raindancewx

It is amusing watching Weather.com. Last night, they had 5-8", and 5-8" for Philadelphia on 12/16 (day and night respectively), then 8-12" and 5-8" before I went to bed, and now 3-5", and 5-8". I get that it is physically possible for Philadelphia to get well over a foot of snow in a La Nina December. We're running at record snow here in a La Nina since 1930-31, and close to record snow for the date (around double average for 10/1-12/12), and Boston had record snow in this part of the pattern in October. My research says higher snowfall La Ninas are much more likely in the NE above 150 ACE (1933, 1995, 2005, 2010, 2017) in the Atlantic, and also that low-solar La Ninas are better than the high-solar La Ninas for snow in Boston in particular (usually around 50"). I'm still skeptical of something that high so early for Philadelphia. It was interesting seeing 1947 on that list for NYC though for top late-Dec snows, that's been a very similar year for national snowfall anomalies since Fall. This is roughly your list of 6"+ daily snows in Philadelphia in December. This is all Decembers from 1931-2019. So forget La Nina, a daily total over a foot in December is just very rare in Philadelphia. The 1932 storm is the one I'd look at hard, as it's the closest date, with some similar ENSO/Atlantic/Solar things going on. Even 6.0"+ in a calendar day is like ~12 days in 90 years for Philadelphia (a couple more would show up if you did storms that started at 6 pm and lasted to 6 am the next day). 22.5" - 12/19/2009 12.4" - 12/24/1966 11.6" - 12/26/2010 10.9" - 12/12/1960 9.2" - 12/19/1945 9.0" - 12/30/2000 8.6" - 12/08/2013 7.4" - 12/19/1948 7.4" - 12/26/1947 7.3" - 12/17/1932 (ahem only other year with a cat 5 hurricane in Nov...) 7.0" - 12/05/2002 6.8" - 12/12/1982

The PDO went much more negative on the traditional Nate Mantua / JISAO index. https://oceanview.pfeg.noaa.gov/erddap/tabledap/cciea_OC_PDO.htmlTable?time,PDO First reading below -1.0 since August 2013. 2020-06-01T00:00:00Z -0.08 2020-07-01T00:00:00Z -0.38 2020-08-01T00:00:00Z -0.28 2020-09-01T00:00:00Z -0.7 2020-10-01T00:00:00Z -0.69 2020-11-01T00:00:00Z -1.12

No, I just added in the top snowfalls at the bottom for the time frame.

For the 28 years I consider La Ninas since 1930-31, Philadelphia has never had more than 12.8 inches of snow in December. Doesn't mean it's impossible, but any models showing that much or more are likely too high. I can see on Weather.com they have 5-8" during the day and then 5-8" during the night on 12/16. I would bet pretty strongly on a different evolution of the storm for the Northeast, just from knowing those stats in Philadelphia. One possible kink in the storm evolution would be the system out here that gets to the NE around 12/14, it's probably going to look somewhat different than depicted once it exits New Mexico. It seems to be trending south and stronger with time. For Boston, NYC, and Philly, the highest snow totals for 12/12-12/31 from 1931-2019 are: Boston: 25.4" (2007) New York City: 29.6" (1947) Philadelphia: 24.1" (2009)

The models haven't been loading today, but the 3-km NAM is in through Saturday at least, and at this range it should be quite good. More decent snows for Western Colorado and Northern New Mexico.

Very close to 1959 so far, although colder in TX, NM and the interior NW. These are the totals from the SW storm - a lot of the SW is now running well above average for precipitation totals winter to date.

12/10 system definitely had some juice with it. GFS is fairly optimistic for Saturday too.

The fetch of subtropical moisture with the current system over the Southwest ended a 110 day rainless period in Phoenix, and a 242 day rainless period in Yuma. There were periods of long dry weather over the Southwest in the 2017-18 La Nina, but it was centered more over Texas & New Mexico, rather than Arizona and Southern California. Amarillo went close to five months without measurable precipitation in 2017-18, and Albuquerque went 96 days without precipitation in that event. A lot of Arizona and New Mexico are now running near average for December precipitation totals, and I'd expect more systems by the end of the month.

That next system on 12/12 looks a bit more promising than it did yesterday along the NM/CO border. Still in the time frame of the most recent early December SOI crash. There should be something around 12/19 too - GFS already has a storm in that time frame, it's +10 days from the SOI crash recently.

The models now say the MJO is in phase five more than neutral. It seemed like there was a strong wave before but with multiple points of strength (which got interpreted by the RMM method as Neutral), and now it is more phase five alone. The Euro has weak progression in phase five for a while now. You can see the emergence into phase five more clearly on the Canadian side of the image.

I got some more snow today. Very light. The local NWS has a lot of trouble estimating lows, but they do real well for highs. But with subtropical moisture initially moving over dewpoints of 5-10, it isn't surprising we hung out in the low 30s for a while with some snow. This La Nina is infinitely better than the 2017-18 La Nina, when the city had three days with 0.1" or more from Oct-May. We're already at three for 10/1-12/10. No rain or snow in the whole state that December, this is already storm two, and I'd imagine there will be another around 12/19 given the SOI drop, and something will probably come through 12/26-12/30 too.

