GaWx

The 12Z EPS is the strongest yet with its +PNA with (in dm at H5) +17 peak anomaly W coast and +15 peak E US. This is much stronger than the 12Z GEFS, which itself was stronger than its own 0Z. The 12Z CMC is similarly strong to the 12Z EPS. My very rough guess as to the 12Z PNA peak on the EPS is +1.5. How strong is that? The peak on 12/26/22 was +1.3. In January, the only ones since 1950 with a daily PNA peak higher were during 2016, 2002, 1995, 1981, 1977, and 1961. So, that is fairly rare in January (~8% of them). The GEFS mean based AO forecast is for ~-1 for then. So, what if the EPS isn't exaggerating the PNA (which it sometimes does) and there is a +1.5 PNA peak combined with a -1 AO? I wonder whether or not that is a significant indicator for cold in the SE in any part of January that has that.

The 3 major ensemble means continue to show a -AO late in their runs along with at least a moderate +PNA. This combo in itself is a positive for a BN dominated period in the SE late this month. Next up: 12Z EPS.

The 0Z EPS held onto the pattern change to +PNA/much colder very nicely. This goes back 4 or so days of runs now. The west coast ridge is the strongest yet. The mean trough is west of the SE allowing for ample moisture and a potential chilly wet period late this month, especially when also considering the progged drop in the AO.

The late 12Z GEFS taken literally (yes I know this is way out in Lala land and thus has limited credibility) says don't give up on the rare chance (only 1 in 25 odds based on last ~140 years) for a 3"+ snow month at RDU during an overall mild winter month. It suggests two Canadian highs may come down late this month with a somewhat split flow bringing in Gulf moisture up against a solid +PNA between the two highs. Again, just fwiw but that's appropriate speculation imo for this thread. Yes, the Canadian source regions will be mild for awhile. But keep in mind that even with that, a solid +PNA is usually cold relative to normals in the South. So, regardless of whether or not there'd actually be a winter storm threat, a pattern change to much colder late month is looking more and more likely. Edit: 12Z EPS is still on board for the colder change/+PNA for ~1/21. The trough axis is slightly further west than the 0Z, which is a good H5 flow for a potential winter storm wet/chilly vs dry/cold).

The Dec 2022 AO ended up way down at -2.7. That's the second lowest AO on record back to 1950 with only 2009's -3.4 lower! Here is the subsequent Jan AO for the strongest Dec -AOs: Strongest Dec -AO/Jan AO 2009: -3.4/-2.6 6th strongest 2022: -2.7/??? 2010: -2.6/-1.7 top 20% 2000: -2.4/-1.0 1995: -2.1/-1.2 top 25% 2005: -2.1/-0.2 1976: -2.1/-3.8 strongest 1985: -1.9/-0.6 1950: -1.9/-0.1 1969: -1.9/-2.4 9th strongest 1952: -1.8/-1.0 The above tells me that there's a good shot at a sub -1 this month though that would probably require a sub -1 for the last week of this month. Implications for February? Not much per these analogs. Whereas a -AO could be argued to be slightly favored over neutral or +, Feb of 2011 went strongly + with a +1.6!

The 0Z EPS is still consistent with its prior runs as regards the developing +PNA late in the run. It actually looks a touch colder thanks to a few more members with pretty strong Arctic highs. Baby steps. Let's see if it remains over the next few runs. If it does and looks like a real deal, the next question will be about its longevity prospects.

