GaWx

The 0Z GFS has a hard freeze in many inland areas and a light freeze all of the way to the SE coast and down to the FL/GA border on March 20th. Some of these would be near record lows. This is the coldest night of the entire run.

And now I can add the 12Z EPS to the colder trends for ~3/18-20. The last few runs have been trending colder for then and this latest one is the coldest of those. It doesn't yet get it as cold as the first one per the mean, but keep in mind that 3/18-20 is still 9-11 days out, when the spread around the mean is much larger due to more uncertainty. This run even has a weak wintry precip signal then in NC. Edit fwiw regarding the new Euro Weeklies: The weeks of 3/20-26 and 3/27-4/2 both have a significant chilly BN signal on them prior to warming up substantially afterward in time for baseball season and the Masters. So, big swings over the next month or so per this run.

I find it interesting that a second round of notable cold is now agreed on by the 12Z model consensus, including GEFS and GEPS, coming in ~3/18 fwiw. Edit: Also, it appears that there could be some snow in parts of N NC and maybe the mountains on Sunday (3/12).

That was definitely major for GA even though not mid to late March. Actually, this map is slightly underdone as there was 4-5"+ in much of the southside of Atlanta as I recall, even including 4.2" officially at KATL.

I think that the last mid to late March major winter event outside of the mountains in GA was the 1993 Storm of the Century. That even brought measurable snow way down here, latest on record! A moderate one was in 1987 and other big N GA ones were in 1983 (trace of sleet even here) and 1971. Plus there were others, including the big one of 3/11/1960. We're sort of overdue fwiw.

I appreciate the favorable mention! I think this stuff is fascinating to follow and hope many like myself learned some new things about SSWs. Regarding the sensible wx impacts from the major SSW: I'd say we actually are about to see this in the form of a lengthy much colder pattern than we have been seeing for most of this winter, especially when considering anomalies, starting this weekend and with no end yet in sight. The start of the colder pattern is admittedly about one week delayed from the average post SSW lag of ~two weeks due to the stubborn strong -PNA. However, no two SSWs are exactly alike and some have taken 3 weeks. Also, there often is warmth for 10-14 days after an SSW. It is still coming even if next week doesn't look quite as cold to me as late last week's EPS runs were suggesting. I mean we're still likely looking at several midmonth freezes at a minimum for many well inland areas. Nothing like that has happened in quite awhile and it has been very rare this entire winter. Whether or not there will also be the typically very hard to predict wintry precip of note accompanying any of this much colder air for any area at any point is still up in the air, of course. Getting notable wintry precip in the SE in mid to late March is never easy in any year, SSW or not. The main and most reliable effect from a downwelled SSW is the cold, which is what I've obviously been emphasizing. Wintry precip is always a bonus. The 2018 major SSW did lead to it in March in much of NC.

The 12Z 3/8 EPS mean has its coldest run yet for the coldest at RDU/ATL with 30/29 on 3/15. But this is still not as impressive to me as the 31/30 from both the 0Z 3/1 and 3/3 runs because those were for 10-11 days out (for earlier days). The 12Z 3/8 run is for only 7 days out, which is much more predictable than 10-11 days out and thus has a much smaller spread of the members. When the spread is bigger, it is more difficult for an ensemble mean of many members to show a strong anomaly since the tendency of the mean is to go towards climo as the model goes out further. Edit: In case there's confusion, the 0Z 3/1 run that had 31/30 wasn't for 3/15 but instead was for 3/12 (11 days out). Likewise, the 0Z 3/3 run that had 31/30 wasn't for 3/15 but instead was for 3/13 (10 days out).

