GaWx
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After a mild period through 3/5 (warmer than the prior run), today's Euro weeklies *fwiw* are significantly colder than the prior run thanks to a stronger and persistent -NAO throughout March. The highlight is in mid-March, when nearly the entire US is BN (3/13-19)(see image below). That is the coldest map for any March week of any Weeklies run to date. I understand if folks doubt this based on this winter so far. Some may laugh at this, which would be understandable. But the major SSW we just had along with, believe it or not, another reversal of the 60N winds at the 10 mb level now being forecasted for about one week from now along with the progged solid -NAO throughout March tell me that a legit cold period is believable. Would that be cold enough for wintry precip in parts of the SE if it were to occur? Who knows but mid March is often still early enough for a state like NC at the least:
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The models are doubting the freezing part other than near the VA border, but regardless, much of NC has a good chance to be 40-45 colder Saturday afternoon vs Thu afternoon! They're projecting dewpoint drops from 60s to 20s during Friday.
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- The closest I could find for GSP to something big is a 79 two days before the 5.1" of 2/26/1982. - But RDU had 6" on 2/18/1989, which was 15 days after an 83 - RDU came very close with 79s three times before other biggies: 7" on 3/24/1983 nine days after, 11.1" on 3/1-2/1980 7-8 days after, and 8" on 3/10/1934 three days after. - ATL: couldn't find 80 prior to a big one but they had 80 nine days before a small one (0.7" of 2/23/1989). - ATL: came very close with 79 seven days prior to 2.7" of 3/2/1980.
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Related to 2018, there have been 15 sub -0.90 March NAOs since 1950. All 15 respective AOs were sub -0.5, which is the threshold I use for -AO. Even more telling is that 12 of the 15 (80%) had a sub -1.0 AO. So, assuming the -NAO will actually persist through all of March, it will be interesting to see whether or not we end up with a -AO despite what current modeling shows.
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Keep in mind that the historically cold and winter storm filled March of 1960 had a PNA of -0.4! But it obviously would be best to have a higher PNA, which could still occur, plus March of 1960 was an extreme anomaly.
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March of 1960, a very cold and wintry March throughout the E US, had a strong -AO and a moderate -NAO. But what may surprise folks is that it had a -PNA (-0.40).
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The GEFS (used in CPC daily updated charts for indices) based NAO forecasts have gone markedly in the -NAO direction over the last 4 day period: - The 2/13 forecast had only 2 members (~6%) with a sub -0.25 NAO (what I consider -NAO) on 2/27. The mean for 2/27 on the 2/13 run was up at +0.5 vs 0 on the 2/16 run and -0.3 on the 2/17 run. - The # of members with a -NAO on 2/27 rose sharply on the 2/14 run from 2 to 8 members (~25%). - The 2/15 and 2/16 forecasts each had ~10 members (~33%) with a -NAO on 2/27. - The 2/17 forecast has ~15 members (~50%) with a -NAO on 2/27. - So, the progression from the 2/13 run to the 2/17 run has been 2 to 8 to 10 to 10 to 15 members with a -NAO on 2/27. - The 2/16 forecast had ~10 members (~33%) with a -NAO on 3/1 vs ~23 (~75%) on the new one - The 2/16 forecast had only 3 members (~10%) with a sub -1 NAO (what I consider a strong -NAO) on 3/1. The 2/17 forecast has 10 (~33%). - The 2/16 forecast had 12 members (~40%) with a +0.25+ NAO (what I consider +NAO) on 3/1 vs only 3 (~10%) on the 2/17. With this trend toward -NAO for 2/27-3/1+ remaining strong even on today's run as per the above, I wonder if the trend will continue to an even stronger -NAO over the next few days of runs as the models get an even better grasp of the downwelling of the major SSW that won't peak til tomorrow (as per the 10 mb 60N winds). Keep in mind that the main period in question (2/26+) is still 9+ days out, which for this uncommon situation (a major SSW with a dip to ~-15 m/s 10 mb winds at 60N) is probably not the easiest for the models to handle well. Also, are the models correct in keeping a +AO with this increasingly strong progged -NAO? Or is the modeled AO going to start dropping if the progged -NAO keeps strengthening? I especially wonder about this since a strong -AO is often the hallmark of a successfully downwelled SSW. What do others think? It remains to be seen whether or not the SE will cool down significantly by early March. Unlike the NE even if the AO ends up also falling, the SE is likely going to also need the PNA to rise (better Pacific than currently). So far, modeling is hardly budging there.