Since 1950 at KATL, there have been five La Niña Febs at one BN or colder out of 21 (2009, 2006, 1996, 1985, and 1971). I decided to check out the PNA, AO, NAO, and EPO for these five to see if there were any tendencies: PNA: -1.0, -0.1, -0.3, -0.5, +0.1; AVG -0.4 AO: -0.7, -0.2, +0.2, -1.4, -0.9; AVG -0.6 NAO: +0.1, -0.5, -0.1, -0.5, +0.2; AVG -0.2 EPO: -24, -34, -20, +75, +85; AVG +16 The coldest Feb (2006) had a PNA of -0.1, an AO of -0.2, an NAO of -0.5, and an EPO of -34. Surprisingly, there was no +PNA tendency for the colder five. The most significant tendency was a -AO. Now I'll compare these averages of the indices for the five colder Febs with that for the warmer 16 Febs: PNA: +0.7, -0.3, -1.0, +0.8, -1.6, +0.5, -0.2, +1.1, -0.1, -1.1, -1.5, -0.2, -0.9, -1.0, -1.7, -1.0 AVG PNA for warmer: -0.5 AO: +1.6, -1.2, +0.1, -0.0, +1.6, +0.9, -0.6, +1.1, +0.5, +3.3, +1.7, +0.2, -0.5, -2.0, -1.5, +0.6 AVG AO for warmer: +0.4 NAO: +1.7, +0.1, +1.6, +0.4, +0.7, +0.7, +0.5, +1.7, +0.3, +2.0, +0.9, -0.6, -0.1, -1.1, -1.1, +0.4 AVG NAO for warmer: +0.5 EPO: -16, +50, -70, +48, -16, +86, -41, +34, +194, -232, +89, +44, +135, +51, +6, +101 AVG EPO for warmer: +29 --------------- The warmest Feb (2018) had a PNA of -1.0, AO of +0.1, NAO of +1.6, and EPO of -70. The average PNA was ~same for warmer vs colder. The average AO was significantly higher at +0.4 vs -0.6 while the average NAO was also significantly higher at +0.5 vs -0.2. The average EPO was just a little bit higher at +29 vs +16. Conclusion: The biggest drivers to determine warmth vs chill for La Niña Febs appear to be AO and NAO. If we could get a sub -1 AO, a sub -0.5 NAO, a PNA of no lower than -0.5, and an EPO no higher than +100, I'd feel there'd be a decent shot at a Feb at one BN or colder.

I prefer BN/lower than average dewpoints for all months of the year. Just add Feb in and this forecast would be perfect for me! The 12Z EPS progressed well vs the last 4 runs. So far so good for cold lovers! Let's see if an actual pattern change in January can hold on the EPS for a change. The prior two attempts (1/9-10 and midmonth) have apparently since failed with the overall mild pattern still not breaking on the models til at least 1/21 or so. The end of the 12Z run shows a solid +PNA with 10-12 dm AN H5 heights Pacific NW and 8-10 dm BN centered over the lower Midwest. BN at the ground is coming into the SE near the end. It still doesn't look very cold yet as there still aren't more than a few members with big Arctic highs coming down, but it is a start and would mean much colder than the preceding warmth if it were to verify.

Today's GEFS based forecast for day 14 has slight improvements vs yesterday's forecast from the perspective of someone who prefers that a colder pattern return in late January: - gets back to a weak +PNA after a -PNA that starts ~1/15 - AO rises to neutral but then falls back to moderately negative; very strong Dec -AO favors sub -1 AO in January. To have a good shot at that, a mainly sub -1 will likely be needed for 1/22-31 - The current moderate to strong +NAO falls back toward neutral Regardless of whether or not a BN dominated period returns at some point during the last 10 days of the month, January will almost certainly end up quite a mild month for the SE as a whole and much of the E US. It has a good shot at being one of the warmest nationwide since 1950 from a population weighted heating degree perspective. So, a near normal Dec and a mild January in the SE US. What will February bring? That's the big question. Any educated guesses?

The period since late Thursday has been very enjoyable for outdoors, specifically for walking in my case. I love these dews in the 30s to 40s along with highs in the 60s and lows in the upper 30s to low 40s. Near normal temps.

Yeah, it is hard to have a cold Feb in a Niña as only 15% of them have been cold. So, having the indices favoring cold is crucial. However, the MJO so far this season has not correlated well with tendencies. (I say tendencies because that's all they are.) This winter's strongest cold to date, 12/23-7, was during phase 5. Also, I just took a look at the MJO/temps during Niña Febs since 1975. What I discovered is that whereas cold Febs were infrequent as expected, I saw virtually no correlation of temps and MJO! For example, the six warmest Febs (1976, 1989, 2000, 2012, 2018, and 2022) had few or no days in phases 4-6! Moreover, two of the coldest Febs (1985 and 2009) had many days in those phases. This is enough data to tell me to not place much emphasis on the MJO to predict how this Feb will turn out. MJO data (see it for yourselves): http://www.bom.gov.au/climate/mjo/

Actually, most Niña February's have not been all out torches (that chance is exaggerated) even though that is a realistic possibility. When I say torch, I mean MA or 5+ AN. ~25% were torches at Atlanta. While that's a relatively high % for getting that warm, that still means 3/4 weren't actually torches. The much higher chance (2/3) is for either just A or N.