A big part of the problem here and in other CFS Twitter posts is that the CFS is strongly cold biased, especially over snowcover. And the CFS often has snowcover when it isn't realistic. It has snowcover when it shouldn't because it is cold biased. So, it becomes a vicious cycle. Just as an example of this strong CFS cold and snow bias since RDU is one of the cities analyzed, I'll analyze all of the major 0Z 3/8 model runs for hour 132 at RDU (for 12Z on Monday 3/13): First of all, RDU NWS has a forecast for RDU Sun night of rain and a low in the lower 40s: "SUNDAY NIGHT RAIN LIKELY. LOWS IN THE LOWER 40S. CHANCE OF RAIN 70 PERCENT." The 0Z 3/8 CFS has ~1" of snowcover as of 12Z on 3/13 because it is cold biased and had a high near 36 the day before with wintry precip falling vs a high from the mid 40s to low 50s on the other models with rain/no snowcover on the other models: https://www.pivotalweather.com/model.php?m=cfs&p=snod-imp&rh=2023030800&fh=132&r=conus&dpdt=&mc= Partially as a result of this snowcover, the 0Z 3/8 CFS has RDU at an unrealistically cold 32 as of 12Z on 3/13: https://www.pivotalweather.com/model.php?m=cfs&p=sfct-imp&rh=2023030800&fh=132&r=conus&dpdt=&mc= Now, I'll compare the CFS' 32 with the 0Z 3/8 runs of the other major models and the already mentioned NWS for 12Z on 3/13 at RDU (they all have rain and no snowcover): Euro 49 UKMET 43 GFS 43 CMC 42 ICON 42 NWS 42 CFS 32

0Z GFS/CMC hard freezes many areas 3/15 and/or 3/16. GFS run is coldest in many runs. Also, 0Z ICON has upper 20s many areas 3/15. 0Z Euro, like 12Z Euro, isn't as cold as it has 30/31 for RDU/ATL on 3/15. Warmer Euro can be traced to it having it ~10 warmer in ND vs GFS/CMC (teens vs single digits) before the cold gets to the SE. 0Z UKMET 144 not quite as cold as the 12Z 156. 0Z EPS 31/31 and 0Z GEFS 31/34 for RDU/ATL on 3/15.

I just noticed that of the 12Z/18Z runs, the 12Z UKMET as of 0Z on 3/14 is the coldest in much of the SE with lower 2M temps, 850s, and 500 mb heights than the other models. That model implies that ATL could have a light freeze on 3/14, which would be a day earlier than other models.

Coldest for each EPS run RDU/ATL: 3/2 0Z: 35/36 on 3/14-15 3/2 12Z: 32/31 on 3/13 3/3 0Z: 31/30 on 3/12-13 (coldest run) 3/3 12Z: 31/31 on 3/13-14 3/4 0Z: 31/31 on 3/13-14 3/4 12Z: 33/34 on 3/13-15 3/5 0Z: 33/36 on 3/13-14 3/5 12Z: 35/37 on 3/14 3/6 0Z: 35/39 on 3/15 (warmest run) 3/6 12Z: 35/38 on 3/15 3/7 0Z: 33/35 on 3/15 3/7 12Z: 32/31 on 3/15 (GEFS 32/34) So, the last two EPS runs have come in significantly colder at ATL (by 7F) and slightly colder at RDU (by 3F). Regardless, I'm still not nearly as impressed with midmonth cold potential compared to how it looked 4-6 days ago. But the overall pattern could still change back to even colder and be similar to how it looked 4-6 days ago being that 3/15 is still 8 days away though I don't consider getting all of the way back to that look to be likely right now. Edit: The 18Z GFS was the coldest in a couple of days on 3/15 with low 30s ATL and upper 20s RDU. Since we're still getting sizable changes from run to run then, there's still a good bit of uncertainty midmonth.

Based on today's 12Z runs, I'm almost there! More specifically the 12Z EPS is similar to the prior run with coldest mean low of 35/38 on 3/15 at RDU/ATL vs 35/39 on the prior run, which means it's just about the same as the least cold run for mid-March so far. That implies probably no more than a couple of light freezes at most. Compare that to 31/30 on both the 3/1 0Z and 3/3 0Z runs, which had implied most likely a couple of lows well down into the 20s/hard freezes (remember that a 51 member ensemble mean is normally higher than the mean coldest of each member) and several highs only in the 40s. The 12Z GFS isn't cold for as long as the prior three runs. The prospect of a lengthy, notable cold period in mid-March is dwindling even more and may soon be on life support. Great/bad news for cold haters/lovers! No matter the final result, it has been fun following the prospects for a colder pattern ever since the mid Feb major SSW was looking likely in early Feb. I learned a lot more than I knew before about SSWs and the potential stratospheric effects on the troposphere and hope that some of y'all learned some new things, too. No matter what, it is evident that the current strong -NAO/-AO very likely resulted from it.

