a shutout and historic KU. Most would be happy with a 4-6”+ event in NYC from late December into early January. Plus several 1-3” 2-4” type snows to put us into double digits by January 10th.

I think the reason models do so much better with primary lows cutting to our west day 6-10 is that they can be off by 500 miles or more and still verify. A primary low can track 50 miles west of NYC and bring mostly rain to the coast. Same goes for a primary cutting near Chicago. But a benchmark storm can’t very from 6-10 day out if the margin of error results in 500 mile track errors. So the normal day 6-10 day model track error isn’t precise enough to handle benchmark tracks. But they have much more leeway in the margin of error for a track to our west.

The models did a good job seeing the primary low cutting from 8-10 days out for around the 15th. It was the 11-15 day runs which had a KU pattern. So that’s what is meant by getting pushed back. Now the threat before Christmas is beginning to get pushed back. Go back to the beginning of this thread in late November when the ensembles had a great looking 11-15 day pattern after December 5-7. That is not to say that we won’t get lucky late December into early January. We just need to see it day 8-10 first in order to lock it in.

I think when the crew on here hears pattern change, the main understanding is that it means 4-6”+ snowstorms will verify. They don’t think of it much in terms of what the teleconnections like the PNA and AO are doing. It can also mean a period of colder temperatures following a warmer one. But many will not call this a full pattern change unless decent snowstorms come with it.

Yeah, 0.6 is the effective skill forecast number on the verification charts.The EPS, GEPS, and GEFS all dip below that number around 10 days out. So that is the range of usefulness. https://www.emc.ncep.noaa.gov/users/verification/global/gefs/ops/geo/

The whole crew here has pretty much figured out through trial and error the the effective model range is about 8-10 days.

Yeah, the ensembles have been trying to improve the Pacific day 11-15 for weeks now. They show a great +PNA and trough near the Northeast. But as we get closer to day 8-10, the OP runs start showing cutters. Someone commented on how the OP GFS has been showing cutters day 11-15 while the GEFS has been cold. We can remember the KU look the ensembles had for Friday only to become a Great Lakes cutter with a secondary passing right over NYC. 360 hr EPS forecast for December 15th 60 hr EPS forecast for December 15th

Yeah, the trough in the West is undercutting the block to the north. So the primary lows track to the Great Lakes. Need a strong Rockies ridge to make the pattern favorable for 4”+ snows in NYC. The lag between the December -4.0 and -4.9 AO and the 4”+ snowstorms has been quite variable. The only thing that we know for sure is that the 4.0”+ snowstorm will take longer than 2000 and 1995. -4.0 to -4.9 December -AO dates and the next 4.0” snow in NYC 12-23-00…12-30…12” 12-18-95….12-19….7.7” 12-26-68….2-9….15.3” 12-13-66…12-24….7.1” 12-31-62….1-26…..4.2” 12-23-50…..None