GaWx

Don, With NYC at +2 for 3/1-11, it will be an even bigger challenge for JB to get his -5 for 3/1-4/15. The latest forecasts seem to suggest that NYC may cool off enough to allow 3/1-20 to almost get back down to right at the 1991-2020 normal. But even if so, that would still require -9 for 3/21-4/15. The 3/21-4/15 cold in both 1956 and 1940 would get him close (to a -4). The last time it almost got to -9 for 3/21-4/15 was right at the turn of the century, the 20th century! That 1900 cold would be just cold enough for him to get his -5 with rounding. The last one to get him -5 without rounding was 1888. So, his chances continue to drop.

Fantasyland 12Z GFS Miller A GOM snowstorm very late 3/21 to early 3/23. Almost entire N half of GA to NW SC to W 2/3 of NC to most of VA gets several inches. Shades of 3/24/1983. Mainly for entertainment but there is still that second batch of cold coming per model consensus and it looks to be coming in with more moisture to play with instead of plunging down like the first one in a few days. This is the extreme of extremes of what can happen. 12Z ICON also has a GOM Miller A but two days earlier. Some snow from that west of the SE US on 3/19.

Per RDU: AS OF 400 AM SUNDAY... ...WINTRY MIX EXPECTED THIS MORNING AND INTO THE EARLY AFTERNOON ACROSS THE NORTHERN PIEDMONT COUNTIES... ...HAVE INCREASED SNOW/SLEET AMOUNTS SLIGHTLY ACROSS THE FAR NORTHERN PIEDMONT COUNTIES... ..IMPACTS STILL EXPECTED TO BE LIMITED CONFIDENCE CONTINUES TO INCREASE THAT A PERIOD OF MIXED WINTRY PRECIP WILL BEGIN LATER THIS MORNING AND COULD LINGER THROUGH MIDDAY/EARLY AFTERNOON ACROSS THE NORTHERN PIEDMONT/I-85 CORRIDOR, BEFORE EVERYTHING CHANGES OVER TO LIQUID RAIN. OVER THE PAST 24-36 HOURS, MODELS ARE LESS SUPPRESSED WRT TO STRONGER MOISTURE TRANSPORT ACROSS THE REGION. THUS, LIQUID EQUIVALENT HAVE INCREASED ACROSS NORTHERN PORTIONS OF THE FORECAST AREA, WITH FAIRLY UNIFORM LIQUID EQUIVALENT OF 0.50-0.66" EXPECTED ACROSS CENTRAL NC. HOWEVER, THE STRONGER SOUTHERLY TRANSPORT FARTHER NORTH CENTRAL NC ALSO MEANS A MODEL SIGNALING OF SLIGHTLY WARMER TEMPS ALOFT, WHICH INTRODUCES INCREASING CHANCES FOR MORE MIXED PRECIP, WHICH INCLUDES SLEET MIXING IN WITH THE RAIN AND SNOW AT TIMES. A FORECAST OF MIXED P-TYPE INHERENTLY INTRODUCES A GOOD DEGREE OF UNCERTAINTY WRT TO ACTUAL SNOW/SLEET AMOUNTS. WITH THAT SAID, THE LAST COUPLE OF RUNS OF THE HI-RES ENSEMBLE PROBABILISTIC FORECASTS CONTINUE TO SHOW LIKELY TO CATEGORICAL CHANCES OF AT LEAST A HALF INCH OF SNOW/SLEET ACROSS THE FAR NORTHERN, I-85 CORRIDOR COUNTIES. WHILE IT'S POSSIBLE TO SEE SOME LOCALIZED HEAVIER AMOUNTS NEAR THE VIRGINIA BORDER(PERSON COUNTY), THE RECENT RECORD WARMTH HAS RESULTED IN VERY WARM GROUND TEMPS. THUS, ACCUMULATIONS SHOULD LARGELY BE CONFINED TO ELEVATED SURFACES SUCH AS ROOF TOPS AND DECKS, WITH LITTLE TO NO IMPACTS ON ROADS AND BRIDGES. * Edit at 5:40 AM: sleet reported in W portion of Triad

How much does sun angle matter anywhere and at any time of year if there is a thick overcast? Anyone know?