I didn't say it is over but that the chance for notable cold had dropped a lot based mainly on the warmer EPS trend and that a cold reversal was needed, stat! Since then, two GFS runs have come out with their longest mid March cold periods in the SE in many runs (3/14-18, 19). RDU to N GA have five straight freezes 3/14-18 along with 5+ days straight of sub 0C at 850 mb. Even down here there's 3.5 straight days of that. That's all quite impressive but they're just two GFS runs out 8-14 days, which get well into fantasy land. So, I'm still monitoring. I really need to see the Euro suite go back colder ASAP! The teens of March are put up or shut up time! That's still 7-13 days out. So, not over yet. Edit for 3/6 0Z EPS: coldest of run at RDU/ATL 35/39 on 3/15 making it even warmer than the prior run, which had been the warmest run for mid March, and thus not good if one prefers the longest/most intense cold. Comparing the 35/39 to the much colder two 31/30 runs of three and five days ago, respectively, doesn't exactly point to an increasing threat of long/strong cold in mid March. What's very odd is that while the GEPS has remained cold for midmonth, the GFS (cold now vs milder earlier) and Euro (milder now vs cold earlier) suites have literally switched positions! I bet that, alone, will cause some cold loving folks to favor the GFS suite over the Euro suite after previously favoring the Euro due to bias. I don't expect to do that and instead expect to continue to consider both with a little more weighting to the better performing Euro.

Here are the coldest mean lows for each EPS run back to the 2/28 0Z run for RDU/ATL, which shows why it is a bad trend for SE cold lovers: 2/28 0Z: 35/36 on 3/13-14 2/28 12Z: 34/34 on 3/14-15 3/1 0Z: 31/30 on 3/12 (coldest tied) 3/1 12Z: 32/31 on 3/12-13 3/2 0Z: 35/36 on 3/14-15 3/2 12Z: 32/31 on 3/13 3/3 0Z: 31/30 on 3/12-13 (coldest tied) 3/3 12Z: 31/31 on 3/13-14 3/4 0Z: 31/31 on 3/13-14 3/4 12Z: 33/34 on 3/13-15 3/5 0Z: 33/36 on 3/13-14 3/5 12Z: 35/37 on 3/14 (warmest run) This is a double whammy because we're now the closest to the crucial dates of 3/12-15 and the coldest means instead of cooling further, which would be expected on an ensemble with 51 members when getting closer if there's big cold coming, have warmed considerably. So, chances of the SE getting a notable cold period in mid March have come down considerably since March 3rd. Because it is getting closer and thus we're running out of time for big ensemble mean changes back in the colder direction (the closer to the days being forecasted, the smaller the average change), the ensembles are going to need to sharply reverse to colder again within the next 3 days or so or that's going to be the ballgame imo. The 3/2 12Z and 3/3 0Z runs did get significantly colder again after a significant warmup on the 3/2 0Z run, but we're three days closer. That reduces the chance considerably for another sizable cold reversal. Not good.

I see nothing on the 12Z 3/5 runs that would likely be encouraging for cold lovers. This is especially compared to some of the really nice much colder runs of 3/2-3. It is almost as if the 3/2-3 runs were troll runs lol. The Euro is especially noteworthy to me with this reversal. 12Z Euro goes all of the way to 12Z on 3/15 and coldest it gets through entire run is middle 30s in ATL and RDU. The 3/3 runs, which only went through 3/13, were well down into the 20s. Edit to add the 12Z EPS to the discouraging trend for SE cold lovers.

The concern for cold lovers, which I think is valid, is that the EPS runs from Feb 27th through 0Z March 3rd were mainly cold in the SE early on March 11th after turning colder on March 10th. But since the 12Z March 3rd run, they've been near normal or warmer for March 10th through early on March 11th. The latest EPS runs don't get it cold til either later on March 11th or on March 12th. So, a 1-2 day delay from the coldest runs. The GEFS was slower to consistently make it cold by then but became that way by the March 1st runs with colder by March 10th. The GEFS' coldest runs were 12Z/18Z of March 2nd. The March 3rd runs were not as cold but still on the cooler side for early on March 11th. Then they suddenly became much warmer (mainly AN) starting with the 0Z March 4th run and have been that way since. The most recent GEFS runs don't make it cold til either March 12th or 13th, a longer delay than the EPS...2-3 day delay vs the coldest runs. Is this another kicking the can or just a little delay? Any educated guesses? We'll know eventually. Aren't forecasting discussions fun? For those interested, go to Tropical Tidbits and look at the maps valid for 6Z on March 11th on the Feb 27th through 0Z/6Z of March 5th runs and see for yourselves as they're still there.