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The GEFS (used in CPC daily updated charts for indices) based NAO forecasts have gone markedly in the -NAO direction over the last 4 day period: - The 2/13 forecast had only 2 members (~6%) with a sub -0.25 NAO (what I consider -NAO) on 2/27. The mean for 2/27 on the 2/13 run was up at +0.5 vs 0 on the 2/16 run and -0.3 on the 2/17 run. - The # of members with a -NAO on 2/27 rose sharply on the 2/14 run from 2 to 8 members (~25%). - The 2/15 and 2/16 forecasts each had ~10 members (~33%) with a -NAO on 2/27. - The 2/17 forecast has ~15 members (~50%) with a -NAO on 2/27. - So, the progression from the 2/13 run to the 2/17 run has been 2 to 8 to 10 to 10 to 15 members with a -NAO on 2/27. - The 2/16 forecast had ~10 members (~33%) with a -NAO on 3/1 vs ~23 (~75%) on the new one - The 2/16 forecast had only 3 members (~10%) with a sub -1 NAO (what I consider a strong -NAO) on 3/1. The 2/17 forecast has 10 (~33%). - The 2/16 forecast had 12 members (~40%) with a +0.25+ NAO (what I consider +NAO) on 3/1 vs only 3 (~10%) on the 2/17. With this trend toward -NAO for 2/27-3/1+ remaining strong even on today's run as per the above, I wonder if the trend will continue to an even stronger -NAO over the next few days of runs as the models get an even better grasp of the downwelling of the major SSW that won't peak til tomorrow (as per the 10 mb 60N winds). Keep in mind that the main period in question (2/26+) is still 9+ days out, which for this uncommon situation (a major SSW with a dip to ~-15 m/s 10 mb winds at 60N) is probably not the easiest for the models to handle well. Also, are the models correct in keeping a +AO with this increasingly strong progged -NAO? Or is the modeled AO going to start dropping if the progged -NAO keeps strengthening? I especially wonder about this since a strong -AO is often the hallmark of a successfully downwelled SSW. What do others think? -------------------------------- 2/13/23 GEFS based NAO prog (see middle graph): look at how + it was at end (for 2/27) with only two members having a -NAO! 2/17/23 GEFS based NAO prog: look at the huge drop in it as of 2/27 vs just 4 days ago with half the members now having a -NAO that day (vs only 2 members 4 days ago) and almost all having a -NAO by 3/1:
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Are you saying that you don't think that the upcoming -NAO is largely due to the current major SSW? The timing is about right with it starting ~10 days after the SSW although I realize that a -AO usually accompanies the -NAO.
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The 0Z GEFS mean has what appears to me the strongest -NAO yet for any GEFS run for late month into early March. It gets started ~2/26 and, intensifies into the first part of March, and persists through the end (3/4). This could very well be mainly due to the current SSW downwelling into the troposphere, which typically takes at least 10-14 days to get started assuming it actually does work its way down.
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The 0Z GEFS mean has what appears to me the strongest -NAO yet for any GEFS run for late month into early March. It gets started ~2/26 and, intensifies into the first part of March, and persists through the end (3/4). This could very well be mainly due to the current SSW downwelling into the troposphere, which typically takes at least 10-14 days to get started assuming it actually does work its way down.
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Fwiw, the 18Z GFS has a 1060 mb high centered in NE SD a week from tomorrow morning with temperatures near -30 F, which would be close to the coldest on record so late in the season. That would be both an alltime record SLP for that region (old record 1056) and an alltime lower 48 Feb SLP record (old record 1059): https://www.wpc.ncep.noaa.gov/research/roth/SLPrecords.html Because that would be a record breaker, this run is stronger by a good margin than previous runs, and no other model has anything nearly that strong, the odds are high that this is too strong with the SLP and the cold. But, regardless, the overall model trends have been for a colder high plunging down into that area 2/23-4. Regarding this high's later effects on the SE (2/24-5), the H5 flow is currently not forecasted to allow it to get that cold (similar to what happened with the early Feb plunge into the NE). But it is expected to lead to much colder than the prior warm days. Some NC locations could have highs on 2/25 40 F colder than those of just two days prior.
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But just 2-3 days later, look out for potential CAD cold! All ensembles have a nice sized Canadian high coming down into the N Plains on 2/24-5 and then the chance for it to wedge down to the SE just afterward. Bathing suits followed closely behind by heavy coats?
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LMAO! If need be, I'm going down with the ship!
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Yeah, it is looking bad for cold lovers for the foreseeable future. If the SSW wild card doesn't produce a big change afterward, winter may very well be over.
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The 18Z GFS fwiw has a -NAO that gets started ~2/26, becomes quite strong the next day, and stays strong through 3/3. One calculation of the NAO is based on the normalized SLP anomaly for a location in the Azores less the same for a location in Iceland. The normal Azores SLP this time of year is ~1020 mb while it is ~1005 in Iceland. So, the Azores SLP averages 15 mb higher than that of Iceland in Feb. But for the period 2/27-3/3, the 18Z GFS has the Azores at only ~1015 (~1/2 SD BN) and Iceland way up at ~1037 (~2 SD AN). So, Iceland is forecasted by this run for this 5 day period to be at a whopping 22 higher than the Azores vs the norm of 15 lower. That is a very strong -NAO, likely way up near -2.5 per this SLP based method. The point of this post is to show that some runs are showing a very strong -NAO already starting near the end of Feb. The 18z GFS is likely way overdone, but it gives an idea of the potential. I expect the 0Z GFS won't be nearly as strong. Edit for 0Z GFS: For the same period, this run looks like it has a pretty neutral NAO with averaged Azores SLP a bit higher than Iceland.
