bluewave

I don’t t think that’s how it actually works. December La Niña snowfall probably acts as more of a marker of what the pattern is able to produce. It’s not a December snowfall causing later seasonal snowfall relationship.That being said, we have still had some nice snowstorms following Decembers in NYC with under 3” back to 1991. Like in 99-00, 07-08, and 21-22. The one early snowfall indicator which has worked for every La Niña winter in NYC going back to the 95-96 has been the December La Niña snowfall. Years with 3 or more inches December have gone on to above normal seasonal snowfall. Every December with under 3” has had a below average snowfall season. This is why December is so important with La Ninas around NYC. 22-23….Dec snow…..T……season….2.3 21-22…..Dec snow….0.2…season…..17.9 20-21…..Dec snow….10.5…season…38.6 17-18……Dec snow….7.7…..season…40.9 16-17…….Dec snow…3.2…..season…30.2 11-12…….Dec snow….0…….season….7.4 10-11…….Dec snow….20.1….season….61.9 08-09…..Dec snow….6.0……season….27.6 07-08…..Dec snow…..2.9…..season….11.9 05-06…..Dec snow…..9.7…….season…40.0 00-01……Dec snow….13.4……season….35.0 99-00…..Dec snow…..T……….season….16.3 98-99…..Dec snow…..2.0……..season….12.7 95-96…..Dec snow…..11.5……..season….75.6

This winter will be a big win in my book if NYC can end the under 4” daily snowfall streak and not surpass 1932. Number of Consecutive Days Snowfall < 4 for NY CITY CENTRAL PARK, NY Click column heading to sort ascending, click again to sort descending. 1 1394 1932-12-16 2 1063 1952-01-27 3 1056 2024-12-20 4 1051 1963-12-22

Hopefully, we can maximize any window we get in early to mid-January.

The biggest uncertainty going forward is how long we will be able to sustain the +PNA. Past December La Niña mismatches generally saw the +PNA reverse at some point in January. These were followed mostly by -PNA Februaries. But if there are other factors behind this +PNA that we haven’t detected, then not sure if we follow past events until we get more model data and verification further into the winter. The EPS weeklies are trying to show this shift in January. But not sure if they are just defaulting to La Niña climo. It’s possible that we still follow the La Niña progression even since this one was so weak. It will be interesting to see how things turn out.

January 2000 stands out as even more impressive a miss especially down in the Carolinas. Since many places that got the heavy snow didn’t even have a storm in the forecast the day before. I don’t think they ever figured out what happened with the models in that one.

Not really in terms of amounts since it was still within .2 spread. This happens all the time with rain. There is no way to narrow that margin down just because the precipitation is frozen rather than liquid. This was still good forecast. January 2015 was a significant model miss for snowfall here. Our greatest recent model error for snow was January 2000 when the storm was forecast to stay offshore and we got 6”+. They still may not know what lead to that model error. Back in the 1970s and 1980s it was more common for models to completely miss storm events. Jan 78 featured a NOAA weather radio forecast of rain heavy at times. Then the next morning we had 15” of snow on the ground. I believe February 1980 had forecasts for 3-6” and the storm missed to our south with nothing here. So while modeling has come a long way since then, a 0.2 error margin may be tough to overcome.

The difference between 0.1 and 2.0 is essentially noise as far as these models are concerned. All through the year we get rainfall events within this margin. But people only notice since it’s snow.

.5 of an inch here just east of HVN.

While it generally means below average seasonal snows, there have still been nice snowstorms following such Decembers. That’s why I am hoping we can maximize the potential January -AO +PNA window. We have seen decent snows following under 3” Decembers in January 2000 and February 2008 to name a few.

The most interesting model trend to me for early to mid-January isn’t the cold showing up, but the stronger blocking NE of Hudson Bay. This could be related to the delayed freeze up. As I have noticed heights beating guidance there all month in the extended forecasts. If we can get a +PNA and -AO at the same time, a place like NYC can see a 4”+ snow event with such a pattern. But there is still an issue of how broad the trough underneath will be. NYC has had issues with broad troughs from west to east across the CONUS in the past during cold patterns. We got a suppressed storm in Jan 19 which gave 12” to DC in a cold pattern with uppers lows in the SW and NE. But my hope is that a possible stronger -AO this time around could compensate and this January window can produce for areas that missed out over the last few winters. New run Old run

The most interesting part of the recent runs which may provide some snowfall hope in January is the much stronger blocking NE of Hudson Bay. This may be a feedback process related to the low sea ice. Chances would increase for a 4”+NYC event January 1-15 if we can get a +PNA and -AO at the same time. Even if we have to deal with a broad trough from the west to the east. New run Old run

Nice .25 of an inch coating here with very light snow falling.

Yeah, the 18-19 winter was when the Northern Stream of the Pacific Jet began to act as the spoiler. It was a great example of a January which averaged close to normal at 32.5° and only delivered a 1.0” snow squall for the month. The heaviest storm that month was a cutter with over 1.00” of rain around the 20th with highs around 60°. Then a strong Arctic outbreak later in the month with near 0° lows.

Steady light snow here with a light dusting on the colder surfaces and 33°.