Yes, indeed, that's common in winter when the average wavelengths are longest. But what causes these west coast troughs (-PNA)? The record warm MC and La Niña are often touted as the main reasons. But also, there's speculation that the SER can be more than just a "forcee" but also do its own forcing at times thanks to the record high AMO of recent years.

I'm looking forward to a walk this evening with the lowest dewpoints in this region in quite some time. We had 80s today with only 40s dewpoints. This reminds me of a midsummer day in Colorado. If all days were similar in summer, I'd enjoy being outside a lot. But alas, these are very rare during summer. Regarding the persistent SER from what I've read from pro mets, it has been dominant the last few years due to a combo of La Niña, record warm MC SSTs favoring longer and stronger MC phases (though in Feb it was short) as well as MC-like forcing even when the MJO isn't there, and the very strong AMO making the Atlantic warmth act like a block in the form of a SER.

0Z GEFS based AO forecasts are still trending lower for Mar 6th and 10th (though at a slower pace as we get closer). The NAO has changed little and remains solidly negative through then. So, the models being less aggressive since yesterday with the change to a colder E US isn't due to the AO/NAO levels: For March 6th: 2/26 run: -0.1 2/27 run: -0.5 2/28 run: -1.3 3/1 run: -1.5 3/2 run: -1.9 3/3 run: -2.2 3/4 run: -2.3 For March 10th: 2/28 run: 0.0 3/1 run: -0.9 3/2 run: -2.0 3/3 run: -2.1 3/4 run: -2.4

0Z GEFS based AO forecasts still trending lower for Mar 6th and 10th though at a slower pace as we get closer For March 6th: 2/26 run: -0.1 2/27 run: -0.5 2/28 run: -1.3 3/1 run: -1.5 3/2 run: -1.9 3/3 run: -2.2 3/4 run: -2.3 For March 10th: 2/28 run: 0.0 3/1 run: -0.9 3/2 run: -2.0 3/3 run: -2.1 3/4 run: -2.4

Thanks for posting. What's amazing is that in addition to plentiful upslope snow for the mountains, the 18Z GFS has not one, not two, but three different systems (in a row) bringing significant wintry precip to parts of the SE (mainly NC to NE GA) during just the period March 13-18! I know it is just the operational GFS way out in the 11-15 but the prospects for the upcoming pattern are so way out there that this kind of thing is actually believable.

The 18Z GFS has a whopping 1060 mb surface high west of Hudson Bay at hour 138! Other runs and models have shown something similar for then. This kind of thing being repeatedly shown and it now being only 6 days out tells me that the pressure may not be overdone. For comparison, the record highest SLP for all of Canada in March is only 1 mb higher and that is way up in northern Nunavut! Also, the highest SLP on record for that area west of Hudson Bay is only ~1058 mb for all months, not just March. That alone is enough to tell me we're likely dealing with a very highly anomalous setup, which may lead to something historic for the US down the road in terms of cold and wintry weather. The map below has the all time record highest SLPs in Canada for all months. Note the 580 just N of NW Manitoba (1058 mb): https://www.wpc.ncep.noaa.gov/research/roth/SLPrecords.html

That's just an ordinary 1060 mb high at hour 138 of the 18Z GFS plunging down. Nothing to see here. Move along.

Not everybody wants the same thing. I try to leave my feelings out of these discussions as much as possible. For example, I don't want tornadoes anywhere near me, but don't complain if they're discussed. Discussing them doesn't make them occur and if anything it increases awareness of the danger. Anyway, the 12Z Euro was unreal but likely overdone as I said. Cold is often dry. Nothing unusual about that at all. Of course, that doesn't mean it will play out that way ultimately.

What Met1985 said. This is literally a wx forecasting discussion thread. Why wouldn't we discuss this stuff, especially with it being so different from almost the entire winter and potentially highly anomalous for mid March? It is what it is. Besides, this discussion may allow some people to prepare for the cold to mitigate potential damage.