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- The GEFS based NAO forecast has slowly been trending toward the negative direction. Yesterday's 0Z GEFS had 8 members (25%) of 31 with a sub -0.25 NAO by Feb 27th. Today's has ~10 of the 31 with sub -0.25 by 2/27. Also, the mean line already gets down to 0 on Feb 26th, which is two days earlier than yesterday's showed. - The end of Feb is still out 13 days, which is still far enough out to make the model skill in the troposphere low and much lower than that for the stratosphere at day 13 (as I showed recently). Thus, confidence in even ensemble means is still low, especially considering that a major SSW is now occurring. - The major SSW is underway with the latest GFS based forecast of 10 mb 60N winds for a dip to an impressive -16 m/s on 2/18 vs -15 on yesterday's forecast. So, the GFS is staying the course. - If there is going to be significant cooling in the E US as a direct result of this SSW, it would likely not get going until at least 10-14 days after it. That would mean starting near the end of Feb at the earliest. This idea on the timing hasn't changed at all since I started talking about this newest SSW. The warmth during it and following it has been expected.
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- The GEFS based NAO forecast has slowly been trending toward the negative direction. Yesterday's 0Z GEFS had 8 members (25%) of 31 with a sub -0.25 NAO by Feb 27th. Today's has ~10 of the 31 with sub -0.25 by 2/27. Also, the mean line already gets down to 0 on Feb 26th, which is two days earlier than yesterday's showed. - The end of Feb is still out 13 days, which is still far enough out to make the model skill in the troposphere low and much lower than that for the stratosphere at day 13 (as I showed recently). Thus, confidence in even ensemble means is still low, especially considering that a major SSW is now occurring. - The major SSW is underway with the latest GFS based forecast for a dip of 60N 10 mb winds to an impressive -16 m/s on 2/18 vs -15 on yesterday's forecast. So, the GFS is staying the course. - If there is going to be significant cooling in the E US as a direct result of this SSW, it would likely not get going until at least 10-14 days after it. That would mean starting near the end of Feb at the earliest. - This idea on the timing hasn't changed at all since I started talking about this newest SSW. The warmth during it and following it has been expected.
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That makes sense. Well, at least for a change the MJO just rocketed through the MC as it took only six days! Looking back at old charts during winter, five days is the fastest to fully traverse both phases 5 and 6 near this amplitude. It more often than not takes longer than six days.
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Based on what I've read by pro mets, near record warm SST anomalies in the Maritime Continent for the last few years have been favoring a SE ridge more by increasing the % of winter days in the MC of the MJO, increasing the magnitude of the MJO when in the MC, and causing tropical forcing patterns to sometimes be MC-like even when the official MJO wasn't in the MC. Therefore, the MC waters would need to cool relative to surrounding waters to reduce the domination of the SE ridge.
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Thank you and lmao! Keep in mind that for the first time this winter there's about to be a major SSW. Don't forget that the prior SSW, which is what got me excited and thus posting a lot more starting in mid January, ended up being a "minor" SSW and it lead to limited effects on the E US. Now this time we have the "real deal" about to occur, which increases the chance to lead to -NAO/-AO/colder E US/increased wintry wx in the E US as compared to the after effects of a minor SSW. Edit: I think it is important to not get so emotionally tied to the outcome. Edit 2: Even stronger indication of -NAO starting Feb 28th on 18Z GEFS vs 12Z
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Great questions: 1. We don't know if it will downwell. However, last week there was a S-G dipole max above 40 mb, which per Dr. Simon Lee, is a positive indicator for it to go down into the troposphere. And the models are suggesting a -NAO in two weeks. 2. Nobody knows the specifics on which areas of the world would be affected the most if it downwells. But if the -NAO that models are hinting at starting in two weeks actually occurs, that would obviously bode well for chances for cold in the E US. 3. The timing varies with each SSW but effects including an -NAO/-AO often start within 1.5-2 weeks of the SSW date, which would be either very late Feb or very early March. So, imho, IF there is to be a colder pattern from this, it would very likely start earlier than mid to late March.
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Same thing
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You're making a good point that can't be ignored. The models' cold bias is important to keep in mind. However, there's one key difference ahead that we haven't seen so far this winter: we now have the unpredictability of the sometimes favorable aftermath of a major SSW to consider. And this is looking to be a solid major with the latest GFS prediction of a -15 m/s 10 mb 60N mean wind now being predicted. While not near the record strongest, which is well down into the -20s, a -15 is no slouch and probably is stronger than the average major SSW by that measure.
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On the 0Z GEFS mean, 25% of the members had a sub -0.25 NAO (what I consider the threshold for an -NAO) already by Feb 27th. With the 12Z GEFS mean being even closer to a -NAO on Feb 27th than was the 0Z mean, I feel that the chance of a -NAO starting in very late Feb as opposed to waiting til the first days of March is increasing. Edit: Remember that nice looking -NAO on the Euro weeklies from last Thursday that then didn't look as good on yesterday's weeklies? The latest trends are heading back to that Thursday weeklies look.
