bluewave

Elevation has been a big help with these marginal set ups since late November.

Taking into climatology into account, the the average maximum temperature during the last week of December has been 50 to 62 since 2011. Time Series Summary for NY CITY CENTRAL PARK, NY Click column heading to sort ascending, click again to sort descending max temperature 12-26 to 12-31 2023-12-31 55 0 2022-12-31 62 0 2021-12-31 55 0 2020-12-31 50 0 2019-12-31 54 0 2018-12-31 60 0 2017-12-31 28 0 2016-12-31 60 0 2015-12-31 61 0 2014-12-31 55 0 2013-12-31 55 0 2012-12-31 39 0 2011-12-31 57 0

We’ll have a shot at 50s to around 60 at some point during the last week of December. Many outside this forum don’t consider milder weather this time of year doom and gloom since it helps lower their winter heating bills. But it isn’t the greatest if you are running a ski resort during the holiday period.

That would be very close to average as recently as the 81-10 climate normals which was 37.5° in NYC. https://www.ncei.noaa.gov/access/us-climate-normals/#dataset=normals-monthly&timeframe=81&location=NY&station=USW00094728

Top 5 warmest last week of December possible for North America.

The hovmoller charts are showing something different. That being said, it’s beyond the more reliable 15 day range. But it still has forcing over the Maritime Continent in early January.

Much different pattern outside the raw PNA values from most of those Decembers which were very snowy around NYC compared to this year. Several of those Decembers were also much colder than this year. So the following Januaries may also not have that much in common. NYC December snowfall 2024 so far….T 2020………….10.5” 1960…………..18.6” 1985…………….0.9” 2005…………..9.7” 2000………….13.4”

The Euro monthly forecast for December is released in the 5th of the month. The other modeling centers get posted around the 1st. Computer model forecasts issued in October into early November have very little skill for the December time frame.

Yeah, NYC has a shot at dropping below 20° for the first time this season before Christmas. Better chance of getting NNW flow at that time which avoids the warmer Great Lakes. But the combined EPS and GEFS temperature forecasts for the week of the 25th to 31st are close to the warmest on record during that period for the entire North American continent averaged out.

Those charts aren’t meant to be used for daily forcing much beyond the 10 day mark. That’s why the 7 day averages are used for the normal daily spread from run to run. You can see on the Hovmoller diagrams the forcing in the days before and after the snapshot you posted has a split look between the EPAC and WPAC. Plus you have to take into account that the models tend to underestimate the signal the further out in time we get. So this gives the appearance of correcting stronger with the convection the closer time we get.

Several of the temperature forecasts have been too cold in the Northeast. Parts of the West have also been verifying warmer. The Southeast may be of the few regions to verify as cold or colder than originally forecast.

The EPS and GEFS continue to show a split forcing pattern and competing influences through at least the end of the more reliable 15 day ensemble range.

The monthly forecasts issued near the start of December weren’t warm. But the main issue this month is that the model forecasts underestimated the strength of the Pacific Jet. So the ridge axis in the means for the whole month will end up further east than forecast. Very active storm track through the Great Lakes with warm ups ahead of the storms with rain and cool downs behind the lows. Very little snowfall near the East Coast with this pattern.

The main thing we need to go right would be for the Pacific Jet to relax enough to prevent all the lows from tracking across the Great Lakes. Even marginally colder patterns like this month are still getting warm storm tracks with the cold arriving after the rains end. So we have very little snow to show for the colder departures relative to the means. The East Coast started getting warmer storm tracks even during colder intervals since the 18-19 winter. Last winter some spots were able to go over 10” in the snowfall department due the El Niño weakening the Northern Branch just enough for the STJ to dominate for a week in February. This wasn’t the case with the La Niña in 22-23. In January 2019 with lows near 0° in NYC and a monthly average near 32.0°, NYC only finished that colder month with an 1 inch of snowfall due to the warm storm track through the Great Lakes.