From RDU: A COMBINATION OF TOP-DOWN AND FORECAST PARTIAL THICKNESS/TRENDS NOMOGRAM FAVOR THE PRESENCE OF A DEEP/NEAR FREEZING ISOTHERMAL LAYER ALOFT AND MELTING OF SNOW IN A SHALLOW, ABOVE FREEZING SURFACE LAYER THAT WILL LIKELY SUPPORT A LARGER AND MORE PROLONGED AREA OF RAIN/SNOW MIX OR EVEN ALL SNOW ACROSS THE NRN NC PIEDMONT SUNDAY MORNING. THAT APPEARS TO BE THE CASE AFTER AN INITIAL PERIOD OF LIGHT RAIN INCOMPLETELY SATURATING/COOLING THE SUB-CLOUD LAYER AT ONSET, THEN BOTH WARMING AND DRYING ALOFT, AND INSOLATION FROM A MID- MARCH SUN ANGLE, DURING THE AFTERNOON, WHICH WOULD FAVOR A CHANGEOVER TO RAIN/DRIZZLE THAT WILL CONTINUE THROUGH SUNDAY NIGHT. ***HOWEVER, MOST/ALL SNOW SHOULD MELT AS IT FALLS*** OWING TO 1) VERY WARM SOIL TEMPERATURES THAT HAVE RANGED FROM MID 40S TO LWR 50S F THE PAST COUPLE OF MORNINGS TO 50S TO NEAR 60 F THIS AFTERNOON, 2) ABOVE FREEZING AIR TEMPERATURES EXCEPT FOR PERHAPS A 2-3 HR PERIOD WHEN HEAVIER PRECIP RATES WOULD FAVOR "MELTING OUT"/ DIABATICALLY- COOLING TO 32-33F AN OTHERWISE ~750-1000 FT AGL ABOVE FREEZING SURFACE LAYER, AND 3) A MID-MARCH/LATE WINTER SUN ANGLE THROUGH THE MORNING-MIDDAY HOURS. ANY ACCUMULATION OF PERHAPS A TENTH TO TWO WOULD BE LIMITED TO ANY PERIOD OF ALL SNOW AND CONFINED TO ELEVATED SURFACES NEAR THE VA BORDER.

Nice beautiful, cool autumn .....ooops, I mean late winter day here. Perfect for a daytime walk with sunshine, 60s, dews in the low 40s, and a pretty sunset as a bonus!

I'm excited for you guys to see a nice fall of snow (mixed with rain). As long as expectations of sticking are kept in check (like expecting none), this looks like fun for y'all.

The 12Z GFS has RDU down to 34 and 100% RH early tomorrow afternoon with rain/snow mix falling. I'm not looking for sticking (though some sticking on nonpaved surfaces in some areas possible), but seeing some flakes falling is the main potential.

The 12Z GFS has RDU down to 34 and 100% RH early tomorrow afternoon with rain/snow mix falling. I'm not looking for sticking (though some sticking on nonpaved surfaces in some areas possible), but seeing some flakes falling is the main potential. Check out the new thread for this.

Not having seen this new thread, I just had posted this in the main one: "More immediately, there's a good chance for snow falling for part of tomorrow for especially northern NC! It would be a challenge for it to stick but not impossible in some areas for some stickage. But it just falling would be quite notable, especially for mid March and this winter's lack of it." Good luck to you guys!

The 6Z GEFS/0Z GEPS has a moderate/modest Miller A ish Gulf storm signal for late 3/19 through 3/20, but that's nearly in fantasyland. Should there be one, it could get interesting for some areas (especially NC) for wintry precip ~3/20 considering how cold the preceding airmass might be. There have been some wintry precip events during 3/19-21 looking back in history. More immediately, there's a good chance for snow falling for part of tomorrow for especially northern NC! It would be a challenge for it to stick but not impossible in some areas for some stickage. But it just falling would be quite notable, especially for mid March and this winter's lack of it.

The 18Z GFS along with recent runs has near record cold over the N Plains 3/17-18. It may be overdone due to snowcover assumptions, but regardless this shows the potential for a very potent cold airmass to come out of Canada then. I'm mentioning this only because most models then have this airmass plunge into the SE shortly after (3/18-20). Some runs, including this one for 3/19-20, have been showing the coldest of this airmass to be colder than the prior one for the SE.

Thanks for that info. I didn't mean in quite a few Marches. I meant it has been quite awhile since we had that (several freezes close together) this winter. The last time Atlanta and Athens had two consecutive freezes was way back in late January, for example.

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.