The end of the last few EPS runs have suggested a redeveloping solid +PNA/cooling SE, but the EPS did that for midmonth and Jan 9-10 and has since backed off of anything sustained. We all have seen this and it is well documented in the forecast threads. The new EPS looks good for potential cold in the SE near 1/21, but we've seen this song and dance before as the Pacific jet has been badly underestimated, quite possibly due to near record Pacific MC SSTs. So, then the EPS backs off. The EPS and others did very well 12/16-27 but that was an exception rather than the rule. Hopefully, ~Jan 21st will be the time for the first sustained pattern change to BN since 12/16-27, but I'm not betting on it at this time. More than likely it will either be just a short cold period or it will back off imho.

My post above based on 3rd year cold ENSO analogs said that the best bet for this DJF was for one mild, one near normal, and one cold. Dec was near normal. Jan, based on actual first week and forecasts through midmonth, will very likely be 7-9 AN for the first half. Combined with more warmth forecast by model consensus to dominate 1/16-1/21 means that Jan will almost certainly end up mild. So, that means one near normal and one mild for DJ. One might then think based on the original best bet to look for Feb to be cold. Maybe that will happen but La Niña climo will make that quite the challenge. I looked at the 34 La Niña Febs over the last 140+ years at ATL: - 18 AN - 11 NN - only 5 BN (15%): 2006, 1934, 1917, 1910, 1904 So, La Nina climo says bet heavily against a cold Feb. But my original best bet suggested one cold month. Will 2023 be one of the unusual cold La Niña Febs? Nobody knows. By the way, Feb of 1895 (3rd year cold ENSO) was record cold, but it was during cold neutral ENSO rather than La Niña.

To an extent it still does

12Z EPS is consistent with the model consensus trend away from a SE winter storm threat during 1/13-15. So, the January of 1933 scenario of warm practically all month surrounding an isolated two day cold that included a big RDU January 13th snow is now looking highly unlikely.

And in case anyone is wondering lol, the 12Z JMA (which also had a SE wintry threat at 12Z yesterday) joins the other models with a warmer solution and no widespread SE wintry threat.

Preliminary 12Z Euro maps suggest to me that this run will be more similar to today's non-wintry 0Z than yesterday's wintry runs. Let's see what it ends up showing.

12Z ICON has no SE wintry precip as the low is way too far north and there's no strong precursor low to lock in the cold from the Canadian high. This allows for way too much warmth ahead of it for SE wintry precip.

0Z EPS has similar change to the 0Z Euro with much weaker or no precursor surface low to help bring down and lock cold air in the SE. Thus, I expect this run won't be as wintry as the last one. Let's see.... Edit: confirmed to have less wintry precipitation

So far (hour 132), the 0Z Euro doesn't look as threatening as the 12Z, mainly because there's no surface low on the way to being the very strong low that the 12Z had that helped lock in cold in the SE ahead of the SE storm, itself. Also, H5 isn't so far as strong for the storm, itself. Going further out, the SE Canadian high is weaker and there's no precursor storm to help bring down cold air into the SE. The H5 for the potential SE storm is weaker, too. Edit: not surprisingly, a big fail on this run

However, this run doesn't have the 970 mb low off SE Canada that the 12Z had and is thus not nearly as cold ahead of the storm. The 12Z looked like it was heading toward a huge mess in the SE after the end. This run isn't as threatening for especially more southern areas.

But the 0Z CMC has the low back way far south on the Gulf coast and thus is back to wintry precip just like two, three, and four runs ago.

0Z GFS and ICON say no widespread SE winter storm 1/13-15. Surface low tracks further north/way too far north and thus way too warm.

To clarify when I say wintry, much of that pivotal accumulation appears to actually be ZR for GA/SC/NC with 850s well above 0C in many cases. This is a major icestorm signature vs the EPS having much more snow in the mix due to a much further south mean surface low track.