Yeah, that was mid-January 2019 when they had the 12” snowstorm around DC which slid to our south. There was an upper low under the +PNA near the Southwest and one near the Northeast. The surface low couldn’t turn the corner up the coast due to the UL near the SW and east of New England. Data for January 12, 2019 through January 13, 2019 Click column heading to sort ascending, click again to sort descending. MD CLARKSBURG 2.1 NE CoCoRaHS 12.2 MD DAMASCUS 3 SSW COOP 11.8 MD ROCKVILLE 2.8 WNW CoCoRaHS 11.5 VA WASHINGTON DULLES INTL AP WBAN 10.6 VA WFO STERLING COOP 10.6 VA OAK HILL 0.3 WSW CoCoRaHS 10.5 VA WASHINGTON REAGAN NATIONAL AIRPORT WBAN 10.2

Almost looks like the super El Niño warmth being forecast for North America last week of December is a lingering effect from last winter but with a faster Northern Stream instead of the STJ due to the competing La Niña influences.

The risk after the 3rd to 5th is that the trough from the west to east is too broad. We haven’t had much luck with these patterns in recent years when there has been lower heights out West under a +PNA ridge in Canada. Even if we have lower heights in the mean here in the East. Hopefully, we can get lucky this time around. But we should keep expectations low with how the fast Pacific flow has been so dominant in recent years.

You have to analyze how each 500 mb pattern forecast is influenced by a faster Pacific Jet. The long range model error in early December was the 500mb heights near Alaska were being eroded from the West by the stronger Pacific Jet. This lead to lower heights than forecast south of Alaska. It also allowed the ridge to drift east at times this month. Remember the models had the ridge anchored along the West Coast all month which didn’t work out. The models are now showing lower heights than forecast over the Western U.S. underneath the +PNA ridge in Canada in early January. So more jet energy is making it through near the U.S West Coast than earlier forecasts. It’s more an undercutting process further south than we initially saw in December. The bottom line is that we are getting lower 500mb heights than forecast out West due to the faster Northern Stream of the Pacific Jet.

I called for a -1 to +1 pattern for this month back in early December which is right on track when you average out the 7 local stations currently at -0.5. A back and forth pattern with warm ups and cool downs. The model forecasts issued on December 1st missed the warmth from mid to late month which I was pointing out three weeks ago with my posts about the Pacific Jet verifying faster than forecast. I also highlighted the mismatch potential back in October with the +PNA we have seen. But that there were competing influences that were different from past La Niña mismatch Decembers.

The trough is there due to the 500mb heights trending weaker with the stronger Pacific Jet influence than forecast. We’ll probably start the month with the trough over the West and warmer than average temps around the area. Then a storm will run to the Great Lakes or hug the coast with colder air rushing behind. The screenshot is the long range model bias showing up. So the spread out trough from west to east across CONUS longer range is smoothing out the storm tracks. When the lows eject from the SW transient Southeast Ridges will emerge in the fast flow allowing the storm tracks to run to the Great Lakes or hug the coast. We have seen this pattern time and time again over the years with low in the West under a Canadian +PNA.

We will be dealing with the same issue to start January that we had December. Models have been underestimating the strength of the Pacific Jet beyond 10 days. So now they are beginning to see a trough the West instead of the ridge several days ago. 500 mb heights over the NEPAC and WNA have been verifying lower than the long range forecasts have been indicating. New run Old run

My guess is that the 2020s will turn out to be the beginning of the shift to below average snowfall. This doesn’t mean that we can’t get above average seasons from time to time. The NYC average snowfall during the 2020s so far is similar to Washington DC from 1961 to 1993. The Boston average snowfall during the 2020s is similar to NYC from 1961 to 1993. So we have seen a shift of sorts this decade with the climate zones shifting north.

That’s the shift which I have been talking about. From 1961 to 1993 NYC had 13 close to average snowfall seasons in the 20s. From 1994 to 2024 NYC only had 3 average seasons in the 20s. So our snowfall pattern since 1994 has been all our nothing with very few average seasons anymore. Nearly all our winters since then were either above or below average. The many above average seasons were masking the winter warming trend. So now it’s harder to get average and above average seasons during the 2020s so far. We are moving toward a below average regime instead of the average pattern which dominated from 1961 to 1994.

Back and forth pattern continues with the coldest temps of the season coming in just before Christmas for more of a seasonal feel. Then we average close to normal around Christmas. Back to above normal to close out the year as we approach New Year’s Eve. So my guess is that the 7 station local average finishes December somewhere in the 0 to -1 range. The current 7 station average for December is -0.5. EWR….-0.1 NYC….-1.0 JFK…..+0.9 LGA…..-1.1 HPN….-0.4 BDR…..-1.3 ISP……-0.8 AVG…..-0.5

My guess is that the La Niña background is making the Northern Stream of the Pacific Jet faster than the models forecast back at the beginning of December. None of the models from three weeks ago had this big jet extension leading to the strong +EPO at the end of December. The other factor has been the +SOI spike in late November lead to the +PNA mismatch pattern for a La Niña background. So we get these ridges in the EPAC and WNA that keep getting weakened by the faster Pacific Jet than forecast.