If you guys want a reason to be optimistic - I had the NAO negative in December. But the Isotherm guy thought it would be negative in January. It looks like December is at least partially right on my end, but that doesn't actually mean his January call is wrong.

I find the 3-km NAM is a pretty good model at short ranges, unlike it's exaggerating cousin the 12-km NAM. Pretty good snows depicted for Western Colorado and Northern New Mexico.

At this point - and I don't expect this to happen - the winter of 1959-60 may be a best case scenario for what 2020-21 can do. You can see a similar look to early December, with a warm NE Pacific, similar north Atlantic, and cold Nino zones - although not a La Nina. The 1959-60 winter gets very cold in 1960. That said, the Indian Ocean, waters by Australia, La Nina, and Western Pacific, not to mention solar activity (very high in 1959-60) and the cold November 1959 - are all very different even if December finishes similarly. The rest of December 1959 does look like what CPC is showing generally for later in the month. I think a warmer version of the January map is possible, but I wouldn't bet on it either.

I went with 0.1-0.4 inches of precipitation for Albuquerque with the coming system. Pretty decent snows are likely for the mountains. There was another SOI crash last night of 10 points, so I'll be on the lookout for a system around 12/19. Something tends to come through in the 12/26-12/30 time frame in NM/CO, so I'd expect two more decent storms after this one as a minimum, so we'll have a shot a wetter than average December.

Next shot at something down here is likely around 12/19. Date Tahiti (hPa) Darwin (hPa) Daily Contribution 30 day Av. SOI 90 day Av. SOI 9 Dec 2020 1013.79 1009.05 5.29 9.93 8.11 8 Dec 2020 1015.26 1010.05 7.73 9.84 8.05 7 Dec 2020 1015.79 1009.15 15.15 10.02 7.90 CPC is pretty confident in a warm period starting 12/17. Texas had near to record warmth in a lot of spots today. Going forward, you'll see some days like that in other parts of the south to help burn off the cold.

Whenever there is some huge surge of subtropical moisture, my default is the 3-km NAM and Euro will eventually have the right sense of the totals, but not until within 48 hours of the event finishing. Until then, I just assume 0.2"-0.6" of liquid equivalent for the NM valleys for each full day of precipitation until proven otherwise, higher or lower.

https://www.weatherbell.com/final-winter-2020-21-forecast Weatherbell updated their winter outlook. This image from above was right, they just labelled it wrong. Weatherbell analog blend changed the from winters starting in 1973, 1988, 2010 (x2), to winters starting in 1973 1974 1975 1995 (x2) 1998 (x2) 1999 2005 2007 (x2) 2010 2017 It's a warmer look. But would support a cold West / hot Southeast in Jan-Feb.

Make it happen Chinook. You guys in Colorado will get at least some snow out of this Thu-Sat period.

The 12-km NAM, Euro, GFS and Canadian now have a pretty big precipitation event out here 12/10-12/12, in the correct time frame after the big SOI crash around 12/1. 3 Dec 2020 1013.45 1007.55 11.31 8.63 7.78 2 Dec 2020 1014.81 1008.05 15.78 8.74 7.73 1 Dec 2020 1014.91 1007.10 21.23 8.99 7.70 30 Nov 2020 1013.51 1006.20 28.18 9.24 7.69 This map from WPC looks pretty good to me. I'd go 0.20-0.60" for most of New Mexico, higher mountains and SW, lower SE (nothing to 0.20"). Arizona could easily get a winter of precipitation in the SE part of the state. This is what the Euro has for days 2-4 (Thursday-Saturday). For a lot of places in the SE AZ / SW NM zone, these totals would be a winter of precipitation if they verified. The 12-km NAM is a lot more optimistic, but I don't really buy these totals, yet.

That's probably the most moisture I've seen a model show in Arizona in at least six months. Not thrilled to see the Euro changing the location of the moisture every 12 hours, but the trend in increasing moisture is nice to see. A lot of it will likely be snow.

I am pretty optimistic for you guys for March. My NAO indicator seems to have worked at least a little bit for December, with the index at least nominally negative. I think the blend I had said -0.2 for December. It had March more negative than December. Nino 4 has been following 2007 (but warmer) all year. If it continues to do that, you do have a strong wet signal for the Ohio Valley in February, and then a weak wet signal for New England. Should still be cold enough for a lot of snow. Nino 4 is likely to finish Dec/Jan the coldest it has been in 5-10 years, maybe more. The warm signal from a cold Nino 4 January doesn't really extend into all of New England either in February. It took until Dec/Jan but the 2007-08 La Nina did see Nino 4 eventually drop below 27.0C, which is very cold for that area, even by 1951-2010 averages. It should happen in the next six weeks. You'll know the effect has kicked in if you see a big snowstorm in say, Kentucky, in mid-January or February. Also a cold signal for you guys in Spring. The correlations aren't super strong, but with Nino 4 set to run ~2C colder in the next few months compared to last year, it should have a big imprint on the patterns. We're not getting jack out here once the Spring correlations kick in, winter is less of an issue though

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