December 2022 was the last time. Climatological Data for NY CITY CENTRAL PARK, NY - December 2022 Click column heading to sort ascending, click again to sort descending. Sum 1369 1016 - - 815 0 5.83 T - Average 44.2 32.8 38.5 -0.6 - - - - 0.0 Normal 44.3 33.8 39.1 - 804 0 4.38 4.9 - 2022-12-01 43 35 39.0 -4.2 26 0 0.00 0.0 0 2022-12-02 47 32 39.5 -3.4 25 0 0.00 0.0 0 2022-12-03 58 45 51.5 8.9 13 0 0.49 0.0 0 2022-12-04 45 38 41.5 -0.8 23 0 0.00 0.0 0 2022-12-05 48 36 42.0 0.0 23 0 0.00 0.0 0 2022-12-06 58 42 50.0 8.3 15 0 1.05 0.0 0 2022-12-07 59 54 56.5 15.1 8 0 0.26 0.0 0 2022-12-08 55 42 48.5 7.4 16 0 0.00 0.0 0 2022-12-09 45 36 40.5 -0.4 24 0 0.00 0.0 0 2022-12-10 39 32 35.5 -5.1 29 0 T 0.0 0 2022-12-11 40 33 36.5 -3.8 28 0 0.32 T 0 2022-12-12 38 28 33.0 -7.0 32 0 0.01 T 0 2022-12-13 40 25 32.5 -7.2 32 0 0.00 0.0 0 2022-12-14 39 31 35.0 -4.5 30 0 0.00 0.0 0 2022-12-15 42 33 37.5 -1.7 27 0 0.51 0.0 0 2022-12-16 44 38 41.0 2.1 24 0 0.85 0.0 0 2022-12-17 44 36 40.0 1.4 25 0 0.00 0.0 0 2022-12-18 40 32 36.0 -2.4 29 0 0.00 0.0 0 2022-12-19 39 31 35.0 -3.1 30 0 0.00 0.0 0 2022-12-20 39 30 34.5 -3.4 30 0 0.00 0.0 0 2022-12-21 40 27 33.5 -4.1 31 0 0.00 0.0 0 2022-12-22 47 35 41.0 3.6 24 0 0.23 0.0 0 2022-12-23 58 8 33.0 -4.1 32 0 1.83 T 0 2022-12-24 15 7 11.0 -25.9 54 0 0.00 0.0 0 2022-12-25 28 14 21.0 -15.7 44 0 0.00 0.0 0 2022-12-26 29 18 23.5 -12.9 41 0 0.00 0.0 0 2022-12-27 35 29 32.0 -4.2 33 0 0.00 0.0 0 2022-12-28 47 33 40.0 4.0 25 0 0.00 0.0 0 2022-12-29 51 40 45.5 9.7 19 0 0.00 0.0 0 2022-12-30 62 46 54.0 18.4 11 0 0.00 0.0 0 2022-12-31 55 50 52.5 17.1 12 0 0.28 0.0 0

We have not had a cold December around the NYC Metro by historical December standards. Since December has experienced such a steep temperature rise over the years, the NYC average is 39.1° now. Years ago the average was in the mid 30s instead of close to 40°. So even a slightly colder finish than 39.1° would be close to average for a December earlier in the 2000s. NYC used to get a December averaging near 32° at least every 5-10 years as recently as 2010. Now we have gone a full 14 years without one. The cold coming in just before Christmas is being preceded and followed by near 60° warmth which wasn’t the case back in the old days when extended cold was much more common. We saw a similar version of this back in 2022 when the cold before Christmas was preceded and followed by near 60° warmth.

The MJO is an important piece of the forcing puzzle which we encounter in winter. This winter it’s been a part of the pattern but not the main driver. Most recent winters it was the main driver. It’s no coincidence that our last really snowy month near the northeast coast was in January 22 which was a solid MJO 8. That forcing configuration was able to weaken the Pacific Jet enough so the ridge axis locked in near the West Coast. That’s the opposite of what we have been seeing this winter as the ridge keeps drifting east instead of staying anchored out west. So we have been getting competing forcing elements between more Nino-like +PNA and La Niña faster Pacific Jet. So if the MJO remains in the background, then we may not see enough influence from a colder phase to really relax the Pacific Jet enough.

Looks like NYC finally has a shot at dropping under 20° this month. Time Series Summary for NY CITY CENTRAL PARK, NY - Month of Dec Click column heading to sort ascending, click again to sort descending. 2024 26 17 2023 25 0 2022 7 0 2021 25 0 2020 20 0 2019 16 0 2018 24 0 2017 9 0 2016 17 0 2015 34 0 2014 24 0 2013 19 0 2012 28 0 2011 22 0 2010 19 0

Back and forth pattern for the rest of the month with the dominant storm track through the Great Lakes. Warm up day 1-5 with a cooldown day 6-10. Then a warm up from around Christmas to the New Years.

The cold has really been muted in the Northeast due to the record warm Great Lakes temperatures along with the delayed freeze up on Hudson Bay. You can see how much colder the 850 mb temperatures were than the surface. Parts of Northern New England are close to average against the warmest 30 year normals. So the +PNA had much less cold to work with than in the past.

The big question is whether we can get a strong enough WWB this winter in order to initiate a large enough oceanic kelvin wave so we go back into El Niño by the summer? If not then we would probably see next winters La Niña stronger than this winter as the SSTs with this one are really struggling against the record warm global background state. Still too early to know which way it will go.

The December pattern this year is a first for any El Nino to La Niña transition regardless of strength.

I was discussing the mismatch potential back in October when the early MJO indicator came in similar to December 2020, 2017, and 2010. But I also mentioned at the time that the La Niña was much weaker than those past instances so there could be issues with the comparison past December La Niña mismatch events. While we got the +PNA this month, the Pacific Jet was much stronger than the previous mismatch Decembers. So the ridge axis keeps shifting into the East instead of staying anchored near the West Coast. The faster Pacific Jet is probably related to the WPAC record warmth and forcing competing with the +AAM influence and EPAC forcing. So it’s as if we have an El Niño pattern interfering or running concurrently with La Niña conditions.

It’s more about the northern stream being too dominant than timing. We were better off last February with the borderline super El Niño. At least some spots were able to go over 10” on the season due to the STJ being able to dominate over the northern stream for about a week.

The coastal could be too suppressed. But even a northward correction in later runs would be an issue along the coast with the low in the Great Lakes instead of a cold high.Very difficult to pull off a good thread the needle along the coast with such a fast northern stream Pacific flow.